A Delcamp

L Maillé

B Rufino

S Mazerat

R Pailler

A Guette

P Weisbecker

H Plaisantin

E Philippe

S Loison

IN-SITU PROCESSING OF CARBON COATINGS ON THE SURFACE OF SiC-BASED FIBERS

Keywords: Carbon, Fibres, Porosity

Carbon coatings have been formed on the surface of free carbon containing Tyranno ZMI and Nicalon SiC-based fibers by reaction with chlorine gas (chlorination) in the temperature range 550-675 °C. The kinetics and growth mechanisms have been investigated for both fibers. Results have shown that, for similar experimental conditions, the reactivity of both fibers displayed noticeable discrepancies. It has thus been observed that the Nicalon fiber was more prone to chlorine-based treatment and displayed a thicker carbon coating than the ZMI fiber. These results have also revealed that, in both case, these thicknesses can be closely monitored by physical parameters such as temperature and aging duration. The nature of the obtained coating has also been investigated by various experimental devices. AES, SEM and TEM analyses have thus evidenced that these carbon coatings were uniform and well adherent whereas AFM showed that the chlorine-based treatments resulted in a nanometer-range roughness at the fiber surface. Adsorption gas analyses also revealed a microporosity coupled with a high specific surface area. The chemical composition of the final products was determined by XPS and Mass Spectroscopy. Finally, the oxidation behaviour which was investigated by thermogravimetric analysis revealed that oxidation occurs at a quite low temperature (225°C).

Introduction

Both their high toughness and non-catastrophic failure behavior render ceramic matrix composite (CMCs) attractive for high temperature structural applications [1][START_REF] Shibuya | 9th Cimtec World Forum on New Materials -Symposium V -Advanced structural fiber composites -Florence[END_REF]. SiCbased fibers have long been considered as the main candidates to provide reinforcement in CMCs because of their high tensile strength, high elastic modulus, good oxidation resistance and thermal stability [START_REF] Pysher | [END_REF]. The advantage of interposing a compliant coating, most often carbon, between the fiber and the matrix in order to provide an adequate bonding and a fair protection of the fibers has also long been established [4][START_REF] Naslain | Langlais International Symposium on Ceramic Matrix Composites[END_REF]. More recently, research works have focused on the in-situ synthesis of these compliant coatings through various treatments.

Some authors [START_REF] Mcnallan | Reaction formed coatings for SiC fibers in ceramic matrix composites, Elevated temperature coatings:Science and Technology III[END_REF][START_REF] Chen | [END_REF] have thus studied the direct synthesis of carbon coating on SiCbased Tyranno ZMI fiber surface, on TiC and/or ZrC powder surfaces [8][9][10], as well as on SiC ceramics [11][12], by chlorine etching. However, no similar studies on standard Nicalon fibers which are of great interest for CMC's applications because of their lower cost and easy availability have yet been reported. This carbon coating should posses all the following distinct features: to provide an individual and uniform coating around the fibers, while avoiding bonding between them; to be stable during the processing of the matrix and of the interphase; to present a good adhesion with both the fibers and the interphase; and to preserve the mechanical properties of the fibers. The objectives of the present work were (i) to study the growth carbon coatings obtained from chlorine etching on the surface of both ZMI and Nicalon fibers (by establishing kinetic laws, correlating these kinetic laws with the nature of the fiber), (ii) to characterize the morphology and the chemistry of these carbon coatings, and (iii) to study their oxidation behavior. The chemical compositions of the as-received fibers are reported in table1.

Materials and experimental procedure

Scanning Electron Microscopy (SEM Hitachi S 4500) and Atomic Force Microscopy (AFM Digital Instrument 3100) characterizations were performed to measure the carbon coating thickness and to characterize the surface morphology (in tapping mode), respectively. The coating microstructure was studied with a Philips CM30ST (LaB6, 300kV) Transmission Electron Microscope (TEM), using samples prepared by mechanical thinning to 20m and ion milling (Gatan Dual Mill). Chemical composition was determined in depth profile by Auger Electron Spectroscopy (VG Microlab 310F) coupled with argon ion sputtering at 4 keV. Gas sorption analysis was carried out using nitrogen as test gas with a commercial instrument (ASAP 2010 from Micromeritics) at -196.15°C. Prior to the analyses, the samples were cleaned at a low pressure (i.e. < 10 -4 Pa) at 120°C for 15 hours. The Specific Surface Area (SSA) was calculated using the Brunauer-Emmet-Teller (BET) equation. The size distribution and the volume of the pores were calculated using the semi-empirical Horvath and Kawazoe (HK) approach [13].

Thermo Gravimetric Analysis (TGA 24 from SETARAM) was used to study the oxidation kinetics of the carbon coatings under dynamic dry oxygen atmosphere (60 mL.min -1 ) within a temperature range of 225-400°C.

Chemical analyses of volatile species were performed by mass spectrometry with a commercial instrument (QMS 200 GSD 301 T2 Thermostar) under dynamic Helium flow (60 mL.min -1 ) within a temperature range of 20-950°C using a ramping rate of 20 °C/min.

X-ray Photoelectron Spectroscopy (XPS) characterizations were performed with a commercial instrument (VG Escalab 220i XL) in order to detect possible absorbed contaminations at the carbon surfaces.

Results and discussions

Chlorine etching process

Carbon coatings were synthesized at the surface of the SiC-based fibers by the extraction of Si from the SiC phase by chlorine according to the following chemical reaction [START_REF] Chen | [END_REF]:

SiC (s) + 2 Cl2 (g) → SiCl4 (g) + C (s) equation 2
As already mentioned in the preceding section, two temperatures, 600°C and 650°C, have been considered for these chlorine-based treatments whereas their durations where varied between 20 to 220 min. Figure 1 displays the average thickness of the carbon coatings thus obtained for both the ZMI and Nicalon fibers as a function of the chlorine etching time.

It may be observed that these thicknesses increase linearly with the treatment duration.

Conseqently, the following growth rate may be proposed:

e = k . t equation 3
Where e is defined as the carbon thickness (nm), k denotes the reaction rate (nm.min -1 ) and t is the treatment duration (min). Correlatively to the data displayed in figure 2, it may be seen that the Nicalon fibers are prone a reaction rate much higher than that of the ZMI fibers.

Besides, the linearity of the kinetic laws suggests that the carbon coating growth is in any case controlled by a surface reaction mechanism, i.e. the diffusion of the reactant species through the layers apparently does not act as a limiting factor. As a result, the carbon coating thicknesses is homogeneous (figure 3), and can be closely controlled by the experimental parameters such as temperature and duration.

An Auger Electron Spectroscopy (AES) depth-profile analysis of a ZMI fiber having a carbon coating thickness of 700 nm has been achieved in order to check the chemical composition of the coating (figure 3a). It may be seen, from figure 4, that both the carbon coating and the fiber core are clearly distinguishable. Results of the AES analysis have confirmed that the coating is solely composed of carbon, i.e. no traces of either silicon or oxygen were detected within the layer. Although, chlorine was not observed, it may be inferred it was likely present in the coating, because, during the Auger Spectroscopy analysis, ion sputtering should have desorbed the adsorbed species.

The thicknesses of the carbon coatings derived from AES analysis are in good agreement with the SEM observations. Figure 5 thus shows the growth rates of the carbon coatings as a function of the chlorine etching temperature for a constant treatment time of 45 min.

For both fibers in temperatures range 550 -675°C, the growth of carbon layer obeyed to Arrhenius law (k = k0e -Ea/RT ), where k0 is the pre-exponential factor, Ea is the activation energy, R is the gas constant and T is the temperature. The apparent activation energy obtained for both fibers are respectively, 202 kJ.mol -1 and 145 kJ.mol -

Chlorine reactivity

An elemental composition analysis has been first performed for both fiber, using an electron probe micro analyser, in order to understand their different reactivity towards chlorine.

Results have evidenced zirconium traces in the ZMI fiber, which is a well known characteristic of this fiber [14]. Yet, the major difference between both fibers resides in the oxygen atomic concentration which is higher in the case of Nicalon fiber. In the temperature range 550-675°C, chlorine is not supposed to react with carbon. Besides, similar experiments conducted on bulk crystalline SiC samples have shown that their reactivity towards chlorine was significantly lower than that of the SiC-based fibers. On the other hand, no reaction between pure silica and chlorine should be expected under similar conditions. The high reactivity of the SiC-based fiber should therefore be related to the presence of mixed tetrahedral environments SiOxC(4-x) (0 ≤ x ≤ 4) in the amorphous oxycarbide phases [15]. As a matter of fact, it has been shown that the introduction of oxygen atoms induces a distortion of the Si tetrahedral environment as well as a polarization effect due to the high electronegativity of oxygen [16][17]. These two phenomena are therefore likely responsible for the increase of reactivity of the SiOxC(4-x) sites with the oxygen content (i.e. x).

As a conclusion, the Nicalon fiber which contains the highest oxygen concentration in Si-O-C amorphous phase, logically exhibits the highest chlorine reactivity, which is besides in accordance with the fact that thus fiber had the lower calculated apparent activation energy.

Carbon coating characteristics

Atomic force spectroscopy analyse have been conducted in order to obtain a morphological characterization of the fiber surface at an extremely low scale. Figure 6 displays the results of AFM analyse performed on a desized ZMI fiber, as well as on two ZMI fibers which had been chlorinated at two different temperatures (550 and 600°C).

These results show that the chlorination treatment has generated a nano-range roughness on the extreme surfaces, which besides tends to increase with the temperature treatment.

In a next step, the carbon coatings porosity and specific surface area (SSA) have been characterized by nitrogen adsorption analysis. Figure 7 thus displays an adsorption and desorption isotherm obtained on Nicalon fibers having a carbon coating thickness of 1m. This investigation was carried out through three parameters, namely the evolution of the SSA calculated using BET equation, the pore volume and the slit pores size distribution calculated using semi-empirical HK approach. After gas sorption analysis, each sample was oxidized at 500°C under pure oxygen atmosphere using a TGA apparatus in order to first determine the weight percent of carbon coating and then to estimate the SSA and pore volume per gram of carbon coating. This isotherm curve presents a type I form, revealing the existence of micropores (øpore < 2 nm) [18].

Besides, the determination of the carbon weight leads to a carbon coating SSA value of about 1600 m 2 .g -1 .

It has also been found that an increase of the chlorine etching temperature led to higher SSA and pore volumes (table 2). This may be related to the fact that, for a constant volume of fiber, the increase of temperature led to a higher carbon extraction during the chlorine-based treatment. This last phenomenon may be explained with a thermodynamic simulation of the chlorine-based treatment.

It may thus be ascertained, from figure 8, that for temperatures lower than 600°C, oxygen localized in the fiber reacts with carbon to produce carbon dioxide (C (s)+O2 (g) = CO2 (g)). Conversely for temperatures higher than 600°C, the predominant reaction becomes the carbon monoxide formation (2C (s)+O2 (g) = 2CO (g)). These results therefore tend to show that, for temperature treatments higher than 600°C, the extraction of oxygen consumes more carbon, resulting thus in a higher pore volume.

Characterization and suppression of the carbon coatings contamination

TGA experiments were achieved under argon atmosphere with a ramping rate of 20°C.min -1 up to 1300°C, in order to degas the carbon coatings. The weight loss evolution derived from these tests presents two interesting temperature domains, 200-400 and 550-1300°C, corresponding respectively to a weight loss of 2 and 23 wt %.

XPS and mass spectrometry analyses were performed on Si-O-C fibers totally transformed into carbon fibers (with a chlorine-based treatment applied at 680°C for 45 minutes), in order to determine the nature of the species responsible for these weight losses. Figure 9 presents an XPS spectrum obtained from a carbon derived ZMI fiber surface. Two peaks correspond to chemisorbed chlorine (200 and 201.5 eV), whereas oxide and nitride functions are also detected at the carbon surface in lower quantity. approximately 23 wt %. contaminants, principally chemisorbed chlorine, but also oxide and nitride functions. It has also been found that, in order to degas small coatings (smaller than 1 µm), it was sufficient to perform heat treatments under argon at 700°C

during one hour. Nevertheless, degassing higher coating thicknesses needed higher temperature treatments, up to 1100°C for an entire transformation into carbon fibers.

Regarding the temperature domain the various volatile species released by SiC-based transformed carbon fibers, three main domains may be observed. The first one concerns aliphatic radicals released between ambient temperature and 350 °C, the second one concerns other aliphatic radicals released between 350°C and 700°C. These first two domains may be connected with the first step of weight loss (i.e. 2 wt %) which are observed in the thermogram displayed in figure 10. The third domain only concerns chlorine thermal desorption occuring between 500°C and the maximum temperature analysis (950°C). This last domain may be connected with the second step of weight loss (23 wt %) observed in figure 11. It should also be pointed out that small quantities of carbon oxide (CO, CO2) were released during analysis revealing therefore a poor functionalization of carbon surface by oxygen group during process and / or storage.

Oxidation behavior

The oxidation behavior of the various carbon coatings has been investigated, after decontamination, by thermogravimetric analysis (TGA) under oxygen atmosphere for a temperature range of 225-400°C.

The results have shown that these carbon coatings were prone to oxidation since 225°C (a relatively low temperature for carbon oxidation) with an apparent activation energy of about 120 kJ.mol -1 which is close to the values reported so far in the literature [19].

This premature oxidation susceptibility of the carbon coatings can be explained by (i) their microporosity coupled with a high specific surface area and (ii) their disordered aspect, as observed by TEM. Thus, the TEM 002 dark field image of one of these carbon coatings exhibited in figure 12a shows a homogeneous phase with no preferential orientation, as confirmed by the uniform lightning of the carbon layer surrounding the fiber. The selected Area Diffraction pattern (SAD) was obtained selecting a 100 nm area in the middle of this carbon layer (figure 12b). This SAD pattern exhibits a broad and diffuse 002 peak with a maximum around 0.366 nm, which is far larger than the d002 spacing usually found in graphite (d = 0.336 nm). Besides two dimensional lattice reflections 10 and 11 can be clearly seen on the diffraction pattern.

Values of the size of the coherent domains along the c-axis, Lc, and of the diameter of these coherent domains, La10, were estimated as 1.7 nm and 5.5 nm, respectively (obtained from the width of the 002 peaks and the 10 band). These results indicate that, inside these in-situ formed coatings, the atoms of carbon are organized into small aromatic domains, randomly oriented and poorly stacked.

Conclusion

Carbon ). The temperature of the chlorine-based treatment appeared to be essential in controlling the carbon coating porosity: the higher was the treatment temperature, the higher were the roughness, the specific surface area and the porosity of the resulting carbon coatings. However, both the porosity and the weak microstructural organization of these carbon coatings resulted in a low oxidation resistance (oxidation beginning at 225°C).

List of figure and table captions

  Two different kinds of SiC-based fibers have been used in the present study: Tyranno ZMI from UBE Industries and Nicalon NLM-202 from Nippon Carbon. After having been desized, these SiC-based fibers were treated under an atmospheric pressure of pure chlorine with a gas flow rate of 500 sccm in the temperature range 550-675°C. The experimental device used for chlorination is represented in figure 1. The chlorine rate was always kept in excess in order to avoid a possible gas depletion. A NaOH allows to capture Cl2 at the exit of the furnace, according to the following chemical

Figure 10

 10 Figure 10 presents the mass spectrometry results obtained for the same sample. Results

  coatings have been in-situ synthesized on the surface of two different SiC-based fibers by chlorine treatment in the temperature range 550-675°C. Various characterizations conducted on this treatment as well as on the resulting carbon coatings have revealed interesting features. Among them and of major importance, it has been shown that homogeneous carbon coatings with a good adhesion to the fiber core can thus be performed on SiC-based fibers in the selected temperature range. It has also been shown that the growth of the carbon coatings follows linear kinetics involving a surface reaction mechanism, the reaction rate being related to the oxygen concentration of the Si-O-C amorphous phase. The NLM-202 Nicalon fiber (O% = 13.8) proved to be more prone to chlorine-based treatment than the Tyranno ZMI fiber (O% = 9.2), as evidenced by a lower apparent activation energy (Ea (Ni) = 143 kJ.mol -1 , Ea (ZMI) = 210 kJ.mol-1

Figure 1 :Figure 2 :

 12 Figure 1: Representation of the reactor used for chlorination

Figure 3 :

 3 Figure 3: Cross-section SEM images obtained on chlorine-etched fibers at

Figure 4 :

 4 Figure 4: Auger Electron Spectroscopy depth-profile of the ZMI fiber chlorine-etched

Figure 5 :

 5 Figure 5: Arrhenius plots obtained for the two SiC-based fibers in the temperature

Figure 6 :

 6 Figure 6: AFM images obtained on desized ZMI fiber; as received (a), after

Figure 7 :

 7 Figure 7: Nitrogen adsorption and desorption isotherms obtained on carbon-coated

Figure 8 :

 8 Figure 8: Plots of the results of thermodynamic calculations for the amount of carbon at

Figure 9 :

 9 Figure 9: XPS spectrum totally transformed in carbon ZMI™ fibers.

Figure 10 :Figure 11 :

 1011 Figure 10: Mass spectrometry spectra totally transformed in carbon ZMI™ fibers Figure 11: TGA under pure argon atmosphere with a temperature rate of 20°C.min -1 .

Figure 12 :

 12 Figure 12: TEM 002 dark field image of the thick carbon coating surrounding the SiC

  Figure 1

Figure 6

 6 Figure 6

Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1

 7891011 Figure 7

Table 1 :

 1 WDS analysis of the two SiC-based fibers used in this study (in atomic percent).

Table 2 :

 2 Evolution of the porosity as a function of the chlorine etching temperature.

for the ZMI and Nicalon fiber.

Acknowledgments

The authors wish to thanks Muriel Alrivie for the preparation of the TEM samples, Michel Lahaye for the Auger spectroscopy analyses. The authors gratefully acknowledge the financial support of the ADEME and SPS for this work.


A Delcamp

L Maillé

S Saint

R Pailler

A Guette

Al-O-N AND Al-O-B-N THIN FILMS APPLIED ON Si-O-C FIBERS

Keywords: A. Coating, A. Ceramic-matrix composites, B. Mechanical properties, Oxidation

Protective coatings (Al-O-N and Al-O-B-N) on Si-O-C fibers (Tyranno ZMI) were applied in order to enhance oxidation resistance under severe thermo-mechanical conditions in the 400-600°C temperature range. The coating process consisted in three steps: (i) the transformation of the Si-O-C fiber surface into microporous carbon; (ii) the impregnation of these carbon microporous layers by an aluminium trichloride (AlCl 3 ) solution and then, (iii) a final heat-treatment under ammonia. Processing parameters were studied in order to select the best conditions. Using these conditions, obtained results have shown that coatings were present around each fiber, with a controlled thickness, and that the mechanical properties of the fibers were preserved. Although, these coatings did not entirely stop the oxygen ingress, it has been shown that they strongly reduced the oxidation of the fiber.

Introduction

New environmental rules as well as economical competition have constrained the aeronautic industry to develop new products with low structural weight and ceramized engines in order to reduce NO x and CO 2 emissions. This is the result of a better combustion and enables also a reduction of aircraft noise.

In order to reach these objectives ceramic matrix composites (CMC) have been selected since they display excellent thermo-mechanical properties as well as low densities [START_REF] Naslain | Introduction aux matériaux composites, 2-Matrices métalliques et céramiques[END_REF].

The aim of the present study was focused on developing low cost raw materials and processes in order to be competitive on the civil aeronautic market. Low cost ceramic fibers such as Tyranno ZMI fibers were therefore selected [START_REF] Shibuya | 9th Cimtec World Forum on New Materials -Symposium V -Advanced structural fiber composites -Florence[END_REF]. When used in an oxidizing environment at elevated temperature, it has been shown that the use of a self healing matrix usually provides an accurate protection of the CMCs against oxidation [START_REF] Naslain | [END_REF]. However, when moderate temperatures are considered, self healing matrices have proved to be not so efficient. Hence, when CMCs exhibit matrix cracks, these cracks are not entirely healed because of the low rate of formation of the oxide. Further oxygen diffuses into the fibers, leading therefore to a catastrophic failure of the CMC [START_REF] Filipuzzi | Oxydation des composites SiC/SiC et de leurs constituents : approche expérimentale, modélisation et influence sur le comportement mécanique[END_REF]. In order to prevent any propagation of the oxygen up to the fibers, and therefore to enhance the lifetime of these materials, it has been decided to set up a protective oxidation barrier on the surface of the Si-O-C fibers. This layer should posses all the following distinct features: to provide an individual and uniform coating around the fibers, while avoiding bonding between them; to prevent any oxygen diffusion to the fibers; to be stable during the processing of the matrix and of the interphase; to present a good adhesion with both the fibers and the interphase; and to preserve the mechanical properties of the fibers. Different materials may be considered regarding these expected properties: refractory oxides such as ZrO 2 or Al 2 O 3 [START_REF] Baklanova | [END_REF][6]; silicon based ceramics such as Si-B-C-N, Si-C-N [7][START_REF] Kern | 27th International Cocoa Beach Conference on Advanced Ceramics Composites, COCOA BEACH -FL, Book Series[END_REF] or Si-Al-C-N [9][10][11], monazite [12][13][14][15], BN [16][17][18][19][20][21] and LaAlO x [22][23][24][25][26].

Two new materials were selected for the present work: Al-O-N and Al-O-B-N [27][START_REF] Gogotsi | US Patent[END_REF][29][30]. As a matter of fact, the aluminium as an element exhibits a low density as well as a stable oxide (Al 2 O 3 ). Besides, most nitrides have attractive refractory properties. Boron was also selected as an adequate element because of an important property of all boron containing species which is to form fluid oxide phases (B 2 O 3 ) over a broad temperature range (600-1200°C) in an oxidizing atmosphere. The Al-O-N-B material could thus be used to design self-healing composites with improved lifetimes. It bases on the in-situ formation of the fluid oxide, which fills the matrix cracks and slows down the in-depth diffusion of oxygen [3;31].

Experimental procedure

Thin coatings deposition

As already mentioned, Tyranno ZMI Si-O-C fibers (from UBE Industry, Japan), were used for this study. The microstructure of this first generation of Si-O-C fibers is composed of -SiC crystallites (2nm), free carbon and a Si-O-C phase resulting from the precursor reticulation step [START_REF] Coustemer | AMAC-CODEMAC Bordeaux-France[END_REF]. The chemical composition of these fibers (Si 37.7 at.%, C 53.4 at.%, O 8.8 at.% and Zr 0.1 at.%) has been determined using an electron probe microanalyser (SX 100 from CAMECA). The average fibre diameter is 13µm.

In order to promote an adequate adhesion between the Al-O-N or Al-O-N-B thin coatings and the fiber surface, microporous carbon layers with a controlled thickness were first produced, using a thermochemical treatment around 600°C with chlorine gas [START_REF] Mcnallan | [END_REF]. This specific treatment leads to an extraction of Si from SiC by chlorine at the fiber surface, according to the following reaction 1 [34]: SiC (s) +2 Cl 2(g) = SiCl 4(g) +C (s) . The microporous carbon thus obtained was found to possess a specific surface area, calculated using the Brunauer-Emmet-Teller equation, of 994m 2 /g. Most of the pores size (> 91%) are smaller than 2nm and the pore volume measures 0.45cm 3 /g (these values were calculated using the semi-empirical Horvath and Kawazoe approach). After this heat treatment, fibers were impregnated with an anhydrous aluminium trichloride (AlCl 3 ) solution.

Only one impregnation was performed during 15 minutes, at ambient temperature, using a batch process. The samples were then heated up to about 120°C in order to evaporate the solvent, and a thermal treatment in an ammonia atmosphere during 60 minutes, at a constant flow rate of 20 sccm and at various temperatures (900, 950 and 1000°C), was finally performed.

In the case of the Al-O-B-N thin coatings, the fiber preforms were impregnated with boron acid prior to the impregnation with an aluminium trichloride solution. A heat treatment at 1150°C during one hour was then performed under an ammonia flow.

Three different types of composite materials were produced and tested: composites C1 only made with a pyrocarbon interface and a SiC matrix, considered as the reference material; composites C2 made with Al-O-N thin coatings and a SiC matrix; and composites C3 made with a pyrocarbon interface, an Al-O-N thin coating and a SiC matrix.

Thin coatings characterization

Both structural, chemical and mechanical characterizations were performed for each kind of coating.

The cross-section morphology was investigated with the aid of a high resolution Scanning Electron Microscopy (SEM, Hitachi S4500). Coating thicknesses as well as the quality of their adhesion to the fibers were studied by the observation of the fracture surfaces.

Microstructural characterizations of the various coatings were performed with a Philips CM30ST (LaB 6 , 300kV) Transmission Electron Microscope (TEM). The chemical composition and thicknesses of the coatings were evaluated using an Auger Electron Spectroscopy (VG Microlab 310 F) coupled with an argon sputter device. The analysis of the atomic content of each element was determined in the depth profile mode.

Room temperature fiber strength was determined by tensile tests performed on single fibers according to a procedure describe in [START_REF] Delcamp | Protection de fibres base SiC pour composites à matrice cérammique[END_REF]. These tests were performed on twenty fibers for each sample, with a gauge length of twenty five millimetres. Each fiber extracted from a tow was fixed on paper frame holders using a hard acrylic resin. Ultimate tensile strength ( r , in GPa), strain to rupture ( r , in %) and Young's modulus (E, in GPa) of the Si-O-C fibers were derived from these tests. Mechanical tests on single ply (2D fiber weaves) with a SiC matrix were also performed.

Static oxidation tests were performed at 600 °C in air during 7000 hours. Silica thicknesses developed at the surface of the fibers were determined by SEM after a few weeks.

Results and discussion

Various processing parameters related to the impregnation and the nitridation have been studied.

The impregnation process

As mentioned before, aluminium compounds were deposited at the fiber surface through the impregnation by an aluminium trichloride solution (AlCl 3 ) of microporous carbon first created at the ZMI fiber surface by chlorination [27-28;36-37]. In this respect, in two fluids were tried to solve the AlCl 3 powder, namely water and ether.

Although water proved to be a good solvent for hydrated AlCl 3 powder, this solution yet caused an important introduction of oxygen into the coating. Conversely, ether also proved to be a good solvent when using anhydrous AlCl 3 powder, and consequently forms a coating with a lower oxygen content. Ether was therefore preferred to impregnate the aluminium trichloride solution.

In a first attempt to impregnate a maximum of aluminium compounds into the microporous carbon, a saturated aluminium trichloride solution was realised. Yet, the high viscosity of this solution did not easily allow the impregnation of the fiber preforms by the aluminium compounds, as illustrated by the high resolution SEM image exhibited in figure 1. This which shows that thin coatings at the fiber surface were not homogeneous, with an excess of matter deposited at the fiber surface. It was therefore then decided to use only thin coatings requiring a lower concentration of aluminium trichloride solution. As expected, a homogeneous microstructure could thus be obtained (figure 2 a) in which no bridge between Si-O-C fibers could be evidenced (figure 2 b). These amorphous coatings, which formed an individual and uniform coverage around each fiber, could thus be expected to efficiently protect the fiber surfaces.

The nitridation process

Nitridations were performed on samples previously impregnated with AlCl 3 solutions at 900°C following the method described by L. Chen et al. [27;36-37], and then extended at 950°C and 1000°C.

The samples thus obtained were also analysed by Auger electron spectroscopy. Results, shown in figure 3, confirm that the impregnation of the aluminium compounds into the microporous carbon network was quite successful. However, the chemical compositions presented in tables 1 and 2 reveal that although all silicon compounds should have been consumed (after equation 1), it was yet possible to observe the presence of this element but in low amounts (figure 3).

Variations in the concentration of the chemical elements (Al, N, O, B and C) present in the coatings were observed according to the nitridation temperature. Besides, these Auger Electron Spectroscopy analyses have also shown that the thickness of the nitrated coatings tend to decrease as the temperature of the nitridation process was increased (figure 3). This last phenomenon can be explained by the reaction occuring between ammonia and carbon which leads to the formation of HCN and to a carbon depletion [START_REF] Van Dijen | [END_REF]. A densification of Al-O-N-B and Al-O-N coatings and therefore a decrease of their thickness, were thus observed after using higher nitridation temperatures. In addition to their densification and concomitant carbon consumption, it has been evidenced that, in the case of Al-O-N thin films, the aluminium and oxygen content increased with the temperature of nitridation, as presented in table 1. These last observations may be explained by a de-mixing process in gradually taking place in the coating as the nitridation temperature was increased, leading to the formation of Al 2 O 3 while liberating nitrogen. Conversely, in the case of the Al-O-N-B coatings, such phenomena were not observed (table 2). Finally, high resolution TEM showed that both thin films were homogeneous, amorphous (no ring patterns of selectedarea electron-diffraction were observed) and adhere to the fibers.

Mechanical properties and oxidation behaviour

Some mechanical properties (Young's modulus, stress and strain to rupture) of single fibers coated with Al-O-N and Al-O-N-B thin films were studied. Results are reported in tables 1 and 2.

Whereas heat treatments performed up to 950°C did not degrade the mechanical properties of the various coated ZMI fibers, both stress and strain to rupture of the fibers coated by Al-O-N and nitrated at 1000°C were found to decrease. This phenomenon can be explained by a reaction between the ammonia and the free carbon contents of the fibre as well as by the aforementioned a de-mixing process occuring in Al-O-N when the temperature of the nitridation process is increased.

On the other hand, stress and strain to rupture of Al-O-N-B coated fibers, nitrated at 1000°C, proved to be stable. Indeed, at this temperature the ammonia reacts preferentially with boron oxide to form boron nitride and water. This formation of boron nitride, which acts as an interphase (confirmed by TEM in figure 4), permits to preserve the mechanical properties of the fibers. Indeed, TEM analyses showed that these Al-O-N-B thin coatings were amorphous, and mainly composed of aluminium, boron, nitrogen and boron compounds with few nano-crystals of BN and Al 4 B 2 O 9 .

Results of the mechanical tests performed on different single ply composites made of treated ZMI fiber in a CVI-SiC matrix are presented in figure 5.

Composites C2 did not present acceptable mechanical properties. Indeed, the observed very limited non linear domains tend to evidence an inherent brittleness (with strains to rupture less than 0.4%). These results can be attributed to a noticeable degradation of the fiber as compared to the reference material C1. Conversely, samples C3 with both an Al-O-N coating and a pyrocarbon interface did not reveal any degradation of the mechanical properties, as compared to the reference material. It may thus be concluded that the pyrocarbon interphase is still necessary to obtain an overall non linear/non brittle behaviour.

Results of static oxidation tests are presented in figure 6

Conclusion

Two different protective coatings, Al-O-N and Al-O-N-B respectively, have been applied

on Si-O-C fibers in order to enhance their oxidation resistance under severe thermomechanical conditions. Aluminium and nitrogen were introduced into a microporous carbon surface by impregnation of an aluminium trichloride solution, followed by a heat treatment and by a nitridation of these thin films (consisting of a thermal treatment in an ammonia atmosphere). Auger spectroscopy and SEM associated to an EDS detector have confirmed the effectiveness of the presence of Al-O-N and Al-O-N-B thin coatings at all the fiber surfaces. These coatings were sufficiently homogeneously distributed to allow the absence of bridging between the fibers. The mechanical compatibility of these applied coatings have been assessed with the help of tensile tests performed on single fibre as well as on composites produced from a 2D fabric containing the fibre with the different coatings mentioned above after impregnation with SiC matrix.

Obtained results have shown that, in both cases, the coatings preserve the mechanical integrity of the fibers, if an additional pyrocarbon interphase is present. Oxidation experiments at 600°C showed that the different coatings considerably decrease oxide scale formation on the fibers which will lead to an increase of the lifetime of the ceramic matrix composites.

List of figure and table captions

  . The usual passive oxidation of SiC based fibers leads to the formation of a silica layer. The presence of Al-O-N or Al-O-N-B coatings has resulted in an important reduction of the silica scale. However, neither Al-O-N nor Al-O-N-B coatings were able to totally prevent the oxygen ingress towards the fiber surface.

Figure 1 :

 1 Figure 1: High resolution SEM image of thin coatings at the fiber surface with a saturation

Figure 2 :

 2 Figure 2: High resolution SEM images of thin coatings with a concentration of aluminium

Figure 3 :

 3 Figure 3: Auger spectroscopy analyses of thin coatings after nitridation at a) 900°C, b)

Figure 4 :

 4 Figure 4: TEM images of Al-O-N-B thin coatings: a) Bright field image; b) High

Figure 5 :

 5 Figure 5: Tensile stress-strain curve of a simple 2-D weave Si-O-C fiber reinforced SiC as

Figure 6 :

 6 Figure 6: Silica thicknesses (x) created at 600 °C in air during forty weeks, on Tyranno

d 1 : 2 :

 12 Thickness of Al-O-N or C coatings; : diameter of fibre; σ r : stress at rupture; E: Young's modulus; ε r : strain to rupture Table Thicknesses and mechanical properties of fiber with Al-O-N-B thin coatings after different nitridation temperatures.

d 2 :

 2 Thickness of Al-O-N or C coatings, diameter of fibre; σ r : stress at rupture; E: Young's modulus; ε r : strain to rupture

Figure 1 27

 1 Figure 1

Table 1 :

 1 Thicknesses and mechanical properties of fiber with Al-O-N thin coatings after different nitridation temperatures.

Table 1

 1

	Fiber	d 1		Chemical composition (at.%)	σ r	E	ε r
		(nm) (µm) C	O	Al N	Si (GPa) (GPa) (%)
	without any treatment	-	13.0						3.2 ± 0.7	210 ± 8	1.5 ± 0.3
	with the carbon coating	560 ± 46	13.0						3.1 ± 0.3	162 ± 6	1.9 ± 0.3
	With Al-O-N
	coating after 900°C nitridation	246 ± 13	12.3 63 15 15	5	2	3.4 ± 0.4	178 ± 22	1.9 ± 0.3
	treatment
	With Al-O-N
	coating after 950°C nitridation	129 ± 12	12.1 11 38 29 16	6	3.2 ± 0.4	184 ± 8	1.7 ± 0.2
	treatment
	With Al-O-N
	coating after 1000°C nitridation	39 ± 15	11.9 10 47 30	9	4	2.5 ± 0.8	198 ± 16	1.3 ± 0.4
	treatment

Table 2

 2

	Fiber	d 2		Chemical composition (at.%)	σ r	E	ε r
		(nm) (µm) C	O	Al N	B (GPa) (GPa) (%)
	without any treatment	-	13.0					3.2 ± 0.7	210 ± 8	1.5 ± 0.3
	with the carbon	560						3.1 ±	162	1.9
	coating	± 46 12.3					0.4	± 6	± 0.3
	with Al-O-N-B
	coating after 1000°C nitridation	180 ± 13	12.2 15 45 17 18	5	3.0 ± 0.4	205 ± 22	1.5 ± 0.2
	treatment
	with Al-O-N-B
	coating after 1050°C nitridation	140 ± 16	12.1 15 25 17 25 18	3.0 ± 0.4	200 ± 21	1.5 ± 0.2
	treatment
	with Al-O-N-B
	coating after 1100°C nitridation	70 ± 10	12.0 15 15 10 30 30	2.8 ± 0.5	185 ± 7	1.5 ± 0.3
	treatment

Acknowledgments

The authors acknowledge the financial support of ADEME for this work and M. Lahaye and P. Weisbecker for Auger electron spectroscopy and Cross Polishing respectively.


L Maillé

C Sant

C Le Paven-Thivet

C Legrand-Buscema

P Garnier

STRUCTURE AND MORPHOLOGICAL STUDY OF NANOMETER W AND W 3 O THIN FILMS

Keywords: RF-sputtering, tungsten thin films, W 3 O structure, W structure, Atomic Force Microscopy, X-Ray diffraction. _____________________________________________________________________________

In this paper, the structure of nanometer tungsten thin films has been correlated to their surface morphology. Films have been deposited by RF-sputtering at a working pressure of 0.5 Pa and with a power density of 1.18 W/cm 2 . Two phases with different morphology has been observed : W 3 O with a nanograins structure is present in the first step of the tungsten growth; and, when the thickness is increased, a pure tungsten Wolfram phase (W) with a lamellar structure appears. We demonstrate that W 3 O is related to a pollution of the target surface between two growth runs. We succeed to suppress this phase and to obtain pure tungsten Wolfram nanolayer, in order to realize [W/WO 3 ] n multilayer.

Introduction

Nanomaterials possess original physical properties. Many searchers study about mechanical [1][2][3][4]; optical [5][6], electrical [4;7] or magnetical [4] properties. On the other hand, tungsten is a material well known for its particular characteristics : high hardness, high melting point, low coefficient of friction, resistance against oxidation and corrosion [1;8]. We purpose the study of the mechanical properties of tungsten nanomaterial.

X-Ray diffraction patterns analysis of tungsten layers often show the existence of two phases [START_REF] Arita | [END_REF][10]. The pure tungsten Wolfram (W) wished; and a second one that was determined to be a low oxidized tungsten phase, W 3 O-A15 also called -W. This unwished phase exists in the very first step of the tungsten films growth.

Our purpose is to realize multilayer structure W/WO x and W/WN x with some nanometers of thickness. In these nanolaminated structures, the individual layer thickness is lower than 50 nm.

Then, the unwished W 3 O phase will dramatically perturb the mechanical properties of multilayers. The oxidized tungsten phase must be absolutely avoided. The objective of this work is to control pure W Wolfram nanolayer elaboration for monolithic layer and multilayer.

Experimental procedure

Thin films deposition

Tungsten films are deposited at room temperature by R.F-sputtering. The working pressure during deposition is 0,5 Pa. The sputtering target (127 mm of diameter) consist in of pure tungsten element metal. Tungsten films are grown using pure argon plasma. WO 3 films are deposited by a reactive R.F-sputtering with an atmosphere composed of an Ar + O 2 (20%) mixture. The target-substrate distance is kept constant at 70 mm. During deposition, the power density on the tungsten target is 1,18 W/cm 2 . <100> silicon wafers are used. A shutter can be placed between the sputter source and the substrate, in order to prevent the layer deposition during the target cleaning process and to allow the multilayer realisation.

Three kinds of samples have been processed :

 Tungsten monolayers without target cleaning and with different thicknesses;

 Tungsten monolayers with a target cleaning before deposition;  Tungsten monolayers with a target cleaning before deposition and with a target cleaning after a fictitious WO 3 layer. The experimental procedure of this third type of sample is described below.

In order to analyse the structure of the tungsten layers in the multilayer [W/WO 3 ] n , we have grown a tungsten layer in the same conditions rather than during the multilayer process. We have first cleaned the target during 1 hour. WO 3 layer (50 nm of thickness) is grown by a reactive RF sputtering (Ar + O 2 (20%) mixture); with the shutter closed. The WO 3 growth is then performed on the shutter and not on the wafer. This layer is then called "fictitious" WO 3 layer. At the end of this fictitious layer, the O 2 reactive gas is removed from the chamber. A target cleaning of the target surface is performed before the W Wolfram film deposition. 1, 3 and 5 minutes have been chosen for target cleaning after the remove of the oxygen gas from the growth chamber. The W layer is then deposited. The so obtained layer is a tungsten one, grown in the same condition rather than during [W/WO 3 ] 1 multilayer growth.

Thin films characterisations

A Bruker D8 Advance X-Ray Diffractometer (XRD) is used to investigate film structure, employing the Cu-K radiation and Ni filter. Film surfaces are observed by Atomic Force Microscopy (AFM tapping mode), using a Digital Instruments 3100. X-Ray reflectometry (XRR) (Bruker D8 Advance) and AFM are used to check the surface's roughness. Thicknesses are simulated by a spectrum fit of XRR if it is smaller than 100nm. Otherwise they are observed on cross section by a Scanning Electron Microscopy (SEM), using a Zeiss DSM982 Gemini SEM.

Results

XRD patterns analysis

X-Ray diffraction patterns analysis performed on very thin tungsten layers show the existence of two phases : W Wolfram (W) and a low oxidized tungsten phase W 3 O-A15 (-W). W is thermodynamically stable form under ordinary conditions. It is a body centered cubic structure with a cell of 3,1648 Å and a density of 19,262 (JCPDS file 4-806). W 3 O has been reported by A. Katagiri et al. to be an intermediate phase before the growth of pure W [11]. It is a cubic structure with a cell of 5,047 Å and a density of 14,662 (JCPDS file 41-1230). The nature of this oxide has been described by several authors [START_REF] Arita | [END_REF][10][11][12][13][14][15]. XRD patterns analysis show that W 3 O peaks are observed in the thin films when thicknesses are smaller than 100 nm. 94 nm and 175 nm respectively. XRD patterns are performed between 2 = 30° to 130°. In figure 1, only the 34°-46° range is presented since no significant peaks were found elsewhere. When the thickness increases, the intensity of W 3 O peaks decreases whereas the intensity of W peaks increases.

Surface morphology

AFM analysis show a change of the surface morphology of these films when the thickness increases (fig. 2). For a 48 nm thick tungsten film, only nanograins are observed (fig. 2a). At the opposite, for a 175 nm thick tungsten films there are only lamellar structures (fig. 2c). Figure 2b (94 nm of thickness) presents an intermediate surface morphology between these two textures. Indeed, the lamellar structure is predominant and some nanograins are still present.

A mass production of layer, using a pure tungsten target with the same growth parameters, has been performed. Same results are obtained with a good reproductibility of the observed phase (XRD results), related to the surface microstructure (AFM studies).

Influence of the target cleaning

To observe the influence of the target cleaning before the tungsten deposition, two films have been grown, with the same thickness (86 nm determinate by XRR). Only target cleaning time changes : 10 min and 60 min.

XRD analysis results are presented in figure 3. The oxidized tungsten W 3 O is the major phase when the target cleaning is 10 minutes (fig. 3a). This last phase disappears totally when the target cleaning time is more important (60 min). In fig. 3b

Growth of a pure W layer after WO 3 layer deposition phase

The objective of this work is to realize multilayer [W/WO 3 ] n with pure W target in order to study the mechanical properties. Three thin films of tungsten with a fictitious WO 3 layer have been performed with different target cleaning period before W layer deposition: 1 min, 3 min and 5 min.

XRD patterns analysis of these three films have been performed. They are similar. For instance, figure 5 represents tungsten thin film XRD diagrams after 1 min of target cleaning between WO 3 "fictitious" film and W film.

The surface microstructure, and the roughness (0,47 nm) are the same for the three experiments (fig. 6). It shows the only presence of lamellar microstructure.

Discussion

XRD patterns analysis (fig. 1) shows the dominant presence of the oxidized tungsten W 3 O peaks with a (002) preferential orientation for all samples thinner than 100 nm and without target cleaning. When the thickness increases, W peak (110) appears until it becomes majority. In fig. 1c) the thickness is too thick to see the peak of the W 3 O intermediate phase, present in the first step of the tungsten growth. For 175 nm thick films, only the W peaks (110) are observed. This suggests that when the growth is carried on, the W 3 O diffraction peak decreases progressively and disappears completely for a layer thickness higher than 175 nm in favour of W (fig. 1c). The oxidized tungsten W 3 O peaks aren't present for all samples thinner than 100 nm and performed with a target cleaning (fig. 3b). These results suggest that the W 3 O phase is related to a target pollution between two growth runs. In sputtering, this phenomenon is currently observed [16]. 60 min of target cleaning at 150 Watt are necessary to remove the oxidized layer formed at the target surface between two runs (few hours). This procedure allows to obtain a pure W layer (fig. 3b), even if the tungsten layer thickness is smaller than 100 nm. Experimental XRD patterns show the only presence of W peaks. These patterns lead to the determination of the tungsten Wolfram thin film cell parameters (a=b=c=3,179 Å).

Concerning the morphology of the surface of the layer, when only W 3 O peaks are observed in XRD patterns, the surface presents the same nanograins morphology (fig. 2a). When both W 3 O and W peaks are present in XRD patterns a lamellar structure with still some nanograins is observed (fig. 2b). When only W peaks are present in XRD patterns, only the lamellar structure is observed (fig. 2c). The same evolution of surface microstructure exists when the time of the target cleaning is changed. With 10 min (fig. 4a), both nanograins and lamellar structures are observed, as for 100 nm fig. 2b). With 60 min, only the lamellar surface structure is observed (fig. 4b), as for 194 nm fig. 2c). It's then possible to correlate the surface morphology thin films to the layer phase composition : W 3 O phase presents a nanograin microstructure whereas pure W phase has a lamellar structure. These results are confirmed by the AFM surface analysis performed on the sample grown with the target cleaning. We observed in fig. 4b WO 3 layers are deposited by a reactive RF-puttering. Then, the target surface is probably oxidized during the WO 3 layer deposition. Nevertheless, during this deposition, the target surface oxidation is much lower than between two runs. Indeed it is continually sputtered and then cleaned by Ar + ions. The principal W 3 O peak (002) at 35,3° is not observed (fig. 5) even with 1 min of target cleaning performed before the W layer growth. Only W (110) peak is represented in the XRD pattern, which indicate that the tungsten layer is constituted of pure W Wolfram.

Then 1 min of target cleaning before the W Wolfram layer growth is enough to remove the oxidized layer to the target surface. This result is compatible with the elaboration of nanolaminated structure.

Conclusion and future outlook

W 3 O phase is observed in tungsten thin films with thicknesses smaller than 100 nm and performed without target cleaning. When the thickness increases, the W 3 O phase decreases progressively to disappear completely for a layer thickness higher than 175 nm, in consideration of W Wolfram. When a 60 min target cleaning is performed, only W Wolfram is present. In this paper we correlated the morphology surface with the stoechiometry layer : W possess a lamellar structure; W 3 O a nanograins structure.

Moreover we demonstrate that it's possible to realize multilayer [W/WO 3 ] n without the presence of W 3 O phase in W films by a 1 min target cleaning performed after WO 3 reactive sputtering deposition and before the W layer growth.

List of Graphics Captions

Figure 1 a

 1 Figure 1 a, b and c present XRD patterns of tungsten thin films with different thickness : 48 nm;

  no more W 3 O peaks are observed and the only one peak present is the (110) W.

Figure 4 a

 4 Figure 4 a and b shows AFM images of tungsten thin films after target cleaning for 10 and 60

  the lamellar morphology correlated to the pure tungsten Wolfram XRD patterns. The objective of our work is to study the mechanical properties of nanolaminated [W/WO 3 ] n multilayers. The structure consists to a periodic stacking of two layers : W Wolfram and WO 3 .

Fig. 1 .

 1 Fig. 1. XRD patterns of tungsten thin films with different thicknesses a) 48 nm , b) 94 nm and c)

Fig. 2 .

 2 Fig. 2. AFM morphology of tungsten films with different thicknesses a) 48 nm, b) 94 nm and c)

Fig. 3 .

 3 Fig. 3. XRD patterns of tungsten thin films (86 nm) after a target cleaning of a) 10 min and b) 60

Fig. 4 .

 4 Fig. 4. AFM images of tungsten thin films (86 nm) after a target cleaning of a) 10 min and b) 60

Fig. 5 .

 5 Fig. 5. XRD pattern of tungsten thin film (50 nm) after 1 min of target cleaning performed

Fig. 6 .Fig

 6 Fig. 6. AFM images of tungsten thin film (50nm) after 1 min of target cleaning performed


Gwenn Menvielle
email: gwenn.menvielle@inserm.fr

Ivana Kulhánová

Josephine Bryere

Guy Launoy

Daniel Eilstein

Cyrille Delpierre

Isabelle Soerjomataram

Joséphine Bryère

Tobacco-attributable burden of cancer according to socioeconomic position in France

Keywords: Cancer epidemiology Tobacco-related cancers, Incidence, Deprivation, PAF, France

Introduction

Smoking is a major preventable cause of morbidity and mortality from cardiovascular disease, chronic obstructive pulmonary disease, lung cancer as well as a range of other cancers, peptic ulcer and various other medical conditions. In 2010 in France it has been estimated that smoking was responsible for about 59,000 deaths in men and 19,000 deaths in women, accounting for 21% and 7% of all deaths respectively. A large part of these deaths are due to cancer: 66% in men and 42% in women. [START_REF] Hill | Trends in tobacco-attributable mortality in France[END_REF] Smoking prevalence has remained stable between 2000 and 2016 in France, with 32% of men and 26% of women smoking in 2016. But this prevalence is socially distributed, with higher tobacco smoking among people from a lower socioeconomic group, and this social gradient in tobacco smoking is increasing. [START_REF] Pasquereau | Tabac et e-cigarette en France: niveaux d'usage d'après les premiers résultats du Baromètre Santé 2016] Tobacco and e-cigarette in France: levels of consumption according to the preliminary results from the 2016 Health Barometer[END_REF][START_REF] Legleye | Widening inequalities in smoking initiation and cessation patterns: a cohort and gender analysis in France[END_REF][START_REF] Bricard | Educational inequalities in smoking over the life cycle: an analysis by cohort and gender[END_REF] It has been shown that a higher level of tobacco smoking partly accounted for the higher incidence of lung cancer in low socioeconomic groups [START_REF] Menvielle | The role of smoking and diet in explaining educational inequalities in lung cancer incidence[END_REF][START_REF] Nkosi | Socioeconomic position and lung cancer risk: how important is the modeling of smoking?[END_REF][START_REF] Mao | Socioeconomic status and lung cancer risk in Canada[END_REF][START_REF] Hart | Social class differences in lung cancer mortality: risk factor explanations using two Scottish cohort studies[END_REF] and that the contribution of smoking to relative inequalities in lung cancer incidence in France ranged from 22% for men with primary education to 31% for men with a high school degree when compared with men with a university degree. [START_REF] Menvielle | The ICARE study group. Quantifying the mediating effects of smoking and occupational exposures in the relation between education and lung cancer[END_REF] In addition to tobacco, other risk factors, such as behavioral, environmental, occupational or psychosocial conditions, are likely to contribute to the excess cancer risk in more deprived groups [START_REF] Mccartney | Why do males in Scotland die younger than those in England? Evidence from three prospective cohort studies[END_REF] . In particular, occupational exposures such as asbestos, silica, or formaldehyde, have been shown to explain a nonnegligible part of socioeconomic inequalities in lung [START_REF] Behrens | Occupational prestige, social mobility and the association with lung cancer in men[END_REF][START_REF] Menvielle | The ICARE study group. Quantifying the mediating effects of smoking and occupational exposures in the relation between education and lung cancer[END_REF] and head and neck cancers [START_REF] Santi | Occupation and educational inequalities in laryngeal cancer: the use of a job index[END_REF][START_REF] Menvielle | Smoking, alcohol drinking, occupational exposures and social inequalities in hypopharyngeal and laryngeal cancer[END_REF] .

In addition to quantifying the relative contribution of risk factors to socioeconomic inequalities in cancer incidence, assessing the absolute number of cancer cases attributable to smoking by socioeconomic position (SEP) at a national level is important for policy makers to assess the population burden of smoking on social inequalities in cancer. However, such assessment based on national data is lacking.

The aim of this study is to estimate the tobacco-attributable cancer burden according to SEP in France by assessing the Population Attributable Fraction (PAF) and the number of cancer cases attributable to smoking by SEP at a national level.

Methods

Data sources

We used data from cancer registries (Calvados, Côte d'Or, Doubs, Finistère, Gironde, Haute-Vienne, Hérault, Isère, Loire-Atlantique, Manche, area of Lille, Bas-Rhin, Haut-Rhin, Saône-et-Loire, Somme, Tarn, and Vendée), which represent approximately 20% of the French population. Cancers diagnosed among people aged 30-74 between 2006 and 2009 were included. We studied the following cancers related to tobacco smoking according to IARC evaluations [START_REF]Humans IWGotEoCRt, personal habits and indoor combustions[END_REF] : lip and oral cavity (ICD-10: C00-C08), oropharynx (C09-C10, C12-C14), nasopharynx (C11), oesophagus (C15), stomach (C16), colon and rectum (C18-C20), liver (C22), pancreas (C25), larynx (C32), trachea, lung and bronchus (C33-C34), cervix uteri (C53), ovary (only mucinous cancer, C56), kidney and renal pelvis (C64-C65), urinary bladder (C67), acute myeloid leukaemia (C92.0). In the rest of the paper, we will refer to this group of cancers as cancers linked to smoking.

As no individual socioeconomic characteristic is available in the French cancer registries, SEP was assessed using an ecological deprivation index which attributes the level of deprivation of his/her place of residence at diagnosis to each case recorded at the registry. We used the French version of the European Deprivation Index (EDI), calculated at the smallest geographical scale available (IRIS, about 2000 inhabitants). The methodology has been described elsewhere. [START_REF] Pornet | Construction of an adaptable European transnational ecological deprivation index: the French version[END_REF] Briefly, from the French part of the European Union Statistics on Income and Living Conditions (EU-SILC), an individual deprivation indicator is first developed from fundamental needs associated in the sample with objective and subjective poverty [START_REF] Townsend | Health and Deprivation: Inequality and the North[END_REF] , then ecological census variables that are the most closely related to the individual deprivation indicator are selected using logistic regressions. A categorical version of the EDI (quintiles calculated at the French level, each quintile includes 20% of the IRIS) was used.

The number of cancer cases was provided by age groups (30-

Calculation of the PAF

Smoking status was not recorded at the cancer registries. Therefore we used an indirect method to assess the population attributable fraction (PAF) of the cancer burden attributable to smoking according to SEP. Although the PAF is a powerful and validated tool to assess the burden of smoking, this is a relatively parsimonious method. By sex, age group (30-59, 60-74) and EDI quintile, we computed the PAF to smoking for each cancer linked to smoking. The number of cancer cases attributable to smoking by EDI quintile was then obtained by multiplying the PAF by the number of cancer cases observed over the period 2006-2009. We then combined these figures to obtain the overall PAF for the age range 30-74.

The PAF was computed using the indirect method proposed by Peto-Lopez, which has been specifically developed to assess the burden of tobacco smoking on cancer incidence. More information can be found elsewhere [START_REF] Parkin | Tobacco-attributable cancer burden in the UK in 2010[END_REF][START_REF] Peto | Mortality from tobacco in developed countries: indirect estimation from national vital statistics[END_REF] . The indirect method assumes that the lung cancer rates in a specific population can be used to approximate the cumulative exposure of tobacco smoking in this population. The PAF and the number of cancer cases attributable to smoking are then computed separately for lung cancer and for the other cancer sites, using two different formulas.

The PAF to smoking for lung cancer incidence was calculated by the following formula: [START_REF] Thun | Lung cancer occurrence in never-smokers: an analysis of 13 cohorts and 22 cancer registry studies[END_REF] . This information was not available by EDI quintile, so we used the same incidence rates among never smokers in all EDI quintiles. P is the observed population size in France by sex, age group and EDI quintile.

𝑃𝑃𝑃𝑃𝑃𝑃 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 = 𝑁𝑁 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 -
For the other cancers linked to smoking, the traditional PAF formula from Levin was used, as incidence rates in never-smokers are not known and the associations with smoking are not as strong as that for lung cancer: 21

𝑃𝑃𝑃𝑃𝑃𝑃 = 𝑝𝑝 𝑜𝑜 (𝐴𝐴𝐴𝐴 -1) 1 + 𝑝𝑝 𝑜𝑜 (𝐴𝐴𝐴𝐴 -1)
ps is a proxy for smoking prevalence in the French population. This proxy reflects the average cumulative exposure to smoking in the population. Assuming the relative risks (RR) of the EPIC study [START_REF] Agudo | Impact of cigarette smoking on cancer risk in the European prospective investigation into cancer and nutrition study[END_REF] apply to the French population, ps is the smoking prevalence that would have been necessary to produce the observed lung cancer incidence in France. ps (by sex, age group and EDI quintile) can be expressed as

𝑝𝑝 𝑜𝑜 = 𝑃𝑃𝑃𝑃𝑃𝑃 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 �1-𝑃𝑃𝑃𝑃𝑃𝑃 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 ��𝑅𝑅𝑅𝑅 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 -1� 18
The adjusted RRs quantified the association between current smoking and cancer incidence for specific cancer sites. The RRs differed by sex and cancer site, but they were the same for all age groups and EDI quintiles. They were obtained from the EPIC study and were adjusted for age, education, body mass index, physical activity, alcohol consumption, total energy intake and consumption of fruits and vegetables [START_REF] Agudo | Impact of cigarette smoking on cancer risk in the European prospective investigation into cancer and nutrition study[END_REF] .

The PAFs by cancer site, age group, sex and EDI quintile are presented in supplementary table 2.

Measure of inequalities in cancer incidence

We quantified the contribution of smoking to socioeconomic inequalities in cancer incidence at the national level. All calculations were performed separately for men and women and by age group (30-59, 60-74). We calculated the total number of cancers linked to smoking by EDI quintile by multiplying the observed incidence rates (IR) with the national population figures (average of the years 2006-2009). Using the PAF, we also computed the number of cancer cases attributable to smoking by EDI quintile.

We considered three hypothetical situations: i) no one ever smoked; ii) the four lower EDI quintiles smoked as the least deprived EDI quintile; iii) and the four lower EDI quintiles had the same IRs as the least deprived EDI quintile. The hypothetical situation i) provided an estimate of the maximal reduction in cancer IR that would be observed if smoking could be totally eliminated. To assess the quantitative burden of smoking on the observed socioeconomic inequalities in cancer incidence in France, we then computed the number of cancer cases that could be prevented in the hypothetical situations ii) and iii). The ratio between the cases prevented in these two hypothetical situations provides an estimate of the potential reduction in cancer burden if inequalities in smoking were to be eliminated. Details for the calculation are available in appendix.

Results

In France, the observed number of cancers linked to tobacco smoking increased with decreasing SEP and was especially high in the most deprived EDI quintile, as the result of both higher IR and larger population size (about 27% both in men and women) in this group (Table 1). The PAF to smoking increased with decreasing SEP from 56% in men living in the least deprived neighborhoods to 70% in men living in the most deprived neighborhoods and from 26% to 38% in women. The higher PAFs observed in men than in women mirror the higher burden of smoking in men. The PAFs differed by age group. Among men and women, the PAF was higher in the younger age groups i.e. 30-59 years. The age difference was pronounced in the more deprived EDI quintiles whereas the PAFs were almost identical in both age groups in the least deprived EDI quintile. The number of cancers attributable to smoking was substantially higher in men (42,613) than in women (9,376) and was highest in the age group 60-74 in men and in the age group 30-59 in women.

IRs by EDI quintile are presented in Figure 1. Both in men and women, IRs for cancers linked to tobacco smoking increased with decreasing SEP. If no one had ever smoked, the IRs would hardly differ by EDI quintile. However, IRs would be only slightly reduced if the four lower EDI quintiles smoked as the least deprived EDI quintile. The socioeconomic gradient would nevertheless still be observed.

Tables 2 and3 assess the contribution of smoking to socioeconomic inequalities in cancer incidence at the national level. If the four lower EDI quintiles had the same IRs as the least deprived EDI quintile, 8,687 cancers cases in men and 2,737 in women could be prevented (Table 2). If the four lower EDI quintiles smoked as the least deprived EDI quintile, 2,388 cancer cases in men and 1,187 cancer cases in women could be prevented, most of them in the age group 30-59. Thus, if inequalities in smoking were eliminated, the excess cancer cases in the 4 most EDI quintiles would be reduced by 27.5%

(2,388/8,687) in men and 43.4% (1,187/2,737) in women. The proportions did not differ by sex in the age group 60-74 (around 30%), but was much higher in women than in men in the age group 30-59 (48.1% versus 26.3%).

The proportion of cancers linked to tobacco smoking that could be prevented if all the population smoked like the least deprived EDI quintile increased with decreasing EDI quintile, and was higher in the younger age group. If in the most deprived EDI quintile smoked like the least deprived EDI quintile, 10% of cancer cases in men and 13% in women could be prevented in the least deprived EDI quintile in the age group 30-59 compared to 5% in men and women in the age group 60-74 (Table 3).

Discussion

Summary of main findings

We investigated the tobacco-attributable cancer burden according to SEP in France using the PAF methodology. This is a relatively crude method and the results should therefore not be interpreted as exact values but rather considered order of magnitude. Our results showed that if socioeconomic inequalities in smoking were removed, a significant part of the excess number of cancer cases in the 4 most deprived EDI quintiles both in men and women would be saved. Indeed, 27.5% of the excess cancers cases in the 4 most deprived EDI quintiles in men and 43.4% in women could be prevented if smoking in these 4 EDI quintiles was similar to that of the least deprived EDI quintile. However, smoking only partly explained socioeconomic inequalities in the incidence of cancers linked to smoking, in particular in men.

Interpretation of the results

The diffusion of smoking within the population follows a process that has been described as the smoking epidemic. [START_REF] Lopez | A descriptive model of the cigarette epidemic in developed countries[END_REF][START_REF] Thun | Stages of the cigarette epidemic on entering its second century[END_REF] Over the whole process, smoking progressively becomes more concentrated among socially deprived groups and this tends to happen first in men and then in women. The contribution of smoking in socioeconomic inequalities in health increases as the smoking epidemic progresses into its final stage. Countries that are at an earlier stage of the smoking epidemic such as France, Italy or Spain will have a smaller contribution of smoking to socioeconomic inequalities than countries that are at the most advanced stage such as North America, Nordic European countries, the UK, and the Netherlands. [START_REF] Bricard | Educational inequalities in smoking over the life cycle: an analysis by cohort and gender[END_REF][START_REF] Kulik | Smoking and the potential for reduction of inequalities in mortality in Europe[END_REF] Consistent with the evolution of the smoking epidemic, we observed an increase in PAF to smoking by decreasing SEP that was more pronounced in the younger age group (30-59), especially in women. Our results, in accordance with the literature, confirm that women in France have only recently reached the last stage of the smoking epidemic.

We estimated that if people in the most deprived EDI quintile had the same tobacco smoking prevalence as those in the least deprived EDI quintile, 10% of cancers linked to smoking in men and 13% in women in the younger age group could have been prevented. The respective figures for the older age group (60-74) were 5%. In a previous paper on socioeconomic inequalities in smoking-related mortality during the 2000s, Kulik et al showed that if everybody smoked like tertiary educated, relative inequalities in lung cancer mortality would be reduced by 10% among French men and by 17% among French women for lung and laryngeal cancer. In total, 6% of lung and laryngeal cancer deaths among French men with up to lower secondary education and 2.5% among French women with up to lower secondary education could be avoided. The percentage was much higher in the UK (30% and 23%). [START_REF] Kulik | Smoking and the potential for reduction of inequalities in mortality in Europe[END_REF] Consistent with the literature [START_REF] Menvielle | The role of smoking and diet in explaining educational inequalities in lung cancer incidence[END_REF][START_REF] Nkosi | Socioeconomic position and lung cancer risk: how important is the modeling of smoking?[END_REF][START_REF] Mao | Socioeconomic status and lung cancer risk in Canada[END_REF][START_REF] Hart | Social class differences in lung cancer mortality: risk factor explanations using two Scottish cohort studies[END_REF][START_REF] Menvielle | The ICARE study group. Quantifying the mediating effects of smoking and occupational exposures in the relation between education and lung cancer[END_REF] smoking only partly explained socioeconomic inequalities in the incidence of cancers linked to tobacco smoking. Several hypotheses could explain these remaining inequalities. Occupational exposures to carcinogens have a high burden on lung, head and neck and bladder cancers especially in men. This is related in particular to higher exposure to asbestos, diesel engine exhaust and silica [START_REF] Brown | Occupational cancer in Britain. Respiratory cancer sites: larynx, lung and mesothelioma[END_REF] . Studies have shown a substantial contribution of occupational exposures to socioeconomic differences in lung and head and neck cancer incidence. [START_REF] Menvielle | The ICARE study group. Quantifying the mediating effects of smoking and occupational exposures in the relation between education and lung cancer[END_REF][START_REF] Santi | Occupation and educational inequalities in laryngeal cancer: the use of a job index[END_REF][START_REF] Menvielle | Smoking, alcohol drinking, occupational exposures and social inequalities in hypopharyngeal and laryngeal cancer[END_REF][START_REF] Menvielle | Occupational exposures contribute to educational inequalities in lung cancer incidence among men: Evidence from the EPIC prospective cohort study[END_REF] As these exposures are disproportionately concentrated in male occupations, this could partly account for the sex difference observed in our data. Other risk factors may also partly account for the higher incidence of cancers linked to tobacco smoking among the low socioeconomic groups because they are more prevalent in these groups (e.g. poor diet or physical inactivity) and/or because, the same level of exposure may be more harmful in these groups (e.g. air pollution). [START_REF] Malon | Compliance with French nutrition and health program recommendations is strongly associated with socioeconomic characteristics in the general adult population[END_REF][START_REF] Menai | Walking and cycling for commuting, leisure and errands: relations with individual characteristics and leisure-time physical activity in a crosssectional survey (the ACTI-Cites project)[END_REF][START_REF] Raaschou-Nielsen | Air pollution and lung cancer incidence in 17 European cohorts: prospective analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE)[END_REF][START_REF] Padilla | Air quality and social deprivation in four French metropolitan areas--a localized spatiotemporal environmental inequality analysis[END_REF][START_REF] Deguen | Neighbourhood Characteristics and Long-Term Air Pollution Levels Modify the Association between the Short-Term Nitrogen Dioxide Concentrations and All-Cause Mortality in Paris[END_REF] These risk factors may also interact, leading to a larger cancer risk in the low socioeconomic groups.

Strengths and limitations

Our estimates are based on population-based cancer registries data and therefore do not suffer from selection bias, known to be associated with people's SEP. We investigated incidence data over the period 2006-2009 because incidence data have not been geocoded for more recent years. The methodology used required lung cancer incidence rates among never smokers by age groups. As these rates were based on very small numbers in the age group 75+ and consequently not reliable, the analyses were restricted to people aged below 75.

In France, cancer registries do not record information on individual SEP. We therefore used an ecological deprivation index. Although this index was computed at the smallest geographical area available, our results may suffer from an ecological bias. [START_REF] Bryere | Assessment of the ecological bias of seven aggregate social deprivation indices[END_REF] The magnitude of the socioeconomic inequalities in health is smaller when SEP is measured with an ecological deprivation index rather than with an individual indicator. Sensitivity analyses using another deprivation index (FDep) [START_REF] Rey | Ecological association between a deprivation index and mortality in France over the period 1997 -2001: variations with spatial scale, degree of urbanicity, age, gender and cause of death[END_REF] yielded close results among men. Among women, the results differed in the oldest age group, with no association between PAF and SEP without any clear explanation for these discrepancies.

The RRs quantifying the relation between smoking and cancer incidence were obtained from the EPIC data, and we assumed that these applied to France. [START_REF] Steenland | An overview of methods for calculating the burden of disease due to specific risk factors[END_REF] This assumption was necessary for practical reasons, as there are no high-quality literature reviews on the impact of smoking on cancer incidence for France. Nevertheless, there is an increasing body of evidence stating that, when the exposure is measured similarly, the RRs are similar across populations in different world regions. 36 Additionally, the relative risks were assumed to be the same for all EDI quintiles. There is nevertheless an increasing body of literature suggesting that socioeconomic factors are biologically embodied and associated with different levels of epigenetic markers that in turn may be associated with a higher risk of diseases including cancer in the lower SEP. [START_REF] Castagné | Biological marks of early-life socioeconomic experience is detected in the adult inflammatory transcriptome[END_REF][START_REF] Kelly-Irving | The embodiment of adverse childhood experiences and cancer development: potential biological mechanisms and pathways across the life course[END_REF] It is thus possible that the effect of the same level of tobacco smoking on cancer incidence may be higher in lower socioeconomic groups and that we slightly underestimated the number of cancer cases attributable to smoking in the more deprived EDI quintiles.

In our analyses, we used lung cancer IRs and RRs of cancer incidence associated with smoking from the European EPIC cohort because of the large differences in the smoking epidemic between the US and France. Sensitivity analyses using data from the CPS-II American survey as in the original publication presenting the Peto-Lopez method led to very similar conclusions.

The PAF calculations based on the indirect method do not require information on smoking in the population, but instead use the absolute lung cancer rates as a proxy of lifetime smoking in the population. Therefore, they do not suffer from reporting bias for tobacco smoking. However the lifetime smoking exposure in the population is derived from lung cancer incidence, and then applied to the other cancer sites, whereas this may not be the most relevant measure on tobacco smoking (and its pattern) for all cancer sites. In addition, this method assumes that lung cancer incidence among non-smokers is fairly small and roughly constant across populations; and hence the incidence (or mortality) rates of lung cancer in a specific population are largely determined by the cumulative exposure of this population to tobacco smoking. These assumptions generally hold in high income populations where lung cancer is rare among non-smokers even in polluted areas. Even radon and asbestos may cause only a small absolute risk among those who have never smoked. [START_REF] Peto | Mortality from tobacco in developed countries: indirect estimation from national vital statistics[END_REF] However, people with a low SEP are more likely to be exposed to other lung cancer carcinogens, namely occupational exposures that are concentrated in manual workers in particular for men, and we therefore cannot rule out that the use of the same lung cancer IR among never smokers for all EDI quintiles let to a slight overestimation of the role of smoking in socioeconomic inequalities in cancer incidence.

Perspectives and conclusion

We observed a substantial burden of smoking in socioeconomic inequalities in cancer incidence in France. Our results by age group, consistently with the available literature on socioeconomic differences in smoking by birth cohort, [START_REF] Bricard | Educational inequalities in smoking over the life cycle: an analysis by cohort and gender[END_REF] showed that this burden is going to dramatically increase in the most deprived groups in the coming years and even decades, especially among women. Our results thus highlight the need for policy efforts to reduce smoking in France in particular in the low socioeconomic groups. The time lag between smoking and cancer onset is of several decades for most cancers linked to tobacco smoking (lung, head and neck, pancreas, bladder, kidney) [START_REF] Sasco | Tobacco smoking and cancer: a brief review of recent epidemiological evidence[END_REF] . Therefore, any decrease in smoking would not affect cancer incidence before at least 20 years. However, other health outcomes such as cardiovascular and respiratory diseases would immediately benefit from a decrease in smoking.

Several tobacco control policies have been implemented in France during the last decade, in particular a gradual price increase since 2000, France now ranking as the European country with the third highest tobacco price (after UK and Ireland). Although the literature suggests that price increase is the most efficient tobacco control policy to decrease socioeconomic inequalities in smoking, [START_REF] Decicca | Cigarette taxes and older adult smoking: evidence from recent large tax increases[END_REF][START_REF] Thomas | Population tobacco control interventions and their effects on social inequalities in smoking: systematic review[END_REF][START_REF] Siahpush | Taxation reduces social disparities in adult smoking prevalence[END_REF] this issue is still heavily discussed [START_REF] Franks | Cigarette prices, smoking, and the poor: implications of recent trends[END_REF][START_REF] Peretti-Watel | Responses to increasing cigarette prices in France: how did persistent smokers react?[END_REF][START_REF] Peretti-Watel | Cigarettes and social differentiation in France: is tobacco use increasingly concentrated among the poor?[END_REF][START_REF] Regidor | Increasing the price of tobacco: economically regressive today and probably ineffective tomorrow[END_REF][START_REF] Breton | Examining tobacco control strategies and aims through a social justice lens: an application of Sen's capabilities approach[END_REF] and the observed trends in smoking rates by SEP in France [START_REF] Pasquereau | Tabac et e-cigarette en France: niveaux d'usage d'après les premiers résultats du Baromètre Santé 2016] Tobacco and e-cigarette in France: levels of consumption according to the preliminary results from the 2016 Health Barometer[END_REF][START_REF] Bricard | Educational inequalities in smoking over the life cycle: an analysis by cohort and gender[END_REF] showed that such strategies largely failed to reduce both smoking rates and social inequalities in smoking. The tobacco control strategies implemented so far mostly focused on modifying individual behaviors. Policies, prevention campaigns and intervention [START_REF] Michie | Development of StopAdvisor: a theory-based interactive internet-based smoking cessation intervention[END_REF] should be implemented in a more comprehensive framework and address the social conditions and determinants where behaviors are rooted. 49 Given the dramatic increase in educational differences in smoking in France among younger generations, such policies are urgently needed to limit the burden of smoking in socioeconomic inequalities in health in the future. In addition, our results show that smoking only partly explains socioeconomic inequalities in the incidence of cancer linked to tobacco smoking. This underlines the urgent need to apprehend the socioeconomic inequalities in cancer as the consequences of many interrelated determinants socially unequally distributed such as behavioral, occupational, environmental and housing exposures, to reduce social inequalities in cancer.

Assumptions taken and potential impact on the results

Computation of the PAF for lung cancer

Assumption: The same lung cancer incidence among non-smokers is observed in all EDI quintiles, hence incidence rates of lung cancer in a specific population are largely determined by the cumulative exposure of this population to tobacco smoking.

Potential impact on the results: Incidence rates among never smokers may be higher among people with a low SEP, because they are more likely to be exposed to other lung cancer carcinogens, namely occupational exposures that are concentrated in manual workers in particular for men.

Therefore, we may have slightly overestimated the PAF for lung cancer in the lower EDI quintiles.

Computation of the PAF for the other cancers linked to smoking Assumption: The RRs quantifying the relation between smoking and cancer incidence were assumed to be the same for all EDI quintiles. There is nevertheless an increasing body of literature suggesting that socioeconomic factors are biologically embodied and associated with different levels of epigenetic markers that in turn may be associated with a higher risk of diseases including cancer in the lower SEP.

Potential impact on the results: It is thus possible that the effect of the same level of tobacco smoking on cancer incidence may be higher in lower socioeconomic groups and the RR may be higher in the lower EDI quintiles.

Therefore, we may have underestimated the PAF in the lower EDI quintiles.

Computation of the final combined PAF for all cancers linked to smoking There is both an overestimation of the PAF for lung cancer and an underestimation of the PAF for the other cancers linked to smoking. It is therefore impossible to say if the final combined PAF for all cancers linked to smoking was overestimated or underestimated in the lower EDI quintiles.

In any case, the least biased PAF will be that computed in the least deprived EDI quintile.

  59, 60-74), sex, cancer site and EDI quintile (see supplementary table 1 for the number of cancer cases observed over the 2006-2009 and the corresponding incidence rates). Due to the small number of cancer cases in some strata, we combined all four years 2006 to 2009. Population data by sex, age group and EDI quintile for the years 2006-2009 were obtained from the French national Statistics Institute.

  Nprevented smoking = Nprevented smoking _EDI=2 + Nprevented smoking _EDI=3 + Nprevented smoking _EDI=4 + Nprevented smoking _EDI=5

		IRobs_EDI=5 * Popu_FranceEDI=5	IRobs_EDI=1 *	Nprevented_EDI=5
			Popu_FranceEDI=5
	Nprevented = Nprevented_EDI=2 + Nprevented_EDI=3 + Nprevented_EDI=4 + Nprevented_EDI=5
	2.2. Number of cancer cases that could be prevented if the 4 most deprived EDI quintiles smoked
	the least deprived EDI quintile.
	EDI	Number of cancer cases	Number of cancer cases that would		Number of cancer
	quintile	observed in France	be observed in France if all EDI		cases that could
			quintiles smoked like the least		be prevented if all
			deprived EDI quintile		EDI quintiles
					smoked like the
					least deprived EDI
					quintile
		(1)	(2)		(1) -(2)
	1	IRobs_EDI=1 * Popu_FranceEDI=1	IRexcess smoking_EDI=1 * Popu_FranceEDI=1	0
	2	IRobs_EDI=2 * Popu_FranceEDI=2	IRexcess smoking_EDI=2 * Popu_FranceEDI=2 Nprevented smoking_EDI=2
	3	IRobs_EDI=3 * Popu_FranceEDI=3	IRexcess smoking_EDI=3 * Popu_FranceEDI=3 Nprevented smoking_EDI=3
	4	IRobs_EDI=4 * Popu_FranceEDI=4	IRexcess smoking_EDI=4 * Popu_FranceEDI=4 Nprevented smoking_EDI=4
	5	IRobs_EDI=5 * Popu_FranceEDI=5	IRexcess smoking_EDI=5 * Popu_FranceEDI=5 Nprevented smoking_EDI=5

This work was funded by the French National Cancer Institute (INCa, grant number 2015-002)

49.

Commission on social determinants of health (CSDH), Closing the gap in a generation. Health equity through action on the social determinants of health. 2008, WHO: Geneva.

Supplementary material: description of the calculations

Tobacco-attributable burden of cancer according to socioeconomic position in France

Gwenn Menvielle, Ivana Kulhánová, Joséphine Bryère, Guy Launoy, Daniel Eilstein, Cyrille Delpierre, Isabelle Soerjomataram

Computation of the incidence rates (IR)

The incidence rates were computed by age group, sex and socioeconomic position (as measured with the European deprivation index (EDI) in quintiles). All incidence rates are computed by age group, sex and EDI quintile.

Observed incidence rate

For each EDI quintile i, IRobs_EDI=i = Nobs_EDI=i / PopuEDI=i Nobs_EDI=i corresponds to the number of cancer cases observed over the 2006-2009 in the area covered by a cancer registry. The figures are presented in Supplementary table 1.

PopuEDI=i is the population living in the area covered by a cancer registry over the period 2006-2009.

Incidence rate that would be observed if no one ever smoked

For each EDI quintile i, IRno smoking_EDI=i = Nno smoking_EDI=i / PopuEDI=i Nno smoking_EDI=i corresponds to the number of cancer cases that are not attributable to tobacco smoking and is computed as follows: Nno smoking_EDI=i = Nobs_EDI=i -AEDI=i, with: PopuEDI=i is the population living in the area covered by a cancer registry over the period 2006-2009.

Incidence rate that would be observed if all EDI quintiles smoked like the least deprived EDI quintile

For each EDI quintile i, IRexcess smoking_EDI=i = Nexcess smoking_EDI=i / PopuEDI=i Nexcess smoking_EDI=i corresponds to the number of cancer cases that would be observed if all EDI quintiles smoked like the least deprived EDI quintile and is computed as follows: Nexcess smoking_EDI=i = Nobs_EDI=i -NexcessEDI=i, with: