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Structure and properties of NiTiC cermet films formed by ion plating
Thin Solid Films, 191 (1990) 69-76 METALLURGICAL AND PROTECTIVE LAYERS
69
STRUCTURE AND PROPERTIES OF Ni-TiC CERMET FILMS FORMED BY ION PLATING A. ISHIDA, K. OGAWA, T. KIMURA AND A. TAKEI National Research Institute for Metals, 2-3-12, Nakameguro, Meguroku, Tokyo (Japan) (Received July 12, 1989; revised March 5, 1990; accepted April 18, 1990)
Ni-TiC cermet films with Ti:Ni ratios of 0.35 and 3 were deposited by activated reactive evaporation. Electron microscopy study revealed that the films were finegrained mixtures of nickel and TiC. Among them a nickel-rich cermet film exhibited good adhesion, high hardness and high ductility.
1. INTRODUCTION Nimmagadda and Bunshahl demonstrated that dispersion of fine TiC particles within nickel grains was produced by activated reactive evaporation (ARE) in the presence of C 2 H 2 using Ni-Ti alloy as an evaporation source. Further, Sarin, Nimmagadda and Bunshah 2'3 reported that dispersion of fine grains of Ti-Ni intermetallics and Ni-Ti solid solution throughout TiC grains was produced by ARE using Ti-10~Ni alloy as an evaporation source. In this study, we report that another type of cermet film which is a fine-grained mixture of nickel and TiC can also be produced by the same ARE process. The microhardness and adhesion behavior of the films were examined. 2.
EXPERIMENTAL DETAILS
The substrate used in this study was a conventional type 304 stainless steel. Coupons of 10 x 10 × 0.8 mm were used for the ion plating, after being polished with 0.05 lain alumina and rinsed in acetone by ultrasonic cleaning. Surface cleaning of the substrate was carried out by argon bombardment immediately prior to deposition. The conditions of argon bombardment were as follows: argon pressure, 1 Pa; substrate bias, - 4 kV; and bombardment time, 600 s. The deposition of Ni-TiC cermet films was carried out using the ion plating apparatus shown in Fig. 1. An Ni-Ti alloy ingot was melted and evaporated in a hearth by an electron beam gun. Carbon was supplied by flowing C2H2 gas at 0.11 Pa. The ionization of vapor species was promoted by a probe electrode which was placed above the hearth and biased positively up to 150 V. The probe current was kept at 2 A during the deposition. The substrate alloy was biased negatively 0040-6090/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
70
A. ISHIDAet al.
Heater
f
.
.
.
.
.
.
.
.
.
.
.
.
.....
1
-X I sus304
Probe electrode "
4 - - C2H 2
i
Ni-Ti
li
Vacuum
/ I~" --~'x
pump
Fig. 1. Schematic illustration of apparatus used forion plating.
(2 kV) with reference to the metal pool. Further, to produce a superior and adherent film, the substrate was heated at 673 K by the tantalum heater. The deposition rate was 0.5 rtm rain- 1 and the final thickness of the films was 7 ~tm. The cermet films used in this study were deposited after several coating runs, since the films of initial runs are known to be unstable in composition 4. Nickel and TiC films of thickness 7 lain were also prepared for comparison. The ion plating condition for the TiC film was the same as those for N i - T i C films. The ion plating of nickel was carried out in the absence o f C2H2 under a probe current of 0.6 A. The deposited films were characterized by X-ray diffraction, electron microprobe analysis and transmission electron microscopy. The concentrations of nickel and titanium were estimated by the Castaing approximation using pure nickel and TiCo.88 as standard specimens in order to obtain a rough estimation of the Ti:Ni ratios of the films. The carbon concentration was not estimated, because the carbon X-ray signal is severely affected by absorption and because there are no available correction methods to determine carbon concentration in a two-phase film with confidence. The thin foils for transmission electron microscopy were prepared by electrolytically polishing only the substrate side of the samples in 909/o acetic acid-10~o perchloric acid solution so that the N i - T i C structure remained in the foils. Vickers microhardness testing was performed at a 20 gfload for cermet films, a nickel film and a TiC film. Scratch tests were also carried out. The equipment used in
Ni-TiC CERMETFILMS
71
the tests had a diamond stylus in the form of a Rockwell C 120 ° cone with a spherical tip radius of 200 pro. The stylus was drawn over the sample surface at a scratch speed of0.17 mm s- 1 with a continuously increasing normal load at 0.82 N s - 1. The mode of damage to the coating was examined by optical microscopy and electron microprobe analysis. 3.
RESULTS
3.1. Compositionoffilms Three films (A-C) were deposited onto stainless steel substrates using various evaporation sources: (A) Ni-25~oTi, (B) N i - 5 0 ~ T i and (C) Ni-75~oTi. Figure 2 shows the X-ray diffractograms of the films. While there are only nickel and stainless steel peaks in Fig. 2(a), TiC peaks appear in Figs. 2(b) and 2(c), in addition to these peaks. N: Ni T: TiC
S : SUS304 sN
s v
I
I
I
(a)
N
I
I
m
I
A
l
I
T .
(b)
T
I
~/bL_
.
.
.
I
.
.
.
.
.
.
I
I
TI T
I 300
.
I
I
I 50*
2e
(c)
S
I
I
I
700
1
90*
(degree, Cu Ka)
Fig. 2. X-ray diffractograms of films: (a) film A, (b) film B and (c) film C.
The results of the electron microprobe analyses for the films are summarized in Table I. It can be seen that both films B and C contain considerable amounts of titanium, while the concentration o f this element is low in film A. The Ti:Ni ratio o f the film is 0.35 for film B, while it is about three for film C. This indicates that cermet films over a wide range of TiC content can be produced by the ARE process. TABLE I CONCENTRATION (wt.%) OF TITANIUM AND NICKEL AND
Film
77
Ni
Ti.Ni
A B C
0.5 20.1 54.5
99.9 70.0 22.0
-0.35 3.0
Ti:Ni
ATOMIC RATIO OF FILMS
A. ISHIDAet al.
72
3.2.
Structure of films As described above, X-ray diffractometry indicated that the films B and C consist of two phases of nickel and TiC. However, an optical microscopy study did not reveal a distinct contrast indicative of such a two-phase structure. Consequently, the thin films for transmission electron microscopy were prepared by electrochemically thinning from the substrate side. Figure 3 shows a bright field micrograph, the electron diffraction pattern and the corresponding dark field micrographs of film B. The bright field micrograph indicates that the film has a fine-grained structure. The diffraction pattern again confirms the existence of nickel and TiC as indicated in Fig. 3(b). In order to delineate the TiC and nickel grains in the film, two dark field micrographs of the same area shown in Fig. 3(a) were obtained from different diffraction rings in Fig. 3(b). Figure 3(c) was taken by placing the objective aperture over the TIC(111) ring as shown in the inset. In this micrograph only the TiC grains are visible and are seen to be distributed uniformly as very small crystallites with an average size of about 5 nm. Figure 3(d) was taken by placing the aperture over a set of rings: TIC(111), Ti(200), Ni(111) and Ni(200), as shown in the inset. Unfortunately the nickel ring could not be isolated from the TiC ring even by the smallest aperture of the transmission electron microscope used, and hence both the nickel and TiC grains are visible in this micrograph. Some of the TiC grains can be seen in both
(a)
I
200rim
!
(b)
(c) (d) Fig. 3. Transmissionelectronphotomicrographsand diffractionpattern of filmB: (a) bright fieldimage, (b) diffractionpattern, (c) and (d) dark fieldimagesof the samearea usingTIC(111)and a set of TiC(111), TIC(200),Ni(l 11) and Ni(200).
N i - T i C CERMETFILMS
73
Figs. 3(c) and 3(d), as demonstrated by the grains specified by X (Fig. 3). In Fig. 3(d) TiC grains appear to be smaller crystallites with the grains size of 5-10 nm. In this figure, in addition to the smaller TiC crystallites, larger crystallites can be seen, which are believed to be nickel. Then, the average size of the nickel crystallites is estimated to be about 15 nm. Figure 4 shows bright field micrographs of the four films; (a) nickel film, (b) film A, (c) film B and (d) film C. These figures show marked differences in grain size between both the nickel film and film A, and both films B and C. (It should be noted that the magnification of the lower photographs is 10 times that of the upper photographs.) This result shows that simultaneous deposition of nickel and TiC produces an extremely fine-grained structure compared with deposition of only nickel. These fine-grained structures of the cermet films are also reflected in their Xray diffractograms; the peaks o f both nickel and TiC observed in Figs. 2(b) and 2(c) are broad, while nickel peaks in Fig. 2(a) are very sharp.
(a)
(b)
I
2 p m
i
(c) (d) ! 20 0 nm ! Fig. 4. Transmissionelectron photomicrographsof films: (a) nickel films, (b) film A, (c) film B and (d) film C. 3.3.
Hardness Microhardness of the deposited films was measured using a Vickers hardness tester. Figure 5 shows the variation in the microhardness as a function o f T i : ( T i ÷ N i ) ratio of the films. The cermet films, films B and C, have very high hardness. It should be noticed that film B has a high hardness in spite o f the low TiC content.
74
A. ISHIDAe t al.
200E Q C "0
1000 o r.,)
:~5
5'0
7~5
100
TI/tNi+Ti) {at%) Fig. 5. Vickers hardness of films as a function of the Ti:(Ni + Ti) ratio.
3.4.
S c r a t c h test
Figure 6 shows the surfaces of the specimens after the scratch test. Clearly film B exhibits far better adhesion than the TiC film; film B was still not removed at a normal load of about 30 N, while the TiC film detached at 4 N. Furthermore, cohesive failure of the TiC coating was observed at the sides of the scratch channel, but no loss of film B was observed. This result indicates that film B has good ductility. However, film C exhibited poor adhesion in contrast to the excellent adhesion of film B. 4.
DISCUSSION
These results indicate that activated reactive evaporation in the presence of C2H2 using Ni-Ti alloy as an evaporation source can produce Ni-TiC cermet films which have a duplex structure of fine grains of nickel and TiC. This structure differs from the dispersion structure reported by Bunshah et al. 1 - 3 The difference in structure between the films could be due to differences in coating conditions such as evaporant composition and substrate temperature. The fine-grained structure probably results from mutual restraint of grain growth during the deposition due to simultaneous deposition of two species which do not react with each other. Film B revealed high hardness, good adhesion and high ductility as shown in Figs. 5 and 6. The high hardness of film B in spite of its low TiC content can be attributed not only to the presence of TiC, but also to grain refinement strengthening of nickel. The strengthening effect of fine grains of nickel has been reported by Spingarn et al. 5 Also, Komiya e t al. 6 have reported that the Hall Petch relation is valid for chromium films with grain sizes ranging from 0.06 to 1 lam. The improved adhesion of film B can be attributed to (1) the improvement in film-substate bonding due to the metallic bond of nickel grains and stainless steel at the film-substrate interface and (2) the increase in ductility of the film, due both to grain refinement and to a large amount of ductile nickel. In contrast, film C, the TiCrich cermet film, exhibited poor adhesion. Although the reason is not clear, degradation in the mechanical properties of film C is a possibility. TiC-rich cermet film seems to be brittle because of the small amount of nickel.
N i - T i C CERMET FILMS
75
(,A)
(B)
(C)
!
!
0.2ram Fig. 6. Scratch tests on films deposited on SUS304: (A) film B(Ti:Ni = 0.35), (B) film C(Ti:Ni = 3) and (C)TiC film. Photographs (a), (b) and (c) show more detail of the scratch channel. The numbers indicate the corresponding normal loads at the left and right ends o f the photographs. 5.
CONCLUSIONS
N i - T i C cermet films over a wide range o f Ti:Ni ratios were deposited by the activated reactive evaporation process. Electron microscopy study revealed that the
76
A. ISHIDA e t al.
d e p o s i t e d films consisted o f fine grains o f nickel a n d TiC. M i c r o h a r d n e s s a n d scratch tests revealed t h a t a nickel-rich c e r m e t film exhibited g o o d a d h e s i o n , high h a r d n e s s a n d high ductility. REFERENCES 1 R. Nimmagadda and R. F. Bunshah, J. Vac. Sci. Technol., 12 (1975) 815. 2 R. Nimmagadda and R. F. Bunshah, J. Vac. Sci. Technol., 13 (1976) 532. 3 V . K . Sarin, R.F. BunshahandR. Nimmagadda, ThinSolidFilms, 40(1977) i83.
4 R. Nimmagadda, A. C. Raghuram and R. F. Bunshah, J. Vac. Sci, TechnoL, 9 (1972) 1406. 5 J, R. Spingarn, B. E. Jacobson and W. D. Nix, Thin Solid Films, 45 (1977) 507. 6 S. Komiya, S. Ono and N. Umezu, Thin Solid Films, 45 (1977) 473.
69
STRUCTURE AND PROPERTIES OF Ni-TiC CERMET FILMS FORMED BY ION PLATING A. ISHIDA, K. OGAWA, T. KIMURA AND A. TAKEI National Research Institute for Metals, 2-3-12, Nakameguro, Meguroku, Tokyo (Japan) (Received July 12, 1989; revised March 5, 1990; accepted April 18, 1990)
Ni-TiC cermet films with Ti:Ni ratios of 0.35 and 3 were deposited by activated reactive evaporation. Electron microscopy study revealed that the films were finegrained mixtures of nickel and TiC. Among them a nickel-rich cermet film exhibited good adhesion, high hardness and high ductility.
1. INTRODUCTION Nimmagadda and Bunshahl demonstrated that dispersion of fine TiC particles within nickel grains was produced by activated reactive evaporation (ARE) in the presence of C 2 H 2 using Ni-Ti alloy as an evaporation source. Further, Sarin, Nimmagadda and Bunshah 2'3 reported that dispersion of fine grains of Ti-Ni intermetallics and Ni-Ti solid solution throughout TiC grains was produced by ARE using Ti-10~Ni alloy as an evaporation source. In this study, we report that another type of cermet film which is a fine-grained mixture of nickel and TiC can also be produced by the same ARE process. The microhardness and adhesion behavior of the films were examined. 2.
EXPERIMENTAL DETAILS
The substrate used in this study was a conventional type 304 stainless steel. Coupons of 10 x 10 × 0.8 mm were used for the ion plating, after being polished with 0.05 lain alumina and rinsed in acetone by ultrasonic cleaning. Surface cleaning of the substrate was carried out by argon bombardment immediately prior to deposition. The conditions of argon bombardment were as follows: argon pressure, 1 Pa; substrate bias, - 4 kV; and bombardment time, 600 s. The deposition of Ni-TiC cermet films was carried out using the ion plating apparatus shown in Fig. 1. An Ni-Ti alloy ingot was melted and evaporated in a hearth by an electron beam gun. Carbon was supplied by flowing C2H2 gas at 0.11 Pa. The ionization of vapor species was promoted by a probe electrode which was placed above the hearth and biased positively up to 150 V. The probe current was kept at 2 A during the deposition. The substrate alloy was biased negatively 0040-6090/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
70
A. ISHIDAet al.
Heater
f
.
.
.
.
.
.
.
.
.
.
.
.
.....
1
-X I sus304
Probe electrode "
4 - - C2H 2
i
Ni-Ti
li
Vacuum
/ I~" --~'x
pump
Fig. 1. Schematic illustration of apparatus used forion plating.
(2 kV) with reference to the metal pool. Further, to produce a superior and adherent film, the substrate was heated at 673 K by the tantalum heater. The deposition rate was 0.5 rtm rain- 1 and the final thickness of the films was 7 ~tm. The cermet films used in this study were deposited after several coating runs, since the films of initial runs are known to be unstable in composition 4. Nickel and TiC films of thickness 7 lain were also prepared for comparison. The ion plating condition for the TiC film was the same as those for N i - T i C films. The ion plating of nickel was carried out in the absence o f C2H2 under a probe current of 0.6 A. The deposited films were characterized by X-ray diffraction, electron microprobe analysis and transmission electron microscopy. The concentrations of nickel and titanium were estimated by the Castaing approximation using pure nickel and TiCo.88 as standard specimens in order to obtain a rough estimation of the Ti:Ni ratios of the films. The carbon concentration was not estimated, because the carbon X-ray signal is severely affected by absorption and because there are no available correction methods to determine carbon concentration in a two-phase film with confidence. The thin foils for transmission electron microscopy were prepared by electrolytically polishing only the substrate side of the samples in 909/o acetic acid-10~o perchloric acid solution so that the N i - T i C structure remained in the foils. Vickers microhardness testing was performed at a 20 gfload for cermet films, a nickel film and a TiC film. Scratch tests were also carried out. The equipment used in
Ni-TiC CERMETFILMS
71
the tests had a diamond stylus in the form of a Rockwell C 120 ° cone with a spherical tip radius of 200 pro. The stylus was drawn over the sample surface at a scratch speed of0.17 mm s- 1 with a continuously increasing normal load at 0.82 N s - 1. The mode of damage to the coating was examined by optical microscopy and electron microprobe analysis. 3.
RESULTS
3.1. Compositionoffilms Three films (A-C) were deposited onto stainless steel substrates using various evaporation sources: (A) Ni-25~oTi, (B) N i - 5 0 ~ T i and (C) Ni-75~oTi. Figure 2 shows the X-ray diffractograms of the films. While there are only nickel and stainless steel peaks in Fig. 2(a), TiC peaks appear in Figs. 2(b) and 2(c), in addition to these peaks. N: Ni T: TiC
S : SUS304 sN
s v
I
I
I
(a)
N
I
I
m
I
A
l
I
T .
(b)
T
I
~/bL_
.
.
.
I
.
.
.
.
.
.
I
I
TI T
I 300
.
I
I
I 50*
2e
(c)
S
I
I
I
700
1
90*
(degree, Cu Ka)
Fig. 2. X-ray diffractograms of films: (a) film A, (b) film B and (c) film C.
The results of the electron microprobe analyses for the films are summarized in Table I. It can be seen that both films B and C contain considerable amounts of titanium, while the concentration o f this element is low in film A. The Ti:Ni ratio o f the film is 0.35 for film B, while it is about three for film C. This indicates that cermet films over a wide range of TiC content can be produced by the ARE process. TABLE I CONCENTRATION (wt.%) OF TITANIUM AND NICKEL AND
Film
77
Ni
Ti.Ni
A B C
0.5 20.1 54.5
99.9 70.0 22.0
-0.35 3.0
Ti:Ni
ATOMIC RATIO OF FILMS
A. ISHIDAet al.
72
3.2.
Structure of films As described above, X-ray diffractometry indicated that the films B and C consist of two phases of nickel and TiC. However, an optical microscopy study did not reveal a distinct contrast indicative of such a two-phase structure. Consequently, the thin films for transmission electron microscopy were prepared by electrochemically thinning from the substrate side. Figure 3 shows a bright field micrograph, the electron diffraction pattern and the corresponding dark field micrographs of film B. The bright field micrograph indicates that the film has a fine-grained structure. The diffraction pattern again confirms the existence of nickel and TiC as indicated in Fig. 3(b). In order to delineate the TiC and nickel grains in the film, two dark field micrographs of the same area shown in Fig. 3(a) were obtained from different diffraction rings in Fig. 3(b). Figure 3(c) was taken by placing the objective aperture over the TIC(111) ring as shown in the inset. In this micrograph only the TiC grains are visible and are seen to be distributed uniformly as very small crystallites with an average size of about 5 nm. Figure 3(d) was taken by placing the aperture over a set of rings: TIC(111), Ti(200), Ni(111) and Ni(200), as shown in the inset. Unfortunately the nickel ring could not be isolated from the TiC ring even by the smallest aperture of the transmission electron microscope used, and hence both the nickel and TiC grains are visible in this micrograph. Some of the TiC grains can be seen in both
(a)
I
200rim
!
(b)
(c) (d) Fig. 3. Transmissionelectronphotomicrographsand diffractionpattern of filmB: (a) bright fieldimage, (b) diffractionpattern, (c) and (d) dark fieldimagesof the samearea usingTIC(111)and a set of TiC(111), TIC(200),Ni(l 11) and Ni(200).
N i - T i C CERMETFILMS
73
Figs. 3(c) and 3(d), as demonstrated by the grains specified by X (Fig. 3). In Fig. 3(d) TiC grains appear to be smaller crystallites with the grains size of 5-10 nm. In this figure, in addition to the smaller TiC crystallites, larger crystallites can be seen, which are believed to be nickel. Then, the average size of the nickel crystallites is estimated to be about 15 nm. Figure 4 shows bright field micrographs of the four films; (a) nickel film, (b) film A, (c) film B and (d) film C. These figures show marked differences in grain size between both the nickel film and film A, and both films B and C. (It should be noted that the magnification of the lower photographs is 10 times that of the upper photographs.) This result shows that simultaneous deposition of nickel and TiC produces an extremely fine-grained structure compared with deposition of only nickel. These fine-grained structures of the cermet films are also reflected in their Xray diffractograms; the peaks o f both nickel and TiC observed in Figs. 2(b) and 2(c) are broad, while nickel peaks in Fig. 2(a) are very sharp.
(a)
(b)
I
2 p m
i
(c) (d) ! 20 0 nm ! Fig. 4. Transmissionelectron photomicrographsof films: (a) nickel films, (b) film A, (c) film B and (d) film C. 3.3.
Hardness Microhardness of the deposited films was measured using a Vickers hardness tester. Figure 5 shows the variation in the microhardness as a function o f T i : ( T i ÷ N i ) ratio of the films. The cermet films, films B and C, have very high hardness. It should be noticed that film B has a high hardness in spite o f the low TiC content.
74
A. ISHIDAe t al.
200E Q C "0
1000 o r.,)
:~5
5'0
7~5
100
TI/tNi+Ti) {at%) Fig. 5. Vickers hardness of films as a function of the Ti:(Ni + Ti) ratio.
3.4.
S c r a t c h test
Figure 6 shows the surfaces of the specimens after the scratch test. Clearly film B exhibits far better adhesion than the TiC film; film B was still not removed at a normal load of about 30 N, while the TiC film detached at 4 N. Furthermore, cohesive failure of the TiC coating was observed at the sides of the scratch channel, but no loss of film B was observed. This result indicates that film B has good ductility. However, film C exhibited poor adhesion in contrast to the excellent adhesion of film B. 4.
DISCUSSION
These results indicate that activated reactive evaporation in the presence of C2H2 using Ni-Ti alloy as an evaporation source can produce Ni-TiC cermet films which have a duplex structure of fine grains of nickel and TiC. This structure differs from the dispersion structure reported by Bunshah et al. 1 - 3 The difference in structure between the films could be due to differences in coating conditions such as evaporant composition and substrate temperature. The fine-grained structure probably results from mutual restraint of grain growth during the deposition due to simultaneous deposition of two species which do not react with each other. Film B revealed high hardness, good adhesion and high ductility as shown in Figs. 5 and 6. The high hardness of film B in spite of its low TiC content can be attributed not only to the presence of TiC, but also to grain refinement strengthening of nickel. The strengthening effect of fine grains of nickel has been reported by Spingarn et al. 5 Also, Komiya e t al. 6 have reported that the Hall Petch relation is valid for chromium films with grain sizes ranging from 0.06 to 1 lam. The improved adhesion of film B can be attributed to (1) the improvement in film-substate bonding due to the metallic bond of nickel grains and stainless steel at the film-substrate interface and (2) the increase in ductility of the film, due both to grain refinement and to a large amount of ductile nickel. In contrast, film C, the TiCrich cermet film, exhibited poor adhesion. Although the reason is not clear, degradation in the mechanical properties of film C is a possibility. TiC-rich cermet film seems to be brittle because of the small amount of nickel.
N i - T i C CERMET FILMS
75
(,A)
(B)
(C)
!
!
0.2ram Fig. 6. Scratch tests on films deposited on SUS304: (A) film B(Ti:Ni = 0.35), (B) film C(Ti:Ni = 3) and (C)TiC film. Photographs (a), (b) and (c) show more detail of the scratch channel. The numbers indicate the corresponding normal loads at the left and right ends o f the photographs. 5.
CONCLUSIONS
N i - T i C cermet films over a wide range o f Ti:Ni ratios were deposited by the activated reactive evaporation process. Electron microscopy study revealed that the
76
A. ISHIDA e t al.
d e p o s i t e d films consisted o f fine grains o f nickel a n d TiC. M i c r o h a r d n e s s a n d scratch tests revealed t h a t a nickel-rich c e r m e t film exhibited g o o d a d h e s i o n , high h a r d n e s s a n d high ductility. REFERENCES 1 R. Nimmagadda and R. F. Bunshah, J. Vac. Sci. Technol., 12 (1975) 815. 2 R. Nimmagadda and R. F. Bunshah, J. Vac. Sci. Technol., 13 (1976) 532. 3 V . K . Sarin, R.F. BunshahandR. Nimmagadda, ThinSolidFilms, 40(1977) i83.
4 R. Nimmagadda, A. C. Raghuram and R. F. Bunshah, J. Vac. Sci, TechnoL, 9 (1972) 1406. 5 J, R. Spingarn, B. E. Jacobson and W. D. Nix, Thin Solid Films, 45 (1977) 507. 6 S. Komiya, S. Ono and N. Umezu, Thin Solid Films, 45 (1977) 473.