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Deposition of wear-resistant BN coatings by r.f. plasma-assisted chemical vapour deposition
272
Thin Solid Films, 228 (1993) 272-275
Deposition of wear-resistant BN coatings by r.f. plasma-assisted chemical vapour deposition J. S m e e t s , V. V a n D e n B e r g h , J. M e n e v e , E . D e k e m p e n e e r
a n d L. D e W i l d e
Materials Division, Vlaamse Instelling voor Technologisch Onderzoek, Boeretang 200, B-2400 Mol (Belgium)
Abstract Thin films of BN have been deposited by r.f. plasma-assisted chemical vapour deposition using B2H6-N2-H2 gas mixtures. The deposited layers were characterized mainly by IR reflection spectroscopy. On the anode, stoichiometric films could only be obtained when a large excess of N 2 was present in the gas phase. For films grown at the r.f. plate, the mixing of B2H 6 and N 2 was less critical. The films were amorphous and had a hexagonal-like short-range order (turbostratic structure). Hardness values up to 9 GPa have been obtained for films grown on stainless steel substrates.
1. Introduction Boron nitride (BN) is one of the most interesting I I I - V compounds because of its unique physical properties. In particular, cubic BN is a material of high potential for engineering applications. It is extremely hard, has a high thermal conductivity and is chemically stable even at high temperatures (up to 1800 °C). Cubic BN can be used for machining iron and steel, where diamond wears out by reacting with iron. Besides these unique mechanical properties, cubic BN is also a very interesting material for high temperature electronic devices [ 1]. In recent years, different techniques have been developed for the deposition of BN thin films from the vapour phase. An excellent overview of the state of the art can be found in ref. 2. Chemically vapour-deposited BN films are typically prepared by using diborane (B2Hr) and ammonia (NH3) as source gases [3]. Stoichiometric films can be obtained only for temperatures above 600 °C. This limits the types of substrate that can be coated. Furthermore, special care should be taken when mixing these reactants in order to prevent the formation of borohydride compounds in the gas lines and the reactor [4]. In this paper, we report the first results on the deposition of boron nitride films prepared at 500 °C by the glow discharge decomposition of B2H 6 and nitrogen (N2) mixtures diluted in hydrogen (H2).
2. Experimental details BN films were deposited in a capacitively coupled r.f. (13.56MHz) parallel-plate plasma-assisted chemical vapour deposition (PACVD) reactor (electrode diame-
0040-6090/93/$6.00
ter, 8 cm). The grounded top electrode could be heated to 600 °C. The temperature of the r.f. electrode was 280 °C when the anode was heated to 500 °C. As precursor gases, B2H 6 (1 vol.% in H2) and N 2 (pure or 1 vol.% in H2) were used. The total gas flow was fixed at 60 standard cm 3 min -1, the pressure at 200 Pa (50 Pa for pure N2) and the r.f. power at 100 W. Films were deposited on Corning 7059 glass and AISI 304 polished stainless steel substrates. The compositional and structural properties of the coatings were determined using Auger electron spectroscopy (AES), X-ray diffraction (XRD) and IR spectroscopy. In the AES measurements, a sintered hexagonal BN specimen was used as a reference. IR spectroscopy was used to study the chemical bonding within the coatings. This technique is now generally accepted to be one of the most sensitive and elegant techniques for the structural evaluation of BN coatings [5]. Since our films were grown on steel substrates, IR reflection spectroscopy (IRRS) was used. IRRS is an ideal tool for analysing thin films adsorbed or grown on metallic substrates [6]. Interference effects, however, can make these spectra rather complicated and confusing, especially in the region of strong adsorption. As pointed out by Allen and Swallow [7], this is caused by the rapidly changing values of the refractive index and the absorption coefficient in the region of mode centres. This can lead to an apparent shift in the band centres and, for sufficiently thick films, one or more interference fringes may be observed near the band centres. Reflectance spectra were measured with unpolarized light. The angle of incident radiation on the sample was 6.5 ° relative to the surface normal. All measurements were done relative to a gold mirror. Mechanical characterization was performed using microhardness measurements (Knoop; 20 g load) and
© 1993-- Elsevier Sequoia. All rights reserved
J. Smeets et al. / a.f. P A C V D o f wear-resistant B N coatings
ball-on-disc wear tests. More details can be found in ref. 8.
3. Results and discussion
3. I. Chemical composition The reactant gas mixture ratio R (=Q(B2H6) / Q(N2), where Q represents the gas flow rate) was varied in order to study its influence on the coating composition. Figure 1 shows the atomic B-to-N ratio normalized to a hexagonal BN standard as a function of R for films grown at the grounded and the r.f. electrode. Nearly stoichiometric BN coatings can be obtained at the anode for low R values (R ~<0.2). Increasing the B2H6 concentration in the gas mixture results in an increased nitrogen deficiency in the boron nitride layers. The coatings grown at the r.f. electrode remain stoichiometric up to R = 1. It even seems that these films are somewhat boron deficient. The difference between anode and cathode is probably due to the ion bombardment (bias voltage, - 2 5 0 V). Our results for the films grown at the anode correspond fairly well to the results of Miyamoto et al. [9] for films grown by the r.f. glow discharge decomposition of B2Hr-NH3-H 2. The carbon content in our films is rather high (the B-to-C AES peak-to-peak height ratio is around 4). The O concentration in our films, on the contrary, is always very low (B-to-O ratio greater than 15). This is probably due to the bombardment of the film with hydrogen, which will bond with oxygen. Stoichiometric films deposited at the
273
anode and cathode were transparent. Non-stoichiometric films were mostly black or brown.
3.2. Structural analysis Figures 2 and 3 show the IR spectra for films deposited at the grounded and r.f. plate respectively as a function of the reactant gas mixture ratio R. Anodic films with R < 0.5 and all films grown at the cathode mainly show IR absorption around 1370 and 800 cm-~.. These two bands correspond to the normal IR modes of hexagonal BN [10]. Both peaks are asymmetric and shifted towards higher and lower wavenumbers respectively in comparison with those for hexagonal BN. XRD measurements showed that the films are amorphous over the whole R range. This clearly shows that the films have a hexagonal-like short-range order, indicative of a so-called turbostratic structure in which layers with a planar network of hexagonal rings are i
,
i
0.1
O.5
4000
3000
2000
1000
Wavenurnber (1/cm)
5
Fig. 2. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H 6 and 1 vol.% N 2 diluted in H 2.
4 i
i
i
l0
2
II"
~
" " ~"~,
1
0
o
l
i
I
2
Reactant
gas .mixture R
Fig. 1. AES peak-to-peak height ratio o f B ( K L L ) to N ( K L L ) vs. reactant gas mixture ratio R for films grown at the anode (E]) a n d the cathode ( A ) using 1 vol.% B2H ~ and 1 vol.% N 2 diluted in H 2.
4000
I
I
I
3000
2000
1000
Waverxm'~er ( "IIcm)
Fig. 3. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the cathode using i vol.% B2H 6 a n d 1 vol.% N a .diluted in H2.
274
J. Smeets et al. / R.f. PACVD of wear-resistant BN coatings
stacked on each other but show random orientation and translation around the layer normal [11]. Besides these peaks, additional absorption occurs around 3400 and 3200cm -j because of N - H stretching and at around 2500 and 920 cm -~ because of B - H stretching and bending vibrations. F o r films grown at the anode with R > 0.5, the I R spectra at around 1400cm -~ are very similar to those reported for cyclic B - N compounds [12]. F o r these high R values, we do not observe any absorption at 3400 cm -~ ( N - H stretching) which indicates that these films contain hardly any nitrogen which corresponds to our AES measurements. These results clearly show that I R measurements are very sensitive to the chemical composition of the films. 3.3. Influence o f gas dilution
Figure 4 shows the I R spectra as a function o f the reactant gas mixture ratio R for films deposited at the grounded plate using pure N2. These spectra clearly show that anodic films are only stoichiometric for very low R values (high N2 dilution). Films grown at the r.f. plate were stoichiometric up to R values of 1. It seems that the region where stoichiometric BN films can be obtained is smaller when pure N2 is used instead of 1 vol.% N2 diluted in H2. The I R absorptions at around 800 c m - ~ suggest that with decreasing R the disorder in the films is decreasing since this peak shifts towards higher wavenumbers. Furthermore, the longitudinal optical in-plane mode o f hexagonal BN at 1600 cm -1 can be distinguished. Figure 4 is a good example of the complexity of IRRS. The band at around 1300 cm -I in the spectra for R = 0.04 and 0.065 for example is not due to an IR-active mode but to interference. Computer simulations showed that the absorption band was lcoated near 1380 c m - 1. A more detailed analysis of the I R spectra using computer simulations will be published elsewhere. i
i
i
0.04
15
10
O
i
i
i
i
i
i
i l l
0.01
i
i
i
0.1 Reoctant
. . . . .
i
1 gas mixture R
Fig. 5. Knoop hardness vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H6 in H2 and pure N 2.
3. 4. M e c h a n i c a l properties
Figure 5 shows the K n o o p hardness vs. the R value for films grown on stainless steel (hardness, 2.2 GPa). The film thickness was around 1 ~tm. Since the film thickness is rather low, we expect that the substrate has its influence on the hardness value. F o r very low R values (less than 0.05) the hardness is rather low probably because these films become more and more hexagonal like, with less disorder as for higher R values. The increase with higher R values can be explained by the turbostratic structure of these films [13]. However, it should be stressed that classical hardness measurements on thin films are very tricky. Furthermore, the interpretation of t h e hardness measurements was often very complicated because of the brittleness of the coating, which led to cracks in the indentations. Therefore no definite conclusion can be drawn from the hardness measurements. The measured friction coefficients were all of the order of 0.35-0.45 (for a relative humidity of 20%). However, the real friction coefficient of the coatings was often difficult to estimate owing to the low loadbearing capacity of the coatings.
4. Conclusions ~.~
0.14
n"
2 : /
4000
3000
2000
Waver~er
1000
( 1/cm)
Fig. 4. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H6 in H2 and pure N 2.
It was demonstrated that P A C V D can be used for the deposition o f B N coatings at 500 °C using B2H 6 and N2 as precursors. Films could be grown on the cathode and the anode. On 'the grounded plate, stoichiometric BN films could only be obtained when a high excess of N2 was used. F o r films grown at the r.f. electrode, the mixing of the source gases was less critical. The films were amorphous with a hexagonal-like short-range order (so-called turbostratic structure). Hardness values up to 9 G P a have been obtained for
J. Smeets et al. / R.f. PACVD o f wear-resistant BN coatings
films grown on stainless steel (four times the substrate hardness). At present the films have a limited adherence, high friction coefficient and low load-bearing capacity.
Acknowledgment This work was partially funded by the Euram programme of the Commission of the European Communities under Contract No. MALE/0011/B.
References 1 0 . Mishima, J. Tanaka, S. Yamaoka and O. Fukunaga, Science, 238 (1987) 181. 2 J. J. Pouch and S. A. Alterovitz (eds), Synthesis and Properties o f Boron Nitride, Trans Tech, Ziirich, 1990.
275
3 M. J. Rand and J. F. Roberts, J. Electrochem. Soc., 115 (1968) 423. 4 A. C. Adams and C. D. Capio, J. Electrochem. Soc., 127(1980) 399. 5 H. Saitoh and W. A. Yarbrough, Diamond Relat. Mater., l (1991) 137. 6 R. G. Greenler, J. Chem. Phys., 44(1966) 310. 7 G. C. Allen and G. A. Swallow, Oxid. Met., 17(1982) 141. 8 J. Meneve, E. Dekempeneer, R. Jacobs, L. Eersels, V. Van den Bergh and J. Smeets, Diamond Relat. Mater., I (1991) 553. 9 H. Miyamoto, M. Hirose and Y. Osaka, Jpn. J. Appl. Phys., 2.2 (1983) L216. 10 R. Geick, C. H. Perry and G. Rupprecht, Phys. Rev., 146 (1966) 543. 11 V. I. Chukalin, N. V. Chukanov, S. V. Gurov, V. N. Troitskii, N. E. Filatova, T. V. Rezchikova and E. P. Domashneva, Russ. Powder Metall., 1 (1988) 81. 12 A. W. Laubengayer, P. C. Moews and R. F. Porter, J. Am. Chem. • Soc., 83 (1961) 1337. 13. B. Rother, H. D. Zscheile, C. Weissmantel, C. Heiser, G. Holzhiiter, G. Leonhardt and P. Reich, Thin Solid Films, 142 (1986) 83.
Thin Solid Films, 228 (1993) 272-275
Deposition of wear-resistant BN coatings by r.f. plasma-assisted chemical vapour deposition J. S m e e t s , V. V a n D e n B e r g h , J. M e n e v e , E . D e k e m p e n e e r
a n d L. D e W i l d e
Materials Division, Vlaamse Instelling voor Technologisch Onderzoek, Boeretang 200, B-2400 Mol (Belgium)
Abstract Thin films of BN have been deposited by r.f. plasma-assisted chemical vapour deposition using B2H6-N2-H2 gas mixtures. The deposited layers were characterized mainly by IR reflection spectroscopy. On the anode, stoichiometric films could only be obtained when a large excess of N 2 was present in the gas phase. For films grown at the r.f. plate, the mixing of B2H 6 and N 2 was less critical. The films were amorphous and had a hexagonal-like short-range order (turbostratic structure). Hardness values up to 9 GPa have been obtained for films grown on stainless steel substrates.
1. Introduction Boron nitride (BN) is one of the most interesting I I I - V compounds because of its unique physical properties. In particular, cubic BN is a material of high potential for engineering applications. It is extremely hard, has a high thermal conductivity and is chemically stable even at high temperatures (up to 1800 °C). Cubic BN can be used for machining iron and steel, where diamond wears out by reacting with iron. Besides these unique mechanical properties, cubic BN is also a very interesting material for high temperature electronic devices [ 1]. In recent years, different techniques have been developed for the deposition of BN thin films from the vapour phase. An excellent overview of the state of the art can be found in ref. 2. Chemically vapour-deposited BN films are typically prepared by using diborane (B2Hr) and ammonia (NH3) as source gases [3]. Stoichiometric films can be obtained only for temperatures above 600 °C. This limits the types of substrate that can be coated. Furthermore, special care should be taken when mixing these reactants in order to prevent the formation of borohydride compounds in the gas lines and the reactor [4]. In this paper, we report the first results on the deposition of boron nitride films prepared at 500 °C by the glow discharge decomposition of B2H 6 and nitrogen (N2) mixtures diluted in hydrogen (H2).
2. Experimental details BN films were deposited in a capacitively coupled r.f. (13.56MHz) parallel-plate plasma-assisted chemical vapour deposition (PACVD) reactor (electrode diame-
0040-6090/93/$6.00
ter, 8 cm). The grounded top electrode could be heated to 600 °C. The temperature of the r.f. electrode was 280 °C when the anode was heated to 500 °C. As precursor gases, B2H 6 (1 vol.% in H2) and N 2 (pure or 1 vol.% in H2) were used. The total gas flow was fixed at 60 standard cm 3 min -1, the pressure at 200 Pa (50 Pa for pure N2) and the r.f. power at 100 W. Films were deposited on Corning 7059 glass and AISI 304 polished stainless steel substrates. The compositional and structural properties of the coatings were determined using Auger electron spectroscopy (AES), X-ray diffraction (XRD) and IR spectroscopy. In the AES measurements, a sintered hexagonal BN specimen was used as a reference. IR spectroscopy was used to study the chemical bonding within the coatings. This technique is now generally accepted to be one of the most sensitive and elegant techniques for the structural evaluation of BN coatings [5]. Since our films were grown on steel substrates, IR reflection spectroscopy (IRRS) was used. IRRS is an ideal tool for analysing thin films adsorbed or grown on metallic substrates [6]. Interference effects, however, can make these spectra rather complicated and confusing, especially in the region of strong adsorption. As pointed out by Allen and Swallow [7], this is caused by the rapidly changing values of the refractive index and the absorption coefficient in the region of mode centres. This can lead to an apparent shift in the band centres and, for sufficiently thick films, one or more interference fringes may be observed near the band centres. Reflectance spectra were measured with unpolarized light. The angle of incident radiation on the sample was 6.5 ° relative to the surface normal. All measurements were done relative to a gold mirror. Mechanical characterization was performed using microhardness measurements (Knoop; 20 g load) and
© 1993-- Elsevier Sequoia. All rights reserved
J. Smeets et al. / a.f. P A C V D o f wear-resistant B N coatings
ball-on-disc wear tests. More details can be found in ref. 8.
3. Results and discussion
3. I. Chemical composition The reactant gas mixture ratio R (=Q(B2H6) / Q(N2), where Q represents the gas flow rate) was varied in order to study its influence on the coating composition. Figure 1 shows the atomic B-to-N ratio normalized to a hexagonal BN standard as a function of R for films grown at the grounded and the r.f. electrode. Nearly stoichiometric BN coatings can be obtained at the anode for low R values (R ~<0.2). Increasing the B2H6 concentration in the gas mixture results in an increased nitrogen deficiency in the boron nitride layers. The coatings grown at the r.f. electrode remain stoichiometric up to R = 1. It even seems that these films are somewhat boron deficient. The difference between anode and cathode is probably due to the ion bombardment (bias voltage, - 2 5 0 V). Our results for the films grown at the anode correspond fairly well to the results of Miyamoto et al. [9] for films grown by the r.f. glow discharge decomposition of B2Hr-NH3-H 2. The carbon content in our films is rather high (the B-to-C AES peak-to-peak height ratio is around 4). The O concentration in our films, on the contrary, is always very low (B-to-O ratio greater than 15). This is probably due to the bombardment of the film with hydrogen, which will bond with oxygen. Stoichiometric films deposited at the
273
anode and cathode were transparent. Non-stoichiometric films were mostly black or brown.
3.2. Structural analysis Figures 2 and 3 show the IR spectra for films deposited at the grounded and r.f. plate respectively as a function of the reactant gas mixture ratio R. Anodic films with R < 0.5 and all films grown at the cathode mainly show IR absorption around 1370 and 800 cm-~.. These two bands correspond to the normal IR modes of hexagonal BN [10]. Both peaks are asymmetric and shifted towards higher and lower wavenumbers respectively in comparison with those for hexagonal BN. XRD measurements showed that the films are amorphous over the whole R range. This clearly shows that the films have a hexagonal-like short-range order, indicative of a so-called turbostratic structure in which layers with a planar network of hexagonal rings are i
,
i
0.1
O.5
4000
3000
2000
1000
Wavenurnber (1/cm)
5
Fig. 2. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H 6 and 1 vol.% N 2 diluted in H 2.
4 i
i
i
l0
2
II"
~
" " ~"~,
1
0
o
l
i
I
2
Reactant
gas .mixture R
Fig. 1. AES peak-to-peak height ratio o f B ( K L L ) to N ( K L L ) vs. reactant gas mixture ratio R for films grown at the anode (E]) a n d the cathode ( A ) using 1 vol.% B2H ~ and 1 vol.% N 2 diluted in H 2.
4000
I
I
I
3000
2000
1000
Waverxm'~er ( "IIcm)
Fig. 3. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the cathode using i vol.% B2H 6 a n d 1 vol.% N a .diluted in H2.
274
J. Smeets et al. / R.f. PACVD of wear-resistant BN coatings
stacked on each other but show random orientation and translation around the layer normal [11]. Besides these peaks, additional absorption occurs around 3400 and 3200cm -j because of N - H stretching and at around 2500 and 920 cm -~ because of B - H stretching and bending vibrations. F o r films grown at the anode with R > 0.5, the I R spectra at around 1400cm -~ are very similar to those reported for cyclic B - N compounds [12]. F o r these high R values, we do not observe any absorption at 3400 cm -~ ( N - H stretching) which indicates that these films contain hardly any nitrogen which corresponds to our AES measurements. These results clearly show that I R measurements are very sensitive to the chemical composition of the films. 3.3. Influence o f gas dilution
Figure 4 shows the I R spectra as a function o f the reactant gas mixture ratio R for films deposited at the grounded plate using pure N2. These spectra clearly show that anodic films are only stoichiometric for very low R values (high N2 dilution). Films grown at the r.f. plate were stoichiometric up to R values of 1. It seems that the region where stoichiometric BN films can be obtained is smaller when pure N2 is used instead of 1 vol.% N2 diluted in H2. The I R absorptions at around 800 c m - ~ suggest that with decreasing R the disorder in the films is decreasing since this peak shifts towards higher wavenumbers. Furthermore, the longitudinal optical in-plane mode o f hexagonal BN at 1600 cm -1 can be distinguished. Figure 4 is a good example of the complexity of IRRS. The band at around 1300 cm -I in the spectra for R = 0.04 and 0.065 for example is not due to an IR-active mode but to interference. Computer simulations showed that the absorption band was lcoated near 1380 c m - 1. A more detailed analysis of the I R spectra using computer simulations will be published elsewhere. i
i
i
0.04
15
10
O
i
i
i
i
i
i
i l l
0.01
i
i
i
0.1 Reoctant
. . . . .
i
1 gas mixture R
Fig. 5. Knoop hardness vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H6 in H2 and pure N 2.
3. 4. M e c h a n i c a l properties
Figure 5 shows the K n o o p hardness vs. the R value for films grown on stainless steel (hardness, 2.2 GPa). The film thickness was around 1 ~tm. Since the film thickness is rather low, we expect that the substrate has its influence on the hardness value. F o r very low R values (less than 0.05) the hardness is rather low probably because these films become more and more hexagonal like, with less disorder as for higher R values. The increase with higher R values can be explained by the turbostratic structure of these films [13]. However, it should be stressed that classical hardness measurements on thin films are very tricky. Furthermore, the interpretation of t h e hardness measurements was often very complicated because of the brittleness of the coating, which led to cracks in the indentations. Therefore no definite conclusion can be drawn from the hardness measurements. The measured friction coefficients were all of the order of 0.35-0.45 (for a relative humidity of 20%). However, the real friction coefficient of the coatings was often difficult to estimate owing to the low loadbearing capacity of the coatings.
4. Conclusions ~.~
0.14
n"
2 : /
4000
3000
2000
Waver~er
1000
( 1/cm)
Fig. 4. IR reflection spectra vs. reactant gas mixture ratio R for films grown at the anode using 1 vol.% B2H6 in H2 and pure N 2.
It was demonstrated that P A C V D can be used for the deposition o f B N coatings at 500 °C using B2H 6 and N2 as precursors. Films could be grown on the cathode and the anode. On 'the grounded plate, stoichiometric BN films could only be obtained when a high excess of N2 was used. F o r films grown at the r.f. electrode, the mixing of the source gases was less critical. The films were amorphous with a hexagonal-like short-range order (so-called turbostratic structure). Hardness values up to 9 G P a have been obtained for
J. Smeets et al. / R.f. PACVD o f wear-resistant BN coatings
films grown on stainless steel (four times the substrate hardness). At present the films have a limited adherence, high friction coefficient and low load-bearing capacity.
Acknowledgment This work was partially funded by the Euram programme of the Commission of the European Communities under Contract No. MALE/0011/B.
References 1 0 . Mishima, J. Tanaka, S. Yamaoka and O. Fukunaga, Science, 238 (1987) 181. 2 J. J. Pouch and S. A. Alterovitz (eds), Synthesis and Properties o f Boron Nitride, Trans Tech, Ziirich, 1990.
275
3 M. J. Rand and J. F. Roberts, J. Electrochem. Soc., 115 (1968) 423. 4 A. C. Adams and C. D. Capio, J. Electrochem. Soc., 127(1980) 399. 5 H. Saitoh and W. A. Yarbrough, Diamond Relat. Mater., l (1991) 137. 6 R. G. Greenler, J. Chem. Phys., 44(1966) 310. 7 G. C. Allen and G. A. Swallow, Oxid. Met., 17(1982) 141. 8 J. Meneve, E. Dekempeneer, R. Jacobs, L. Eersels, V. Van den Bergh and J. Smeets, Diamond Relat. Mater., I (1991) 553. 9 H. Miyamoto, M. Hirose and Y. Osaka, Jpn. J. Appl. Phys., 2.2 (1983) L216. 10 R. Geick, C. H. Perry and G. Rupprecht, Phys. Rev., 146 (1966) 543. 11 V. I. Chukalin, N. V. Chukanov, S. V. Gurov, V. N. Troitskii, N. E. Filatova, T. V. Rezchikova and E. P. Domashneva, Russ. Powder Metall., 1 (1988) 81. 12 A. W. Laubengayer, P. C. Moews and R. F. Porter, J. Am. Chem. • Soc., 83 (1961) 1337. 13. B. Rother, H. D. Zscheile, C. Weissmantel, C. Heiser, G. Holzhiiter, G. Leonhardt and P. Reich, Thin Solid Films, 142 (1986) 83.