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Growth of diamond films on spherical surfaces by hot filament CVD
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Diamond and Related Materials 7 ( ! 998) 129- ! 32
Growth of diamond films on spherical surfaces by hot filament CVD B. L u n n *, D . A . W r i g h t , L . Y . Z h a n g Deparonent of Engineering Design and Manufacture, University of Hull, Hull, HU6 7RX, UK Received 21 June 1997; accepted 11 September 1997
Abstract
A growth system has been designed and built for depositing diamond films onto spherical surfaces; the initial experiments have been carried out using 1.5 cm tungsten carbide (6 % Co) balls, as substrates. The films are deposited by hot filament chemical vapour deposition (HFCVD), using an hydrogen/methane mixture as the source. The ball is heated (typically to 925 C) by tantalum filaments, which also provide the source of atomic hydrogen. Provision is made to support and rotate the ball simultaneously either continuously, or in a programmed manner. It has proved possible to coat balls uniformly and reproducibly with good quality diamond fioatings, typically 3 ~tm thick. Raman measurements confirm that the coatings are indeed diamond, but with some graphite incorporation. The soft impressor method, first presented at ICNDST 5 (1996), has been uged to investigate the mechanical properties of the coating system. Results obtained, using both single loading and fatigue measurements, indicate that the films do protect the balls and failure of the system occurs as the result of substrate failure. © 1998 Elsevier Science S.A. Keywords: Diamond; Coating; Spheres; H F C V D
I. Introduction
2. Growth
The unique mechanical properties of diamond [1] make it an attractive material for wear resistant and chemically stable coatings, particularly since it has proved possible to deposit polycrystalline films on a variety of substrates under a wide range of conditions. There are several applications, e.g. in balls and ball valves, for which an ability to coat spherical surfaces becomes paramount. However, in order to grow films of uniform thickness, it is important to provide good temperature uniformity over the substrate area, a problem that becomes more acute when the substrate is spherical. Hot filament chemical vapour deposition (HFCVD) has been used extensively, in recent years, to deposit diamond films and has the advantages of simplicity and potential for scaling up. The present paper reports preliminary re:. ,Its on the H F C V D diamond coating of tungsten carbide (6% Co) balls and on the mechanical properties of the coating system. These properties have been evaluated by using the soft impressor technique [2-5].
2.1. Ewerhm,nt
* Correspo~ding author. 0925-9635/98/$19.00 ~.©1998 Elsevier Science B.V. All rights reserved. PI! S0925-9635( 97 )00205-7
It is well known [6] that the presence of Co has adverse effects on the deposition and adhesion of diamond films. Therefore, the balls are etched, for 10 min in 1:1 HNO3/HF, to remove the near surface Co and to provide voids that key the film to the substrate [7]. This process removes the Co to a depth of greater than 3 lam, as determined by EDX measurements. The balls are then abraded, prior to growth, with 0-2 I.tm (mesh size) synthetic diamond particles and cleaned, ultrasonically, in methanol. One of the key features of the present growth system is the method of supporting and rotating the ball on a tepee-like tripod of alumina rods (Fig. 1). Two of the rods are rotated independently (up to 200 rpm), whilst the third rod is free to idle; rotation of the rods can be either continuous or intermittent. The other key feature is the arrangement of the heater filaments, to provide uniform heating of the spherical substrate. These filaments also act as a source of atomic hydrogen. The temperature of the substrate can be varied by changi~g the radial position of the filaments, prior to a growth
130
B. Lutm et aL
Diamomt ami Rehtted hhtterials 7 ( 19981 129-132
Fig. !. Ball support and rotation system.
experiment. Temperature measurements are made with an optical pyrometer (Ircon SAI0+); the emissivity setting for the substrate material was previously determined by calibrating the pyrometer against the output of a thermocouple embedded in the centre of ~ test ball. The films are deposited from a 0.5% CH4/H2 gas mixture, at a substrate temperature of ---925 C . with the Ta filaments at 2200 :C (the filaments are carburized, with the same gas mixture, prior to growth). Tile total flow rate and the total gas pressure are 100 sccm and 20Torr (2.7 kPa), respectively. Mass flow controllers (MKS) are used to control the gas flows and the chamber pressure is controlled by pumping through a simple needle valve. These conditions yield an average growth rate of 0.15 ~tm/h. A range of rotation conditions has been explored. When the two rods are rotated continuously in the opposite sense, at the same speed, the ball itself rotates and precesses resulting in "'grooved" coatings. The coatings described in the present paper are produced using intermittent rotation; the rods are contra-rotated continuously, for the first 5 minutes of growth and thereafter, for one minute every half hour, at 30 rpm.
Fig. 2. SEM micrograph of bali, showingcoating coverage.
3. Results The films produced are unitbrm and continuous over the whole surface of the ball (Fig. 2). Growth appears to take place in two stages: examination of films 1-2 ~tm thick, shows that the growth is granular, whilst films 3 - 4 p m thick are faceted (Fig. 3). The black spots (Fig. 2) are not holes in the film, but areas that charge in the SEM to a lesser extent than the rest of the coating. We believe that these regions are contaminated by tl3e
Fig. 3. SEM micrograph of the surface morphology of a -,-3tam thick coating.
B. Lunn et al. / Diamondand Related Materials 7 (1998) 129-132
loosely adhering diamond film that forms on the alumina support rods during the run. Raman spectroscopy verifies that the films are indeed diamond, but with some non-diamond components. The uniformity and reproducibility of the growth is con.firmed by examining several regions of each ball.
4. Evaluation of the coating quality When conventional rigid indenters are used to evaluate hard ceramic coatings, the extent of deformation almost always leads to the propagation of cracks-usually radial cracks for pyramidal indenters or riag cracks for spherical indenters. The essential difference with the soft impressor technique is that it enables the measurement of a threshold mean contact pressure to initiate failure - ~,ither through the formation of a ring crack or an impression due to plastic deformation. This measurement can be achieved by using an impressor, initially in the form of a cone with an included angle of 120° and made from a softer material than the specimen, under a known normal load [8,9]. As the load is applied, the conical tip of the softer impressor deforms plastically, the cone becomes blunter until its flattened tip is large enough to support the applied load elastically. The mean pressure produced is then determined by dividing the normal load by the measured circular contact area. The soft impressor technique has three particularly useful advantages when it is used to evaluate the coated system. First, a high level of surface finish is unnecessary, as the blunted tip of the impressor will conform intimately to the surface topography of the specimen. Secondly, the mean contact pressure can be controlled, by using impressor materials over a range of hardness values, to be just sufficient to initiate failure - - i.e. a threshold mean pressure can be measured for each failure event. Thirdly, the depth of the deformed zone is approximately equal to the radius of the circular contact [5] and, consequently, adjusting the normal load for a given impressor allows the strain to be accommodated entirely within the coating, or transmitted to the interface or into the substrate material. In addition, cumulative fatigue processes can be studied using repeated load cycles on a given contact area [4,5,8]. Preliminary evaluation has been carried out, of uncoated tungsten carbide balls and of balls coated with ---3 lam diamond films, by measurements involving both single loading and multiple fatigue loading cycles.
4.1. Single load measurements In earlier (unpublished) work on the basal plane of a tungsten carbide single crystal, a threshold contact pressure of 8+0.3 GPa formed an impression by plastic
131
flow. A similar pressure produced a smooth plastic impression in a tungsten carbide ball, as supplied, but the same pressure caused disintegration and "crumbling" of the surface after it had been prepared for coating. To cause failure of the coated ball, it was necessary to apply a mean pressure o f approximately 16.0 + 0.3 GPa, via a cubic boron nitride impressor. This failure consisted of an impression, with a discontinuous ring crack at the periphery of the contact area. The crack was located below the coating and was probably caused by the onset of plastic deformation of the substrate. However, since the threshold mean pressure to initiate a ring crack in a single cryst d diamond is of the same order [5], it is unlikely that any further coating improvement could be made.
4.2. Fatigue loading Fatigue measurements [4, 5,10] on the diamond coated balls were made with an hardened silver steel impressor at a load of 39.2 N, giving an initial mean contact pressure of about 8.0 GPa - - i.e. half the pressure that caused the failure beneath the cubic boron nitride impressor after a single load cycle. No failure was observed after 20 000 load cycles but, after 40 000 cycles under the same conditions, delayed fracture (about 24 h after the test was completed) caused iocalised delamination [Fig. 4(a)]. It is interesting to observe that both the coating and the interface appear to have been stronger than the substrate material, in that the <:rack causing the delamination propagated entirely through the tungsten carbide. This was confirmed by EDX measurements on the substrate side of the delaminated film [ Fig. 4{ b)], which showed the presence of tungsten carbide, but not cobalt. Thus, again, the failure of the coated system was as the result of substrate failure. EDX measurements on the delaminated region of the substrate were just able to detect the presence of cobalt. This result can be explained by the fact that the average thickness of the substrate that delaminates is !.5 lain, hence allowing the electron beam to sample to a greater depth, relative to the original surface of the substrate. Delayed fracture of this type has been observed alter soft impressor fatigue testing of natural diamond crystals and tentatively attributed to a stress corrosion effect [5]. This phenomenon requires the co-existence of a stress, applied or residual and a structure that is unstable in a given corrosive atmosphere. Diamond is thermodynamically unstable at normal atmospheric pressures and temperatures and often there is a significant level of residual stress from both natural and synthetic growth processes. A major indicator of embrittlement, due to stress corrosion, is that, whilst the level of stress may be insufficient to propagate a crack initially, the delay in fracture is due to time dependent diffusion of the
132
B, Lunn et al. / Diamond ami Related Materials 7 (1998) 129-.132
well prove to have significant advantages over most competitor materials for applications requiring both erosion and corrosion resistance - - e.g. ball valves in sea water. However, it should be noted that the surface preparation of the substrate material, prior to coating, will have reduced the mechanical integrity of this region. Thus, it will be necessary to address the pre-growth treatment of the ball surface - - a well known phenomenon with tungsten carbide [11].
Acknowledgement The authors would like to thank De Beers Industrial Diamond Division Limited for a grant to the laboratory and EPSRC/ROPA for a grant and research assistantship to one of us (LYZ). They would also like to thank Prof. CA Brookes ( Univ. of Hull) for discussions on the soft impressor results and for his general support for this projec'.. Thanks also to Garry Robinson for the SEM micrographs.
References
(b)
Fig. 4. (a) SEM micrograph of a delaminated 3 lain thick coating The darker region is the substrate. (b) Substrate-side view of the lilm, ,~h~wn m I~) illu~tratfllg the t:oltt|lln~r n,dure of the Iihn.
relevant reactive element(s). Clearly, this ph?nomenon deserves further study, with respect to tungsten carbide.
5. Conclusions The results of the present work have demonstrated that it is possible to coat spheres with good quality diamond films. The initial soft impressor measurements indicatL: that the mechanical integrity of the diamond coated tungsten carbide balls is relatively good and may
[I ] J.E. Field (Ed.), The Properties of Natural and Synthetic Diamonds, Academic Press, London, 1992. [2] C.A. Brookes, E.J. Brookes, L.Y. Zhang, in: Proceedings of the 2nd International Conference on the Application of Diamond Films and Related Materials, Tokyo, 1993, p. 737. [3] C.A. Brooke,.. L.Y. Zhang, P.W. May, Diamond Relat. Mater. 6 (1997) 348. [4] C.P. Waddington, L.Y. Zhang, E.J. Brookes, C.A Brookes, Surlace Engng 13 (1997) 3. [31 I,.Y. Zhang, Phl) Thesis, University of Hull, Ilull, UK, 1997. [6] A.K. Mehlm,m, S. Berger, A. Fayer, S. Dirnlield, M. Bamberger, Y. Avigal, A. Hoffmann, R. Porath, Diamond Relat. Mater. 3 (1994) 805. [7] C. Tsai, J.C. Nelson, W.W. Gerberich, J. Heberlein, E. Pfender, Diamond Relat. Mater. 2 ( 1993' 617. [81 C.A. Brookes, P. Green, Nature 246(IO7..".) !19. [91 C.A Brookes, M.P. Shaw, Nature 263 (1976) 760. [10] C.A. Brookes, E.J. Brookes, L.Y. Zhalig, in: R.C. Bradt, C.A. Brookes, J.L. Routbort (Eds.), Plastic Deformation of Ceramics, Plenum Press, New York, 1995. [Ill M.B. Guseva, V.G. Babaev, V.V. Khvostov, G.M. Lopez Ludena, A.Yu. Bregadze, I.Y. Konyashin, A.E. Alexenko, Diamond Relat. Mater. 6 (1997) 89.
~. . . .
!!!!!i!!
....
D| OND AND REL T|D TER|A
ELSEVIER
Diamond and Related Materials 7 ( ! 998) 129- ! 32
Growth of diamond films on spherical surfaces by hot filament CVD B. L u n n *, D . A . W r i g h t , L . Y . Z h a n g Deparonent of Engineering Design and Manufacture, University of Hull, Hull, HU6 7RX, UK Received 21 June 1997; accepted 11 September 1997
Abstract
A growth system has been designed and built for depositing diamond films onto spherical surfaces; the initial experiments have been carried out using 1.5 cm tungsten carbide (6 % Co) balls, as substrates. The films are deposited by hot filament chemical vapour deposition (HFCVD), using an hydrogen/methane mixture as the source. The ball is heated (typically to 925 C) by tantalum filaments, which also provide the source of atomic hydrogen. Provision is made to support and rotate the ball simultaneously either continuously, or in a programmed manner. It has proved possible to coat balls uniformly and reproducibly with good quality diamond fioatings, typically 3 ~tm thick. Raman measurements confirm that the coatings are indeed diamond, but with some graphite incorporation. The soft impressor method, first presented at ICNDST 5 (1996), has been uged to investigate the mechanical properties of the coating system. Results obtained, using both single loading and fatigue measurements, indicate that the films do protect the balls and failure of the system occurs as the result of substrate failure. © 1998 Elsevier Science S.A. Keywords: Diamond; Coating; Spheres; H F C V D
I. Introduction
2. Growth
The unique mechanical properties of diamond [1] make it an attractive material for wear resistant and chemically stable coatings, particularly since it has proved possible to deposit polycrystalline films on a variety of substrates under a wide range of conditions. There are several applications, e.g. in balls and ball valves, for which an ability to coat spherical surfaces becomes paramount. However, in order to grow films of uniform thickness, it is important to provide good temperature uniformity over the substrate area, a problem that becomes more acute when the substrate is spherical. Hot filament chemical vapour deposition (HFCVD) has been used extensively, in recent years, to deposit diamond films and has the advantages of simplicity and potential for scaling up. The present paper reports preliminary re:. ,Its on the H F C V D diamond coating of tungsten carbide (6% Co) balls and on the mechanical properties of the coating system. These properties have been evaluated by using the soft impressor technique [2-5].
2.1. Ewerhm,nt
* Correspo~ding author. 0925-9635/98/$19.00 ~.©1998 Elsevier Science B.V. All rights reserved. PI! S0925-9635( 97 )00205-7
It is well known [6] that the presence of Co has adverse effects on the deposition and adhesion of diamond films. Therefore, the balls are etched, for 10 min in 1:1 HNO3/HF, to remove the near surface Co and to provide voids that key the film to the substrate [7]. This process removes the Co to a depth of greater than 3 lam, as determined by EDX measurements. The balls are then abraded, prior to growth, with 0-2 I.tm (mesh size) synthetic diamond particles and cleaned, ultrasonically, in methanol. One of the key features of the present growth system is the method of supporting and rotating the ball on a tepee-like tripod of alumina rods (Fig. 1). Two of the rods are rotated independently (up to 200 rpm), whilst the third rod is free to idle; rotation of the rods can be either continuous or intermittent. The other key feature is the arrangement of the heater filaments, to provide uniform heating of the spherical substrate. These filaments also act as a source of atomic hydrogen. The temperature of the substrate can be varied by changi~g the radial position of the filaments, prior to a growth
130
B. Lutm et aL
Diamomt ami Rehtted hhtterials 7 ( 19981 129-132
Fig. !. Ball support and rotation system.
experiment. Temperature measurements are made with an optical pyrometer (Ircon SAI0+); the emissivity setting for the substrate material was previously determined by calibrating the pyrometer against the output of a thermocouple embedded in the centre of ~ test ball. The films are deposited from a 0.5% CH4/H2 gas mixture, at a substrate temperature of ---925 C . with the Ta filaments at 2200 :C (the filaments are carburized, with the same gas mixture, prior to growth). Tile total flow rate and the total gas pressure are 100 sccm and 20Torr (2.7 kPa), respectively. Mass flow controllers (MKS) are used to control the gas flows and the chamber pressure is controlled by pumping through a simple needle valve. These conditions yield an average growth rate of 0.15 ~tm/h. A range of rotation conditions has been explored. When the two rods are rotated continuously in the opposite sense, at the same speed, the ball itself rotates and precesses resulting in "'grooved" coatings. The coatings described in the present paper are produced using intermittent rotation; the rods are contra-rotated continuously, for the first 5 minutes of growth and thereafter, for one minute every half hour, at 30 rpm.
Fig. 2. SEM micrograph of bali, showingcoating coverage.
3. Results The films produced are unitbrm and continuous over the whole surface of the ball (Fig. 2). Growth appears to take place in two stages: examination of films 1-2 ~tm thick, shows that the growth is granular, whilst films 3 - 4 p m thick are faceted (Fig. 3). The black spots (Fig. 2) are not holes in the film, but areas that charge in the SEM to a lesser extent than the rest of the coating. We believe that these regions are contaminated by tl3e
Fig. 3. SEM micrograph of the surface morphology of a -,-3tam thick coating.
B. Lunn et al. / Diamondand Related Materials 7 (1998) 129-132
loosely adhering diamond film that forms on the alumina support rods during the run. Raman spectroscopy verifies that the films are indeed diamond, but with some non-diamond components. The uniformity and reproducibility of the growth is con.firmed by examining several regions of each ball.
4. Evaluation of the coating quality When conventional rigid indenters are used to evaluate hard ceramic coatings, the extent of deformation almost always leads to the propagation of cracks-usually radial cracks for pyramidal indenters or riag cracks for spherical indenters. The essential difference with the soft impressor technique is that it enables the measurement of a threshold mean contact pressure to initiate failure - ~,ither through the formation of a ring crack or an impression due to plastic deformation. This measurement can be achieved by using an impressor, initially in the form of a cone with an included angle of 120° and made from a softer material than the specimen, under a known normal load [8,9]. As the load is applied, the conical tip of the softer impressor deforms plastically, the cone becomes blunter until its flattened tip is large enough to support the applied load elastically. The mean pressure produced is then determined by dividing the normal load by the measured circular contact area. The soft impressor technique has three particularly useful advantages when it is used to evaluate the coated system. First, a high level of surface finish is unnecessary, as the blunted tip of the impressor will conform intimately to the surface topography of the specimen. Secondly, the mean contact pressure can be controlled, by using impressor materials over a range of hardness values, to be just sufficient to initiate failure - - i.e. a threshold mean pressure can be measured for each failure event. Thirdly, the depth of the deformed zone is approximately equal to the radius of the circular contact [5] and, consequently, adjusting the normal load for a given impressor allows the strain to be accommodated entirely within the coating, or transmitted to the interface or into the substrate material. In addition, cumulative fatigue processes can be studied using repeated load cycles on a given contact area [4,5,8]. Preliminary evaluation has been carried out, of uncoated tungsten carbide balls and of balls coated with ---3 lam diamond films, by measurements involving both single loading and multiple fatigue loading cycles.
4.1. Single load measurements In earlier (unpublished) work on the basal plane of a tungsten carbide single crystal, a threshold contact pressure of 8+0.3 GPa formed an impression by plastic
131
flow. A similar pressure produced a smooth plastic impression in a tungsten carbide ball, as supplied, but the same pressure caused disintegration and "crumbling" of the surface after it had been prepared for coating. To cause failure of the coated ball, it was necessary to apply a mean pressure o f approximately 16.0 + 0.3 GPa, via a cubic boron nitride impressor. This failure consisted of an impression, with a discontinuous ring crack at the periphery of the contact area. The crack was located below the coating and was probably caused by the onset of plastic deformation of the substrate. However, since the threshold mean pressure to initiate a ring crack in a single cryst d diamond is of the same order [5], it is unlikely that any further coating improvement could be made.
4.2. Fatigue loading Fatigue measurements [4, 5,10] on the diamond coated balls were made with an hardened silver steel impressor at a load of 39.2 N, giving an initial mean contact pressure of about 8.0 GPa - - i.e. half the pressure that caused the failure beneath the cubic boron nitride impressor after a single load cycle. No failure was observed after 20 000 load cycles but, after 40 000 cycles under the same conditions, delayed fracture (about 24 h after the test was completed) caused iocalised delamination [Fig. 4(a)]. It is interesting to observe that both the coating and the interface appear to have been stronger than the substrate material, in that the <:rack causing the delamination propagated entirely through the tungsten carbide. This was confirmed by EDX measurements on the substrate side of the delaminated film [ Fig. 4{ b)], which showed the presence of tungsten carbide, but not cobalt. Thus, again, the failure of the coated system was as the result of substrate failure. EDX measurements on the delaminated region of the substrate were just able to detect the presence of cobalt. This result can be explained by the fact that the average thickness of the substrate that delaminates is !.5 lain, hence allowing the electron beam to sample to a greater depth, relative to the original surface of the substrate. Delayed fracture of this type has been observed alter soft impressor fatigue testing of natural diamond crystals and tentatively attributed to a stress corrosion effect [5]. This phenomenon requires the co-existence of a stress, applied or residual and a structure that is unstable in a given corrosive atmosphere. Diamond is thermodynamically unstable at normal atmospheric pressures and temperatures and often there is a significant level of residual stress from both natural and synthetic growth processes. A major indicator of embrittlement, due to stress corrosion, is that, whilst the level of stress may be insufficient to propagate a crack initially, the delay in fracture is due to time dependent diffusion of the
132
B, Lunn et al. / Diamond ami Related Materials 7 (1998) 129-.132
well prove to have significant advantages over most competitor materials for applications requiring both erosion and corrosion resistance - - e.g. ball valves in sea water. However, it should be noted that the surface preparation of the substrate material, prior to coating, will have reduced the mechanical integrity of this region. Thus, it will be necessary to address the pre-growth treatment of the ball surface - - a well known phenomenon with tungsten carbide [11].
Acknowledgement The authors would like to thank De Beers Industrial Diamond Division Limited for a grant to the laboratory and EPSRC/ROPA for a grant and research assistantship to one of us (LYZ). They would also like to thank Prof. CA Brookes ( Univ. of Hull) for discussions on the soft impressor results and for his general support for this projec'.. Thanks also to Garry Robinson for the SEM micrographs.
References
(b)
Fig. 4. (a) SEM micrograph of a delaminated 3 lain thick coating The darker region is the substrate. (b) Substrate-side view of the lilm, ,~h~wn m I~) illu~tratfllg the t:oltt|lln~r n,dure of the Iihn.
relevant reactive element(s). Clearly, this ph?nomenon deserves further study, with respect to tungsten carbide.
5. Conclusions The results of the present work have demonstrated that it is possible to coat spheres with good quality diamond films. The initial soft impressor measurements indicatL: that the mechanical integrity of the diamond coated tungsten carbide balls is relatively good and may
[I ] J.E. Field (Ed.), The Properties of Natural and Synthetic Diamonds, Academic Press, London, 1992. [2] C.A. Brookes, E.J. Brookes, L.Y. Zhang, in: Proceedings of the 2nd International Conference on the Application of Diamond Films and Related Materials, Tokyo, 1993, p. 737. [3] C.A. Brooke,.. L.Y. Zhang, P.W. May, Diamond Relat. Mater. 6 (1997) 348. [4] C.P. Waddington, L.Y. Zhang, E.J. Brookes, C.A Brookes, Surlace Engng 13 (1997) 3. [31 I,.Y. Zhang, Phl) Thesis, University of Hull, Ilull, UK, 1997. [6] A.K. Mehlm,m, S. Berger, A. Fayer, S. Dirnlield, M. Bamberger, Y. Avigal, A. Hoffmann, R. Porath, Diamond Relat. Mater. 3 (1994) 805. [7] C. Tsai, J.C. Nelson, W.W. Gerberich, J. Heberlein, E. Pfender, Diamond Relat. Mater. 2 ( 1993' 617. [81 C.A. Brookes, P. Green, Nature 246(IO7..".) !19. [91 C.A Brookes, M.P. Shaw, Nature 263 (1976) 760. [10] C.A. Brookes, E.J. Brookes, L.Y. Zhalig, in: R.C. Bradt, C.A. Brookes, J.L. Routbort (Eds.), Plastic Deformation of Ceramics, Plenum Press, New York, 1995. [Ill M.B. Guseva, V.G. Babaev, V.V. Khvostov, G.M. Lopez Ludena, A.Yu. Bregadze, I.Y. Konyashin, A.E. Alexenko, Diamond Relat. Mater. 6 (1997) 89.