Gustavo Palácio

Damien Boyer

Sandrine Therias

Sandra H Pulcinelli

Rachid Mahiou

Geneviève Chadeyron

Celso V Santilli
email: cv.santilli@iq.unesp.br

Accelerated ultraviolet aging of structural and luminescent properties of the ureasil-polyether hybrid materials U-PEO:Eu 3+ and U-PPO:Eu 3+

Keywords: Organic-inorganic hybrid materials, polyether, sol-gel, photooxidation, photoluminescence



Formate was the main photoproduct formed during the ultraviolet aging of ureasil-polyether hibrid (U-PEO and U-PPO) .

 In Eu 3+ doped hybrid (U-PEO:Eu 3+ and U-PPO:Eu 3+ ) a concomitant decreases of the Eu 3+ complexed to urea bridges, the spatially correlated siloxane nodes, and the Judd-Ofelt parameter Ω 2 (R) were observed during the photodegradation process  The sensitivity of structural features and PL properties arising from the photoinduced degradation of ureasil-polyether films could be exploited in the fabrication of new transparent, flexible, and biocompatible UV dosimeters and sensors.

Introduction

The combination of lanthanide compounds and organic-inorganic hybrids (OIHs) provides the benefits of the intrinsic characteristics of sol-gel derived host materials (such as processability, mechanical, thermal, and chemical stability [START_REF] Kataoka | Drastically Improved Durability and Efficiency of Silicon Solar Cells Using Hyper-Stable Lanthanide Coordination Polymer Beads[END_REF][START_REF] Druvari | Polymeric Quaternary Ammonium-Containing Coatings with Potential Dual Contact-Based and Release-Based Antimicrobial Activity[END_REF][START_REF] Abohalkuma | Corrosion protection of carbon steel by special phosphonic acid nanolayers[END_REF], and biocompatibility) and the outstanding luminescent properties of lanthanide compounds (such as line-like emissions, long luminescence lifetimes, large Stokes shifts, and high luminescence quantum yields) [START_REF] Yan | Recent progress in photofunctional lanthanide hybrid materials[END_REF][START_REF] Feng | Hybrid Materials Based oon Lanthanide Organic Complexes : a review[END_REF]. Research into lanthanide-doped OIH sol-gel materials emerged about thirty years ago, with the use of this soft chemistry route to prepare homogeneous and transparent materials with tunable composition and tailored properties [START_REF] Faustini | History of Organic-Inorganic Hybrid Materials: Prehistory, Art, Science, and Advanced Applications[END_REF][START_REF] Parola | Optical Properties of Hybrid Organic-Inorganic Materials and their Applications[END_REF][START_REF] Sanchez | Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market[END_REF]. In general, the lanthanide ions (Ln) embedded in an OIH host matrix are linked to matrix moieties by specific interactions such as covalent, coordination, electrovalent, dipolar, and hydrogen bonds [START_REF] Binnemans | Lanthanide-Based Luminescent Hybrid Materials[END_REF][START_REF] Binnemans | Interpretation of europium (III) spectra[END_REF]. The existence of specific guest-host (Ln-OIH) coordination sites can overcome the disadvantages of concentration quenching, clustering or aggregation of emitters, inhomogeneity, or poor chemical stability [START_REF] Yan | Recent progress in photofunctional lanthanide hybrid materials[END_REF][START_REF] Feng | Hybrid Materials Based oon Lanthanide Organic Complexes : a review[END_REF][START_REF] Binnemans | Lanthanide-Based Luminescent Hybrid Materials[END_REF][START_REF] Li | Multifunctional Ionogels Incorporated with Lanthanide (Eu3+, Tb3+) Complexes Covalently Modified Multi-Walled Carbon Nanotubes[END_REF].

An important example of the easy tunability of the guest-host interaction site is provided by the Ln-OIH family classified as di-ureasils. These materials present urea bridges between a silicate backbone and polyether (PE) macromers of different molecular weights, known as U-PE [START_REF] Dahmouche | Structural modelling of Eu3+-based siloxane-poly(oxyethylene) nanohybrids[END_REF][START_REF] Molina | Structure and luminescence of Eu3+-doped class I siloxane-poly(ethylene glycol) hybrids[END_REF]. The hybrids display an intrinsic broad green-blue emission from the urea bridges, as well as emission originating from electron-hole recombination in the siliceous domains of the inorganic backbone [START_REF] Fernandes | Highly Photostable Luminescent Poly ( E -caprolactone ) siloxane Biohybrids Doped with Europium Complexes[END_REF][START_REF] Nunes | Structure and photoluminescent features of di-amide crosslinked alkylenesiloxane hybrids[END_REF][START_REF] Huang | Encapsulating a Ternary Europium Complex in a Silica / Polymer Hybrid Matrix for High Performance Luminescence Application[END_REF]. The energy transfer from the di-ureasil emitter centers to incorporated Eu 3+ ions results in white luminescence constituted of Eu 3+ red emission in addition to the broad green-blue emission arising from the hybrid host matrix [START_REF] Ferreira | Energy-Transfer Mechanisms and Emission Quantum Yields In Eu3+ -Based Siloxane-Poly ( oxyethylene ) Nanohybrids[END_REF]. Furthermore, the emission color can be tuned across the CIE chromaticity diagram by changing the excitation wavelength, the PE chain length, or the amount of incorporated europium salt [START_REF] Li | Multifunctional lanthanide coordination polymers[END_REF]. The two latter dependencies are especially related to the higher efficiency of energy transfer to Eu 3+ coordinated to the carbonyl groups (O carb ) of the urea bridges, which are located near the hybrid host emitting centers, as compared to Eu 3+ coordination to ether oxygens of the PE chain [START_REF] De | Eu3+ Coordination in an Organic/Inorganic Hybrid Matrix with Methyl End-Capped Short Polyether Chains[END_REF]. This interesting characteristic has resulted in a large number of studies focused on the design and synthesis of U-PE:Ln 3+ for applications in different areas such as chemical sensing, bio-imaging, dye-sensitized photoelectrochemical cells, and organic light-emitting devices (OLEDs) [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF][START_REF] Wei | Advances in luminescent lanthanide complexes and applications[END_REF][START_REF] Xu | Recent advances in near-infrared emitting lanthanide-doped nanoconstructs: Mechanism, design and application for bioimaging[END_REF][START_REF] Wang | Selective fluorescent sensing and photodegradation properties of Tb(III)-based MOFs with different bulky backbone ligands[END_REF][START_REF] Lan | Nanoscale metal-organic frameworks for phototherapy of cancer[END_REF][START_REF] Zhou | Double protected lanthanide fluorescence core@shell colloidal hybrid for the selective and sensitive detection of ClO[END_REF].

A difficulty of long term application is that organic materials and polymers, such as those used in OIHs, are degraded during their life cycle. Depending on the application, these materials are subjected to environmental aging factors including UV light, adverse temperature, and the atmosphere [START_REF] Giancaterina | Photochemical evolution of poly ( ether ether ketone )[END_REF]. Degradation and oxidation of the polymers have several serious effects on their chemical, physical, and mechanical properties including molecular weight, tensile strength, impact strength, and creep, among others [START_REF] Randy | Photodegradation and photo-oxidation of synthetic polymers[END_REF]. Changes in mechanical properties result from reactions that either reduce the size of molecules or lead to crosslinking between polymer chains to form rigid three-dimensional structures [START_REF] Rouillon | Is carbonyl index a quantitative probe to monitor polypropylene photodegradation ?[END_REF]. In this way, chain scission and oxidation reactions can be intimately coupled in polymer degradation and can affect the stability of the OIH:Ln 3+ complex, causing loss of the Ln 3+ optical properties [START_REF] De | Eu3+ Coordination in an Organic/Inorganic Hybrid Matrix with Methyl End-Capped Short Polyether Chains[END_REF]. The most common mechanisms of polymer degradation are photooxidation and thermooxidation [START_REF] Lubineau | A review of strategies for improving the degradation properties of laminated continuous-fiber / epoxy composites with carbon-based nanoreinforcements[END_REF][START_REF] White | Weathering of polymers : mechanisms of degradation and stabilization , testing strategies and modelling[END_REF][START_REF] Iring | Thermal Oxidation of Polyethylene and Polypropylene: Effects of Chemical Structure and Reaction Conditions on the Oxidation Process[END_REF]. These types of degradation are often triggered by macroradical formation from the abstraction of hydrogen atoms, leading to scission of the main chain [START_REF] Caulfield | Some Aspects of the Properties and Degradation of Polyacrylamides[END_REF]. In the case of polyethers such as polyethylene oxide (PEO) and polypropylene oxide (PPO), degradation proceeds by random chain scission, resulting in several oxidation products: esters, aldehydes, and formates [START_REF] Morlat | Phototransformation of water-soluble polymers . I : photoand thermooxidation of poly(ethylene oxide) in solid state[END_REF][START_REF] Morlat | Phototransformation of water-soluble polymers . Part II : photooxidation of poly ( ethylene oxide ) in aqueous solution[END_REF][START_REF] Hassouna | Photochemical behaviour of poly (ethylene oxide) (PEO) in aqueous solution : Influence of iron salts[END_REF][START_REF] Hassouna | Influence of water on the photodegradation of poly ( ethylene oxide )[END_REF]. The photooxidation and thermooxidation mechanisms are quite similar, differing in terms of the amounts of the main oxidation products. Thermooxidation produces equal amounts of esters and formates, whereas photooxidation generates formates and esters in a ratio of 5:1 [START_REF] Vijayalakshmi | Photocatalytic Degradation of Poly (ethylene oxide) and Polyacrylamide[END_REF].

In recent work [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF] we have demonstrated that specific coordination of Eu 3+ to the carbonyl groups (O carb ) of the urea bridges and of Li + to the ether-type oxygen (O eth ) can be exploited to prepare U-PE materials with multifunctional properties, namely photoluminescence (PL) and ionic conduction. This emerging dual functionality was achieved by the co-incorporation of Eu 3+ and Li + into OIH materials based on U-PEO and U-PPO with similar molecular weights (1900 g mol -1 for PEO and 2000 g mol -1 for PPO) using a metal molar ratio fixed at [O carb ]/[Eu 3+ ] = 3 and [O eth ]/[Li + ] = 15. Although the degradation of PEO and PPO is well documented [START_REF] Morlat | Phototransformation of water-soluble polymers . I : photoand thermooxidation of poly(ethylene oxide) in solid state[END_REF][START_REF] Morlat | Phototransformation of water-soluble polymers . Part II : photooxidation of poly ( ethylene oxide ) in aqueous solution[END_REF][START_REF] Hassouna | Photochemical behaviour of poly (ethylene oxide) (PEO) in aqueous solution : Influence of iron salts[END_REF][START_REF] Hassouna | Influence of water on the photodegradation of poly ( ethylene oxide )[END_REF], the photodegradation and the effect of photo ageing on the PL properties of Eu 3+ doped materials (U-PEO:Eu and U-PPO:Eu) have not been reported. The aim of this study was therefore to investigate the effect of the ultraviolet light irradiation time (λ ≥ 300 nm) on the photodegradation of U-PE film prepared with polymer of different nature and molecular weight, and doped with several [O carb ]/[Eu 3+ ] molar ratios. Special attention was paid to correlate the aging on the PL properties of U-PEO:Eu 3+ and U-PPO:Eu 3+ materials with the modification of the chemical environment of Eu 3+ induced by photo degradation of PE chains. The photodegradation studies were performed under air at atmospheric pressure using an accelerated aging device with λ > 300 nm (SEPAP12/24). The effects of aging time on the photodegradation products, the OIH nanostructure, and changes of the Eu 3+ complexation site were investigated using Fourier transform infrared (FTIR) spectroscopy, small-angle X-ray scattering (SAXS), and photoluminescence techniques.

Experimental

Synthesis of the materials

The OIH materials were synthesized by the sol-gel method, according to a well-known procedure [START_REF] Santilli | Controlled Drug Release from Ureasil -Polyether Hybrid Materials[END_REF].

In brief, the ureasil (U) cross-linking agent IsoTrEOS (3-(isocyanatopropyl)triethoxysilane, Sigma-Aldrich, 95% purity, CAS #24801-88-5) was covalently bonded to both ends of the different PE macromer by reacting the terminal amino-propyl groups, followed by the sol-gel reaction involving the Si(OEt) 3 hydrolysis, followed by the condensation reaction to form ureasil cross-linking nodes. This procedure was adopeted to prepare U-PEO with mw 1900 g.mol -1 (Jeffamine ® ED-2003), (CAS #65605-36-9) and UPPO with mw 230 (Jeffamine ® D-230), 2000 (Jeffamine ® D-2000) and 4000 g.mol -1 (Jeffamine ® D-4000 (XTJ-510)) (CAS #9046-10-0) using as reactant Jeffamine ® ED-2003, Jeffamine ® D-230, Jeffamine ® D-2000, and Jeffamine ® D-4000, respectively. The doping with different Eu 3+ loading was achieved by suitable mass of europium(III) chloride hexahydrate (Fluka, Aldrich, 99.9% purity, CAS #13759-92-7) dissolved into the OIH precursor ethanoic solution in order to prepare U-PEO:Eu 3+ and U-PPO:Eu 3+ samples with [O carb ]/[Eu 3+ ] of 0.75, 1.5, 3.0, 4.5, and 6.0. Films of 50 μm thick were prepared by casting OIH sol on a Teflon mould and drying at room temperature (25 °C).

Accelerated ultraviolet aging

The accelerated photodegradation was performed in the presence of air, at 60 °C, in a SEPAP 12/24 device. This apparatus was designed for the study of polymer photodegradation during artificial aging corresponding to moderately accelerated conditions [START_REF] Morlat-Therias | Polymer / carbon nanotube nanocomposites : Influence of carbon nanotubes on EVA photodegradation[END_REF]. The device was equipped with four medium-pressure mercury lamps (Mazda MA 400) in a vertical position at one axis of the chamber and the samples were placed on a rotating carousel 13 cm in diameter, which could hold 24 samples. Wavelengths below 300 nm were filtered using a glass envelope. The OIH samples were exposed in the form of rectangular films (2 x 3 cm) with thickness of 50 µm.

Characterization techniques

The FTIR spectra of the OIH films were recorded with a Nicolet 760-FTIR spectrometer operating in transmission mode and controlled with OMNIC software. Spectra were obtained with accumulation of 32 scans and 4 cm -1 resolution.

The absolute quantum yield (AQY) efficiencies and the emission spectra (upon excitation at 365 nm) were determined using a C9920-02G PL-QY system (Hamamatsu). This consisted of a 150 W monochromatized Xe lamp, an integrating sphere (with Spectralon ® coating, Ø = 3.3 inches), and a high sensitivity CCD spectrometer for detecting the entire spectral luminescence (the emission spectra were integrated from 576 to 728 nm for the europium-based compounds). The automatically controlled excitation wavelength range was from 250 to 500 nm, with a resolution bandwidth better than 5 nm. Absolute photoluminescence (PL) quantum yield excitation spectra were obtained by scanning the excitation and monitoring the overall luminescence of Eu 3+ .

The photooxidized samples were submitted to NH 3 chemical treatment in order to identify the photoproducts. The irradiated films were exposed to NH 3 gas in all-Teflon vessels, at room temperature, in a simple flow system that could be sealed off to permit the reaction to proceed. In parallel to these treatments, it was observed that the unoxidized samples without Eu 3+ loading did not react with the gas. This procedure, which has been described in a previous paper [START_REF] Hassouna | Influence of water on the photodegradation of poly ( ethylene oxide )[END_REF], was performed for the photooxidized OIH materials without and with Eu 3+ loading.

The nanostructures of the as-prepared OIH and photooxidized samples were analyzed by small-angle X-ray scattering (SAXS) measurements performed at the SAXS1 beamline of the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil). The beamline was equipped with a 2D Pilatus 300k detector located 910.9 mm from the sample, recording the image of the scattering intensity, I(q), as a function of the modulus of the scattering vector, q = 4π/λsin(ε/2), where ε is the X-ray scattering angle. The data were normalized considering the varying intensity of the direct X-ray beam, the detector sensitivity, and the sample transmission. The intensity of the parasitic scattering due to the cell windows and vacuum was subtracted from the total scattering intensity.

The Differential Scanning Calorimetry (DSC) measurements were carried out using a Q100 analyzer (TA Instruments. The samples were heated at the rate 5 °C/min in the range -90 °C -100 °C, nitrogen was used for the gas purge, supplied at a flow rate of 50 mL/min. The melting enthalpy (ΔH m ) was recorded during the first heating run. The degree of crystallinity (D C ) was calculated from the relationship between ΔH m and the standard melting enthalpy for 100 % crystalline PEO (ΔH p ) (196.4 J/g), following the equation: D C (%) = (ΔH m / ΔH p ) * 100% [START_REF] Zaldivar | Thermal properties , nanoscopic structure and swelling behavior of chitosan /( ureasilpolyethylene oxide hybrid ) blends[END_REF].

Results and Discussion

Eu 3+ complexation in the U-PEO and U-PPO host OIHs

In order to determine the [O carb ]/[Eu 3+ ] ratios that provided the best optical efficiencies for the two polyether hybrid materials (U-PEO, with mw = 1900 g.mol -1 , and U-PPO, with mw = 2000 g.mol -1 ), the absolute quantum yields (AQY) were determined as a function of the amount of Eu 3+ , using UV excitation at 365 nm. The results are shown in Figure SI.1 (Supplementary Information). Regardless of the polyether type, the highest AQY values were obtained for [O carb ]/[Eu 3+ ] = 3, with values of 2.3% and 8.9% for U-PEO:Eu 3+ and U-PPO:Eu 3+ , respectively. The photoluminescence properties of OIH materials doped with different Eu 3+ concentrations were studied by Ribeiro et al. [START_REF] Ribeiro | Study of Hybrid Silica-Polyethyleneglycol Xerogels by Eu3+ Luminescence Spectroscopy[END_REF] and Bermudez et al. [START_REF] De Zea | Coordination of Eu3+ Ions in Siliceous Nanohybrids Containing Short Polyether Chains and Bridging Urea Cross-links[END_REF]. The results obtained in these studies were similar and indicated that for lower doping concentrations, the dried gels presented distributions of Eu 3+ typical of an amorphous environment, while a crystalline-like environment of Eu 3+ was observed for higher concentrations. Decreasing radiative decay rates and spectral variations suggested that the transformation of the surrounding Eu 3+ environment from amorphous-like to crystalline-like led to quenching of the Eu 3+ PL features. Considering the semicrystalline and amorphous states of U-PEO:Eu 3+ and U-PPO:Eu 3+ [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF], respectively, this explanation also supports the strong difference of AQY found for the OIHs prepared with polymers of different natures.

Photoluminescence (PL) studies were performed with the OIHs prepared with PE chains of different molecular weights (PPO of 230, 2000, and 4000 g.mol -1 , and PEO of 1900 g.mol -1 ), containing the optimized amount of Eu 3+ ([O carb ]/[Eu 3+ ] = 3). The PL emission spectra recorded upon UV excitation at 365 nm are displayed in Figure 1a. All the spectra exhibited narrow emission bands arising from f-f transitions of Eu 3+ ions from the 5 D 0 excited state to 7 F J (J = 0-4) ground states. The 5 D 0  7 F 2 transition located at 611 nm led to the strongest emission intensity, conferring a red emission feature to the samples. The broad band in the green-blue spectral range could be ascribed to the convolution of the emission from NH groups of urea bridges and the emission originating from electron-hole recombination in the siliceous domains in the inorganic backbone [START_REF] Carlos | Nanoscopic Photoluminescence Memory as a Fingerprint of Complexity in Self-Assembled Alkyl / Siloxane Hybrids *[END_REF][START_REF] Carlos | White-Light Emission of Amine-Functionalized Organic / Inorganic Hybrids : Emitting Centers and Recombination Mechanisms[END_REF][START_REF] Nobre | Eu3+-Assisted Short-Range Ordering of Photoluminescent Bridged Silsesquioxanes[END_REF], which have already been observed for unloaded matrices [START_REF] Nobre | Energy Transfer and Emission Quantum Yields of Organic -Inorganic Hybrids Lacking Metal Activator Centers[END_REF] of similar hybrids [START_REF] Fernandes | Highly Photostable Luminescent Poly ( E -caprolactone ) siloxane Biohybrids Doped with Europium Complexes[END_REF][START_REF] Nunes | Structure and photoluminescent features of di-amide crosslinked alkylenesiloxane hybrids[END_REF]. The latter feature was much stronger for U-PPO:Eu 3+ of high molecular weight (2000 and 4000 g.mol -1 ), compared to U-PEO:Eu 3+ of 1900 g.mol -1 and U-PPO:Eu 3+ of 230 g.mol - 1 , indicating a significant sensitization of the organic part upon excitation at 365 nm. This suggested that the Eu 3+ atoms in U-PPO:Eu 3+ of lower chain length were present in a crystallinelike environment similar to that observed for U-PEO:Eu 3+ . This effect of the nature and chain length of the PE was in agreement with the PL excitation spectra showing a broad band centred at 365 nm for the U-PPO:Eu 3+ hybrids of high molecular weight (2000 and 4000 g.mol -1 ), when monitoring the Eu 3+ 5 D 0  7 F 2 transition at 611 nm (Figure 1b). This band was associated with the ligand-to-metal charge-transfer (LMCT) and occurred in the same energy region of the 7 F 0 → 5 L 6 (at 395 nm) and 7 F 0 → 5 D 2 (at 465 nm) transitions, ascribed to the intra-configurational transitions of Eu 3+ [START_REF] Ferreira | Energy-Transfer Mechanisms and Emission Quantum Yields In Eu3+ -Based Siloxane-Poly ( oxyethylene ) Nanohybrids[END_REF]. This feature was consistent with an antenna effect mechanism [START_REF] Binnemans | Interpretation of europium (III) spectra[END_REF], indicating efficient transfer from the high energy levels of the ligands to high excited states of the Eu 3+ ions. This antenna effect was lower for U-PEO:Eu 3+ and for U-PPO:Eu 3+ of low molecular weight (230 g.mol -1 ). Valuable information concerning the dependency of the chemical environment of the Eu 3+ cations on the nature and chain length of the PE could be obtained from the FTIR analysis of the hydrogen bonding signature of this OIH for amide I and amide II, located in the carbonyl regions (1800-1500 cm -1 ). The FTIR spectra of 50 μm thick OIH films, either undoped or doped with Eu 3+ at [O carb ]/[Eu 3+ ] = 3, are compared in Figure 2. Irrespective of the nature and chain length of the PE, the spectra of all the undoped OIHs presented an envelope composed of three components in the amide I region, at 1717-1721 (weak), 1678-1694 (average), and 1634-1641 cm -1 (intense), together with a single intense band at 1559-1564 cm -1 in the amide II region [START_REF] Nunes | Structure and photoluminescent features of di-amide crosslinked alkylenesiloxane hybrids[END_REF][START_REF] De | Eu3+ Coordination in an Organic/Inorganic Hybrid Matrix with Methyl End-Capped Short Polyether Chains[END_REF][START_REF] Bermudez | Optically Functional Di-Urethanesil Nanohybrids Containing Eu3+ Ions[END_REF][START_REF] Gonçalves | Infrared and Raman Spectroscopic Investigation of Eu3+-Doped Mono and Di-Urethanesil Hybrid Siliceous Materials[END_REF]. Comparison of the spectra evidenced that the changes induced by Eu 3+ doping could be classified into two families. In the first, the incorporation of Eu 3+ in the high molecular weight hybrids (U-PEO:Eu 3+ and U-PPO:Eu 3+ of 2000 and 4000 g.mol -1 ) led to suppression of the two clearly separated bands and the appearance of a single and very intense band at 1608 cm -1 , indicative of complexation of the Eu 3+ atom by the resonant urea linkage [START_REF] De | Eu3+ Coordination in an Organic/Inorganic Hybrid Matrix with Methyl End-Capped Short Polyether Chains[END_REF][START_REF] Gonçalves | Di-Urethane Cross-linked Poly(oxyethylene)/Siloxane Nanohybrids Doped With Eu(CF3SO3)3[END_REF]. On the other hand, for the U-PPO:Eu 3+ with short polymer chain (230 g.mol -1 ), the suppression of the amide I and amide II envelope was not observed, suggesting that the Eu 3+ atoms were unable to disrupt the strongly hydrogen-bonded urea-urea aggregates [START_REF] Fernandes | Di-ureasil hybrids doped with LiBF4: Spectroscopic study of the ionic interactions and hydrogen bonding[END_REF], resulting in a negligible quantity of free urea groups available for Eu 3+ coordination. This could provide an explanation for the lower LMCT efficiency observed for U-PPO:Eu 3+ with shorter chains, but did not apply for U-PEO:Eu 3+ . In the latter case, the lower LMCT efficiency could be a consequence of the low amount of disordered U-PEO:Eu 3+ -urea associations resulting from the semicrystalline state of the U-PEO matrix [START_REF] De | Eu3+ Coordination in an Organic/Inorganic Hybrid Matrix with Methyl End-Capped Short Polyether Chains[END_REF]. Given these distinct classes of PL properties, an in-depth structural/optical study of the U-PEO:Eu 3+ and U-PPO:Eu 3+ (2000 g.mol -1 ) was performed in order to elucidate the modifications induced by exposure to UV light, as discussed below.

Effect of photooxidation on structural and nanostructural properties

The chemical modifications resulting from exposure of the undoped and Eu 3+ -doped U-PEO and U-PPO hybrid samples to UV light irradiation resulted in important modifications of the FTIR spectra (Figure 3). The more important feature is the appearance of new bands in the carbonyl region at 1800-1500 cm -1 . The spectra for the undoped U-PEO (Figure 3a) and U-PPO (Figure 3b) showed an intense band with a maximum at 1725 cm -1 , together with a shoulder at 1750 cm -1 . Previous studies of the photochemical behaviour of unhybridized polyethylene oxide (PEO) showed that these bands could be ascribed to the photooxidation products of the PEO chains, identified as formate (1725 cm -1 ) and ester (1750 cm -1 ) [START_REF] Morlat | Phototransformation of water-soluble polymers . I : photoand thermooxidation of poly(ethylene oxide) in solid state[END_REF][START_REF] Morlat | Phototransformation of water-soluble polymers . Part II : photooxidation of poly ( ethylene oxide ) in aqueous solution[END_REF][START_REF] Hassouna | Photochemical behaviour of poly (ethylene oxide) (PEO) in aqueous solution : Influence of iron salts[END_REF][START_REF] Hassouna | Influence of water on the photodegradation of poly ( ethylene oxide )[END_REF]. In the case of U-PPO (Figure 3b) the shoulder at 1750 cm -1 was less evident, suggesting a predominance of decomposition to formate product. For OIH:Eu 3+ samples the band at around 1725 cm -1 (Figures 3c and3d for U-PEO and U-PPO, respectively) was developed upon irradiation with faster rate for U-PEO:Eu 3+ and comparable rate for U-PPO, U-PEO and the U-PPO:Eu 3+ . This feature suggests that the semicrystalline nature of the U-PEO coupled to the Eu 3+ doping plays an import role on the photodegradation kinetics.

Moreover, slight modifications of the amide I (1635 cm -1 ) and amide II (1560 cm -1 ) bands of the urea linkage were observed during exposure of U-PEO and UPPO to UV light for 10 h. Decreased intensity and splitting of the bands indicated an increasing participation of amide groups in hydrogen-bonded aggregates [START_REF] Binnemans | Interpretation of europium (III) spectra[END_REF]. In the case of U-PEO:Eu 3+ and UPPO:Eu 3+ , there was a decrease of the band at 1608 cm -1 , which was assigned to a fingerprint of complexation of the Eu 3+ ions by the urea bridges [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF]. This suggested that photooxidation of the OIH:Eu 3+ samples led to a continuous dissociation of Eu 3+ ions from the organic-inorganic interfaces of the U-PEO and U-PPO matrices. In order to identify the U-PE oxidation products, NH 3 derivatization treatment of the photoaged films was performed according to the procedure described previously [START_REF] Morlat | Phototransformation of water-soluble polymers . I : photoand thermooxidation of poly(ethylene oxide) in solid state[END_REF], [START_REF] Morlat | Mechanisms of photooxidation of polyglycerol[END_REF]. The FTIR spectra of the photooxidized U-PEO:Eu 3+ and U-PPO:Eu 3+ films before and after ) and to the dissociation of urea complexed to Eu 3+ ions. The NH 3 reaction confirmed that the both Eu 3+ -loaded polyether hybrid materials produced similar photooxidation products. The evolution of the OIH and OIH:Eu 3+ nanostructures, as a function of the UV light exposure time, was evidenced from the SAXS curves shown in Figure 4. The main effect observed following Eu 3+ incorporation and photoaging was alteration of the peak attributed to the spatial correlation of regularly spaced siloxane cross-linked nodes [START_REF] Dahmouche | Small-Angle X-ray Scattering Study of Sol-Gel-Derived Siloxane-PEG and Siloxane-PPG Hybrid Materials[END_REF]. After loading with Eu 3+ , this peak was shifted from 2.15 to 1.66 nm -1 for U-PEO and from 1.51 to 1.44 nm -1 for U-PPO, corresponding to increases of the average correlation distance between adjacent siloxane nodes (D = 2π/q max ). Following Eu 3+ incorporation this distance is displaced from 2.92 to 3.78 nm for U-PEO and from 4.16 to 4.36 nm for U-PPO. The concomitant increased intensity of the correlation peak evidenced the increase of the electron density between the inorganic and the polymeric phases. These phenomena were due to the increase of the free volume of the polymeric matrix caused by the breaking of H-bonding associations, together with the complexation of Eu 3+ by the carbonyl oxygen of the amide group [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF]. In the case of U-PEO:Eu 3+ , the correlation peak intensity decreased as the UV exposure time increased. This could be ascribed to two effects, with (i) Eu 3+ -O carb dissociation decreasing the electronic density contrast between the inorganic and organic moieties, and (ii) scission of the PEO chains leading to a decrease of the number of spatially correlated siloxane nodes. It can be seen from the curves in Figure 4a that after 3 h of UV exposure, the I(q max ) intensity was lower for U-PEO:Eu 3+ , compared to the undoped U-PEO host. In contrast, even after 10 h of UV exposure, the intensity of the U-PPO:Eu 3+ correlation peak remained higher, compared to the U-PPO matrix, as shown in Figure 4b. This difference revealed an important role played by the nature of the polymer (semicrystalline or amorphous; hydrophilic or hydrophobic), in terms of the sensitivity to photoaging. For semicrystalline and hydrophilic polymers (such as PEO), photoaging starts in the amorphous phase, due to the easy diffusion of oxygen through the unordered chains [START_REF] Shen | Comparison of solution intercalation and melt intercalation of polymer-clay nanocomposites[END_REF]. Accordingly, the observed decrease of the electron density contrast between the siloxane nodes and the polymeric matrix indicated a more compact structure resulting from increase of the crystalline phase fraction [START_REF] Molina | Ureasilpolyether hybrid blend with tuneable hydrophilic / hydrophobic features based on U-PEO1900 and U-PPO400 mixtures[END_REF], caused by the scission of polyether chains in the amorphous phase. However, the U-PEO:Eu 3+ nanostructure was preserved, as evidenced from the maintenance of the spatial correlation between the regularly spaced siloxane cross-linked nodes.

In order to understand the nanostructural modifications caused by the photoaging, it is important to note that after 10 h of UV exposure, the absorbance (A) value for the band at 1725 cm -1 (see Figure 3), ascribed to the photoaging products, was lower for U-PPO:Eu 3+ (0.44) compared to U-PEO:Eu 3+ (0.83). This indicated that the more intense I(q max ) decrease observed for U-PEO:Eu 3 was due to degradation of the polyether chains. However, the effect of chain scission on the nanostructure of the amorphous U-PPO:Eu 3+ could be seen from the slight displacement of the q max position from 1.44 to 1.37 nm -1 , due to the formation of more compact structures [START_REF] Molina | Ureasilpolyether hybrid blend with tuneable hydrophilic / hydrophobic features based on U-PEO1900 and U-PPO400 mixtures[END_REF]. In contrast, no systematic changes were observed in the U-PEO:Eu 3+ correlation peak position following UV exposure, suggesting that the average correlation distance between two adjacent siloxane nodes of the already compact structure was less influenced by this effect, due to the semicrystalline nature of the material. Despite this complexity, the evolution of the growth of the relative area (A t /A 10 ) of the formate band at 1725 cm -1 , as well as the decreases of the amide band at 1608 cm -1 and the SAXS peak (S t /S 0 ), could be described by exponential growth (y t /y 10 = 1 + exp(k.t)) or decay (y t /y 0 = 1 -exp(k.t)) laws, as shown in Figure 5. This approach evidenced that for U-PPO:Eu 3+ , all the events occurred concomitantly, with comparable time constants (k) (Table 1), suggesting the existence of correlations between the processes occurring at nanoscopic and molecular length scales.

In contrast, for U-PEO:Eu 3+ , the different time constant values indicated the existence of a faster process that was probably governed by the facilitated transport of the oxidative species through the interface and the amorphous moieties of the semicrystalline U-PEO. This hypotheses was confirmed from the fusion enthalpies values founded from DSC measurements, with evidenced a decrease from 21% to 16% of the U-PEO crystalline phase fraction upon Eu 3+ addition (Figure SI.3a). This decrease of crystallinity degree favors the diffusion of oxygen through the unordered chains, making ease the initiation of photooxidative degradation of the amorphous phase [START_REF] Kaczmarek | The influence of transition metal salts on photo-oxidative degradation of poly ( ethylene oxide ) a[END_REF]. This fast degradation process leads to formation of low molecular weight products which can diffuse out the polymer matrix [START_REF] Vijayalakshmi | Photocatalytic Degradation of Poly ( ethylene oxide ) and Polyacrylamide[END_REF][START_REF] Kaminska | Cobalt ( II ) chloride catalysed oxidative degradation of poly ( ethylene oxide ) by a short wavelength UV-radiation[END_REF], decreasing the mass of amorphous polymer and increasing the crystallinity degrees during the first 3 h of UV light irradiation. The correlation between the increase of the crystallinity degree and the deaccelerated growth of formate production observed in this initial stage (Figure SI. 3a andb) is consistent with the explanation based on the facilitated transport of oxidative species. In addition, the second increases of the format production rate coupled with a pronounced decrease of crystallinity degrees was observed in the advanced stage of photodegradation of U-PEO:Eu 3+ . These correlated features observed in the early and advanced stages are driven by the association of europium salt with the ether type oxygens atoms occurring at the initial addition? of europium salt and also by the continuous photo dissociation of urea-Eu 3+ complexes. (single fitting image) Evolution of the relative areas of the SAXS correlation peak (S t /S 0 ), the formate formation band at 1725 cm -1 (A t /A 10 ) 1725 , the Eu 3+ -OIH decomplexation fingerprint band at 1608 cm -1 (A t /A 0 ) 1608 , and the Judd-Ofted parameter ratio (Ω 2 (R)t/(Ω 2 (R) 0 ), as a function of the time of photooxidation, for (a) U-PEO:Eu 3+ and (b) U-PPO:Eu 3+ . The continuous and dashed lines show the best fits of the exponential decay (y t /y 0 = 1-exp(k.t)) and growth (y t /y 10 = 1 + exp(k.t)) laws, respectively.

Effect of photooxidation on luminescence properties

The effects of the photodegradation on the photoluminescence (PL) properties of the U-PEO:Eu 3+ and U-PPO:Eu 3+ samples were investigated in the visible range. The photooxidation of the polymer and the decomplexation of Eu 3+ ions from the hybrid host matrix could be confirmed by following the excitation (Figure 6) and emission (Figure 7) spectra recorded as a function of the UV irradiation time. As shown in Figure 1b, the antenna effect between the organic ligand and the Eu 3+ ions was only efficient for the U-PPO:Eu 3+ sample, resulting in a broad band centred at 365 nm in the excitation spectrum. However, the excitation spectra for the photooxidized U-PPO:Eu 3+ (Figure 6b) revealed a dramatic decrease of intensity after 1 h of exposure to UV irradiation. This was accompanied by increased intensities of the bands located at 380, 395, and UV-Irradiation time (h) 5 D 2 , respectively [START_REF] Ferreira | Energy-Transfer Mechanisms and Emission Quantum Yields In Eu3+ -Based Siloxane-Poly ( oxyethylene ) Nanohybrids[END_REF]. These results were consistent with degradation of the organic moiety and consequent dissociation of Eu 3+ ions from the hybrid host matrix. The low intensity found for U-PEO:Eu 3+ could be explained by the nature of the polymer, especially the hydrophilic properties, with the water molecules that are typically found in interaction with the PEO backbone acting to quench the Eu 3+ photoluminescence [START_REF] Wang | Molecular Engineering of Host Materials for High-Performance Phosphorescent OLEDs : Zig-Zag Conformation with 3D Gridding Packing Mode Facilitating Charge Balance and Quench Suppression[END_REF]. The emission spectra recorded with excitation at 365 nm (Figure 7) revealed decreased intensities of both the broad band at 425 nm and the narrow lines ( 5 D 0 → 7 F J ). The decreased intensity of the blue band could be attributed to photooxidation of the polyether chains, which decreased the number of spatially correlated siloxane nodes, as revealed by the reduced area of the SAXS peak shown in Figure 5. This hindered the complexation of Eu 3+ to urea bridges, as observed for the shorter chain U-PPO:Eu 3+ (Figure 2a). The photodegradation of the organic moieties led to OIH-Eu 3+ dissociation at the organic-inorganic interface. These phenomena could The luminescence intensity ratio, 𝑅 = ( 5 D 0 → 7 F 2 )/( 5 D 0 → 7 F 1 ), is known as the asymmetry ratio. Higher values of 𝑅 indicate lower symmetry around the trivalent europium ions. The Judd-Ofelt parameter (Ω 2 ) and 𝑅 provide similar information about the physical nature of the bonding between Eu 3+ ions and the surrounding ligands, explaining the short range effects on the local structure around the Eu 3+ ions [START_REF] Ofelt | Intensities of Crystal Spectra of Rare-Earth Ions[END_REF][START_REF] Su | Lanthanide Anionic Metal -Organic Frameworks Containing Semirigid Tetracarboxylate Ligands: Structure, Photoluminescence, and Magnetism[END_REF][START_REF] Law | Structural Characterization of Shielded Isomeric Europium Complexes with Metal -Metal Contact[END_REF]. Similar values of the Ω 2 (R) parameter (4) were observed for the non-irradiated PEO:Eu 3+ and PPO:Eu 3 samples, indicating that the Eu 3+ atoms occupied very similar coordination (semicrystalline or amorphous) [START_REF] Yi | Reverse Lyotropic Liquid Crystals from Europium Nitrate and P123 with Enhanced Luminescence E ffi ciency[END_REF]. These Ω 2 (R) values were higher than those observed elsewhere for Eu 3+ incorporated in several oxide glasses and polymers, including PEO and PPO [START_REF] Binnemans | Lanthanide-Based Luminescent Hybrid Materials[END_REF], indicating that the symmetry of the Eu 3+ sites was distorted [START_REF] Palácio | Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials[END_REF] , [START_REF] Yi | Reverse Lyotropic Liquid Crystals from Europium Nitrate and P123 with Enhanced Luminescence E ffi ciency[END_REF]. However, after 10 h of UV exposure, the Ω 2 (R) values decreased from 4 to 3 and 2 for U-PPO:Eu 3+ and U-PEO:Eu 3+ , respectively. These decreases occurred continuously during the irradiation time, following trends comparable to the decreasing intensity of the broad blue band. This behaviour suggested that the photooxidation of the polyether chains led to modification of the surrounding Eu 3+ ligands, resulting in decreases of the asymmetry parameter.

The most noteworthy finding was the good correlation between the nanoscopic features and the local environment around the Eu 3+ emitter, as evidenced by the comparable temporal evolution of the relative SAXS peak area values (S t /S 0 ) and Ω 2 (R) t /Ω 2 (R) 0 (Figure 5). These two functions were described by exponential decay laws with comparable time constants (Table 1), clearly evidencing the sensitivity of the photoluminescence to the elimination/formation of the hydrogenbonded urea-polyether array during photooxidation of the PEO and PPO chains. These nanoscopic properties arising from the photoinduced degradation of ureasil-polyether can be exploited in the development of new transparent, flexible, and biocompatible UV dosimeters and sensors.

Table 1: Time constants (k) of the fitted exponential growth or decay law curves (y t /y 10 = 1 ± exp(k.t)) for the relative area of the SAXS correlation peak, the Judd-Ofelt parameter ratio (Ω 2 (R) t /(Ω 2 (R) 0 ), the Eu 3+ -OIH decomplexation fingerprint band at 1608 cm -1 , and the formate formation band at 1725 cm -1 (undoped in brackets), as a function of photooxidation time, for U-PEO:Eu 3+ and U-PPO:Eu 3+ .

Relative area from

Time constant K (h -1 ) U-PEO:Eu 3+ U-PPO:Eu 3+ SAXS 0.25 0.26 PL (Ω 2 (R)) 0.39 0.15 IR @1608 cm -1 0.97 0.07 IR @1725 cm - 0.45 (0.33) 0.10 (0.10)

Conclusions

Different ureasil-polyether materials based on polyethylene oxide (U-PEO) and polypropylene oxide (U-PPO), undoped and loaded with Eu 3+ , were successfully prepared by the sol-gel route in the forms of monoliths and thin films (50 μm thick). The FTIR spectra confirmed the preferential complexation of Eu 3+ by the carbonyl group (O carb ) of the urea linkage. The best photoluminescence properties were found for a doping level equivalent to [O carb ]/[Eu 3+ ] = 3. The PL results evidenced loss of the ligand-to-metal charge transfer when the Eu 3+ cations were unable to disrupt the strong hydrogen-bonded urea/urea aggregates formed using shorter chain U-PPO (230 g.mol -1 ) and semicrystalline U-PEO (1900 g.mol -1 ).

Formate was the main photoproduct formed following exposure of U-PEO and U-PPO to artificial UV light, revealing a photooxidation process quite similar to the observed for conventional PEO and PPO materials. In addition, the urea linkages present at the organicinorganic interface are weakly dependent on the time of UV light exposure as demonstrated by slight modifications of the amide bands.

The Eu 3+ addition to semicrystalline U-PEO allows to an increase of the amorphous phase with accelerate initial formation of the formate, the main photooxidation product. The photoinduced dissociation of Eu 3+ complexed to the urea and consequent association to the ether oxygen atoms of the polymer chains favors the propagation of depolymerisation reactions in the advanced stages.

The continuous recovery of the hydrogen bond of the amide structure evidenced the Eu 3+ decomplexation induced by exposure to intense UV light. This induces the decreased intensity of the broad green-blue band, as revelled by PL analyses conducted as a function of the UV exposure time.

The photooxidation of the polyether chains resulted in concomitant decreases of the spatially correlated siloxane nodes and the Judd-Ofelt parameter, Ω 2 (R). The kinetic evolution of these parameters could be described by exponential laws with comparable time constants, evidencing the sensitivity of the Eu 3+ photoluminescence to the elimination/formation of the hydrogenbonded urea-polyether array during the photooxidation of the PPO chains. In contrast, for U-PEO:Eu 3+ different time constant values were obtained, due to the existence of a faster process governed by the facilitated transport of the oxidative species through the amorphous moieties of the semicrystalline U-PEO.

Figure 1 :

 1 Figure 1: (2-columm fitting image) (a) Emission and (b) excitation spectra before photooxidation (t (h) = 0) for U-PEO:Eu 3+ (mw = 1900 g.mol -1 ) and U-PPO:Eu 3+ of different molecular weights (mw = 230, 2000, and 4000 g.mol -1 ), with [Ocarb]/[Eu 3+ ] = 3.

1 Figure 2 :

 12 Figure 2: (2-columm fitting image) Effect of Eu 3+ doping on the amide I and II regions of the FTIR spectra of OIH films for (a) U-PEO (mw = 1900 g.mol -1 ) and for U-PPO with mw of (b) 230, (c) 2000, and (d) 4000 g.mol -1 .

Figure 3 :

 3 Figure 3: (2-columm fitting image) Room temperature IR spectra from 1800 to 1500 cm -1 of the OIH films during photooxidation: (a) U-PEO, (b) U-PPO, (c) U-PEO:Eu 3+ , and (d) U-PPO:Eu 3+ .

  10 h reaction with NH 3 are shown in Figure SI.2. After NH 3 treatment, the FTIR spectrum of the photooxidized film showed a decrease of the absorption band at 1725 cm -1 , together with the new absorption bands at 1690, 1640, and 1565 cm -1 , related to the formation of amide from the reaction of ester such as formate with NH 3 (HC=O-O-CH 2 -+ NH 3 → 2HN-C=O-O-CH 2 -

Figure 4 :

 4 Figure 4: (single fitting image) SAXS curves, as a function of UV exposure time, for the Eu 3+ -doped hybrids compared to the undoped ones: (a) U-PEO and (b) U-PPO

Figure 5 :

 5 Figure5: (single fitting image) Evolution of the relative areas of the SAXS correlation peak (S t /S 0 ), the formate formation band at 1725 cm -1 (A t /A 10 ) 1725 , the Eu 3+ -OIH decomplexation fingerprint band at 1608 cm -1 (A t /A 0 ) 1608 , and the Judd-Ofted parameter ratio (Ω 2 (R)t/(Ω 2 (R) 0 ), as a function of the time of photooxidation, for (a) U-PEO:Eu 3+ and (b) U-PPO:Eu 3+ . The continuous and dashed lines show the best fits of the exponential decay (y t /y 0 = 1-exp(k.t)) and growth (y t /y 10 = 1 + exp(k.t)) laws, respectively.

Figure 6 :

 6 Figure 6: (single columm fitting image) Excitation spectra of (a) U-PEO:Eu 3+ and (b) U-PPO:Eu 3+ exposed to UV irradiation for different times.

Figure 7 :

 7 Figure 7: (single columm fitting image) Emission spectra recorded at RT for (a) U-PEO:Eu 3+ and (b) U-PPO:Eu 3+ exposed to UV irradiation for different times.

  be detected from the change of the intensity of the so-called "hypersensitive" transition[START_REF] Carlos | Local coordination of Eu(III) in organic / inorganic amine functionalized hybrids[END_REF][START_REF] Sanchez | Molecular Engineering of Functional Inorganic and Hybrid Materials[END_REF], 5 D 0 → 7 F 2 , as a function of the UV irradiation time.

Acknowledgements

This work was supported by the Brazilian agency CNPq (grants 142495/2013-1, 203031/2015-6, and 401679/2013-6). This study is a collaborative research project of the members of the National Institute on Advanced Eco-Efficient Cement-Based Technologies (FAPESP INCT 2014 50948-3; 465593/2014-3). The authors thank the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas for use of the SAXS1 beamline.


Kacou Dingui

Noël Brunetière
email: noel.brunetiere@univ-poitiers.fr

Jean Bouyer

Mohand Adjemout

Surface texturing to reduce temperature in mechanical seals

Keywords: mechanical seal, surface texture, friction, temperature, infrared thermography

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction.

Mechanical seals are composed of two annular flat rings in contact and relative motion to ensure sealing of rotating shafts. The flat geometry of the seal faces is not favorable to the generation of a fluid film by hydrodynamic lubrication [START_REF] Lebeck | Parallel Sliding Load Support In the Mixed Friction Regime. Part 1 -The Experimental Data[END_REF][START_REF] Lebeck | Parallel Sliding Load Support In the Mixed Friction Regime. Part 2 -Evaluations of the Mechanisms[END_REF][START_REF] Brunetière | Lubrication Mechanisms Between Parallel Rough Surfaces[END_REF]. In order to reduce wear and friction, it could be interesting to develop features to help in the lubrication of the faces.

More than fifty years ago, Anno et al. [START_REF] Anno | Microasperity Lubrication[END_REF] showed that creating micro-pillars on one of the seal faces promotes fluid film generation. Two years later [START_REF] Anno | Load Support and Leakage from Microasperity-Lubricated Face Seals[END_REF], they showed that micro-dimples are also efficient while maintaining a low leakage level.

In this pioneering work, photo-etching was used to create the surface pattern, which make the industrial application difficult. Three decades later, thanks to the development of laser technology, Etsion et al. [START_REF] Etsion | Increasing Mechanical Seals Life with Laser-Textured Seal Faces[END_REF] used laser surface texturing (LST) to create dimples and confirmed the results of Anno et al [START_REF] Anno | Load Support and Leakage from Microasperity-Lubricated Face Seals[END_REF]. LST appears to be a very convenient method for surface modification [START_REF] Etsion | State of the Art in Laser Surface Texturing[END_REF] allowing the application of surface texture to many lubricated components including journal bearings, piston rings and seals [START_REF] Gropper | Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings[END_REF][START_REF] Rosenkranz | Surface Texturing in Machine Elements -A Critical Discussion for Rolling and Sliding Contacts[END_REF].

Even if many texture manufacturing techniques are now available [START_REF] Coblas | Manufacturing textured surfaces: State of art and recent developments[END_REF], the spherical cap or cylindrical dimples are probably the most used type of surface texture in experimental studies [START_REF] Yu | Frictional Characteristics of Mechanical Seals with a Laser-textured Seal Face[END_REF][START_REF] Wang | The Lubrication Effect of Micro-Pits on Parallel Sliding Faces of SiC in Water[END_REF][START_REF] Wang | Improving the Anti-Seizure Ability of SiC Seal in Water with RIE Texturing[END_REF][START_REF] Wan | The effect of laser surface texturing on frictional performance of face seal[END_REF][START_REF] Yan | Significance of Dimple Parameters on the Friction of Sliding Surfaces Investigated by Orthogonal Experiments[END_REF][START_REF] Wang | Experimental Study of Two-phase Mechanical Face Seals with Laser Surface Texturing[END_REF]. These types of surface texture exhibit significant friction reduction. However, the theory is not able to demonstrate the positive effect of surface texture.

Numerical simulations based on the Reynolds equation coupled with a mass-conserving cavitation algorithm show that spherical or cylindrical dimples are inefficient for hydrodynamic load generation [START_REF] Qui | On the Prediction of Cavitation in Dimples Using a Mass-Conservative Algorithm[END_REF][START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF] because the amount of load generated is very small compared to the load that must be bear by the seal [START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF]. These dimples work as single isolated bearings without any collective effect between the cavities [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF]. In fact, it was shown that the hydrodynamic load generation observed in experiments is due to the interaction between dimples and surface roughness [START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF] and that dimples cannot generate a sufficient load on their own. Using other texture shapes (square, pyramidal or triangular shapes) with different orientation can slightly improve their performance [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF][START_REF] Meng | Lubrication film flow control by oriented dimples for liquid lubricated mechanical seals[END_REF]. It was show by Adjemout et al. [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF] that a great improvement can be achieved if a collective effect between the dimples is generated. The most effective solution they found is composed of triangular dimples placed base to base. They showed that thanks to this texture, it is possible to reduce friction and contact temperature by 50% [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF].

In the present paper, the experimental work started in paper [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF] is continued with different shapes of surface texture.

The idea is to compare the optimal triangular dimples to usual dimples shapes (square dimples, cylindrical cavities, other triangle configuration) which are less efficient according to the lubrication theory. The operating temperatures in the sealing interface with the different surface texture patterns are measured by infrared thermography and compared to find out which one is the best solution.

Material and methods.

2.1. Seals and surface texture.

The mechanical seal used in this work is an unbalanced seal with a rotating ring made of stainless steel. This ring is textured by ion-etching technique through a mask, carburized and coated with Diamond Like Carbon (DLC) (figure 1).

More details on the surface texturing technique can be found in reference [START_REF] Adjemout | Influence of the real dimple shape on the performance of a textured mechanical seal[END_REF]. The main characteristics of the seal are given in table 1. Four texture patterns (presented in figure 1) have been tested as well as a smooth surface. The geometrical parameters of the texture patterns are based on numerical simulations results [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF][START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF]. These surfaces were measured with a white light interferometer TalySurf CCI 6000. An example of surface measurement is presented in figure 2 and the main parameters of the surfaces are presented in table 2. The groove size is its diameter or its width depending on the geometry of the texture. The surface roughness indicated in table 2 was measured on the top surface out of the dimples. It can be noted that the grooves are very shallow (less than one micron) but at least two times the roughness height Sq. The groove depth and size are based on values used in numerical studies [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF][START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF] but show a slight deviation due to manufacturing uncertainty [START_REF] Adjemout | Influence of the real dimple shape on the performance of a textured mechanical seal[END_REF].

Due to the polishing process a slight curvature of the seal ring surface can be seen in the central part of the sealing area (figure 2). Examples of radial surface profiles measured on the five different surfaces are presented in figure 3. A polynomial fit (thick line) is added to highlight the curvature of each profile. The radius of curvature of the surface varies from 0.44 to 0.93 m leading to submicron height variations of the same order of magnitude as the roughness height and texture depth.

Test rig

A cross section of the test cell is presented in figure 4. Two identical mechanical seals are installed back to back in the test cell to ensure an axial force balance. The seals are fed by water at 0.5 MPa and 40°C by an hydraulic equipment. An electrical motor is used to rotate the seals at speeds varying from 500 to 3000 rpm. The operating conditions are summarized in table 3. The stators of the two seals have been replaced by sapphire disks to allow visualization of the seal interface. Several thermocouples are used to measure the fluid temperature, the ambient temperature and sapphire surface temperature. The leakage of the seals is collected by two drains and measured by a weighting system. To ensure an accurate leakage measurement, several drops of fluid need to be collected. During the tests of about 10 minutes per operating condition, the leakage levels were too small (leakage volume varying between zero or one drop of water) to ensure a reliable measurement. The parameter is thus not presented in the results section.

The sapphire has a lower thermal conductivity than the silicon carbide generally used for the seal seat material (35 W.m -1 .K -1 compared to 150 W.m -1 .K -1 ) and higher temperature levels are expected in the sealing interface. However, the sapphire has values of the thermal expansion coefficient and the elastic modulus very close to that of the silicon carbide, leading to small face deformations compared to the seal counter-face [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF] as it the case with silicon carbide. It is thus expected that results obtained with the sapphire disk, even if higher in terms of temperature, are similar to what would be obtained in a real seal composed of a silicon carbide face.

Infrared thermography

An infrared camera FLIR SC7000 is placed in front of the test cell to capture the radiative flux emitted by the seal rings (figures 4 and 5). Since the sapphire disk is not fully transparent and the seal rings are reflective, the radiative flux captured by the camera is composed of an ambient emission, the disk emission and the seal ring emission (figure 6). The total radiative flux thus depends on the seal rings emissivity, sapphire disk transmissivity and refraction indices. The seal face temperature is finally obtained using the camera calibration curve. The seal face temperature is calculated on a radial band (blue rectangle in figure 5) after averaging in the circumferential direction, giving thus a radial temperature profile.

Since the water film between the faces is very thin (a micron or less in thickness), it is assumed, as in paper [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF], that it is fully transparent and at the same temperature as the seal faces. The water film is therefore not taken into account in the balance of radiative fluxes. The full calibration and calculation processes are described in the following section.

According to Reungoat and Tournerie, the total radiative flux, I cam , recorded by the camera in the configuration of figure 6 is:

𝐼 𝑐𝑎𝑚 = (𝑅 1 -+ 𝑇 1 2 𝜌 3 1-𝜌 3 𝑅 1 + ) 𝐼 𝑎𝑚𝑏 + (𝐸 1 -+ 𝐸 1 + 𝑇 1 𝜌 3 1-𝜌 3 𝑅 1 + ) 𝐼 1 + 𝜀 3 𝑇 1 1-𝜌 3 𝑅 1 + 𝐼 3 (1)
Where I amb , I 1 and I 3 are respectively the black body radiation emitted by the environment, the sapphire disk and the seal face, respectively. The coefficients with index 1 are related to the sapphire disk and index 3 to the seal face. Because of the Kirchoff law, the following relations exist between the coefficients [START_REF] Reungoat | Temperature Measurement by Infrared Thermography in a Lubricated Contact: Radiometric Analysis[END_REF]:

𝜀 3 + 𝜌 3 = 1 (2) 𝑅 1 -+ 𝐸 1 -+ 𝑇 1 = 1 (3)
𝑅 1 + + 𝐸 1 + + 𝑇 1 = 1 (4)
The coefficients can be expressed as:

𝑇 1 = (1-𝜌 𝑎𝑖𝑟/1 )(1-𝜌 1/2 )𝜏 1 1-𝜌 𝑎𝑖𝑟/1 𝜌 1/2 𝜏 1 (5) 𝑅 1 -= 𝜌 𝑎𝑖𝑟/1 +𝜏 1 2 𝜌 1/2 (1-2𝜌 𝑎𝑖𝑟/1 ) 1 -𝜌 𝑎𝑖𝑟/1 𝜌 1/2 𝜏 1 (6)
𝑅 1 + = 𝜌 1/2 +𝜏 1 2 𝜌 𝑎𝑖𝑟/1 (1-2𝜌 1/2 ) 1-𝜌 𝑎𝑖𝑟/1 𝜌 1/2 𝜏 1 (7)
The reflection coefficients at media interfaces can be calculated from the refraction indices of the media [START_REF] Reungoat | Temperature Measurement by Infrared Thermography in a Lubricated Contact: Radiometric Analysis[END_REF]:

𝜌 𝑎𝑖𝑟/1 = (𝑛 𝑎𝑖𝑟 -𝑛 1 ) 2 (𝑛 𝑎𝑖𝑟 +𝑛 1 ) 2 (8)
𝜌 1/2 = (𝑛 1 -𝑛 2 ) 2 (𝑛 1 +𝑛 2 ) 2 (9)
The refraction indices of the media can be found in the literature and are given in table 4. The only missing coefficients are the emissivity of the rotor  3 and the transmissivity of the sapphire disk  1 .

Camera calibration

The first step consists in the camera calibration. The camera is placed in front of a black body. Since there is no reflection, the camera records the radiative flux I 0 emitted by the black body. By varying the temperature T of the black body, it is possible to obtain the calibration curve of the camera:

𝐼 𝑐𝑎𝑚 = 𝐼 0 (𝑇) (10)

Seal face emissivity

The rotational seal ring is placed on a monitored heating system. The radiative scheme of figure 6 is simplified since media 1 and 2 are removed. The radiative equation is thus:

𝐼 𝑐𝑎𝑚 = (1 -𝜀 3 )𝐼 𝑎𝑚𝑏 + 𝜀 3 𝐼 3 = (1 -𝜀 3 )𝐼 0 (𝑇 𝑎𝑚𝑏 ) + 𝜀 3 𝐼 0 (𝑇 3 ) (11)
Knowing the calibration curve of the camera, the ambient temperature T amb and the rotor temperature T 3 , it is possible to calculate the emissivity  3 of the rotor. The value of this coefficient is presented in figure 7 as a function of the surface temperature.

Sapphire transmissivity

To determine the transmissivity of the sapphire disk, it is placed between the camera and a black body. Referring to figure 6, it is equivalent to replace medium 2 by air and solid 3 by a black body with an emissivity equal to 1. Equation ( 1) is strongly simplified in this case:

𝐼 𝑐𝑎𝑚 = 𝑅 1 -𝐼 𝑎𝑚𝑏 + 𝐸 1 -𝐼 1 + 𝑇 1 𝐼 3 = 𝑅 1 -𝐼 0 (𝑇 𝑎𝑚𝑏 ) + 𝐸 1 -𝐼 0 (𝑇 1 ) + 𝑇 1 𝐼 0 (𝑇 3 ) (12)
Knowing the calibration curve of the camera, the ambient temperature T amb , the sapphire temperature T 1 and the black body temperature T 3 , it is possible to calculate the transmissivity  1 of the sapphire disk. The value of this coefficient is presented in figure 8 as a function of the black body temperature.

Temperature calculation

Knowing the calibration curve of the camera and the radiative coefficients of the media, it is possible to calculate the rotor temperature by solving equation ( 13):

𝐼 𝑐𝑎𝑚 = (𝑅 1 -+ 𝑇 1 2 𝜌 3 1-𝜌 3 𝑅 1 + ) 𝐼 0 (𝑇 𝑎𝑚𝑏 ) + (𝐸 1 -+ 𝐸 1 + 𝑇 1 𝜌 3 1-𝜌 3 𝑅 1 + ) 𝐼 0 (𝑇 1 ) + 𝜀 3 𝑇 1 1-𝜌 3 𝑅 1 + 𝐼 0 (𝑇 3 ) (13)
A thermocouple is used to measure T amb and a second one is stuck on the external surface of the sapphire disk to measure its external surface temperature T 1 ext . To solve the problem, it is assumed that the average sapphire temperature is the average of the rotor temperature and its external temperature:

𝑇 1 ≈ 0.5(𝑇 1 𝑒𝑥𝑡 + 𝑇 3 ) (14)
It is then possible to solve equation [START_REF] Wang | Improving the Anti-Seizure Ability of SiC Seal in Water with RIE Texturing[END_REF]. This equation being non-linear because of the dependence of the coefficients to the temperature and because of the calibration curve, the Newton method is used to obtain the rotor surface temperature.

Results

Each test was performed three times to ensure repeatability of the measurements. The results presented are an average of the results of the three tests.

Figure 9 presents radial temperature profiles obtained with the five different surface textures at rotational speeds of 1500, 2500 and 3000 rpm. The profiles are the average of the three tests carried out and the error bars indicate the minimum and maximum temperature values recorded during the three experiments. Generally speaking, whatever is the surface, the temperature at the interface increases with the rotating speed, as expected. Moreover, the temperature is lower at outer radius where the sealed fluid, which ensures cooling of the seal faces, is located. For most of the surfaces, the highest temperature is observed at the outlet side of the sealing interface. This is due to the slight curvature appearing during polishing (see figures 2 and 3) providing lowest film thickness close to the middle of the sealing interface (see ref [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF]). The smooth and triangle 2 samples are the same as those tested in reference [START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF]. It can be seen that very close temperature values are found, showing the repeatability of the tests. For the four textured surfaces (figure 9 a -d), it can be seen that the seal face temperature is significantly reduced compared to the smooth surfaces (figure 9 e). This shows the beneficial effect of texture to reduce friction in the contact.

To make the comparison easier, the average temperature rise in the contact for each surface is presented as a function of speed in figure 10. The benefit of using surface texture can be clearly seen. At 3000 rpm, the surface temperature rise can be divided by 2 to 3 depending on the surface patterns used.

The reduction in sealing interface temperature is generally explained by an increase in fluid film thickness due to the hydrodynamic pressure generated by the dimples. During the tests, the seal leakage was collected. However, it was not possible to measure any leakage for all the samples due to the sensitivity limit of the measuring method. It means that, even if the film thickness is increased, the leakage remains small enough so that it evaporates before reaching the weighting system used for leakage measurement. The effect of film thickness changes and temperature variations on the seal leakage are so small that they cannot be measured during the tests.

Discussion

These results first show that even very shallow grooves (submicron depth between 0.22 and 0.31 µm, see table 2) can significantly improve the behavior of a mechanical seal as shown in recent papers [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF][START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF]. However, it was shown in several papers [START_REF] Qui | On the Prediction of Cavitation in Dimples Using a Mass-Conservative Algorithm[END_REF][START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF][START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF] that usual dimples such as spherical caps, cylindrical dimples, square shaped texture are not efficient because they cannot provide sufficient lift-off to completely balance the load applied to the seal. This is due to the fact that each dimple works as a single bearing and without any collective effect. The effective friction reduction that is, however, experimentally observed [START_REF] Anno | Load Support and Leakage from Microasperity-Lubricated Face Seals[END_REF][START_REF] Etsion | Increasing Mechanical Seals Life with Laser-Textured Seal Faces[END_REF][START_REF] Etsion | State of the Art in Laser Surface Texturing[END_REF][START_REF] Gropper | Hydrodynamic lubrication of textured surfaces: A review of modeling techniques and key findings[END_REF][START_REF] Rosenkranz | Surface Texturing in Machine Elements -A Critical Discussion for Rolling and Sliding Contacts[END_REF][START_REF] Yu | Frictional Characteristics of Mechanical Seals with a Laser-textured Seal Face[END_REF][START_REF] Wang | The Lubrication Effect of Micro-Pits on Parallel Sliding Faces of SiC in Water[END_REF][START_REF] Wang | Improving the Anti-Seizure Ability of SiC Seal in Water with RIE Texturing[END_REF][START_REF] Wan | The effect of laser surface texturing on frictional performance of face seal[END_REF][START_REF] Yan | Significance of Dimple Parameters on the Friction of Sliding Surfaces Investigated by Orthogonal Experiments[END_REF][START_REF] Wang | Experimental Study of Two-phase Mechanical Face Seals with Laser Surface Texturing[END_REF]] is, according to numerical simulation, due to the interaction of surface roughness with the dimples [START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF].

The surface texture composed of triangular dimples placed base to base was proposed in paper [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF] to create a collective effect between the dimples and provide sufficient lift-off to reach full hydrodynamic regime. These textures (triangle 1 and triangle 2 in figure 1 and table 2) were expected to give better results due to better texture arrangements.

The present experimental results show that it is not the case. It is probably due to the fact that the interaction with roughness has a stronger effect than the texture shape itself. Indeed, in papers [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF][START_REF] Adjemout | Friction and temperature reduction in a mechanical face seal by a surface texturing: comparison between TEHD simulations and experiments[END_REF] it was shown that the film thickness obtained with triangular dimples is a fraction of microns, close to the surface roughness standard deviation Sq measured on the samples of this study (see table 2).

Even if all the dimple types provide at least 50% of temperature decrease compared to smooth surfaces, some of them lead to lower operating temperature. Knowing the average size of the dimples, it is possible to calculate the local area density a (the ratio of the dimple area over the unit cell area) and the total area density A calculated using the full sealing interface (table 4). There is a difference between the two values because the surfaces are not fully textured, depending on the texture type (see figure 1). Additional parameters such as the surface curvature R, the aspect ratio and the roughness to depth ratio are also reported in table 5. The average temperature rise measured at the highest speed is also given in table 5.

For each geometrical parameter presented in table 5 a correlation coefficient with the temperature rise has been calculated and presented in figure 11.

Based on this analysis, it appears that the total area density A of the texture is the most significant influencing parameter.

An increase of the texture area leads to a decrease in temperature. In previous research works on textured seals [START_REF] Wang | The Lubrication Effect of Micro-Pits on Parallel Sliding Faces of SiC in Water[END_REF][START_REF] Wang | Improving the Anti-Seizure Ability of SiC Seal in Water with RIE Texturing[END_REF][START_REF] Wan | The effect of laser surface texturing on frictional performance of face seal[END_REF][START_REF] Yan | Significance of Dimple Parameters on the Friction of Sliding Surfaces Investigated by Orthogonal Experiments[END_REF],

the dependence of friction and temperature on texture area density A is not monotonous as in the current work. It is probably due to the fact that a much larger interval was explored compared to the present work and that deeper grooves were used. An important parameter is also the roughness height Sq. The higher the roughness, the lower the temperature.

This conclusion is in line with the findings of paper [START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF] showing that the performance is reached thanks to an interaction between the roughness and the texture. It is confirmed by the strong influence of the ratio of the roughness height Sq to the groove depth h g . Finally, the groove size or related parameters have limited effect on the surface temperature. The surface curvature R also has a limited effect on the results, as indicated by its low correlation with the temperature rise.

Its effect will be ignored in the next part.

A simple explanation of the effect of the texture area ratio A can be proposed. If it assumed that the film thickness out of the dimples is h 0 and that the dimple depth is h g , the power P dissipated by viscous friction can be expressed as follows:

𝑃 = 𝜇𝑉 2 𝑆 ( 1-𝐴 ℎ 0 + 𝐴 ℎ 0 +ℎ 𝑔 ) = 𝜇𝑉 2 𝑆 ℎ 0 (1 -𝐴 ℎ 𝑔 ℎ 0 +ℎ 𝑔 ) ( 15
)
where µ is the fluid viscosity, V the sliding speed and S the sealing area. The dissipated power is a linear decreasing function of A. If it is assumed the heat convection coefficients vary in a limited range and that the heat conduction coefficients of the materials are constant, the temperature rise is approximately proportional to the dissipated power and thus a linear decreasing function of the texture area ratio A. The measured average temperature rise at 3000 rpm is presented as a function of the total area density in figure 12 for each surface texture. The smooth surface result has also been included in the figure showing that the linear relationship extends to total texture area density when A equals zero.

However, this linear relation can only be obtained if the film thickness, the groove depth and the fluid viscosity do not vary significantly from one case to the other. If the viscosity variations are limited to less than 4% based on the temperature measurements, the groove depth changes of about 30% between the different seal rings (table 2) and it is not possible to estimate the variation of h 0 . Moreover the surface curvature and the thermal deformation will affect the fluid film geometry [START_REF] Adjemout | Numerical analysis of the texture effect on the hydrodynamic performance of a mechanical seal[END_REF]. A numerical analysis would be necessary to get a better understanding of the experimental results and a better identification of the mechanisms providing the friction and temperature reduction. More particularly, the interaction of surface roughness with surface texture can be analyzed as well as real seal materials.

Conclusion

This paper presents an experimental study of textured mechanical face seal. The seal faces temperature is measured by infrared thermography through a transparent sapphire disk used as the stator of the seals. Different texture shapes were used with a depth varying from 0.22 to 0.31 µm and a total area ratio covered by grooves in the range 0.18-0.25. These very shallow grooves allow to decrease the temperature of the sealing interface of at least 50%. The performance appears to be independent of the texture shape but related to the total area density and the surface roughness height. These experimental findings confirm the numerical results from [START_REF] Brunetière | Numerical Analysis of a Surface-Textured Mechanical Seal Operating in Mixed Lubrication Regime[END_REF] showing that interaction of roughness and surface texture is a key parameter in the behavior of textured mechanical face seal.

Fig. 1 Fig. 2

 12 Fig. 1 Textured seal ring

Fig. 3 FigFig. 5 a

 35 Fig. 3 Examples of radial profiles of the surfaces with a polynomial fit (thick line)

Fig. 6

 6 Fig. 6 Radiative scheme of the problem: Test condition: 1 = sapphire disk, 2 = water film, 3 = rotational ring -Transmissivity calibration: 1 = sapphire disk, 2 = air, 3 = black body.

Fig. 7 Fig. 8 Fig. 9 Fig. 10

 78910 Fig. 7 Emissivity of the rotor as a function of its temperature

Fig. 11

 11 Fig. 11 Correlation between the texture parameters and the temperature rise

Table 1

 1 Mechanical seal characteristics

	Parameter	Value
	Outer radius R o (mm)	28.8
	Inner radius R i (mm)	25.8
	Closing force (at 0.5 MPa)	316.6 N
	Table 2 Seal surface characteristics

Acknowledgments

This work was supported by the ANR under the reference ANR-11-RMNP-0008 and by the French government program "Investissements d'Avenir" (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01).

This work was partially presented and published at the BHR Group 25th International Conference on Fluid Sealing, 4-5 March 2020, Manchester. BHR Group is thank for allowing the authors to publish this work in Tribology Online.