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Relations between deposition conditions, microstructure and mechanical properties of amorphous carbon-metal films
Vacuum 70 (2003) 181–185
Relations between deposition conditions, microstructure and mechanical properties of amorphous carbon-metal films V.V. Uglovaa,*, V.M. Anishchika, Y. Pauleaub, A.K. Kuleshova, F. Thie" ryb, J. Pelletierb, S.N. Dubc, D.P. Rusalskya a
Solid State Physics Department, Belarussian State University, Pr. F. Scoriny 4, 220080 Minsk, Belarus b CNRS-LEMD, 25 Rue des Martyrs, 38042 Grenoble Cedex 9, France c Institute for Superhard Materials, 2 Avtozavodskaya Street, 07074 Kiev, Ukraine
Abstract In this paper, structural state, relation between ‘‘disordered’’ and graphite carbon clusters, and mechanical properties of two types of metal-carbon have been investigated. The first type of hydrogen-free a-C, (Cu, Ni, Zr) film is formed using the cathodic arc-vacuum deposition (CAVD) by applying the negative bias potential of 20 kV to the sample. The second one of a-C:H, Cu coatings is obtained by means of the plasma-enhanced chemical vacuum deposition (PECVD) in the mixture of Ar+CH4 gases. It is established that as a result of PECVD of copper in the Ar+CH4 gas mixture aC:H, Cu films with the concentration of C from 0 to 75 at.% depending on CH4 content are formed. The percentage of graphite clusters increases the Yong module and wear resistance decreases with the carbon concentration increase up to 60% and more. Hydrogen-free a-C (Cu,Ni) films obtained by CAVD have higher hardness and wear resistance, higher Id =Ig relation than a-C:H, Cu films. During a-C, Zr film deposition one can observe the formation of ta-C carbon having a high concentration of sp3-bonded carbon that provides the highest hardness (up to 15 GPa) and wear resistance of a composite film. r 2003 Published by Elsevier Science Ltd. Keywords: Composite carbon-metal coatings; Grain size; Raman spectroscopy; Hardness; Friction coefficient
1. Introduction A considerable interest in the studies of carbon and composite metal-carbon coatings deposited in a vacuum arises from significant changes in the physical properties of coatings depending on the structure state and elemental composition [1–3]. Metal incorporations allow for the additional
*Corresponding author. Tel.: 375-17-226-58-34; fax: 375-17226-59-40. E-mail address: [email protected] (V.V. Uglova).
modification of the structure and properties of carbon films. The doping into carbon coatings with Ti, Cr, W, C allows an important increase of hardness, wear reduction improvement of adhesion to various types of substrates [4–6]. However, the influence of doping with metals weakly interacting with carbon on the structure and properties of metal-carbon coatings is not well known. The role of assisting ion beam energy and density on the formation of the structure and physical properties of deposited carbon films with different hydrogen content has not been established yet.
0042-207X/03/$ - see front matter r 2003 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 6 3 9 - 5
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
The aim of the present work was to study the correlation of the structure, elemental composition, sp2/sp3 type carbon cluster ratio with mechanical properties (hardness, friction, wear) for two types of Me-C coatings. The first type of carbon coatings (a-C:H, Cu) contains hydrogen. The coatings were formed using PECVD with Cu target sputtering in the mixture of Ar+CH4 reactive gases. The second type of hydrogen-free coatings (aC, Me) was formed using the cathodic arc vacuum deposition (CAVD) with high negative bias.
C (a-C, Cu)
100 CONCENTRATION, at. %
182
80 60 Ni
Fe (a-C, Zr)
40 Cu
Zr
20 0 0
50
100 150 DEPTH, nm
(a)
1200
D peak, 1350 cm-1
2. Experimental
200
250
G peak, 1560 cm-1
INTENSITY, a.u.
900 a-C, Zr
600
a-C, Ni
300
a-C, Cu a-C
0 1200
1400 1600 RAMAN SHIFT, cm-1
(b)
FRICTION COEFFICIENT
The hydrogen-free coatings (a-C, Me) are formed on Si (1 0 0) and AISI M2 substrates using a CAVD with high negative bias of 20 kV applied to the sample [6]. The ion flow assistance dose was 1017 ion/cm2. The thickness of coatings of all the types was E200 nm. The pressure in vacuum chamber was 103 Pa. The a-C:H, Cu copper-carbon composite films have been deposited on Si (1 0 0) substrates using a microwave PECVD process of carbon from Ar– CH4 mixtures associated with sputter deposition of metal from a copper target [7]. The thickness of coatings was varied from 0.2 to 0.5 mm. Mechanical testing was carried out using Nano Indenter II with Berkovich indenter. The load was varied in the range 0.25–50 mN. The composition of films was obtained from the RBS measurements using 2 MeV He ions or 1 MeV protons. The incidence and detection angles were 901 and 1601, respectively. The Raman spectra of coatings were measured using a Spex 1403 spectrometer. The excitation was performed by argon laser at 488 nm wave length with the power on the sample of 0.3– 0.35 W. The velocity of friction tests (pin on surface) was 4 mm/s, the pin was made of BK-8 hard alloy (87.5 HRC), and the load was of 0.5 and 1 N.
Fe (a-C, Ni) Fe (a-C, Cu)
C (a-C, Ni) C (a-C, Zr)
(c)
0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10
1800
a-C, Ni a-C, Zr
a-C, Cu
a-C
0
2
4
6 8 10 12 14 SLIDING DISTANCE, m
16
18
Fig. 1. (a) AES depth profile of the (a-C, Me) coatings deposition by CAVD on AISI M2 (’—a-C, Cu; ~—a-C, Zr; m—a-C, Ni); (b) Raman spectra of (a-C, Me) coating; and (c) friction coefficient vs. sliding distance of (a-C, Me) coatings.
3. Result and discussion 3.1. a-C, (Cu, Ni, Zr) coatings For the hydrogen-free (a-C, Me) films, obtained by the CAVD method, the depth
distribution of Ni, Zr, Cu is inhomogeneous (Fig. 1(a)). Raman scattering spectra (RS) from (a-C, Me) films and the results of their mathematical treatment are given in Fig. 1(b) and Table 1. The
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
183
Table 1 The results of mathematical treatment for the RS spectra of a-C, Me (CAVD) and a-C:H, Cu (PECVD) coatings Concentration CH4,%
D peak (cm1)
G peak (cm1)
FWHM of D peak (cm1)
FWHM of G peak (cm1)
Id =Ig
a-C; MeðCAVDÞ a-C a-C, Cu a-C, Zr a-C, Ni
1393 1383 1290 1399
1556 1546 1526 1567
381 308 179 297
151 132 216 105
2.33 2.52 0.24 3.51
a-C : H; CuðPECVDÞ 60 80 100
1337 1357 1350
1551 1560 1557
170 228 280
101 106 135
1.25 1.13 0.71
spectra from a-C and a-C, Me films are characterized by the presence of two smeared peaks: E1340 cm1 (D peak) and E1550 cm1 (G peak). This kind of spectrum with two smeared D and G peaks is characteristic of the so-called ‘‘diamondlike carbon’’(DLC) films [1,8]. The a-C film, obtained by CAVD compared with the typical DLC films is characterized by: (a) G-peak shift from 1580 cm1 position for crystalline graphite to lower values; and (b) Id =Ig relation is more than 2. A comparative analysis of the spectrum parameters obtained from the DLC films, containing a different number of defects and different inhomogeneity from those given in literature [8,9] leads to a conclusion that the a-C film has a high degree of dimensional inhomogeneity of carbon clusters. Cu incorporation into the a-C carbon affects the parameters of RS insignificantly. The (a-C, Ni) film with a higher metal concentration is characterized by Id =Ia increase, reduction of FHWM of the G line and a small G-peak shift. This confirms the increase of the number of more ordered clusters in the graphite structure of carbon clusters. The zirconium incorporation influences the characteristics of RS spectra in a special way. G peak is noticeably shifted from its initial position to a lower wave length region, FHWM of G peak increases and the Id =Ig ratio becomes 0.24. Such RS spectra parameters mean that a large part of carbon clusters in a-C, Zr is fixed by sp3 bonds [1,2]. The concentration of ordered
graphite clusters is small. This carbon film the called ta-C in literature [1,2]. The hardness of (a-C, Me) samples (Table 2) exceeds the hardness of (a-C:H, Cu) films. a-C, Zr films have the maximum hardness of B15 GPa. The changes of the friction coefficients of a-C and (a-C, Cu) depending on the sliding path are similar (Fig. 1(c)). The friction coefficient for these films increases with the increasing sliding distance. If the film has a uniform hardness depth distribution and the indentor does not change its form while rubbing, the increase of friction coefficient reflects a gradual increase of the real contact area [10]. If the indentor remains in the film during rubbing, a quicker film wear corresponds to a quicker growth of the real contact area and, accordingly, to a quicker increase of the friction coefficient with the increasing sliding distance. For the (a-C, Zr) films, the friction coefficient does not practically change with increasing the sliding distance although the magnitude of friction coefficient is larger for the (a-C, Zr) films than for the a-C and (a-C, Cu) films because of higher film hardness.
3.2. a-C:H, Cu coatings The concentration of copper and carbon atoms in the films as determined by the RBS measurements was found to be dependent on CH4 concentration in the gaseous phase (Fig. 2(a)). The carbon content in the films increased progres-
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
Hardness (H) (GPa )
3 4 3 4
2.5 2.2 1.9 2 2.3 2.5
0.2 0.1 0.6 0.5 0.2 0.6
sively from 60 to 75 at.% as the CH4 concentration in the gaseous phase increased from 70% to 100%. Raman scattering spectra from the a-C:H, Cu films, containing 60–100% of CH4 are shown in Fig. 2(b). From the parameters at the RS spectra parameters for the (a-C:H, Cu) films obtained by mathematical treatment (Table 1), the decrease of Id =Ig ratio was observed. The position of G peak remained practically the same. Thus, carbon clusters of a graphite composition predominate in the film structure. Table 2 shows the change in the hardness of the a-C:H, Cu films depending on the hydrocarbon concentration. As follows from the experimental data, hardness changes are in the range of 1.9– 2.6 GPa and do not depend on the concentration of hydrocarbon in the gaseous mixture. Fig. 2(c) shows the results of the (a-C:H, Cu) films friction tests depending on their composition. By applying optical methods of investigating wear tracks for the (a-Ca-C:H, Cu) films taking into account the dependence of the friction coefficient on the increasing sliding distance, one can distinguish three regions. Let us consider the dependence of the Cu film on Si substrate as an example (Fig. 2(c)). The region on the graph of the sliding distance from 0 to 4, where the friction
20
40
60
80
100
100
C Cu
100
80
80
60
60
40
40
20
20
0
0 0 20 40 60 80 100 CH4 CONCENTRATION IN GAS PHASE, %
(a) 3200
G peak, 1560 cm-1
2800 INTENSITY, a.u.
a-C:H, Cu (PECVD) Concentration CH4 (%) 0 20 40 60 80 100
4 5 15 10
2400
D peak, 1350 cm-1
2000
100% CH4 60% CH4
1600
80% CH4
1200 800 400 0 1200
1400 1600 RAMAN SHIFT, cm-1
(b)
1800
0.8 FRICTION COEFFICIENT
a-C, Me (CAVD) Composition a-C a-C, Cu a-C, Zr a-C, Ni
DH (GPa)
0
C CONCENTRATION, at.%
Table 2 The hardness of a-C, Me (CAVD) and a-C:H, Cu (PECVD) coatings for the depth penetration of 50–100 nm
Cu CONCENTRATION, at.%
184
80% CH4 100% CH4
0.7 0.6
0.5 60% CH4 0.4 0.3
Cu/Si
0.2
20% CH4
0.1 0 (c)
2
8 6 4 SLIDING DISTANCE, m
10
Fig. 2. (a) Composition of (a-C:H, Cu) coatings deposited by PECVD vs. the concentration of CH4 in the gas phase; (b) Raman spectra of (a-C:H, Cu) coatings; and (c) friction coefficient vs. the sliding distance of (a-C:H, Cu) coatings.
coefficient increases nearly linearly with the increase in the sliding distance, corresponds to the indentor friction only in the Cu film. A transition region from 4 to 6 corresponds to the
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
indentor friction both in the Cu film and on a very smooth Si surface. In the region above 6, the friction coefficient exhibits a great deal of magnitude, which corresponds to the penetration of a larger part of the indentor into Si. Thus, the magnitude of the sliding distance characterizes the wear resistance of the film. On the basis of the obtained results, one can conclude that the wear resistance of the film decreases with increasing carbon concentration. There is only one exception from this dependence, namely, a composite film, obtained at the CH4 concentration of 60%, which shows the highest wear resistance.
4. Conclusions It is found that (a-C:H, Cu) coatings (PECVD) with C concentration up to 25 at.% have hardness up to 2.6 GPa and good wear resistance. With the increasing carbon concentration more than 60 at% percentage of graphite clusters increases, wear resistance decreases. Plasticity of Cu grains and low hardness of a-C:H structure are the reasons for low hardness of (a-C:H, Cu) composite coatings. (a-C, Me) coatings (CAVD) have a higher hardness and wear resistance than (a-C:H, Cu)
185
coatings. The change of carbon structure in the case of CAVD process takes place by means of assisting ion flux with higher energy than that of the PECVD process. During the (a-C, Zr) film deposition one can observe the formation of ta-C carbon, which has a high concentration of sp3 fixed carbon, that, in its turn provides the highest hardness up to 15 GPa and wear resistance of a composite film.
References [1] Robertson J. Thin Solid Films 2001;383:81–8. [2] Schultrich B, Shiebe HG, Drescher D, Ziegle H. Surf Coat Technol 1998;52:1097–101. [3] Pauleau Y, Mounier E. Diamond Relat Mater 1997;9:1182–91. [4] Gahlin R, Larsson M, Hedengist P. Wear 2001;249:302–9. [5] Harry E, Rouzand A, Juliet P, Pauleau Y. Surf Coat Technol 1998;1–3:286–9. [6] Uglov VV, Kuleshov AK, Rusalsky DP, Onate JI, Yang Si-Ze. Wear-resistant metal-carbon composite coating. Surf. Coat. Technol. 2000;128–129:150–155. [7] Pichot M, Pelletier J. Microwave Excited Plasma, Amsterdam: Elsevier, 1992. p. 419. [8] Theye ML, Paret V, Sadki A. Diamond Relat Mater 2001;10:182–90. [9] Dillon RO, Woollam JA. Physical Rev B 1984;6:3482–9. [10] Wang FD, Kato K. Wear 2002;252:210–9.
Relations between deposition conditions, microstructure and mechanical properties of amorphous carbon-metal films V.V. Uglovaa,*, V.M. Anishchika, Y. Pauleaub, A.K. Kuleshova, F. Thie" ryb, J. Pelletierb, S.N. Dubc, D.P. Rusalskya a
Solid State Physics Department, Belarussian State University, Pr. F. Scoriny 4, 220080 Minsk, Belarus b CNRS-LEMD, 25 Rue des Martyrs, 38042 Grenoble Cedex 9, France c Institute for Superhard Materials, 2 Avtozavodskaya Street, 07074 Kiev, Ukraine
Abstract In this paper, structural state, relation between ‘‘disordered’’ and graphite carbon clusters, and mechanical properties of two types of metal-carbon have been investigated. The first type of hydrogen-free a-C, (Cu, Ni, Zr) film is formed using the cathodic arc-vacuum deposition (CAVD) by applying the negative bias potential of 20 kV to the sample. The second one of a-C:H, Cu coatings is obtained by means of the plasma-enhanced chemical vacuum deposition (PECVD) in the mixture of Ar+CH4 gases. It is established that as a result of PECVD of copper in the Ar+CH4 gas mixture aC:H, Cu films with the concentration of C from 0 to 75 at.% depending on CH4 content are formed. The percentage of graphite clusters increases the Yong module and wear resistance decreases with the carbon concentration increase up to 60% and more. Hydrogen-free a-C (Cu,Ni) films obtained by CAVD have higher hardness and wear resistance, higher Id =Ig relation than a-C:H, Cu films. During a-C, Zr film deposition one can observe the formation of ta-C carbon having a high concentration of sp3-bonded carbon that provides the highest hardness (up to 15 GPa) and wear resistance of a composite film. r 2003 Published by Elsevier Science Ltd. Keywords: Composite carbon-metal coatings; Grain size; Raman spectroscopy; Hardness; Friction coefficient
1. Introduction A considerable interest in the studies of carbon and composite metal-carbon coatings deposited in a vacuum arises from significant changes in the physical properties of coatings depending on the structure state and elemental composition [1–3]. Metal incorporations allow for the additional
*Corresponding author. Tel.: 375-17-226-58-34; fax: 375-17226-59-40. E-mail address: [email protected] (V.V. Uglova).
modification of the structure and properties of carbon films. The doping into carbon coatings with Ti, Cr, W, C allows an important increase of hardness, wear reduction improvement of adhesion to various types of substrates [4–6]. However, the influence of doping with metals weakly interacting with carbon on the structure and properties of metal-carbon coatings is not well known. The role of assisting ion beam energy and density on the formation of the structure and physical properties of deposited carbon films with different hydrogen content has not been established yet.
0042-207X/03/$ - see front matter r 2003 Published by Elsevier Science Ltd. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 6 3 9 - 5
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
The aim of the present work was to study the correlation of the structure, elemental composition, sp2/sp3 type carbon cluster ratio with mechanical properties (hardness, friction, wear) for two types of Me-C coatings. The first type of carbon coatings (a-C:H, Cu) contains hydrogen. The coatings were formed using PECVD with Cu target sputtering in the mixture of Ar+CH4 reactive gases. The second type of hydrogen-free coatings (aC, Me) was formed using the cathodic arc vacuum deposition (CAVD) with high negative bias.
C (a-C, Cu)
100 CONCENTRATION, at. %
182
80 60 Ni
Fe (a-C, Zr)
40 Cu
Zr
20 0 0
50
100 150 DEPTH, nm
(a)
1200
D peak, 1350 cm-1
2. Experimental
200
250
G peak, 1560 cm-1
INTENSITY, a.u.
900 a-C, Zr
600
a-C, Ni
300
a-C, Cu a-C
0 1200
1400 1600 RAMAN SHIFT, cm-1
(b)
FRICTION COEFFICIENT
The hydrogen-free coatings (a-C, Me) are formed on Si (1 0 0) and AISI M2 substrates using a CAVD with high negative bias of 20 kV applied to the sample [6]. The ion flow assistance dose was 1017 ion/cm2. The thickness of coatings of all the types was E200 nm. The pressure in vacuum chamber was 103 Pa. The a-C:H, Cu copper-carbon composite films have been deposited on Si (1 0 0) substrates using a microwave PECVD process of carbon from Ar– CH4 mixtures associated with sputter deposition of metal from a copper target [7]. The thickness of coatings was varied from 0.2 to 0.5 mm. Mechanical testing was carried out using Nano Indenter II with Berkovich indenter. The load was varied in the range 0.25–50 mN. The composition of films was obtained from the RBS measurements using 2 MeV He ions or 1 MeV protons. The incidence and detection angles were 901 and 1601, respectively. The Raman spectra of coatings were measured using a Spex 1403 spectrometer. The excitation was performed by argon laser at 488 nm wave length with the power on the sample of 0.3– 0.35 W. The velocity of friction tests (pin on surface) was 4 mm/s, the pin was made of BK-8 hard alloy (87.5 HRC), and the load was of 0.5 and 1 N.
Fe (a-C, Ni) Fe (a-C, Cu)
C (a-C, Ni) C (a-C, Zr)
(c)
0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10
1800
a-C, Ni a-C, Zr
a-C, Cu
a-C
0
2
4
6 8 10 12 14 SLIDING DISTANCE, m
16
18
Fig. 1. (a) AES depth profile of the (a-C, Me) coatings deposition by CAVD on AISI M2 (’—a-C, Cu; ~—a-C, Zr; m—a-C, Ni); (b) Raman spectra of (a-C, Me) coating; and (c) friction coefficient vs. sliding distance of (a-C, Me) coatings.
3. Result and discussion 3.1. a-C, (Cu, Ni, Zr) coatings For the hydrogen-free (a-C, Me) films, obtained by the CAVD method, the depth
distribution of Ni, Zr, Cu is inhomogeneous (Fig. 1(a)). Raman scattering spectra (RS) from (a-C, Me) films and the results of their mathematical treatment are given in Fig. 1(b) and Table 1. The
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
183
Table 1 The results of mathematical treatment for the RS spectra of a-C, Me (CAVD) and a-C:H, Cu (PECVD) coatings Concentration CH4,%
D peak (cm1)
G peak (cm1)
FWHM of D peak (cm1)
FWHM of G peak (cm1)
Id =Ig
a-C; MeðCAVDÞ a-C a-C, Cu a-C, Zr a-C, Ni
1393 1383 1290 1399
1556 1546 1526 1567
381 308 179 297
151 132 216 105
2.33 2.52 0.24 3.51
a-C : H; CuðPECVDÞ 60 80 100
1337 1357 1350
1551 1560 1557
170 228 280
101 106 135
1.25 1.13 0.71
spectra from a-C and a-C, Me films are characterized by the presence of two smeared peaks: E1340 cm1 (D peak) and E1550 cm1 (G peak). This kind of spectrum with two smeared D and G peaks is characteristic of the so-called ‘‘diamondlike carbon’’(DLC) films [1,8]. The a-C film, obtained by CAVD compared with the typical DLC films is characterized by: (a) G-peak shift from 1580 cm1 position for crystalline graphite to lower values; and (b) Id =Ig relation is more than 2. A comparative analysis of the spectrum parameters obtained from the DLC films, containing a different number of defects and different inhomogeneity from those given in literature [8,9] leads to a conclusion that the a-C film has a high degree of dimensional inhomogeneity of carbon clusters. Cu incorporation into the a-C carbon affects the parameters of RS insignificantly. The (a-C, Ni) film with a higher metal concentration is characterized by Id =Ia increase, reduction of FHWM of the G line and a small G-peak shift. This confirms the increase of the number of more ordered clusters in the graphite structure of carbon clusters. The zirconium incorporation influences the characteristics of RS spectra in a special way. G peak is noticeably shifted from its initial position to a lower wave length region, FHWM of G peak increases and the Id =Ig ratio becomes 0.24. Such RS spectra parameters mean that a large part of carbon clusters in a-C, Zr is fixed by sp3 bonds [1,2]. The concentration of ordered
graphite clusters is small. This carbon film the called ta-C in literature [1,2]. The hardness of (a-C, Me) samples (Table 2) exceeds the hardness of (a-C:H, Cu) films. a-C, Zr films have the maximum hardness of B15 GPa. The changes of the friction coefficients of a-C and (a-C, Cu) depending on the sliding path are similar (Fig. 1(c)). The friction coefficient for these films increases with the increasing sliding distance. If the film has a uniform hardness depth distribution and the indentor does not change its form while rubbing, the increase of friction coefficient reflects a gradual increase of the real contact area [10]. If the indentor remains in the film during rubbing, a quicker film wear corresponds to a quicker growth of the real contact area and, accordingly, to a quicker increase of the friction coefficient with the increasing sliding distance. For the (a-C, Zr) films, the friction coefficient does not practically change with increasing the sliding distance although the magnitude of friction coefficient is larger for the (a-C, Zr) films than for the a-C and (a-C, Cu) films because of higher film hardness.
3.2. a-C:H, Cu coatings The concentration of copper and carbon atoms in the films as determined by the RBS measurements was found to be dependent on CH4 concentration in the gaseous phase (Fig. 2(a)). The carbon content in the films increased progres-
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
Hardness (H) (GPa )
3 4 3 4
2.5 2.2 1.9 2 2.3 2.5
0.2 0.1 0.6 0.5 0.2 0.6
sively from 60 to 75 at.% as the CH4 concentration in the gaseous phase increased from 70% to 100%. Raman scattering spectra from the a-C:H, Cu films, containing 60–100% of CH4 are shown in Fig. 2(b). From the parameters at the RS spectra parameters for the (a-C:H, Cu) films obtained by mathematical treatment (Table 1), the decrease of Id =Ig ratio was observed. The position of G peak remained practically the same. Thus, carbon clusters of a graphite composition predominate in the film structure. Table 2 shows the change in the hardness of the a-C:H, Cu films depending on the hydrocarbon concentration. As follows from the experimental data, hardness changes are in the range of 1.9– 2.6 GPa and do not depend on the concentration of hydrocarbon in the gaseous mixture. Fig. 2(c) shows the results of the (a-C:H, Cu) films friction tests depending on their composition. By applying optical methods of investigating wear tracks for the (a-Ca-C:H, Cu) films taking into account the dependence of the friction coefficient on the increasing sliding distance, one can distinguish three regions. Let us consider the dependence of the Cu film on Si substrate as an example (Fig. 2(c)). The region on the graph of the sliding distance from 0 to 4, where the friction
20
40
60
80
100
100
C Cu
100
80
80
60
60
40
40
20
20
0
0 0 20 40 60 80 100 CH4 CONCENTRATION IN GAS PHASE, %
(a) 3200
G peak, 1560 cm-1
2800 INTENSITY, a.u.
a-C:H, Cu (PECVD) Concentration CH4 (%) 0 20 40 60 80 100
4 5 15 10
2400
D peak, 1350 cm-1
2000
100% CH4 60% CH4
1600
80% CH4
1200 800 400 0 1200
1400 1600 RAMAN SHIFT, cm-1
(b)
1800
0.8 FRICTION COEFFICIENT
a-C, Me (CAVD) Composition a-C a-C, Cu a-C, Zr a-C, Ni
DH (GPa)
0
C CONCENTRATION, at.%
Table 2 The hardness of a-C, Me (CAVD) and a-C:H, Cu (PECVD) coatings for the depth penetration of 50–100 nm
Cu CONCENTRATION, at.%
184
80% CH4 100% CH4
0.7 0.6
0.5 60% CH4 0.4 0.3
Cu/Si
0.2
20% CH4
0.1 0 (c)
2
8 6 4 SLIDING DISTANCE, m
10
Fig. 2. (a) Composition of (a-C:H, Cu) coatings deposited by PECVD vs. the concentration of CH4 in the gas phase; (b) Raman spectra of (a-C:H, Cu) coatings; and (c) friction coefficient vs. the sliding distance of (a-C:H, Cu) coatings.
coefficient increases nearly linearly with the increase in the sliding distance, corresponds to the indentor friction only in the Cu film. A transition region from 4 to 6 corresponds to the
V.V. Uglova et al. / Vacuum 70 (2003) 181–185
indentor friction both in the Cu film and on a very smooth Si surface. In the region above 6, the friction coefficient exhibits a great deal of magnitude, which corresponds to the penetration of a larger part of the indentor into Si. Thus, the magnitude of the sliding distance characterizes the wear resistance of the film. On the basis of the obtained results, one can conclude that the wear resistance of the film decreases with increasing carbon concentration. There is only one exception from this dependence, namely, a composite film, obtained at the CH4 concentration of 60%, which shows the highest wear resistance.
4. Conclusions It is found that (a-C:H, Cu) coatings (PECVD) with C concentration up to 25 at.% have hardness up to 2.6 GPa and good wear resistance. With the increasing carbon concentration more than 60 at% percentage of graphite clusters increases, wear resistance decreases. Plasticity of Cu grains and low hardness of a-C:H structure are the reasons for low hardness of (a-C:H, Cu) composite coatings. (a-C, Me) coatings (CAVD) have a higher hardness and wear resistance than (a-C:H, Cu)
185
coatings. The change of carbon structure in the case of CAVD process takes place by means of assisting ion flux with higher energy than that of the PECVD process. During the (a-C, Zr) film deposition one can observe the formation of ta-C carbon, which has a high concentration of sp3 fixed carbon, that, in its turn provides the highest hardness up to 15 GPa and wear resistance of a composite film.
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