M Al Kharboutly

G Veryasov

P Gaval

A Verchere

C Camp

E A Quadrelli

J Galipaud

B Reynard

M Cobian

T Le Mogne

C Minfray

Mo(VI) dithiocarbamate with no pre-existing Mo-S-Mo core as an active lubricant additive

Mo(VI) dithiocarbamate with no pre-existing Mo-S-Mo

core as an active lubricant additive

Introduction

Energy saving has become a critical societal stake that stimulates drastic technological changes in the transportation and energy sectors [START_REF] Holmberg | The impact of tribology on energy use and CO2 emission globally and in combustion engine and electric cars[END_REF]. In the field of tribology, reducing friction losses over extended durations is a key challenge. Several solutions have been developed, in particular Transition Metal Dichalcogenides (TMDs), like MoS2, have been known as efficient solid lubricants for more than fifty years [START_REF] Singer | Hertzian stress contribution to low friction behavior of thin MoS2 coatings[END_REF][START_REF] Donnet | Super-low friction of MoS2 coatings in various environments[END_REF][START_REF] Fleischauer | Comparison of oxidation and oxygen substitution in MoS2 solid film lubricants[END_REF]. MoS2 is used in tribological contacts as coating but it can also be formed in situ through tribochemical reactions from molecular lubricant additives [START_REF] Spikes | Friction Modifier Additives[END_REF][START_REF] De Feo | Ageing impact on tribological properties of MoDTC-containing base oil[END_REF][START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Grossiord | MoS2 single sheet lubrication by molybdenum dithiocarbamate[END_REF][START_REF] Komaba | Kinetic study on lubricity of MoDTC as a friction modifier[END_REF][START_REF] Okubo | In Situ Raman Observations of the Formation of MoDTC-Derived Tribofilms at Steel/Steel Contact Under Boundary Lubrication[END_REF]. Among molecular lubricant additives effective under boundary lubrication for steel-steel contacts, the "all-in-one" types of molecules, that is the ones which provide the molybdenum and the sulfur atoms within a single molecule, are commonly used. Molybdenum DiThioCarbamates (MoDTC) are representative examples of these types of molecules (Figure 1).

Even if the mechanism of action of MoDTC has been widely studied, [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Grossiord | MoS2 single sheet lubrication by molybdenum dithiocarbamate[END_REF][START_REF] Komaba | Kinetic study on lubricity of MoDTC as a friction modifier[END_REF][START_REF] Deshpande | Tribocatalytic behaviour of a TiO2 atmospheric plasma spray (APS) coating in the presence of the friction modifier MoDTC: a parametric study[END_REF][START_REF] Onodera | Development of a quantum chemical molecular dynamics tribochemical simulator and its application to tribochemical reaction dynamics of lubricant additives[END_REF][START_REF] Peeters | Characterization of Molybdenum Dithiocarbamates by First-Principles Calculations[END_REF][START_REF] Rai | Transient processes of MoS2 tribofilm formation under boundary lubrication[END_REF][START_REF] Komaba | The effect of temperature on lubrication property with MoDTC-containning lubricant : temperature dependence of friction coefficient and tribofilm structure[END_REF] the understanding of the chemical pathway allowing the generation of MoS2 sheets from the MoDTC molecule is still attracting substantial interest because of the potential insight in novel molecule design. Concerning the elementary steps of MoDTC decomposition, several approaches are found in literature. Initially, only the sulfur atoms present in the core of dimeric molecules, that is as bridging atoms across molybdenum two centers as in (R2NCS2) [Mo(O)(µ-S)2Mo(O)] (S2CNR2) were considered able to contribute to the formation of MoS2 sheets [START_REF] Grossiord | MoS2 single sheet lubrication by molybdenum dithiocarbamate[END_REF][START_REF] Peeters | Characterization of Molybdenum Dithiocarbamates by First-Principles Calculations[END_REF].

Later on, it was also proposed that the sulfur atoms from the thiocarbamate ligand, (R2NCS2)

[Mo(O)(µ-S)2Mo(O)] (S2CNR2) could be involved in the generation of MoS2 sheets, either by prior linkage isomerism, as in (R2NCOS) [Mo(S)(µ-S)2Mo(O)] (S2CNR2) [START_REF] Deshpande | Tribocatalytic behaviour of a TiO2 atmospheric plasma spray (APS) coating in the presence of the friction modifier MoDTC: a parametric study[END_REF][START_REF] Onodera | Development of a quantum chemical molecular dynamics tribochemical simulator and its application to tribochemical reaction dynamics of lubricant additives[END_REF] or by cleavage of carbon-sulfur bonds of the ligand, schematized as (R2NC) [(S2Mo(O)(µ-S)2Mo(O)S2)] (CNR2) [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Coffey | Molybdenum(VI) imidodisulfur complexes formed via double sulfur-carbon bond cleavage of dithiocarbamates[END_REF]. All these approaches suggest that core [MonSm] fragments called MoSx intermediate products [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF] are needed at some stage in generating well-organized MoS2 lamellar sheets during friction.

In this work, the performance of a molecular precursor MoDTC 1, containing only one molybdenum atom and no pre-formed core [MonSm], is compared with that of a well-established "all-in-one" MoDTC 2 molecule, which contains preformed [Mo(µ-S)2Mo] core classically used in engine lubrication. We show that the tribological properties of molecule MoDTC 1 are competitive with those of well-established MoDTC 2 and show better performances at high contact pressures.

Experimental methods

Lubricant additive synthesis

Two different MoDTC molecules were synthetized (Figure 1). MoDTC 1 has no pre-existing core [Mo-(µ-S)-Mo], nor terminal sulfur atoms (Mo=S) and is in oxidation state Mo(+VI).

MoDTC 2 contains the [Mo(µ-S)2Mo] sulphided core and is in oxidation state Mo(+V). Molecule MoDTC 2 is used here as a representative example of the classical molecules used in engine lubrication.

The lubricant composition is prepared in a beaker on a stirrer hot plate by adding a weight percentage of 0.3 w% of each molecule of MoDTC in the synthetic base oil PAO4 at a temperature of 50°C, whilst stirring for 30 min. Solutions were transparent after this mixing procedure showing the good solubility of the additives in base oil.

Note that the atomic concentration of Mo in lubricants containing MoDTC 1 or MoDTC 2 is in the same range since the contribution of Mo to the mass of MoDTC1 (23 wt%) is extremely close to that in MoDTC2 (21 wt%). Thus the difference in molybdenum atomic content in both lubricants is not significant enough to explain potential differences in tribological behaviors.

The content of sulfur atoms is a bit larger in MoDTC 1 than in MoDTC 2. As both molecules don't have the same atomic S/Mo ratio (4 for MoDTC 1 and 3 for MoDTC 2), it is anyway not possible to have the exact same content of molybdenum and sulfur in each lubricant.

• Synthesis method of MoDTC 1:

The complex was prepared according to literature [START_REF] Farmer | Lubricating compositions containing sulfurized oxymolybdenum dithiocarbamates[END_REF]. Under air, ammonium molybdate tetrahydrate [(NH4)6Mo7O24] • 4 H2O (3.20 g, 2.59 mmol) was dissolved in 25 mL of distilled water. 3.05 g (13.54 mmol) of sodium diethyldithiocarbamate trihydrate, dissolved in 10 mL of distilled water, were then slowly added over the course of 30 min under vigorous stirring, during which time the colour of the solution gradually turned from yellow to orange and then red with the formation of a precipitate. The red precipitate was isolated on a fritted glass connected to a vacuum Erlenmeyer flask then washed with distilled water (3x10 mL). After washing, the sample was dried under vacuum. Yield, 2.10 g (56 %). Anal. Calcd for MoC10H20N2O2S4 (MW= 424.48 g.mol -1 ): C 28.30, H 4.75, N 6.60; Found C 27.94, H 5.09, N 6.51%.

• Synthesis method of MoDTC 2:

The complex was prepared according to the literature [START_REF] Farmer | Lubricating compositions containing sulfurized oxymolybdenum dithiocarbamates[END_REF]. Ammonium molybdate tetrahydrate [(NH4)6Mo7O24] • 4 H2O (4.22 g, 3.41 mmol) was dissolved in 20 mL of distilled water. 30 mL of dimethylformamide (DMF) were poured into the solution and stirred vigorously. Then 9 g of dioctylamine (37.27 mmol) and 30 mL of DMF were added at room temperature. The reaction medium was cooled in an ice-water bath and finally, 3 mL of carbon disulphide CS2 (49.65 mmol) were added dropwise over a period of 15 min. The addition of CS2 resulted in a gradual colour change of the suspension from white to yellow, then orange and finally purple-red. The solution was then refluxed at 105°C under argon until the colour changed from purple-red to green. The resulting viscous residue was collected on a fritted glass connected to a vacuum Erlenmeyer flask, then washed with acetone (3x50 mL).

After washing, a yellow precipitate appeared. After washing, the sample was dried under vacuum Yield, 1.10 g (10 %). Anal. Calcd for Mo2C34H68N2O2S6 (MW = 921.19 g.mol -1 ): C 44.33, H 7.44, N 3.04; Found C 41.13, H 7.10, N 3.78%.

The XPS characterizations of MoDTC 1 and MoDTC 2 are reported in supplementary information (SI 1) and corroborate the structures reported in Figure 1.

Figure 1: Synthesis of the MoDTC derivatives 1 and 2 (only one linkage isomer represented).

Friction experiments

Balls and flats counterfaces used for tribotests are both in steel AISI52100. The hardness is around 800 HV. The roughness of the flats and balls are respectively Ra = 12 ± 5 nm and Ra = 40 ± 1 nm. The base oil used is a PAO4. A reciprocating ball-on-flat tribometer [START_REF] Guibert | Design and manufacturing of a reciprocating linear tribometre[END_REF] was used for tribotests. No external sulfur supply was added to the system. During the test, normal force (strain gage), tangential force (piezoelectric sensor and bi-blades) and temperature (thermocouple type K) were recorded.

Standard conditions of boundary lubrication were as follow: average sliding speed of 56 mm/s, temperature of 100°C and an initial Hertz maximum pressure of 1 GPa (Load of 17 N).

For other tests, the temperature was varied as well as load in order to vary the initial Hertz maximum contact pressure. The contact conditions are summed up in Table1. As tests were repeated at least four times, average steady state friction coefficients and their standard deviations were calculated from the values obtained from the different repeatability tests.

After each test, wear scar diameters were measured by optical microscopy.

Characterization of tribofilms

All samples were cleaned with n-heptane for 10 min in ultrasonic bath before characterization by XPS, Raman spectroscopy and FIB-TEM.

XPS analyses were carried out on a PHI 5000 Versaprobe II apparatus from ULVAC-PHI Inc.

A monochromatized AlK source (1486.6 eV) was used with a spot size of 20 µm. A charge neutralization system is used to limit charge effect. The remaining charge effect was corrected fixing the C-C bond contribution of C1s peak at 284.8 eV. Spectra of Mo3d5/2 and S2p regions were obtained using a pass energy of 23.5 eV. All the peaks were fitted with CasaXPS software using a Shirley background. Quantification was carried out using the transmission function of the apparatus and angular distribution correction for a 45° angle. Sensitivity factors were extracted from [START_REF] Wagner | Empirical atomic sensitivity factors for quantitative analysis by electron spectroscopy for chemical analysis[END_REF], they integrate cross section and escape depth correction.

Raman spectra were collected using a LabRam HR800 spectrometer from Horiba Scientific.

Two different laser wavelengths were used 785 nm (MoDTC 1) and 532 nm (MoDTC 2 and tribofilms). Typical laser powers of 5 mW were used.

FIB sections were performed across tribofilms for TEM observations. A platinum layer was deposited as a protective layer on tribofilm before nanomachining with Ga + ions. Subsequent thinning was carried out to ensure that lamella is thin enough for TEM observations.

The TEM observations were carried out on JEOL2010F equipped with EDS and operating at an accelerating voltage of 200 kV.

Tribofilm compositions could slightly vary in terms of composition regarding position on the wear track. Because of reciprocating oscillatory motion, the contact conditions are most severe at the extremity of wear tracks, where the change of friction directions occurs. The Raman and XPS analyses where done close to these area as the amount of MoS2 was the most important.

Then, some discrepancies in the results between XPS and Raman analyses could be related to the fact that the size of the analyzed area was slightly different for each technics (20 µm of diameter for XPS analyses and around 1 µm for Raman analyses) and to the fact that XPS spectroscopy is more surface sensitive than Raman. nm on HRTEM images, consistent with MoS2 lamellar sheets observed in tribofilms [START_REF] Komaba | The effect of temperature on lubrication property with MoDTC-containning lubricant : temperature dependence of friction coefficient and tribofilm structure[END_REF].

Results

Low friction with

Figure 3: a) and b) TEM images of FIB-cross section of tribofilm (flat) obtained with MoDTC 1 after 18 000 cycles of rubbing under standard contact conditions c) EDX spectrum recorded on the MoS2 sheets area.

The XPS analysis of the Mo3d and the S2p transitions carried out on the tribofilms obtained after 18 000 cycles with MoDTC 1 at 100°C is reported in Figure 4; details of the fits can be found in Table 2.

Two main molybdenum contributions are found: one with a Mo3d5/2 peak around 232.1 eV (Mo3d (C), pink) corresponding to a Mo(+VI) environment related to MoO3 or FeMoO4 [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Deshpande | Tribological behaviour of TiO2 Atmospheric Plasma Spray (APS) coating under mixed and boundary lubrication conditions in presence of oil containing MoDTC[END_REF][START_REF] Spevack | A Raman and XPS investigation of supported molybdenum oxide thin films. 2. Reactions with hydrogen sulfide[END_REF] and another one with a Mo3d5/2 peak around 229.0 eV (Mo3d (B), blue) attributed to a Mo(+IV) environment that could correspond to MoS2 [START_REF] De Feo | Ageing impact on tribological properties of MoDTC-containing base oil[END_REF][START_REF] Deshpande | Tribological behaviour of TiO2 Atmospheric Plasma Spray (APS) coating under mixed and boundary lubrication conditions in presence of oil containing MoDTC[END_REF] or MoS3 [START_REF] Benoist | X-ray photoelectron spectroscopy characterization of amorphous molybdenum oxysulfide thin films[END_REF][START_REF] Fu | 2D amorphous MoS3 nanosheets with porous network structures for scavenging toxic metal ions from synthetic acid mine drainage[END_REF] The characteristic Raman analyses of the tribofilm obtained at 100°C under standard conditions with MoDTC 1 are presented in Figure 5. The two MoS2 characteristic Raman shifts 𝐸 2𝑔 1 and 𝐴 1𝑔 at 373 cm -1 and 405 cm -1 , respectively, are detected, in agreement with Raman analyses of MoDTC tribofilms [START_REF] Okubo | In Situ Raman Observations of the Formation of MoDTC-Derived Tribofilms at Steel/Steel Contact Under Boundary Lubrication[END_REF][START_REF] Rai | Transient processes of MoS2 tribofilm formation under boundary lubrication[END_REF][START_REF] Khaemba | A methodology for Raman characterisation of MoDTC tribofilms and its application in investigating the influence of surface chemistry on friction performance of MoDTC lubricants[END_REF]. The presence of amorphous MoS3 compound [START_REF] Oumahi | MoS2 formation induced by amorphous MoS3 species under lubricated friction[END_REF] could explain the background shape near 𝐸 2𝑔 1 and 𝐴 1𝑔 . The presence of Fe(MoO4) is suggested by the presence of the Raman peak at 925 cm -1 [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF] and is consistent with the Mo (+VI) oxide contribution detected by XPS at 232.3 eV on Mo3d peak.

MoDTC 2 Tribofilm characterization

Raman (Figure 5) and XPS (Figure 6 and Table 3) characterizations were also performed on tribofilm obtained with MoDTC 2 at 100°C and 1 GPa for comparison with MoDTC 1. Both results show the presence of MoS2 and FeMoO4 for MoDTC 2 tribofilm, like for MoDTC 1. This is in accordance with literature works since XPS analyses of tribofilm obtained with MoDTC often propose the presence of two main contributions attributed to Mo+IV and Mo+VI [START_REF] Komaba | The effect of temperature on lubrication property with MoDTC-containning lubricant : temperature dependence of friction coefficient and tribofilm structure[END_REF][START_REF] Morina | ZDDP and MoDTC interactions and their effect on tribological performance -Tribofilm characteristics and its evolution[END_REF].

The presence of molybdenum oxysulfide (purple contribution B' in Figure 6 and table 3) is also detected for MoDTC 2 tribofilm by XPS and was previously suggested in literature [START_REF] De Feo | Ageing impact on tribological properties of MoDTC-containing base oil[END_REF][START_REF] Deshpande | Tribological behaviour of TiO2 Atmospheric Plasma Spray (APS) coating under mixed and boundary lubrication conditions in presence of oil containing MoDTC[END_REF].

Effect of temperature

Friction and wear results

The friction performances of molecules MoDTC 1 and MoDTC 2 have been studied at three different temperatures (T = 20°C, 100°C and 180°C at V = 56 mm/s, PHertzmax = 1 GPa, Figure 7).

The optimal tested temperature for both molecules is 100°C as a durable and low friction regime is achieved in both cases (cf §3.1.1). At 20°C, MoDTC 2 performs only marginally worse than at 100°C, while MoDTC 1 significantly underperforms (µ = 0.074 vs 0.099, respectively). Considering the overall monotonous decrease of the friction coefficients over the whole experiment (Total number of cycles = 18 000) for both molecular lubricants, it might be possible that neither system has reached steady state by the end of the experiment. Given the more pronounced decrease of the friction coefficient (both in terms of starting absolute value and in terms of overall drop) for MoDTC 1 than for MoDTC 2, the induction process seems more pronounced for MoDTC 1 than for MoDTC 2.

At 180°C, the loss of friction reduction capabilities is observed after a few thousand cycles for both MoDTC 1 and MoDTC 2 and very high friction coefficient are observed for most of the experiment, leading to high (apparent) steady state coefficients.

The wear behaviors obtained with the two molecules over the three temperatures appear to follow the same trend as the apparent steady state coefficients described above (cf Figure 8).

Figure 7: Coefficient of friction at 20°C (blue), 100°C (orange) and 180°C (yellow) for a) molecule MoDTC 1 b) molecule MoDTC 2. The tests were performed at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

Representative curves of the friction coefficient as the function of number of cycles are shown as well as average steady state friction coefficients calculated from several tests with the related standard deviation. Figure 8: Width of ball wear obtained at 20°C, 100°C and 180°C for a) molecule MoDTC 1 b) molecule MoDTC 2. The tests were performed at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s. The red line represents the Hertz diameter (size of the contact before friction).

Tribofilm characterizations with MoDTC 1 at 20°C, 100°C and 180°C (XPS and

Raman)

The XPS analyses of the Mo3d and the S2p transitions carried out on the tribofilms obtained with MoDTC 1 at different temperatures after 18 000 cycles are reported in Figure 9; details of the fits can be found in Table 4. The fit explanation of the tribofilm obtained at 100°C is detailed in § 3.1.2.

For the tribofilm at 20°C, only one Mo environment is found around 232.1 eV (Mo3d (C), pink) corresponding to a Mo(+VI) environment related to MoO3 or FeMoO4. Significant amount of SOx species is found but no evidence of MoS2 is detected.

For the tribofilm at 180°C, two Mo environments are found. The main one with a Mo3d5/2 peak around 232.1 eV (Mo3d (C), pink) corresponds to a Mo(+VI) environment related to MoO3 or FeMoO4. The Mo3d (B') contribution (229.9 eV, purple) at binding energy higher than MoS2 but lower than a typical Mo(+VI) signal could correspond to oxysulfide species MoOxSy [START_REF] De Feo | Ageing impact on tribological properties of MoDTC-containing base oil[END_REF][START_REF] Deshpande | Tribological behaviour of TiO2 Atmospheric Plasma Spray (APS) coating under mixed and boundary lubrication conditions in presence of oil containing MoDTC[END_REF]. SOx species are also detected.

Table 4: Peak fitting details related to Figure 9 e.i. XPS analyses of tribofilms obtained with MoDTC 1 after 18 000 cycles of rubbing at 20°C, 100°C and 180°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s Figure 9: XPS analyses (Mo3d) carried out of tribofilms (flats) obtained with MoDTC 1 after 18 000 cycles of rubbing at 20°C. 100°C and 180°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s (see text for further details).

The Raman analyses of the MoDTC 1 tribofilms obtained at 20°C, 100°C and 180°C are presented in Figure 10.

At 20°C, the observed peaks are mostly related to iron oxides (Fe2O3 and Fe3O4), like at 180°C [START_REF] Faria | Raman microspectroscopy of some iron oxides and oxyhydroxides[END_REF] and molybdenum based species like MoO3 are poorly detected [START_REF] Dieterle | Raman spectroscopy of molybdenum oxides[END_REF]. Atomic ratio Mo/Fe calculated from XPS data (Mo/Fe20°C = 0.5 -Mo/Fe100°C = 2.9 -Mo/Fe180°C = 0.9) suggests that the amount of molybdenum compared to iron in tribofilms is much smaller at 20°C and 180°C The comparison of tribofilm composition of MoDTC 1 and MoDTC 2 at 100°C was presented in §3.1.3. As found in literature [START_REF] Komaba | The effect of temperature on lubrication property with MoDTC-containning lubricant : temperature dependence of friction coefficient and tribofilm structure[END_REF][START_REF] Morina | ZDDP and MoDTC interactions and their effect on tribological performance -Tribofilm characteristics and its evolution[END_REF], the Mo+IV contribution (MoS2) increases in tribofilms obtained at higher temperatures, up to 80°C-100°C. Rai et al. [START_REF] Rai | Transient processes of MoS2 tribofilm formation under boundary lubrication[END_REF] and Komaba et al. [START_REF] Komaba | The effect of temperature on lubrication property with MoDTC-containning lubricant : temperature dependence of friction coefficient and tribofilm structure[END_REF] have also suggested that such temperatures have an effect on the structural organization of the MoS2 layers.

For temperatures higher than 100°C, only few data are found in literature [START_REF] Morina | ZDDP and MoDTC interactions and their effect on tribological performance -Tribofilm characteristics and its evolution[END_REF].

20°C 100°C 180°C MoS2 Fe(MoO4)
3.3 Effect of contact pressure

Friction results

Friction results obtained at 100°C for the two molecules MoDTC 1 and MoTDC 2 at different initial maximum Hertz pressures (from 0.6 to 2.3 GPa) are reported in Figure 11 (V = 56 mm/s. T = 100°C, 18 000 cycles). Average steady state friction coefficients obtained from repeated tests and observed standard deviations are reported.

At low contact pressures, the friction behavior at the steady state is similar for the two molecules within standard deviations. Above an initial maximum Hertz contact pressure of 1.9

GPa, significant differences in friction behavior appear between the two molecules: MoDTC 1 still achieves a low friction regime whereas MoDTC 2 does not.

Effect of temperature without contact pressure and shear

To test if the decomposition of both MoDTC molecules toward MoS2 can be thermally induced (with no other concurrent reagent), we performed Raman analyses after thermal degradation for both molecules at different temperatures in air and under argon atmosphere (Figure 14) in the absence of steel material. At high temperature (160°C for open air tests and 120°C for argon atmosphere tests), the molecule MoDTC 1 is decomposed but no MoS2 formation is observed. In contrast, Raman spectra obtained from 260°C both in air and Ar atmosphere with MoDTC 2 exhibits Raman peaks around 400 cm -1 . This is not compatible with MoO3 and MoO2 [START_REF] Dieterle | Raman spectroscopy of molybdenum oxides[END_REF] but that could be related to Mo-S vibrations. The 𝐸 2𝑔 1 and 𝐴 1𝑔 peaks are found at slightly lower positions than standard MoS2 analyzed at ambient temperature, but this may be due the temperature dependence of the peak position with temperature [START_REF] Sahoo | Temperature-dependent Raman studies and thermal conductivity of few-layer MoS2[END_REF] or to the fact that it is more a MoSx compound rather than MoS2 [START_REF] Seo | Monomeric MoS42--Derived Polymeric Chains with Active Molecular Units for Efficient Hydrogen Evolution Reaction[END_REF]. The decomposition in MoS2 of MoDTC closed to MoDTC 2 structure by thermal degradation was already found in literature [START_REF] Sakurai | The Synthesis of Di-μ-thio-dithio-bis (dialkyldithiocarbamates) Dimolybdenum (V) and Their Effects on Boundary Lubrication[END_REF]. Concerning the oxidation state of molybdenum atom within the molecule, systems with a Mo(+VI) molybdate core have been described but they need to be blended with other additives to form MoS2 sheets [START_REF] Oumahi | MoS2 formation induced by amorphous MoS3 species under lubricated friction[END_REF]. Among the potential reductive sources, the organic oil or the organic ligands could play a chemical reducing role, but the formation of iron-molybdate involving both iron coming from the steel surface and molybdenum from the additive supports the idea of an active role of steel in these phenomena. This is further corroborated by the fact that MoS2 is not observed upon thermal heating of MoDTC powder in the absence of steel (results §3.4)

and by the fact that molybdate is the main detected phase under less severe conditions at low contact pressure (Figures 12 and13). Therefore, it can be suggested that the steel-steel contact lubricated with oil environment is reducing enough to reduce Mo(+VI) in MoDTC 1 into Mo(+IV), in the absence of other additives.

The two MoDTC molecules display a very different response during thermal degradation.

Additive MoDTC 2 seems to degrade into MoSx compounds in both air and argon atmospheres near 260°C. This is in agreement with reported mechanism on thermal degradation of such molecules containing the [Mo(µ-S)2Mo] sulphided core as MoDTC 2 [START_REF] Sakurai | The Synthesis of Di-μ-thio-dithio-bis (dialkyldithiocarbamates) Dimolybdenum (V) and Their Effects on Boundary Lubrication[END_REF]. Conversely, as shown on Figures 14, MoS2 is not formed from MoDTC 1 during thermal degradation. This suggests a strong role of mechanical stresses and a chemical role of iron. Consistently, we have evidenced the formation of MoS2 from MoDTC 1 only in wear tracks of tribotests (Figures 3 and4). As shown above, mechanical stresses for MoDTC 2 action does not seem necessary for forming MoS2, as it seems to form in thermal degradation experiments, but they may positively add to thermal degradation and/or chemical role of steel surface.

To summarize, MoDTC 1 is an interesting molecular additive to reduce friction in boundary lubricated steel-steel contacts. This efficient molecule achieves high performance up to high contact pressures by accessing mechanistic pathways that could be distinct from those of classical molecules such as MoDTC 2 or molybdate [START_REF] Oumahi | MoS2 formation induced by amorphous MoS3 species under lubricated friction[END_REF]. We ruled out the formation of MoS2 by a mechanism based solely on the thermal degradation of MoDTC 1. The absence of core sulfur in molecule MoDTC 1 also rules out (at least in the initial stages) the chemical degradation mechanism proposed by Grossiord et al. [START_REF] Grossiord | MoS2 single sheet lubrication by molybdenum dithiocarbamate[END_REF] where only sulfur atoms present in the core of the molecule contribute to the MoS2 generation. The necessary involvement of the sulfur atoms in the ligands of MoDTC 1 remains compatible with at least two proposed mechanisms: the formation of a Linkage Isomer -MoDTC [START_REF] Deshpande | Tribocatalytic behaviour of a TiO2 atmospheric plasma spray (APS) coating in the presence of the friction modifier MoDTC: a parametric study[END_REF][START_REF] Onodera | Development of a quantum chemical molecular dynamics tribochemical simulator and its application to tribochemical reaction dynamics of lubricant additives[END_REF], or the cleavage of carbonsulfur bonds of the ligand [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Coffey | Molybdenum(VI) imidodisulfur complexes formed via double sulfur-carbon bond cleavage of dithiocarbamates[END_REF]. Then, MoSx intermediate products seems needed before generating well-organized MoS2 lamellar sheets during friction as it was proposed previously [START_REF] Khaemba | New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study[END_REF][START_REF] Oumahi | MoS2 formation induced by amorphous MoS3 species under lubricated friction[END_REF].

Conclusion

In this work, the tribological properties and factors influencing the formation of MoS2 from to what was classically assumed, the presence of peripheral thiocarbamate ligands can be sufficient to provide the required sulfur source in these "all-in-one" types of molecules. This molecule is not only competitive with "classical" MoDTC used in engine lubrication, containing sulfur in the core of the molecule, but is even more powerful at high contact pressures.

On the mechanistic point of view, this work confirms that the chemical pathway from MoDTC to MoS2 is not only thermal. Pressure and shear take an important role in the tribochemical reactions route leading to the formation of MoS2. A substantial contribution of the surface, as mentioned in the discussion section, remains an important parameter to consider as well.

Those results give insights in molecule design and lubricant formulation to improve MoS2 sheets generation from several types of contact conditions

MoDTC ( 1 ) under standard boundary lubricated conditions 3 . 1 . 1

 1311 Friction results with MoDTC (1) MoDTC 1 has been tested under our standard conditions of boundary lubrication (T = 100°C, V = 56 mm/s, PHertzmax = 1 GPa, 18 000 cycles). The evolution of the friction coefficient as function of the number of cycles is presented in Figure 2.a as well as statistical data processing from several measurements. The average steady state friction coefficient for MoDTC 1 is 0.063 ± 0.001 that is within the range obtained with the reference molecule MoDTC 2 (Figure 2.b -µMoDTC 2 = 0.0644 ± 0.009). The values for the two molecules are statistically similar and in the expected range for the formation of MoS2 in a boundary lubricated contact [6,15]. It can be also noticed that MoDTC 1 requires longer induction time to reach the low friction steady state regime than MoDTC 2. No loss of friction reduction capabilities is observed over the full experiment (18000 cycles) for each molecule.

Figure 2 : 1 2 3. 1 . 2

 21212 Figure 2: Coefficient of friction at 100°C for a) molecule MoDTC 1 b) molecule MoDTC 2.The tests were performed at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s. Representative curves of the friction coefficient as the function of number of cycles are shown as well as average steady state friction coefficients calculated from several tests with the related standard deviation.

Figure 5 :

 5 Figure 5: Raman analyses on tribofilms (flats) obtained with MoDTC 1 and MoDTC 2 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

Figure 6 :

 6 Figure 6: XPS analyses (Mo3d) carried out of tribofilms (flats) obtained with MoDTC 2 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

  The lowest wear is found at 100°C for both molecules MoDTC 1 and MoDTC 2, with MoDTC 1 still marginally outperforming MoDTC 2 (width of the wear = 180 µm vs. 222 µm, respectively). Both wear (MoDTC 1 and MoDTC 2 at 100°C) are just slightly superior to Hertz diameter (red line) which represent the size of the contact before friction without any wear. The loss in performance at 20°C is more noticeable for MoDTC 1 than for MoDTC 2 (width of the wear = 558 µm vs. 299 µm, respectively). The poorest performances are observed at 180°C for both lubricants (width of the wear = 620 µm for MoDTC 1 and 650 µm for MoDTC 2).

Figure 10 :

 10 Figure 10: Raman analyses on tribofilms (flats) obtained with MoDTC 1 after 18 000 cycles of rubbing at 20°C. 100°C and 180°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

Figure 11 :

 11 Figure 11: Average steady state friction coefficient for different Initial Hertz maximum contact pressures for molecule MoDTC 1 and molecule MoDTC 2. The tests were performed at a temperature of 100°C and an average sliding speed of 56 mm/s. Average steady state friction coefficients are calculated from several tests with the related standard deviation.

Figure 13 : 1 than

 131 Figure 13: Raman analyses on tribofilms (flats) obtained with MoDTC 1 after 18 000 cycles of rubbing at different initial maximum Hertz pressure, at 100°C and at an average sliding speed of 56 mm/s.

Figure 14 :

 14 Figure 14: Raman analyses performed on pure additives in different atmosphere and at different temperatures: a) MoDTC 1 in air (laser wavelength 784 nm) and b) MoDTC 2 in air (laser wavelength 532 nm) c) MoDTC 1 under argon atmosphere (laser wavelength 784 nm) and d) MoDTC 2 under argon atmosphere (laser wavelength 532 nm).

{

  [MoO2](S2CNEt2)2} (MoDTC 1) and {[Mo2(S/O)4](S2CNOc2)2} (MoDTC 2) were investigated.Unexpectedly, MoDTC 1, comprising Mo(+VI) and containing sulfur atoms only in its thiocarbamate ligands, is able to form MoS2 sheets in lubricated steel/steel contacts under boundary lubrication conditions. This experimental study therefore demonstrates that, contrary

Table 1 : Contact conditions of reciprocating ball-on-flat tribotests.

 1

	Thiocarbamate ligand
	Core

Table 2 : Peak fitting details related to Figure 4 e.i. XPS analyses of tribofilms obtained with MoDTC 1 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s. Figure 4: XPS analyses (Mo3d) carried out of tribofilms (flats) obtained with MoDTC 1 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

 2

	Mo3d				S2p
	Intensity / CPS			2x Intensity / CPS
	240	235	230	225	166	164	162	160	158
		Binding Energy / eV		Binding Energy / eV

Table 3 : Peak fitting details related to Figure 6 e.i. XPS analyses of tribofilms obtained with MoDTC 2 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 1 GPa and an average sliding speed of 56 mm/s.

 3

	5

Table 5 : Peak fitting details related to Figure 12 e.i. XPS analyses of tribofilms obtained with MoDTC 1 after 18 000 cycles of rubbing at 100°C at an initial maximum Hertz pressure of 0.8 GPa, 1 GPa, 1.4 GPa, 2 GPa and an average sliding speed of 56 mm/s Figure 12: XPS analyses (Mo3d peak) carried out of tribofilms (flats) obtained with MoDTC 1 at various initial maximum Hertz pressure after 18 000 cycles of rubbing at 100°C and an average sliding speed of 56 mm/s.

 5

	x2 Intensity / CPS

Acknowledgments

The authors thank "Institut Carnot I@L" for financial support (Projet MoST AAP2016). The FIB cross section have been done at Manutech-USD in Saint-Etienne (France). Electron Beam Microscopy have been done at the "Centre des technologies et des microstructures", Claude Bernard University of Lyon (France). The Raman facility at LGL-TPE ENS de Lyon is also supported by INSU-CNRS and by the LABEX Lyon Institute of Origins (ANR-10-LABX-0066) of the Université de Lyon within the program "Investissements d'Avenir" (ANR-11-IDEX-0007) of the French government operated by the National Research Agency (ANR).

Tribofilms characterization

The XPS and Raman characterizations performed on tribofilms obtained with MoDTC 1 at different contact pressures are shown respectively in Figure 12/Table 5 and Figure 13.

The interpretation of the XPS data follows the results presented in §3.1.2. For all tested contact pressures, beside the main contributions (Mo3d (B), blue) attributed to MoS2 or MoS3, Mo (+VI) oxide contributions attributed to Fe(MoO4) is also found in significant amount (namely, Mo3d (C), pink and 925 cm -1 Raman peak). At high contact pressures, Mo3d (B) is the main contribution, and Mo3d (C) dominates at low contact pressure (0.8 GPa). The low binding energy Mo3d (A) contribution is detected mainly at low contact pressures (0.8 GPa and 1 GPa) and would correspond to defective MoS2. No oxysulfide-type molybdenum is evidenced (binding energies expected between Mo3d (B) and Mo3d (C)) [START_REF] De Feo | Ageing impact on tribological properties of MoDTC-containing base oil[END_REF][START_REF] Deshpande | Tribological behaviour of TiO2 Atmospheric Plasma Spray (APS) coating under mixed and boundary lubrication conditions in presence of oil containing MoDTC[END_REF][START_REF] Benoist | X-ray photoelectron spectroscopy characterization of amorphous molybdenum oxysulfide thin films[END_REF].

Concerning the S2p spectra, two main characteristic sulfur environments are found with no clear dependence on contact pressure: a sulfide-type environment at low binding energies (S2p3/2 < 164 eV) and a SOx type environment at higher binding energies (S2p3/2 > 166 eV) characteristic of oxidized sulfur.

In Raman spectra, different components, MoS2, Fe(MoO4), amorphous MoS3 and carbonaceous matrix [START_REF] Espejo | MoDTC tribochemistry in steel/steel and steel/diamond-like-carbon systems lubricated with model lubricants and fully formulated engine oils[END_REF] are identified. It is difficult to further interpret the observed differences as they may be related to the localization of the analyzed area within the heterogeneous wear track, rather than to systematic dependence on contact pressure.


Pascal Paradis
email: pascal.paradis.2@ulaval.ca

Bernard Dussardier
email: bernard.dussardier@unice.fr

Vincent Fortin
email: vincent.fortin@copl.ulaval.ca

Réal Vallée
email: rvallee@copl.ulaval.ca

Martin Bernier
email: martin.bernier@ulaval.ca

Towards Mid-IR Pulsed All-Fiber Lasers Based on Saturable Absorbers

Keywords: fiber, laser, ultrafast, components, nonlinear

We present our preliminary design and propositions towards an all-fiber mode-locked linear laser cavity operating at 2.8 microns based on a heavily erbium-doped fluoride fiber bounded by a semiconductor saturable absorber mirror and a fiber Bragg grating generates stable pulses as short as 15 ps.

I. INTRODUCTION

Mid-infrared (mid-IR) pulsed fiber lasers are promising for biomedical applications such as dental tissue ablation, cardiovascular surgery and vitreoretinal surgery due to the strong water absorption peak at 2.94 µm [START_REF] Zhu | Pulsed fluoride fiber lasers at 3 μm [Invited[END_REF], [START_REF] Amini-Nik | Ultrafast Mid-IR Laser Scalpel: Protein Signals of the Fundamental Limits to Minimally Invasive Surgery[END_REF]. The emission spectrum of these lasers can also reach the fundamental rotation-vibrational resonances of many atmospheric pollutants such as methane [START_REF] Tittel | Mid-Infrared Laser Applications in Spectroscopy[END_REF]. Thus, they would enable advantageous field applications such as remote sensing with mid-IR spectroscopy.

In recent years, a multitude of ultrafast fiber lasers operating near 3 µm have been reported. Ring lasers using nonlinear polarization evolution generated the shortest pulses while those using saturable absorber (SA) emitted pulses as short as 6 ps [START_REF] Antipov | High-power mid-infrared femtosecond fiber laser in the water vapor transmission window[END_REF], [START_REF] Hu | Stable, self-starting, passively mode-locked fiber ring laser of the 3 μm class[END_REF]. A simpler linear mode-locked fiber laser based on a semiconductor saturable absorber mirror (SESAM) and a fiber Bragg grating (FBG) was also reported, producing 60 ps pulses limited by the narrow FBG bandwidth [START_REF] Haboucha | Fiber Bragg grating stabilization of a passively mode-locked 28 μm Er3+: fluoride glass fiber laser[END_REF]. A more traditional mode-locked fiber based on a SESAM and a broadband output coupler generated 25 ps pulses [START_REF] Tang | Watt-level passively mode-locked Er^3+-doped ZBLAN fiber laser at 28 μm[END_REF]. However, the complexity, building cost and alignment requirements of these lasers restrict their field deployment and limit their usage to laser experts.

In this paper, we present a mode-locked erbium-doped fluoride fiber laser based on a fiber Bragg grating (FBG) and a SESAM, generating 15 ps pulses at 2.825 µm with 150 mW average output power at a repetition rate of 55 MHz as a step towards ultrafast allfiber lasers emitting in the mid-IR.

II. EXPERIMENTAL SETUP

The laser setup, as illustrated in fig. 1a is made of a 2 m-long 7%mol Er 3+ -doped fluoride fiber (Le Verre Fluoré) as the gain medium for the laser cavity bounded by a SESAM (BATOP GmbH) and a fiber Bragg grating (FBG) written using femtosecond pulses through the fiber's protective coating to preserve its original robustness [START_REF] Habel | Femtosecond FBG Written through the Coating for Sensing Applications[END_REF]. The gain fiber is backward pumped with a 976 nm pump diode from (Alfalight, B33125) to simplify the intracavity alignment and achieve higher output power than in a forwardpumping scheme. The FBG has a peak reflectivity of 65% at 2790.9 nm with a FWHM of 3.2 nm. The aspherical ZnSe lenses have focal lengths of 25 mm at the output and 12.5 mm inside the cavity, and each of them transmits around 95% of the signal at 2.8 µm. The fiber tips are angle-cleaved at 8° to eliminate parasitic reflections thus achieving stable mode-locking and the residual pump power is removed by the dichroic mirror (DM) to avoid damaging the SESAM.

Moreover, we propose the use of a fiber-based saturable absorber mirror (SAM) and a pump combiner, as shown in fig. 1b, to achieve a robust pulsed all-fiber linear laser emitting around 2.8 µm.

III. RESULTS

This mode-locked laser cavity generates 15 ps pulses with a time-bandwidth product of 0.380, i.e., close to the Fourier-Transform limit as shown in fig. 2. The mode-locked regime is self-starting without any Q-switched regime at lower pump power if the alignment of the intracavity components is optimal. The mode-locked regime is stable with one pulse per round trip between 87.5 mW and 157.5 mW of average output power. At higher power, the mode-locked regime became unstable with two pulses per round trip with longer pulse duration. Moreover, the emission spectrum is centered on the FBG's minimum transmission (i.e. on its maximum reflectivity) which indicates that the duration and the spectrum can be controlled by the FBG's properties in this design if the total cavity dispersion is low enough in the anomalous regime and the SA's recovery time is short enough, its modulation depth is not too shallow and the saturation fluence is high enough.

IV. CONCLUSION

Fig. 1 .

 1 Fig. 1. a) Experimental setup of the mode-locked Er 3+ -doped fiber laser based on a SESAM and an FBG. b) Proposed experimental setup for the mode-locked Er 3+ -doped fiber laser based on a SA fiber and an FBG. (CMS: cladding mode stripper, DM: dichroic mirror, M: gold mirror)

We present a simple mode-locked fiber laser design based on a SESAM and a FBG that allows control of the duration and the bandwidth of the pulses. While being much simpler than ultrafast ring lasers operating in Mid-IR with a saturable absorber, this laser design was able to generate 15 ps pulses close to the Fourier-transform limit. During the presentation, different new configurations of fiber-based SA mirror will be discussed to greatly improve the robustness and reliability of ultrafast mid-IR lasers.


Pascal Paradis
email: pascal.paradis.2@ulaval.ca

Vincent Fortin

Bernard Dussardier

Réal Vallée

Martin Bernier

Novel schemes are proposed for building an all-fiber mode-locked laser cavity operating at 2.8 microns based on a heavily erbium-doped fluoride fiber bounded by a semiconductor saturable absorber mirror and a fiber Bragg grating.

Introduction

Mid-infrared (mid-IR) pulsed fiber lasers are promising for laser surgery due to the strong water absorption peak at 2.94 µm [START_REF] Zhu | Pulsed fluoride fiber lasers at 3 µm [Invited[END_REF]. These lasers would also be advantageous for field applications such as remote sensing because their emission spectrum can reach the fundamental rotation-vibrational resonances of many atmospheric pollutants such as methane [START_REF] Tittel | Mid-Infrared Laser Applications in Spectroscopy[END_REF]. In recent years, a multitude of ultrafast fiber lasers operating near 3 µm have been reported. Ring cavity lasers using nonlinear polarization rotation evolution generated pulses as short as 180 fs [START_REF] Antipov | High-power mid-infrared femtosecond fiber laser in the water vapor transmission window[END_REF], while those using a saturable absorber (SA) emitted pulses as short as 6 ps [START_REF] Hu | Stable, self-starting, passively mode-locked fiber ring laser of the 3 µm class[END_REF]. A simpler linear mode-locked fiber laser based on a semiconductor saturable absorber mirror (SESAM) and a fiber Bragg grating (FBG) was also reported, producing 60 ps pulses limited by the narrow FBG bandwidth [START_REF] Haboucha | Fiber Bragg grating stabilization of a passively mode-locked 28 µm Er 3+ : fluoride glass fiber laser[END_REF]. A more traditional mode-locked fiber laser based on a SESAM and a broadband output coupler generated 25 ps pulses [START_REF] Tang | Watt-level passively mode-locked Er 3+ -doped ZBLAN fiber laser at 28 µm[END_REF]. However, the building cost and alignment requirements of these lasers restrict their field deployment and limit their usage by laser experts.

In this paper, we present a mode-locked erbium-doped fluoride fiber laser, based on an FBG and a SESAM, generating 15 ps pulses at 2.825 µm with 150 mW average output power at a repetition rate of 55 MHz as a step towards ultrafast all-fiber lasers emitting in the mid-IR.

Experimental Setup

The laser setup, as illustrated in Fig. 1(a), is made of a 2 m-long 7%mol Er 3+ -doped fluoride fiber (Le Verre Fluoré) as the gain medium for the laser cavity bounded by a SESAM (BATOP GmbH) and an FBG written through the fiber's protective coating using femtosecond pulses to preserve its original robustness [START_REF] Habel | Femtosecond FBG Written through the Coating for Sensing Applications[END_REF]. The gain fiber is backward pumped with a 976 nm pump diode (Alfalight, B33125) to simplify the intracavity alignment and achieve higher output power than in a forward-pumping scheme. The FBG has a maximum reflectivity of 65% at 2790.9 nm with a FWHM of 3.2 nm. The aspherical ZnSe lenses have focal lengths of 25 mm at the output and 12.5 mm inside the cavity, and each of them transmits around 95% of the signal at 2.8 µm. The fiber tips are angle-cleaved at 8°to eliminate parasitic reflections thus achieving stable mode-locking and the residual pump power is removed by the dichroic mirror (DM) to avoid damaging the SESAM. Moreover, we propose the use of a fiber-based saturable absorber mirror (SAM) and a pump combiner to achieve a robust pulsed all-fiber linear laser emitting around 2.8 µm as shown in Fig. 1(b).

Results

This mode-locked laser cavity generates 15 ps pulses with a time-bandwidth product of 0.380, i.e., close to the Fourier-Transform limit as shown in Fig. 2. The mode-locked regime is self-starting without any Q-switched regime at lower pump power if the alignment of the intracavity components is optimal. The mode-locked regime is stable with one pulse per round trip between 87.5 mW and 157.5 mW of average output power. At higher power, the mode-locked regime becomes unstable with two pulses per round trip with longer pulse duration. Moreover, the emission spectrum is centered on the FBG's minimum transmission (i.e., on its maximum reflectivity) which indicates that the duration and the spectrum can be controlled by the FBG's properties in this design if the total cavity dispersion is low enough in the anomalous regime and the SA's recovery time is short enough, its modulation depth is not too shallow and the saturation fluence is high enough.

Conclusion

We have presented a simple mode-locked fiber laser design based on a SESAM and an FBG that allows control of the duration and the bandwidth of the pulses. While being much simpler than ultrafast ring lasers operating in Mid-IR with a saturable absorber, this laser design was able to generate 15 ps pulses close to the Fourier-transform limit. During the presentation, different new configurations of fiber-based SA mirror will be discussed to greatly improve the robustness and reliability of ultrafast mid-IR lasers.

Fig. 1 .Fig. 2 .

 12 Fig. 1. a) Experimental setup of the mode-locked Er 3+ -doped fiber laser based on a SESAM and an FBG. b) Proposed setup for the mode-locked Er 3+ -doped fiber laser based on a SA fiber and an FBG. (CMS: cladding mode stripper, DM: dichroic mirror, M: gold mirror, L1: silica lens f=11 mm)

© 2 0 2 0 T h e Au t h o r ( s )


Hélène Lotz
email: helene.lotz@cea.fr

Charly Carrière

Christian Bataillon

Emmanuel Gardes

Isabelle Monnet

Eddy Foy

Michel L Schlegel

James J Dynes

Delphine Neff

Florence Mercier

Philippe Dillmann

Investigation of steel corrosion in MX80 bentonite at 120°C Running title: Steel corrosion in MX80 bentonite at 120°C

Keywords: Steel, anoxic corrosion, MX80 bentonite, multiscale analysis, magnetite, iron silicate

Corrosion experiments were performed on two metallic substrates, a ferritic-pearlitic steel (P285NH) and a ferritic one (Armco), in silicate environment during 30 days at 120°C. Corrosion products were characterized in terms of morphology (SEM, TEM), composition (EDS) and structure (µ-Raman, SAED, XANES). Results show a nanometric inner layer made of compact and adherent nano-crystallized magnetite with, locally a thickness up to several micrometers due to the metal microstructure. An outer layer of Fe-rich phyllosilicate, smectite and serpentine, more porous than the inner one and poorly adherent is observed around both samples.

Introduction

Deep geological disposal is being studied in many countries as a long-term (>10 000 years) answer to the management of high-level radioactive waste [START_REF]ANDRA report[END_REF]. The host rocks are selected according to their properties and their availability in each country. In France, a Callovo-Oxfordian claystone (Cox) located 500 m deep in the east of the Paris basin, is currently being investigated. The French National Radioactive Waste Management Agency (Andra) plans to confine the radionuclides in Cox using a multi-barrier system made up of a glass matrix embedded in a stainless steel container, confined in a carbon steel overpack [START_REF]ANDRA report[END_REF][START_REF] Dillmann | [END_REF].

These waste packages would be stored in boreholes lined with steel tubes in the clay host rock.

After closure of the system, the packages are expected to experience mostly anoxic conditions.

The low permeability of the clay host could delay the water ingress in contact with the packages.

The longevity of such a storage system is assessed by predictive models based on identified long-term corrosion mechanisms of the metallic containers. For decades, several studies have examined steel generalized corrosion in argillite media and in carbonated clay pore water [START_REF] Schlegel | [END_REF][4][5][6][7][8][9][10][11][12]. In the geochemical conditions of the repository, i.e. an anoxic and carbonated environment, the formation of Fe-rich stable phases, namely iron oxides, carbonates, sulphates and silicates was observed [13,14]. The Corrosion Product Layers (CPL) can be described as follows: an internal magnetite oxide layer, an external layer consisting of Fe-rich phyllosilicates sometimes with iron carbonates, and a transformed medium layer with iron carbonate. Each phase has its own physico-chemical properties influencing the corrosion mechanisms of steel and the alteration rate. For example, the presence of an oxide layer at the metal/corrosion product layer (M/CPL) interface could decrease the corrosion rate [15]. Moreover, studies from the oil and gas industry demonstrated that carbonate corrosion products also slow down the corrosion rate by moderating ion diffusion through the CPL [16]. On the contrary, formation of non-expansive, or swelling Fe-rich silicates, such as serpentine or smectite respectively, was shown to change the permeability properties around the steel surface [17].

Studies performed on Iron / Clay interactions without carbonate buffer have shown the formation of magnetite and Fe-rich silicate [18][19][20]. These studies were performed in chloride solution or distilled water at 80-90°C on powder in order to maximize the contact surfaces, and aimed to understand the processes of clays destabilization (Cox without carbonate minerals, MX80, montmorillonite, saponite…) by iron. However, they have not scrutinized the metallic interface as well as the CPL layout. Yet, more investigations focusing on that issue are needed to understand the interplay between silicate transformation and iron corrosion mechanism, like the formation (or not) of a protective oxide interfacial layer at the M/CPL interface.

This study considers steel corrosion in MX80 bentonite and in Cox porewater without carbonate minerals to simulate a compact silicate environment in absence of a carbonate buffer.

Two types of metallic substrates are studied, a ferritic-pearlitic steel and a ferritic one, in order to investigate potential influence of the steel microstructure and composition on the corrosion behaviour. The morphology, composition and structure of the corrosion products are determined using a multiscale approach (macro-micro-nano). Their properties are discussed and the role of the corrosion product layers is explained.

Materials and methods

In this work, a steel, P285NH, and a pure iron, Armco, were studied. Their compositions are given in Table 1. P285NH is a ferritic-pearlitic steel which microstructure is made of two elements: α-Fe grains and of pearlite grains formed by a succession of α-Fe and cementite Fe 3 C lamellas. Armco is a pure ferrite containing only α-Fe grains. The comparison with these two steels is essential to highlight the effect of cementite on the corrosion processes. The metallic coupons were mechanically polished under ethanol with SiC paper (up to grade 4000). Between each polishing grade, samples were ultrasonically cleaned for 2 min in ethanol to remove SiC grains.

The corrosion experiment was performed in an autoclave at 120°C for 30 days. This high temperature was chosen in order to increase the reaction kinetics. The samples were simultaneously immersed in Cox pore water synthetic solution mixed with 30% wt. of MX80 bentonite to simulate a simplified compact clay environment. Compositions of the Cox synthetic pore water without carbonated compounds [21] and MX80 bentonite [14] are given in Table 2 and3, respectively. After sample immersion, the autoclave was evacuated to remove oxygen and reach anoxic conditions. Upon completion of the experiment, the samples were dried using a stream of dry air and then stored under primary vacuum.

Prior to analyses, samples were embedded in epoxy resin (EpoFix, Struers) then cut with a precision diamond saw (Minitom, Struers). The cross-sections were ground with SiC paper (up to grade 4000) under ethanol, and then polished with 1 µm diamond paste on a velvet disk.

Cross-section were etched with Nital in order to reveal the metal microstructure in the light of the corrosion products. Observation of the corrosion systems were performed both before and after etching.

Field Emission Gun-Scanning Electron Microscope (JEOL SEM 7001F) was used to investigate the interfaces in backscattered electron (BSE) and secondary electron (SE) modes.

The scanning electron microscope (SEM) is coupled to a silicon drift detector (SDD) to obtain the elemental composition by Energy Dispersive X-ray Spectroscopy (EDS). Analyses were performed at 10 kV on metallized samples. Aztec software, developed by Oxford Instruments was used for data acquisition and for data treatment.

Raman analyses were performed on an Invia Renishaw microspectrometer equipped with a frequency-doubled Nd-YAG green laser (532 nm wavelength) and with a high groove density grating (2400 gr/mm) associated with a coupled charge device (CCD) detector. The beam was focused using a Leica optical microscope. At 50x magnification the beam lateral size is 1.2 μm and has a probed depth of about 1 µm. Laser power was filtered at 0.1 mW to avoid any damage of the corrosion products. The spectrometer energy was calibrated by setting the Raman line of a silicon wafer to 520.5 cm -1 . Spectra were acquired using Wire 3.4 software with a resolution of 2 cm -1 between 200 and 1200 cm -1 and dwell time of 60 s.

Thin foils (~100 nm and ~1 µm thick) were extracted from the samples to analyse the Metal/Corrosion Product Layer (M/CPL) interface using a SEM coupled with Focused Ga-Ion Beam FEI Nanolab 660. Lamella thinning was performed first using 30 kV Ga ions, followed by final polishing at decreasing energies of 5, 2 and 1 kV to remove damaged and Ga-implanted layers on each side of the lamellas [22], and finally stored in sealed jars under primary vacuum.

The 100 nm and 1 µm thick foils were used for transmission electron microscopy (TEM) and Scanning Transmission X-ray Microscopy (STXM) analyses at Si K-edge, respectively.

For TEM analysis, a JEOL 2010 F equipped with a diode EDAX X-ray microanalysis was operated at 200 kV. Complementary Selected Area Electron Diffraction (SAED) as well as dark field imaging on characteristic diffraction spots were used to identify solid phases.

Interplanar spacings were calculated with ImageJ software, and then compared to d hkl values from Joint Committee on Powder Diffraction Standards (JCPDS) cards.

The chemical environment of iron silicates was characterized using synchrotron radiation using STXM at the Si K-edge on the 10ID-A beamline at the Canadian Light Source (CLS). Analyses and data processing, achieved with aXis2000 software, have already been described in the literature [23,24]. Experimental spectra decomposition was done using a least square method with standard reference spectra from a previously created database [22].

Results

Corrosion distribution around the metallic samples

The corroded interface can be divided into two parts for both steel nuances (figure 1).

Firstly, an inner corrosion product layer (iCP), light grey contrast in the BSE, seems compact and adherent to the metallic surface. Secondly, a discontinuous layer of external corrosion product (eCP) with various darker grey contrasts appears porous and poorly adherent to the iCP. The eCP morphology appears heterogeneous with disjointed chunks. The characterisation of the two corrosion product layers will be presented in the next two paragraphs.

Characterization of the iCP

SEM micrographs in SE mode were obtained after metallographic etching of the crosssections in order to reveal the possible connection between the metallic microstructure and the corrosion products (Fig. 2). A continuous inner layer of 500 nm average thickness (Fig. 2AB) and up to 25 µm locally (Fig. 2CD) in contact with the metal is evidenced for both samples. For P285NH, local corrosion penetrations are commonly observed and can be correlated with the presence of pearlite grains in the metal microstructure (Fig. 2C). However, this is not systematic because some contiguous pearlite grains around the metal are not associated with such local penetrations (Fig. 2A). For Armco, the local penetrations of corrosion products are rarer (three per cross section of diameter 2.5 cm) but deeper (up to 25 µm) than for P285NH and do not seem to be related to the metal microstructure (Fig. 2D). SEM-EDS investigations of the iCP detected only Fe and O as major elements. Raman spectroscopy was performed on the iCP of both metallic samples. All spectra present an intense band at 669 cm -1 with two weaker bands at 550 cm -1 and 305 cm -1 (Fig. 3), characteristic of magnetite Fe 3 O 4 [25].

This magnetite layer was investigated by TEM on thin foils collected on each sample in order to highlight its crystalline properties. Diffraction patterns collected by SAED close to the metal interface confirmed the presence of magnetite (JCPDS 19-0629; Fig. 4A andtable 4).

Interestingly, the SAED pattern of the magnetite displays almost continuous rings, meaning magnetite domains are small compared to the lateral size of the electron beam. TEM images collected on P285NH (Fig. 4B) and Armco (Fig. 4C) showed that the magnetite grain size is less than 30 nm diameter. Moreover, TEM acquisitions performed at the M/CPL interface indicate that the oxide layer is compact, non-porous at nanoscale and adherent to the metal (Fig. 4C)

Characterisation of the eCP

The external CPL appears darker (in BSE mode) than the oxide layer (Fig. 5), suggesting the presence of lighter elements. Besides, corrosion products seem poorly adherent to the metal and appear porous as they are infiltrated with some resin (in black on Fig. 1 and5). They extend from 40 µm to 60 µm in the clay matrix. EDS measurements (Fig. 5, Table 5) show the presence of mainly Fe and Si containing phase, associated with O, Al and Mg. Interestingly, in the eCP layers of both samples, two Fe-Si-O rich phases with distinct compositions are identified.

Firstly, homogeneous concentrations of Fe (17 at.%) and Si (18 at.%), with Al (10 at.%) and Mg (1-1.5 at.%) is compatible the presence of trioctahedral phyllosilicate corresponding to the serpentine group [26] (see points 1 and 3 for P285NH and Armco respectively). Secondly, phases with Si content (21 at.%) higher than Fe (5-6 at.%) with Al (7 at.%) and Mg (1-2 at %) suggest the presence of smectite type minerals (points 2 and 4 for P285NH and Armco respectively) [START_REF] Schegel | EUROCORR'17[END_REF]. Due to the weak Raman scattering of these phases, their investigation by Raman spectroscopy was not possible [START_REF] Schlegel | [END_REF]. Hence, High-Resolution Transmission Electron Microscopy (HR-TEM) was used. HR-TEM micrographs revealed foliated aggregates (Fig. 7), compatible with the morphology of phyllosilicate particles [START_REF] Grauby | [END_REF]. In both eCP, HR-TEM analyses reveal characteristic d 001 distances of 13 Å (Fig. 6AB) and 7.3 Å (Fig. 6CD), compatible with hydrated smectite and serpentine respectively [29][30][31][32][33][34].

To assess the local crystallochemical organization of the Fe-Si-O rich phases, and to narrow the iron silicate identification, STXM spectra were collected at the Si K-edge. Reference spectra of MX80 bentonite, montmorillonite and quartz were collected to discriminate iron silicate formed in the present study and silicates from MX80 bentonite (fig. 7A). Two different spectra were obtained in the eCP of both samples, i.e. for P285NH and Armco (Fig. 7B and7C). The first spectrum (Fig. 7B) presents a major peak at 1846.0 eV (peak A), an oscillation at 1856.3 eV (C) and an additional broad band at 1861.9 eV (D). The second spectrum (Fig. 7C) also displays the A and D contributions at 1846.0 eV and 1861.9 eV respectively, but presents an intense additional shoulder at 1851.4 eV (peak B) and a smaller oscillation at the C position at 1856.6 eV. Note that for the two different spectra, the major peak (A) and the broad band (D) correspond to the Si fourfold coordinated by O and the contribution of neighbouring O respectively [35,36]. The spectrum on Fig. 7A is qualitatively compared with the Si K-edge spectra database collected by Carrière et al. [22]. The presence of the A, C and D contributions identifies the smectite group (excepting saponites), and excludes chlorites, mica and serpentines-kaolinites [7,22]. The qualitative approach conducted on the second spectrum (Fig. 7C) excludes the smectites and the chlorites group, and can promote serpentine group.

Then, these two experimental spectra were decomposed by a linear regression using the reference spectra of selected minerals. The best fits (Fig. 7B and Table 6) are obtained with the combination of only two reference phases: about 90 % of Garfield nontronite, an Fe(III)-rich smectite, and 10 % of amorphous SiO 2 . The good quality of the fit is attested with R² = 0.99 and χ² = 0.01. Note that the amorphous contribution generated by the amorphization of each face of the FIB foils, during thinning process [START_REF] Schlegel | [END_REF]37], is not the only feature to explain this amorphous part. Indeed the Si: Fe: Al ratio impacts the nontronite crystallinity [38,39].

Regarding the other spectra collected in the eCP layer of P285NH and Armco (Fig. 7C), no satisfactory spectral decomposition was obtained. Although, it may be a serpentine (SEM, TEM analyses), no phase from the database can be associated to the experimental spectra. In conclusion, two independent analytical techniques demonstrate that the whole eCP layers of P285NH and Armco consist of precipitated Fe-Si-O phases identified as smectite (assimilated to nontronite) and serpentine.

Discussion

To summarise the experiments and results, two metallic substrates, a ferritic-pearlitic steel (P285NH) and a pure ferrite (Armco) were corroded during 720 h in MX80 bentonite medium with a synthetic water simulating the Cox pore water in equilibria with argillite. Carbonate species were not added to the experiment in order to study the silicate contribution in the corrosion processes. Due to their absence, the experiment is made without pH buffer, which could also affect the silicate contribution. As there is no pH buffer, the pH of the solution depend on the pH of H 2 O, which varies with the temperature. At 120°C, the pH of a Fe-H 2 O system is expected to be around 5.95 [40].

The CPL cross-sections of both samples were characterized with a multiscale analytical methodology that revealed similar corrosion facies made of a bi-layer structure. The inner layer is made of a compact and continuous oxide layer, which is adherent to the metallic substrate.

The external one, made of iron-rich phyllosilicates, is porous and not adherent to the inner layer.

A schematic illustration of the CPL morphology and structure is presented on figure 8 depending on the metal microstructure.

iCP : magnetite

Magnetite Fe 3 O 4 was identified by Raman spectroscopy in contact with both metallic surfaces. The oxide is present as a continuous layer of several hundreds of nanometers to, locally, up to 25 µm around the metallic substrate. The oxide layer appears adherent to the metal, nano-crystallized, compact with no visible porosity even at nanoscale. The metal microstructure does not influence the nature, crystallinity and compactness of the iCP.

The magnetite layer morphology differs in P285NH and Armco samples. For P285NH, Figure 2C clearly shows that local penetrations in the metal are observed through pearlite grains. Although not all of the pearlite grains are associated with such penetrations. This suggests the presence of local galvanic coupling on perlite grains on the sample [41]. For some pearlite grains, coupling occurs between the ferrite lamellas and the cementite lamellas as both components are electrical conductors. Ferrite from pearlite grains is oxidized while cementite acts as a cathode where water reduction can occur, promoting an enhanced corrosion current [42]. Thus, cementite runs as a cathodic site and the consequence is the heterogeneous thickness of the magnetite layer.

For Armco, some local penetrations are also present but are significantly rarest (maximum three per cross section of diameter 2.5 cm). These later can be related either to the metal microstructure or to stochastic effects of the corrosion propagation. To conclude, the metal microstructure influences the morphology and thickness of the iCP. The presence of pearlite in the metal microstructure favours local penetration and heterogeneous thickness of the iCP.

The presence of magnetite at the M/CPL interface was already demonstrated in several studies of steel corrosion in Cox medium [START_REF] Schlegel | [END_REF][4][5]7,9,13] as well as in MX80 silicate environment media [11,12]. In the literature, when studied at a nanoscale, the oxide layer appears with thickness ranging from several dozen to several hundred of nm [43].

Two hypotheses can be considered for magnetite formation. The first one assumes that magnetite formation occurs by co-precipitation of Fe 3+ and Fe 2+ either directly [44] or by Schikorr reaction [45]. Magnetite would then form on the surface of the metal. However, E-pH diagram of Fe-H 2 O system shows that between 100 and 150°C, at pH less than 6, magnetite is not stable [START_REF] Chivot | Collection science et technique[END_REF]. Note that these diagrams do not include local pH variation around the metallic samples.

The second one assumes that the oxide layer grows to the detriment of metal by solid-state diffusion of oxygen as stated in the PDM and DPCM model [START_REF] Macdonald | [END_REF]48]. With this hypothesis, the formation of the oxide layer is linked to a steel passive layer growth [15]. The higher oxide thickness measured in this experiment compared to simulation [15] and other experimental studies [43,49] may be linked to the slow kinetics of montmorillonite dissolution [50], which gives time for magnetite to grow.

In corrosion modelling, the sum of the anodic and cathodic current must be null (𝑗 𝑒 + 𝑗 𝐹𝑒 𝐼𝐼𝐼

). This is the coupled current condition at the α-Fe/Fe 3 O 4 interface (Fig. 9B). In + 𝑗 𝑣𝑜 = 0 presence of pearlite grains, cementite is known as preferential cathodic sites i.e. the kinetic constant of water reduction on cementite is higher than on magnetite [51]. Then, electron current runs through cementite since the water reduction at the cementite/H 2 O interface is faster than at the magnetite/H 2 O interface (Fig. 9A). Thus, the coupled current condition is no more fulfilled at the α-Fe/Fe 3 O 4 interface. These preferential cathodic paths will activate the oxidation of the α-Fe/Fe 3 O 4 interface between the cementite lamellas. This induce heterogeneous corrosion. In presence of pearlite grains, corrosion of α-Fe runs like in « anodic potentiotatic mode » (anodic enhancement) since all or a major part of the electronic current doesn't cross the magnetite layer. Thus, the electrochemical condition (coupled current equation) at the α-Fe/Fe 3 O 4 interface is not the same with and without cementite in the metal microstructure.

Whatever the formation process, in liquid [44] or in solid state [52], the magnetite grains crystallinity is not affected by this corrosion mechanism. As no porosity was observed on the oxide layer at nanoscale, its water permeability can be questioned as well as its impact on the corrosion rate.

eCP : Fe-rich phyllosilicate

For Armco and P285NH, the eCP differs from the iCP by the presence of Si. Up to the magnetite layer, a diffuse, heterogeneous and very porous area containing iron rich silicate is observed on SEM-EDS images. The comparison between XANES reference spectra of MX80 bentonite, montmorillonite and quartz (Fig. 7A) with spectrum acquired around the metallic sample shows spectral differences. Thereby, phases observed in the eCP correspond to the transformation of MX80 bentonite from the environment that have incorporated iron ions to form iron-rich phyllosilicate: nontronite-like smectite, and serpentine type. Phyllosilicate formation depends on geochemical parameters such as (but not limited to) temperature, pressure, pH, oxic/anoxic medium, and Fe/clay ratio. Serpentine and/or smectite formation was already observed in steel/clay interaction experiments [START_REF] Schlegel | [END_REF]5,31,38,[53][54][55][56]. Literature suggests a competition between these two groups. For example, Wyoming bentonite aging for several months in contact with iron up to 150°C under neutral pH, resulted in the formation of serpentines for an iron/clay mass ratio > 0.5 [31]. However, for a mass ratio equals to 0.1, and/or under alkaline conditions (pH = 10-12), smectite were identified. Other results, obtained with an iron/clay mass ratio higher than 0.2 at 80°C in presence of FoCa7 bentonite, likewise hinted at smectite transformation to serpentines [55]. Indeed, natural or synthetic smectites (nontronite in particular) are preferentially formed at lower temperatures, neutral pH in reducing conditions [22,38,56,57], and are destabilized above 80°C [54,58,59]. Furthermore, their destabilization was systematically observed for pH higher than 7 at 80°C, in Fe/clay ratio equals to 1 [18]. However, whatever the amount/nature of precipitated phyllosilicate, the formation of such family of phases leads to a local pH decrease [60][61][62]. Hence, the following of the three parameters: temperature, pH and Fe/clay ratio controls smectite/serpentine transformation. In the present study, both serpentine and smectite neoformation were identified in P285NH and Armco.

The thickness of the iCP as well as the porosity and poorly adherent properties of the eCP contrast with experiments where the corrosion environment contain carbonate competing with silicate. Indeed, in these studies, it seems that Si-rich phases form closer to the metal interface than Fe carbonate such as siderite or chukanovite, which could explain their compactness [START_REF] Schlegel | [END_REF][4][5]7,10]. The absence of carbonate in the environment precludes the formation of a carbonate layer such as siderite that could tend to favour the formation of a fully compact CPL from the early stages of corrosion.

Smectite is a swelling clay [63], which could influence ion exchange. After 720 h experiment, due to its porous and poorly adherent properties, smectite would not influence diffusion processes. The oxide inner layer seems compact and adherent to the metal, which may have an impact on the corrosion mechanism. Further analysis of these corrosion products, for example a D 2 O immersion [64], may help to better identify their properties in the light of diffusion processes, specifically for the magnetite layer.

Conclusion

In this study, the CPL formed at the interface of two metallic substrates in contact with MX80 bentonite was characterized at the micro-and nano-scales. The CPL is made of a magnetite-rich iCP and a Fe-rich phyllosilicate-rich eCP for both substrates. The metal microstructure has an impact only on the iCP morphology.

The magnetite layer appears continuous with 500 nm average thickness and variations of thickness up to 25 µm related to metal microstructures, in particular with pearlite grains for P285NH. Its polycrystalline nature was highlighted with nanometric grains diameters. The magnetite layer is compact, non-porous at nanoscale and adherent to the metal. Several hypotheses on its formation were discussed.

Moreover, two types of Fe-rich phyllosilicates coexist in a porous and poorly adherent eCP: smectite (nontronite type) and serpentine. Smectite is swelling clay that could reduce the diffusion processes between iron and its environment. However, after 720 h experiment, the eCP morphology may enable easy ionic flux from the environment to steel.

The morphology of the CPL is modified compared to CPL that formed in presence of carbonate / silicate competition, probably because of the absence of carbonate buffer and iron carbonate formation. The presence of carbonate leads to a more compact CPL around the metallic substrate, which may help to reduce the interaction between steel and its environment from the early stages of corrosion. However, this silicate environment favours the magnetite growth. Further study of the CPL water permeability would evidence the impact of these phases on the ionic exchanges between steel and the corrosion environment.

[62] B.M.J. Thien, N. Godon, A. Ballestero, S. Gin, A. Ayral, J. Nucl

Figure 1 -Figure 2 -

 12 Figure 1-SEM micrographs in BSE obtained on P285NH (A) and Armco (B) cross-section.The white lines correspond to the interface between the iCP and the eCP

Figure 3 -Figure 4 -

 34 Figure 3-Raman spectra acquired at the M/CPL interfaces in the iCP of P285NH and Armco

Figure 5 -

 5 Figure 5-SEM micrographs in BSE and EDS chemical mapping of the corrosion layer on P285NH (left) and Armco (right). The white lines delimit the iCP.

Figure 6 -

 6 Figure 6-HR-TEM micrographs obtained on P285NH sample in eCPL. B and D correspond to zoom obtained on A and B, respectively

Figure 7 -

 7 Figure 7-XANES Si K-edge reference spectra obtained on MX80 bentonite, montmorillonite, quartz (A) and experimental spectra (black) acquired on the eCP of P285NH and Armco (B and C) with associated fit, nontronite and amorphous SiO 2 (grey).

Figure 8 -Figure 9 -

 89 Figure 8-Schematic representation of the iron-clay corrosion interface after 720 h at 120 °C for P285NH (left) and Armco (right)

Table 1 -

 1 . Mater. 2012, 427, 297. [63] D.A. Laird, Appl. Clay Sci. 2006, 34, 74. [64] M. Saheb, P. Berger, L. Raimbault, D. Neff, P. Dillmann, J. Nucl. Mater. 2012, 423, 61 Chemical composition of the two low-alloy carbon steels used in the corrosion

					experiment (wt. %)
	Elements	C	Si	Mn	S	P	Cr	Mo	Ni	Cu	Fe
	P285NH 0.15 0.25 1.03 0.001 0.006 0.081 0.021 0.067 0.073 Base
	Armco	0.002 0.009 0.33 0.019 0.005 -	-	-		0.03	Base

Table 2 -

 2 Chemical composition of Cox synthetic solution without carbonated compound

	Compounds	Concentration [mmol.L -1 ]
	Na 2 SO 4	10
	KCl	0.96
	CaCl 2	10
	MgCl 2	2.5
	SrCl 2	0.14
	NaCl	14.7

Table 3 -

 3 Mineral composition of MX80 Bentonite

	Mineral	Dry proportion (vol. %)
	Na/Ca-Montmorillonite	84.35
	Quartz	7.03
	Albite	3.49
	Biotite	2.78
	Microdine	1.07
	Calcite	0.97

Table 4 -

 4 Inter-planar distance (d) of magnetite Fe 3 O 4 extracted from the diffraction pattern of

	Fig. 4A
	d hkl (Å)
	4.9
	3.0
	2.6
	2.1
	1.7
	1.6
	1.5

Table 5 -

 5 Chemical composition of Region of Interest (RoI) identify on Fig.5for P285NH (1 and 2) and Armco(3 and 4), at. %

	RoI	.1	.2	.3	.4
	O	56	61	60	66
	Si	18	21	18	21
	Fe	17	5	17	6
	Al	7	10	4	7
	Mg	1	1.5	1	1
	Na	1	1.5	n.d.	1

Table 6 -

 6 Best FIT parameters for the linear regression performed on XANES Si K-edge

		spectrum
	Sample	Contribution for linear regression	R²	χ²
	P285NH 89% nontronite + 11% SiO 2	0.99	0.01
	Armco	93% nontronite + 7% SiO 2	0.99	0.01

Acknowledgements

This work was partially supported by the ANR funding Investissements d'avenir ANR-11-EQPX-0020 (Equipex GENESIS), by the Fonds Européen de Developpement Régional and by the Région Basse-Normandie.

The CLS is supported by NSERC, CIHR, NRC, the Province of Saskatchewan, WEDC and the University of Saskatchewan.


Tristan Guillot
email: tristan.guillot@oca.eu

Leigh N Fletcher
email: leigh.fletcher@le.ac.uk

Revealing Giant Planet Interiors Beneath the Cloudy Veil

Observations from the Juno and Cassini missions provide essential constraints on the internal structures and compositions of Jupiter and Saturn, resulting in profound revisions of our understanding of the interior and atmospheres of Gas Giant planets. The next step to understand planetary origins in our Solar System requires a mission to their Ice Giant siblings, Uranus and Neptune.

Juno and Cassini unveil the depths of Jupiter and Saturn

Legend has it that Zeus (Jupiter) shrouded himself beneath a cloudy veil to hide his mischiefs, and that his wife Hera (Juno) was able to lift these clouds to confront him. The Juno mission, in orbit around Jupiter since 2016, indeed was able to lift some of the veil surrounding that planet. With a slightly different approach, the Cassini mission did the same at Saturn. But the task is proving more challenging than initially anticipated: Jupiter and Saturn are not eager to see all of their inner secrets so easily discovered.

Thanks to its special polar orbit, Juno has been able to measure Jupiter's gravity field with unprecedented accuracy, more than 2 orders of magnitude better than what had been achieved previously [START_REF] Bolton | Jupiter's interior and deep atmosphere: The initial pole-topole passes with the Juno spacecraft[END_REF][START_REF] Iess | Measurement of Jupiter's asymmetric gravity field[END_REF], a milestone for research on interior models of this archetypal giant planet. This led to the resolution of a long-standing mystery: would the strong banded pattern of zonal winds in their atmosphere (with speeds exceeding 100m/s) be limited to a shallow atmospheric layer, or would they penetrate deep into the interior? Juno has revealed that Jupiter's zonal winds extend down to about 3000 km below the visible clouds and that differential rotation becomes small at greater depths [START_REF] Kaspi | Jupiter's atmospheric jet streams extend thousands of kilometres deep[END_REF][START_REF] Guillot | A suppression of differential rotation in Jupiter's deep interior[END_REF]. Similar measurements from the Grand Finale orbits of Cassini (the last part of Cassini's mission before its final plunge into the giant planet in 2017) revealed that, as anticipated from the Juno results, Saturn's deep winds must extend deeper, to about 9000 km [START_REF] Iess | Measurement and implications of Saturn's gravity field and ring mass[END_REF][START_REF] Galanti | Saturn's Deep Atmospheric Flows Revealed by the Cassini Grand Finale Gravity Measurements[END_REF]. This shows that atmospheric zonal winds extend relatively deep, until hydrogen becomes conductive enough to be dragged by the interior magnetic field into uniform rotation. This new insight helps us to understand the interiors, magnetic fields, and atmospheric dynamics both on our Gas Giants, and also on giant exoplanets.

The accurate gravity field determination also led to the realization that Jupiter's envelope is inhomogeneous [START_REF] Wahl | Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core[END_REF][START_REF] Debras | New Models of Jupiter in the Context of Juno and Galileo[END_REF]: It contains more heavy elements somewhere within its metallic interior, about halfway inside the planet, than in its outer envelope. The presence of this so-called diluted core tells us that Jupiter, despite being internally hot and therefore mostly convective, is unable to fully mix its heavy elements.

A few years before, Cassini's discovery of free oscillations of Saturn through perturbations of its rings (9) led to a similar surprise: in order to explain the complex splitting of the modes, interactions between different types of oscillations is needed, indicating the existence of both the usual convective region where p-modes (for which the restoring force is pressure) propagate and a deep stable region where g-modes (with gravity as the restoring force) are present [START_REF] Fuller | Saturn ring seismology: Evidence for stable stratification in the deep interior of Saturn[END_REF]. In that case as well, the hypothesis of a fully convective interior led way to a more complex structure. Thus, the interiors of these planets are not as simple as previously thought. They are only partially mixed. For Jupiter, the reason certainly lies in the initial conditions: if the planet formed with a large diluted core, only partial mixing would have occurred, with large composition gradients remaining present throughout the planet's lifetime [START_REF] Vazan | Jupiter's evolution with primordial composition gradients[END_REF]. However, the causes of such a large diluted core are not clear, and the possibility that the planet witnessed a giant impact early in its history is, at present, the most likely explanation [START_REF] Liu | The formation of Jupiter's diluted core by a giant impact[END_REF]. In Saturn, the situation is different: the presence of a large stable region may be the result of an extensive helium phase separation leading to the formation of an almost pure-helium core [START_REF] Mankovich | Evidence for a Dichotomy in the Interior Structures of Jupiter and Saturn from Helium Phase Separation[END_REF]. A detailed characterization of the deep interiors will require seismology. But progress will come also from a better coupling of interior, dynamo and atmospheric models constrained by the Juno and Cassini measurements.

Ever-Changing Atmospheres

Giant planet atmospheres provide a lens through which we can catch a glimpse of the hidden depths. Peering through clearings in the clouds, visible and infrared observations reveal a complex story involving composition changes driven by vertical and horizontal motions [START_REF] Fletcher | How well do we understand the belt/zone circulation of Giant Planet atmospheres?[END_REF]. Conventional theoretical models predict the formation of clouds above a well-defined condensation level, with uniform mixing below that point. Radiometric observations, which penetrate through the clouds and can probe very deep compared to visible or infrared observations, provide a strikingly different picture. In fact, ground-based observations from the Very Large Array 30 years ago showed that ammonia is depleted across most of Jupiter except near its equatorial zone [START_REF] De Pater | Peering through Jupiter's clouds with radio spectral imaging[END_REF]. Juno's MicroWave Radiometer (MWR) further demonstrated that Jupiter's ammonia has a variable abundance as a function of depth and latitude down to at least 200km below the cloud tops, far beneath the expected cloud base [START_REF] Bolton | Jupiter's interior and deep atmosphere: The initial pole-topole passes with the Juno spacecraft[END_REF][START_REF] Li | The distribution of ammonia on Jupiter from a preliminary inversion of Juno microwave radiometer data[END_REF]). Juno's key contributions have been revealing the great depth of the ammonia depletion and the fact that it affects most of the planet.

With hindsight, the strong spatial variability in the sub-cloud layers is not altogether unexpected -the spatial distribution of disequilibrium species (like phosphine, arsine, germane, and para-hydrogen) and lightning (indicating the vigor of moist convection) all hint that the strength of vertical mixing changes significantly with latitude, primarily on the length scales of the belts and zones [START_REF] Fletcher | How well do we understand the belt/zone circulation of Giant Planet atmospheres?[END_REF]. But whereas lightning may be a feature only of the weather layer and the water clouds, disequilibrium species are being dredged from their quench levels, at great depths where temperatures exceed 1000 K. Thus, results from Juno suggest that the cloud-top bands are merely the tip of the iceberg, with atmospheric circulation (horizontal winds and vertical motions) penetrating to great depths.

The unexpected behavior of the volatiles has made constraining Jupiter's deep water abundance even more challenging. Because the signature of water is much smaller than that of ammonia, uncertainties caused by the spatial variability in ammonia severely affect our ability to constrain water. By focusing on the MWR data at the equator where ammonia is well-mixed, the deep water abundance could be constrained to between 1.0 and 5.1 times the solar (O/H) value [START_REF] Li | The water abundance in Jupiter's equatorial zone[END_REF]. The uncertainty remains large however, and other regions of the planet have not been explored. We should expect that the distribution of condensing species will be highly variable, with depth and latitude, on the other giant planets as well.

Jupiter and Saturn have one more trick to shield their secrets from view: they are variable with time. Jupiter's bands can expand, contract, fade away and re-appear with spectacular storms, over well-defined multi-year time periods that we are yet to fully understand [START_REF] Antuñano | Jupiter's Atmospheric Variability from Long-Term Ground-based Observations at 5 microns[END_REF]. Saturn exhibits enormous storm outbursts that contribute a significant fraction of the planet's energy budget [START_REF] Sanchez-Lavega | The Great Saturn Storm of[END_REF]. Do these episodes reflect changes in the deep interior, or are they a consequence of shallower weather-layer processes? As Juno's mission continues, and as Europe prepares the Jupiter Icy Moons Explorer (JUICE) for arrival in 2029, we may finally answer these questions and understand the complex interplay between interior and atmosphere.

Uranus and Neptune hold the keys

The last few decades have thus provided tremendous leaps in our understanding of the Gas Giants, whilst the Ice Giants Uranus and Neptune remain poorly explored and mysterious, out in the distant solar system. Ice Giant volatiles show strong equator-to-pole gradients, being massively depleted over the poles and enriched at the equator [START_REF] Sromovsky | Methane depletion in both polar regions of Uranus inferred from HST/STIS and Keck/NIRC2 observations[END_REF]. Clouds are organized into banded patterns, but these are not apparently reflected in wind and temperature contrasts [START_REF] Fletcher | How well do we understand the belt/zone circulation of Giant Planet atmospheres?[END_REF]. Storms erupt and drift in latitude, and episodic outbursts may teach us about convection in environments where strong density gradients serve to stabilize atmospheric layers, maybe even separating them from the deeper interiors.

Uranus and Neptune's abundant methane clouds and storms have properties similar to those of water clouds in Jupiter and Saturn in terms of abundance and heat content. However, the methane clouds are located at much lower optical depths and are thus much easier to access and study. A mission to Uranus or Neptune including an orbiter with deep remote sensing capability and a probe would enable mapping the deep atmospheric temperature and composition while having a fixed, reliable reference profile. It would thus fully characterize the deep atmosphere of an Ice Giant and constrain its interior structure. This would be key to understand the mechanisms that govern the physics of clouds and storms in planets with hydrogen atmospheres. Indeed, the expanding census of planets beyond our Solar System suggest that Ice-Giant-sized worlds are a common endpoint of the planet formation process, such that future exploration of the Ice Giant atmospheres and interiors, and how they differ from Jupiter and Saturn, is the vital next step in our exploration of the Solar System, filling in the missing link between Gas Giants and terrestrial worlds.

We have begun lifting the veil on the interiors and atmospheres of Jupiter, Saturn. Doing so on Uranus and Neptune is within reach with an ambitious robotic mission to the Ice Giants. It is needed to understand the origin of the Solar System and to analyze with confidence data obtained for the numerous planets with hydrogen atmospheres in our Galaxy.

Figure 1 :

 1 Figure 1 : The four giant planets shown to scale. From closest to furthest (or right to left) respectively: Jupiter (Juno perijove 6, Credit:NASA/SwRI/MSSS/Gerald Eichstädt/Seán Doran); Saturn during the great storm of 2010-2011 (Cassini, Credit: NASA/JPL-Caltech/Space Science Institute); Uranus and Neptune (Hubble, Credits: NASA/ESA/A. Simon (NASA Goddard Space Flight Center), and M.H. Wong and A. Hsu (University of California, Berkeley)).

Acknowledgements:

T. Guillot was supported by the Centre National d'Études Spatiales and a fellowship from the Japan Society for the Promotion of Science. L.N. Fletcher was supported by a Royal Society Research Fellowship and European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, grant agreement No 723890) at the University of Leicester.

Author Contributions:

TG wrote the parts of this article focussing on planetary interiors; LNF wrote parts of this article discussing planetary atmospheres. Both authors reviewed and edited this article.

Competing Interests:

The authors declare no competing interests in the preparation of this article.