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Comparison of wear and corrosion behaviors of Cr and CrN sputtered coatings
Surface and Coatings Technology 115 (1999) 230–233 www.elsevier.nl/locate/surfcoat
Comparison of wear and corrosion behaviors of Cr and CrN sputtered coatings J. Stockemer a, *, R. Winand a, P. Vanden Brande b a Universite´ Libre de Bruxelles, Department of Metallurgy, CP 165/71, 50 av. F. Roosevelt, B 1050 Brussels, Belgium b Centre de Recherches et De´veloppements du Groupe Cockerill-Sambre, Campus Universitaire du Sart Tilman, Bd. de Colonster B57, 4000 Lie`ge, Belgium Received 8 July 1998; accepted 24 April 1999
Abstract Cr and CrN coatings were prepared by d.c. magnetron sputtering in a pure Ar and a N –Ar atmospheres, respectively. The 2 N –Ar gas composition was chosen in order to allow the production of stoichiometric CrN deposits at the highest possible 2 deposition rate. The columnar growth, low friction coefficient and high hardness of CrN were confirmed for deposits produced at a low temperature. The electrochemical behavior of CrN sputtered deposits in chloride solutions seemed to be similar to the behavior of Cr sputtered deposits. Moreover, it was shown that an anodic polarization treatment of the deposit increases the corrosion resistance of a chromium coating in the presence of chloride ions. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction
2. Experimental
Chromium coatings are well known for their high hardness, allowing a low wear rate, but also for their good corrosion and oxidation resistances. It was previously shown that these properties can be improved by the introduction of nitrogen in the deposit [1–7]. CrN is normally harder than a Cr deposit, and the presence of nitrogen changes the tribological behavior [1–3] and the stability of the deposit [4–7]. The corrosion behavior of Cr and CrN coatings has been compared in deoxidized H SO solution [6,7], but these reports do not 2 4 specify which deposit composition will resist best against aqueous electrochemical corrosion in the presence of chloride ions, a common environment in many practical situations (near the seaside or during the winter). In the case of a good stability, such a layer could be applied by d.c. reactive sputtering [2,3] on decorative pieces and used by the automotive industry. The objective of this paper is to compare the structure, wear and corrosion resistances of Cr and CrN coatings produced by d.c. magnetron sputtering in order to determine which composition offers the best compromise between the tribological properties and corrosion resistance.
Chromium and chromium nitride layers were prepared by non-reactive and reactive (in a mixture of Ar and N gas) d.c. magnetron sputtering of a 90-mm2 diameter target, respectively. All coatings were deposited onto precleaned substrates: a silicon substrate for the deposition rate determination, 430 stainless steel for the layer tribological testing and 304 stainless steel for the characterization and corrosion properties of the deposits. The total gas pressure was maintained at a value of 3×10−3 Torr (0.4 Pa). The variable process parameters were: the applied power on the target, the gas composition and finally the deposition time. The deposition rate was derived from thickness measurements, and the elemental composition of the film was determined by Auger electron spectroscopy (AES). The structure of the coating and the grain size were determined by optical microscopy and transmission electron microscopy ( TEM ) observations, respectively. The texture of the coating was determined by X-ray diffraction ( XRD) on 3.8-mm-thick films. The deposit hardness was derived from Vickers measurements of a film thicker than 20 mm. The applied loads were 0.015 and 0.1 kg for the Cr and CrN coatings, respectively. The wear resistance was investigated from a ball-on-disc resistance test. The wear fraction, which is the measure of the worn volume per force unit (force
* Corresponding author. Tel.: +32-2-650-30-25; fax: +32-2-650-36-53. E-mail address: [email protected] (J. Stockemer)
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 17 7 - 2
J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
applied on the alumina ball ) and per contact length unit, was measured for different force values (force applied on an alumina ball — 6.35 mm in diameter — in contact with the coating) at a relative speed of 0.157 m/s. This value is independent of the contact force and the test time and it is used to compare the wear behavior of the deposits. The deposit corrosion resistance was tested in a 0.5 M chloride solution (Cl typical concentration in sea water) by measurement of the electrochemical polarization curves. These curves were established in a chloride deoxidized acid solution (H SO 0.5 M–NaCl 0.5 M ), from the potential value 2 4 −100 mV/ENH ( ENH: normal hydrogen electrode) to −350 mV/ENH (total cathodic current, see Fig. 3) and from −350 mV/ENH to +1300 mV/ENH (above the breakdown potential ) at a constant scan speed (1 mV/s).
3. Results and discussion 3.1. Deposition rate and composition Fig. 1 shows the AES analysis of a stoichiometric CrN compound deposited in a pure N atmosphere (a 2 nuclear method was used for the analysis to be certain that the compound was composed of stoichiometric CrN ). Hence, the stoichiometric CrN compound is characterized by a N/Cr peak-to-peak intensity ratio of 0.77±0.04. The N/Cr peak-to-peak intensity ratio of all the coatings deposited in a mixture of Ar and N were 2 compared to that of the stoichiometric CrN to determine which gas composition leads to CrN. This was confirmed by XRD spectra. The deposition rate decreases with increasing N 2 content (Fig. 2). As is well known, this behavior can be attributed first to the nitrogen target poisoning, which decreases the sputtering yield, but also to the intrinsically lower Cr sputtering yield by nitrogen compared to argon because nitrogen is lighter than argon. The gas composition was chosen to produce stoichiometric CrN coatings at the highest possible deposition rate. In our experimental configuration, the gas composition giving the highest deposition rate was a mixture of 60% N and 40% Ar. This is power-independent between 2 600 and 1000 W. 3.2. Structure X-ray diffraction spectra confirmed the chemical composition of the deposit and indicated several preferential orientations. Hence, Cr deposits forward the growth of {200} and {211} planes parallel to the substrate surface, whereas stoichiometric CrN deposits show no preferential orientation. These results are totally independent of the target power. The deposits show a columnar structure, irrespective of their chemical composition when produced below
231
250°C. This is coherent with the structure observed on the diagram of Thornton (based on the diagram of B.A. Movchan and A.V. Demchishin for evaporated films [8,9]) for the same gas pressure and substrate temperature conditions. The grain size determined by TEM samples examination was always lower for the CrN coating (5 nm) than for the Cr coating (30 nm). 3.3. Tribological properties and corrosion resistance 3.3.1. Hardness and wear resistance Table 1 summarizes the hardness and the wear results from coated and uncoated stainless-steel substrates. The hardness of the CrN coating is higher than the hardness of the uncoated substrate. The increase in hardness is linked to a better wear resistance. The friction coefficient derived from the ball-on-disc test performed at 2 N was found to be 28 and 0.45 for the CrN and Cr coatings, respectively. These results indicate clearly that the wear resistance of the CrN coating is due to its high hardness and low friction coefficient. 3.3.2. Corrosion resistance The corrosion resistance of the protected and unprotected substrate was obtained from polarization curves measured in chloride deoxidized acid solution (H SO 0.5 M–NaCl 0.5 M ). 2 4 Fig. 3 shows the polarization curve of a CrN-coated sample. The left-hand side of the diagram (C ) corresponds to a total cathodic current (the main reaction is the reduction of H+), and the right-hand side (A) corresponds to a total anodic current (the main reaction is the oxidation of the surface layer). Four quantities are defined on this curve: U and j are the passivtion tion ation potential and the passivation current density, respectively, j is the minimal current density, and min j represents the value of the maximal current density 400 measured in a given potential interval of 400 mV. This quantity is introduced because j is only available in min a very small potential interval. Table 2 summarizes and compares the values of the four quantities defined above in Fig. 3 obtained from several polarization curves measured on coated and uncoated 304 stainless-steel substrates. It is worth noting that the passivation current density depends on the initial sample surface state (oxidized or not), which explains the large spread of values obtained for each type of sample (stainless steel, Cr or CrN coatings). The values in Table 2 clearly indicate that the Crand CrN-coated stainless steels exhibit better electrochemical behaviors than the uncoated steel. However, the presence of nitrogen in the deposit does not seem to improve the corrosion resistance of the coating. The chemical composition of the passivation layer
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J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
Fig. 1. Standard stoichiometric CrN AES analysis (Cr: 529 eV, N: 379 eV ).
resolved by AES and XPS showed the presence of Cr O for the Cr-coated steel and the presence of a 2 3 chromium oxi-nitride for the CrN-coated steel.
The polarization curve of the Cr-coated steel was compared with the polarization curve of a bulk Cr sample. This latter result showed lower j and j min 400 values (between 0.5 and 2 mA/cm2). Moreover, these values ( j and j ) are comparable. This is not the min 400 case for the Cr-coated steel. The difference observed between the current densities measured on the Cr-coated steel and bulk Cr sample may be explained by the presence of a columnar structure in the deposit absent in the bulk sample. The deposit real surface area in contact with the electrolyte is increased comparatively to the projected apparent surface area. As the calculated current density is the Table 1 Vickers hardness and wear fraction of coated and uncoated stainlesssteel substrates
Fig. 2. Deposition rate vs. gas composition at constant pressure and constant target power.
Vickers hardness (kg/mm2) Wear fraction (mm3/N.m)
Stainless-steel 430 substrate
Cr
CrN
230±30 10−7
1100±150 10−7
3000±200 10−10
J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
233
Fig. 3. Polarization curve of a CrN-coated sample. The left-hand side of the diagram (C ) corresponds to a total cathodic current (the main reaction is the reduction of H+), and the right-hand side (A) corresponds to a total anodic current (the main reaction is the oxidation of the surface layer). Table 2 Values of the four quantities defined on the polarization curves
U (mV/ENH ) tion j (mA/cm2) tion j (mA/cm2) min j (mA/cm2) 400
Stainless-steel 304 substrate
Cr
CrN
−20/−30 600/1000 50/80 70/120
−20/−30 50/300 4/12 5/20
−40/−50 20/400 4/10 5/30
ratio between the measured current and the projected apparent surface area sample, the calculated current density is higher than the real current density. In order to equal the real surface area and the apparent area, the structure of the Cr-coated sample must be denser. This was confirmed by the fact that deposits oxidized by anodic polarization, and whose columnar structure was partially filled by Cr O , presented current density values 2 3 almost similar to those observed for bulk Cr samples. Due to the lack of information related to the polarization curve of bulk CrN, the same reasoning is not applicable on CrN polarization curves.
4. Conclusions The structure, tribological and corrosion resistance properties of Cr and stoichiometric CrN deposits were compared. Each type of deposit shows a columnar structure due to the low substrate temperature during deposition
(<250°C ). This is in agreement with the Thornton diagram. The CrN deposit shows a better wear resistance than Cr, owing to its higher hardness and lower friction coefficient. The polarization curves of the Cr- and CrN-coated steels indicate a similar behavior. It also seems clear that an anodic polarization treatment of the deposit increases the corrosion resistance, of the chromium at least. The CrN deposit electrochemical behavior is more complex to interpret because the polarization curve of bulk stoichiometric CrN is still unknown, and, hence, it was not possible to separate the stainless-steel substrate and the coating contributions from the polarization curves observed experimentally. To circumvent this problem, the electrochemical behavior of a bulk CrN sample, for instance a CrN coating deposited on an inert substrate like platinum or glass, must be studied.
References [1] S.J. Bull, D.S. Rickerby, Surf. Coat. Technol. 43–44 (1990) 732–744. [2] J.P. Terrat et al., Surf. Coat. Technol. 45 (1991) 59–65. [3] J.P. Terrat, M. Cartier, Galvano-Organo-Traitements de surface 665 (1996) 315–319. [4] B. Navinsek et al., Surf. Coat. Technol. 74–75 (1995) 155–161. [5] B. Navinsek, P. Panjan, Surf. Coat. Technol. 59 (1993) 244–248. [6 ] M. Taguchi, J. Kurihara, Mater. Trans. 32 (12) (1991) 1170–1176. [7] M. Tagushi, H. Takahashi, Mater. Trans. 35 (5) (1994) 356–362. [8] G. Reiners et al., Surf. Coat. Technol. 54 (55) (1992) 273–278. [9] A. Richardt, A.M. Durand, Le vide les couches minces — les couches dures, Editions IN FINE, Paris, 1994.
Comparison of wear and corrosion behaviors of Cr and CrN sputtered coatings J. Stockemer a, *, R. Winand a, P. Vanden Brande b a Universite´ Libre de Bruxelles, Department of Metallurgy, CP 165/71, 50 av. F. Roosevelt, B 1050 Brussels, Belgium b Centre de Recherches et De´veloppements du Groupe Cockerill-Sambre, Campus Universitaire du Sart Tilman, Bd. de Colonster B57, 4000 Lie`ge, Belgium Received 8 July 1998; accepted 24 April 1999
Abstract Cr and CrN coatings were prepared by d.c. magnetron sputtering in a pure Ar and a N –Ar atmospheres, respectively. The 2 N –Ar gas composition was chosen in order to allow the production of stoichiometric CrN deposits at the highest possible 2 deposition rate. The columnar growth, low friction coefficient and high hardness of CrN were confirmed for deposits produced at a low temperature. The electrochemical behavior of CrN sputtered deposits in chloride solutions seemed to be similar to the behavior of Cr sputtered deposits. Moreover, it was shown that an anodic polarization treatment of the deposit increases the corrosion resistance of a chromium coating in the presence of chloride ions. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction
2. Experimental
Chromium coatings are well known for their high hardness, allowing a low wear rate, but also for their good corrosion and oxidation resistances. It was previously shown that these properties can be improved by the introduction of nitrogen in the deposit [1–7]. CrN is normally harder than a Cr deposit, and the presence of nitrogen changes the tribological behavior [1–3] and the stability of the deposit [4–7]. The corrosion behavior of Cr and CrN coatings has been compared in deoxidized H SO solution [6,7], but these reports do not 2 4 specify which deposit composition will resist best against aqueous electrochemical corrosion in the presence of chloride ions, a common environment in many practical situations (near the seaside or during the winter). In the case of a good stability, such a layer could be applied by d.c. reactive sputtering [2,3] on decorative pieces and used by the automotive industry. The objective of this paper is to compare the structure, wear and corrosion resistances of Cr and CrN coatings produced by d.c. magnetron sputtering in order to determine which composition offers the best compromise between the tribological properties and corrosion resistance.
Chromium and chromium nitride layers were prepared by non-reactive and reactive (in a mixture of Ar and N gas) d.c. magnetron sputtering of a 90-mm2 diameter target, respectively. All coatings were deposited onto precleaned substrates: a silicon substrate for the deposition rate determination, 430 stainless steel for the layer tribological testing and 304 stainless steel for the characterization and corrosion properties of the deposits. The total gas pressure was maintained at a value of 3×10−3 Torr (0.4 Pa). The variable process parameters were: the applied power on the target, the gas composition and finally the deposition time. The deposition rate was derived from thickness measurements, and the elemental composition of the film was determined by Auger electron spectroscopy (AES). The structure of the coating and the grain size were determined by optical microscopy and transmission electron microscopy ( TEM ) observations, respectively. The texture of the coating was determined by X-ray diffraction ( XRD) on 3.8-mm-thick films. The deposit hardness was derived from Vickers measurements of a film thicker than 20 mm. The applied loads were 0.015 and 0.1 kg for the Cr and CrN coatings, respectively. The wear resistance was investigated from a ball-on-disc resistance test. The wear fraction, which is the measure of the worn volume per force unit (force
* Corresponding author. Tel.: +32-2-650-30-25; fax: +32-2-650-36-53. E-mail address: [email protected] (J. Stockemer)
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 17 7 - 2
J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
applied on the alumina ball ) and per contact length unit, was measured for different force values (force applied on an alumina ball — 6.35 mm in diameter — in contact with the coating) at a relative speed of 0.157 m/s. This value is independent of the contact force and the test time and it is used to compare the wear behavior of the deposits. The deposit corrosion resistance was tested in a 0.5 M chloride solution (Cl typical concentration in sea water) by measurement of the electrochemical polarization curves. These curves were established in a chloride deoxidized acid solution (H SO 0.5 M–NaCl 0.5 M ), from the potential value 2 4 −100 mV/ENH ( ENH: normal hydrogen electrode) to −350 mV/ENH (total cathodic current, see Fig. 3) and from −350 mV/ENH to +1300 mV/ENH (above the breakdown potential ) at a constant scan speed (1 mV/s).
3. Results and discussion 3.1. Deposition rate and composition Fig. 1 shows the AES analysis of a stoichiometric CrN compound deposited in a pure N atmosphere (a 2 nuclear method was used for the analysis to be certain that the compound was composed of stoichiometric CrN ). Hence, the stoichiometric CrN compound is characterized by a N/Cr peak-to-peak intensity ratio of 0.77±0.04. The N/Cr peak-to-peak intensity ratio of all the coatings deposited in a mixture of Ar and N were 2 compared to that of the stoichiometric CrN to determine which gas composition leads to CrN. This was confirmed by XRD spectra. The deposition rate decreases with increasing N 2 content (Fig. 2). As is well known, this behavior can be attributed first to the nitrogen target poisoning, which decreases the sputtering yield, but also to the intrinsically lower Cr sputtering yield by nitrogen compared to argon because nitrogen is lighter than argon. The gas composition was chosen to produce stoichiometric CrN coatings at the highest possible deposition rate. In our experimental configuration, the gas composition giving the highest deposition rate was a mixture of 60% N and 40% Ar. This is power-independent between 2 600 and 1000 W. 3.2. Structure X-ray diffraction spectra confirmed the chemical composition of the deposit and indicated several preferential orientations. Hence, Cr deposits forward the growth of {200} and {211} planes parallel to the substrate surface, whereas stoichiometric CrN deposits show no preferential orientation. These results are totally independent of the target power. The deposits show a columnar structure, irrespective of their chemical composition when produced below
231
250°C. This is coherent with the structure observed on the diagram of Thornton (based on the diagram of B.A. Movchan and A.V. Demchishin for evaporated films [8,9]) for the same gas pressure and substrate temperature conditions. The grain size determined by TEM samples examination was always lower for the CrN coating (5 nm) than for the Cr coating (30 nm). 3.3. Tribological properties and corrosion resistance 3.3.1. Hardness and wear resistance Table 1 summarizes the hardness and the wear results from coated and uncoated stainless-steel substrates. The hardness of the CrN coating is higher than the hardness of the uncoated substrate. The increase in hardness is linked to a better wear resistance. The friction coefficient derived from the ball-on-disc test performed at 2 N was found to be 28 and 0.45 for the CrN and Cr coatings, respectively. These results indicate clearly that the wear resistance of the CrN coating is due to its high hardness and low friction coefficient. 3.3.2. Corrosion resistance The corrosion resistance of the protected and unprotected substrate was obtained from polarization curves measured in chloride deoxidized acid solution (H SO 0.5 M–NaCl 0.5 M ). 2 4 Fig. 3 shows the polarization curve of a CrN-coated sample. The left-hand side of the diagram (C ) corresponds to a total cathodic current (the main reaction is the reduction of H+), and the right-hand side (A) corresponds to a total anodic current (the main reaction is the oxidation of the surface layer). Four quantities are defined on this curve: U and j are the passivtion tion ation potential and the passivation current density, respectively, j is the minimal current density, and min j represents the value of the maximal current density 400 measured in a given potential interval of 400 mV. This quantity is introduced because j is only available in min a very small potential interval. Table 2 summarizes and compares the values of the four quantities defined above in Fig. 3 obtained from several polarization curves measured on coated and uncoated 304 stainless-steel substrates. It is worth noting that the passivation current density depends on the initial sample surface state (oxidized or not), which explains the large spread of values obtained for each type of sample (stainless steel, Cr or CrN coatings). The values in Table 2 clearly indicate that the Crand CrN-coated stainless steels exhibit better electrochemical behaviors than the uncoated steel. However, the presence of nitrogen in the deposit does not seem to improve the corrosion resistance of the coating. The chemical composition of the passivation layer
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J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
Fig. 1. Standard stoichiometric CrN AES analysis (Cr: 529 eV, N: 379 eV ).
resolved by AES and XPS showed the presence of Cr O for the Cr-coated steel and the presence of a 2 3 chromium oxi-nitride for the CrN-coated steel.
The polarization curve of the Cr-coated steel was compared with the polarization curve of a bulk Cr sample. This latter result showed lower j and j min 400 values (between 0.5 and 2 mA/cm2). Moreover, these values ( j and j ) are comparable. This is not the min 400 case for the Cr-coated steel. The difference observed between the current densities measured on the Cr-coated steel and bulk Cr sample may be explained by the presence of a columnar structure in the deposit absent in the bulk sample. The deposit real surface area in contact with the electrolyte is increased comparatively to the projected apparent surface area. As the calculated current density is the Table 1 Vickers hardness and wear fraction of coated and uncoated stainlesssteel substrates
Fig. 2. Deposition rate vs. gas composition at constant pressure and constant target power.
Vickers hardness (kg/mm2) Wear fraction (mm3/N.m)
Stainless-steel 430 substrate
Cr
CrN
230±30 10−7
1100±150 10−7
3000±200 10−10
J. Stockemer et al. / Surface and Coatings Technology 115 (1999) 230–233
233
Fig. 3. Polarization curve of a CrN-coated sample. The left-hand side of the diagram (C ) corresponds to a total cathodic current (the main reaction is the reduction of H+), and the right-hand side (A) corresponds to a total anodic current (the main reaction is the oxidation of the surface layer). Table 2 Values of the four quantities defined on the polarization curves
U (mV/ENH ) tion j (mA/cm2) tion j (mA/cm2) min j (mA/cm2) 400
Stainless-steel 304 substrate
Cr
CrN
−20/−30 600/1000 50/80 70/120
−20/−30 50/300 4/12 5/20
−40/−50 20/400 4/10 5/30
ratio between the measured current and the projected apparent surface area sample, the calculated current density is higher than the real current density. In order to equal the real surface area and the apparent area, the structure of the Cr-coated sample must be denser. This was confirmed by the fact that deposits oxidized by anodic polarization, and whose columnar structure was partially filled by Cr O , presented current density values 2 3 almost similar to those observed for bulk Cr samples. Due to the lack of information related to the polarization curve of bulk CrN, the same reasoning is not applicable on CrN polarization curves.
4. Conclusions The structure, tribological and corrosion resistance properties of Cr and stoichiometric CrN deposits were compared. Each type of deposit shows a columnar structure due to the low substrate temperature during deposition
(<250°C ). This is in agreement with the Thornton diagram. The CrN deposit shows a better wear resistance than Cr, owing to its higher hardness and lower friction coefficient. The polarization curves of the Cr- and CrN-coated steels indicate a similar behavior. It also seems clear that an anodic polarization treatment of the deposit increases the corrosion resistance, of the chromium at least. The CrN deposit electrochemical behavior is more complex to interpret because the polarization curve of bulk stoichiometric CrN is still unknown, and, hence, it was not possible to separate the stainless-steel substrate and the coating contributions from the polarization curves observed experimentally. To circumvent this problem, the electrochemical behavior of a bulk CrN sample, for instance a CrN coating deposited on an inert substrate like platinum or glass, must be studied.
References [1] S.J. Bull, D.S. Rickerby, Surf. Coat. Technol. 43–44 (1990) 732–744. [2] J.P. Terrat et al., Surf. Coat. Technol. 45 (1991) 59–65. [3] J.P. Terrat, M. Cartier, Galvano-Organo-Traitements de surface 665 (1996) 315–319. [4] B. Navinsek et al., Surf. Coat. Technol. 74–75 (1995) 155–161. [5] B. Navinsek, P. Panjan, Surf. Coat. Technol. 59 (1993) 244–248. [6 ] M. Taguchi, J. Kurihara, Mater. Trans. 32 (12) (1991) 1170–1176. [7] M. Tagushi, H. Takahashi, Mater. Trans. 35 (5) (1994) 356–362. [8] G. Reiners et al., Surf. Coat. Technol. 54 (55) (1992) 273–278. [9] A. Richardt, A.M. Durand, Le vide les couches minces — les couches dures, Editions IN FINE, Paris, 1994.