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Protective coatings of amorphic diamond on fragile and sensitive substrates
Su@we and Coatings Technology. 54/55 (1992)581-585
581
Protective coatings of amorphic diamond on fragile and sensitive substrates T. J. Lee, H. Park, J. H. You, F. D a v a n l o o a n d C. B. C o l l i n s * Centerfor Quantum Electronics, University of Texas at Dallas, PO Box 830688. Richardson. TX 75083-0688 (USA)
Abstract Amorphic diamond filmscan be grown with a laser plasma dischargesourcein an ultrahigh vacuumenvironmentwithoutthe use of any catalyst. This technique produces fihns that adhere more readily to substrates. Describedhere is a study of the bonding and properties realized in one such example, the deposition of amorphic diamond on germanium. Measurements with Rutherfordbackscattering spectrometryand transmission electronmicroscopyshowed that the diamondcoatingsdepositedfromlaser plasmas were bonded to tile germanium substrates through an interracial layer 200-300 A thick. Resistanceto wear was estimated with a modified sandblaster and it was shown that a coating of only 1.4 ~.tmof amorphicdiamond can increasethe lifetimeof samples by a factor of better than 12. Results of other mechanicalmeasurementssuch as hardness and frictiontests are also described.
1. Introduction Recent microstructural studies of amorphic diamond films showed them to be composed of hard, dense nodules. Grain sizes were on the order of 200-1000 A and the diamond character was attested by the agreement of morphology, high density, optical properties, soft X-ray spectroscopy and lack of appreciable hydrogen [-1]. A range of properties was found that correlated with the packing density of the diamond nodules. The better the packing, the closer the mechanical properties approached those of crystalline diamond. The principal conclusion was that material prepared with a laser plasma source had a structure which agreed closely with the theoretical predictions I-2.]. Amorphic diamond film may be the unique product of the laser ablation of carbon. It has been produced by the acceleration and quenching of a laser-produced plasma of C 3+ and C 4+ on a cold substrate [1, 3--5]. No other source has been reported for this material and to date the quenching of C + beams on a substrate has been shown to produce only i-C or defected graphite 1-1, 6]. The importance of this amorphic diamond material has been suggested by recent reports of its unique mechanical properties I-7, 8.]. In those studies a beam-bending method was used to measure the internal stress of nominally produced amorphic diamond film and a relatively low value of compressive stress was found. The dependence of stress on the laser intensity at the graphite ablation target indicated *Author to whom correspondenceshould be addressed.
0257-8972/92/$5.00
that the stress was largely due to the graphitic content and that it could be reduced further by increasing the sp 3 fraction of the films. Analyses of the interfaces of amorphic diamond film on several substrates showed significant interracial layers I-7, 8]. This was encouraged by the highly localized but intense levels of energy density created by impact of the C 3+ and C 4+ ions. The mechanical properties of hardness, Young's modulus and stiffness were obtained and it was shown that a coating of laser plasma diamond can protect substrates and increase the lifetime against abrasive wear from particulate or rain impacts [7, 9]. Amorphic diamond has the unique properties of being hard but resilient, extremely resistant to chemical attacks and transparent in the IR region of the spectrum. These properties, together with the room temperature growth environment, make this material suitable for abrasion and corrosion protection and for the antireflective coating of sensitive and fragile optical materials. In this paper we report details of the adhesion and mechanical properties of amorphic diamond deposited on germanium substrates. Analyses of the interface by Rutherford-backscattering spectrometry (RBS) and transmission electron microscopy (TEM) indicated significant intermixing of the substrate and the coating. Encouraged by the high kinetic energies of the laser plasma ions, these enhanced diffusions of substrate and coating were concluded to be the cause of the good adherence between the amorphic diamond films and the germanium substrates. The results of this study showed that both the mechanical performances and the lifetimes against abrasion were strongly enhanced by the bonding
~_~1992- ElsevierSequoia.All rights reserved
582
T, J. Lee et al. / Protective coatings of amorphic diamond
of a few microns' thickness of amorphic diamond to g e r m a n i u m surfaces.
2. Deposition method The preparation of amorphic diamond by a laser plasma discharge has been realized with a Q-switched N d : Y A G laser [3, 4]. Figure 1 shows a schematic representation of the deposition system reported earlier and reproduced here for convenience. In our deposition system the laser delivers 250-1400 m] to a graphite feed stock in an ultrahigh vacuum system at a repetition rate of 10 Hz. For the production of films with nominal qualities, the beam is focused to a diameter chosen to keep the intensity on the target near 5 x 10 t~ W cm -2 a n d the graphite target is moved so that each ablation occurs from a new surface. A high current discharge confined to the path of the laser-ignited plasma is used to heat and process the ion flux further. Discharge current densities typically reach 106 A c m - 2 through the area of the laser focus. As seen in Fig. 1, a planetary drive system for rotating substrates within the core of the plasma, where they are exposed only to ions, ensures the simultaneous deposition of uniform layers of amorphic diamond over several substrate disks 30 mm in diameter. The combination of laser ablation and confined electric discharge has been shown to produce unhydrogenated diamond films with unique nodular structure and with more diamond-like properties than achieved with a laser ablation source of traditional design [1-1. With this system we have produced over 2000 different films of thicknesses varying fror0 0.1 to 6.0 pm. Films arc grown over areas of 100 cm 2 on a variety of substrates, including fragile IR substrates such as germanium and ZnS.
UHVEhamber
5ubslrate ...... Planelary Drive Discharge ...... Electrode
....
The hardness and Young's modulus of amorphic diamond film on a silicon substrate have been measured by a nano-indenter system at Oak Ridge National Laboratory [7]. The average values of hardness and Young's modulus from 20 separate naeasurements were 37 G P a and 369 G P a respectively. This seemed to be consistent with a composition o f 75% d i a m o n d reported for nominally produced a m o r p h i c d i a m o n d films [ I ] . Independent hardness measurements were made at Harwell Laboratory on an amorphic diamond film 0.97 I,tm thick deposited on a polished germanium singlecrystal substrata. The hardnesses of both coating and substrate were measured by a Vickers indenter and results are shown in Fig. 2. To obtain the true coating material hardness, a computer-modeling code was used to account for the indentation size effect and to deconvolute the coating hardness from that of the substrata [I0, I l l . Values for Vickers hardness in the range 3500-4000 k g f m m -2 were extracted. These values were consistent and agreed well with hanD-indentation measurements [7]. Scratch testing with loads varying between 100 gf and 5 kgf was performed at Harwell L a b o r a t o r y as well on the same sample of amorphic d i a m o n d on germanium. No evidence of adhesive failure was detected a n d the coefficient of friction remained relatively constant with load until the substrate was crushed at around 40 GPa.
4. lnterfacial layer formation The protective ability of any thin film depends upon the strength of the bond joining the film to the substrata material. In "bulk", amorphic d i a m o n d is in compressive stress to an extent that correlates with the a m o u n t of
2500
.............High Refleclien ........ Turning Mirror ....
.....'
Focusing ...... Optics
3. Hardness and friction tests
,
_]
................Window
o 6e
r
.g
. ~ oa .
qJ
1000 500 0
Plasma ...... Plume '""i
............. Rotatable 5raphile Target
Fig, I. Schematic representation of the laser plasma discharge source used to deposit thin film amorphic diamond.
9 Coating /
1500
H lASER
C 3+ and C 4+ ions ............. ol 1 KeY
2000
Substrate
t 100
t 200
1 300
l 400
I 500
600
Loud (gins.) Fig. 2. Vickers hardness as a function of load for substrate and coating-substrate composite. (Measurements courtesy of A. M. Jones, C. J. Bedell and G. Dearnaley of Harwell Laboratory, UK.)
T. J. Lee et al. / Protectice coatings o f amorphic diamond
sp a content between nodules [7]. The effect of compressive stress in a tilm is to generate a force tending to "pop" it off the substrate to which it is applied. To produce a successful coating, a greater opposing force must be provided to hold the film on the substrate. Our method of preparing amorphic diamond is capable of producing such a force by naturally forming an interlayer between the film and ahnost any substrate. Recently we reported [7, 8] that the impact of C 3+ and C 4+ ions carrying kiloelectronvolt energies from the laser plasma did form interracial layers of disturbed compositions of 100-300 ~ thickness on several materials. RBS was an important diagnostic tool in that work and a profile of elemental composition vs. depth below the original surface could be obtained [7, 8]. Figure 3 shows comparable data for an amorphic diamond film deposited on a germanium substrate. While an effective interlayer of about 2.5 x 10 t7 cm -2 can be readily seen, the mixing of the C and Ge atoms has continued to much greater depths. Figure 4 shows a tapered cross-section through an interface on germanium obtained by thinning a sample from the back side through the substrate and then imaging it with TEM. Although the sample suffered
583
some distortion during the thinning, the interfacial layer is clearly apparent. This seems to be in good agreement with what was inferred from RBS measurements. Our earlier studies of interlayer formation of amorphic diamond on several substrates such as silicon and titanium have shown that the adhesion is strongly promoted by chemical bonding and atomic interdiffusion [7, 8]. Formation of carbide has been confirmed and microcrystals of TiC precipitated in the interracial layer have been identified. However, in the case of germanium the situation may be different. No germanium carbide compound is found in Ge-C binary alloys [-12]. Recently the existence of metastable compounds has been reported at the a-C: H - G e interface [13-1. This was attributed to the impingement of C,~H,, ions with energies in the range 200-400 eV. The interlayer thicknesses formed were much smaller than the intermixing observed in the case of amorphic diamond as shown in Figs. 3 and 4. The fact that amorphic diamond was condensed from plasmas of multiply charged ions carrying kiloelectronvolts of kinetic energies, together with superior adhesion to the germanium substrates, seemed to indicate that metastable phases were formed in the boundaries of amorphic diamond-germanium as well.
Energy ( MeV ) Z.O
I0
5. Protective coatings of amorphie diamond on germanium substrates
7_5
ZO
1oo ~0 60 40 ZO 0
200
250
350 Chonnel
300
400
450
i
i
-... i
r
200 400 600 000 fO00 Oepth Below Diamond L o v e r ( xfO 15 cm -2 )
Fig. 3. (Left) RBS measurements of amorphic diamond film on germanium substrate. The solid curve plots the computer-generated fit obtained by considering interracial Ge-C layers. The edges in the data correspond to the different elements as indicated. (Right) Plot of the concentration of the C atom component as a I'unction of depth into the interracial layer used to generate the solid curve fit to the RBS data. (Measurements courtesy of J. C. Pivin of CNRS Laboratory, Orsay.)
Fig. 4. Tapered cross-section through an interracial layer thinned and then imaged by TEM.
It can be seen from the results of previous sections that amorphic diamond is only half as hard as natural diamond. However, there is another aspect of importance in mechanical applications in which it is better. Natural diamond, while hard, is also very brittle and so is not particularly resistant to the impact of high speed particles. The problem is that a rigid coating with a high Young's modulus of elasticity (YME) will pass the shock of particulate impact through to the substrate largely unattenuated. To dissipate such shocks, protective coatings with lower YME are necessary. Clearly, amorphic diamond should be much better than natural diamond in its ability to dissipate the shock of impact. Recent reports [7, 8] have described the use of a convenient system to quantitatively estimate the increase in lifetime against particulate erosion of silicon and titanium afforded by different thicknesses of amorphic diamond films. In that work a jet of compressed gas carried glass beads 20-40 I-tin in diameter thro~lgh a venturi to impact upon a l cm z target region. Test samples were prepared from films of amorphic diamond a few microns thick which were chemically bonded to silicon and titanium substrates. It was found that the increase in lifetime against a given level of erosion could be expressed by a factor F: F(Si) oc (35)', F(Yi)cc (8.7)"
(1)
T. d. Lee et al. / Protective coatings of amorphic diamond
584
where m (I.Lm) is the thickness of the amorphic diamond film. It is difficult to quantify the resistance to damage imparted by a thin film coating. In this work we again [7, 8] used 1og[(I-Io)/I~], where I is the scattered laser intensity from the damaged film, Io is the same quantity measured before damage and I~ is the intensity from a standard scattering object for calibration. Of the possible angles of incidence, 20 ~ has been found to work best. At this low angle of incidence the scattering from the rims of the pits chipped by impact of the larger particles provided a better contrast between damaged and undamaged regions of film. Figure 5 shows a plot o f the relative scattering from a bare germanium substrate and from one with 1.4 lain of amorphic diamond as a function of the time of exposure to a flux of 20-40 I.tm glass beads driven from the venturi at 40 Ibf in-2. Differences in the details of the damage mechanisms between uncoated germanium and amorphic diamond-germanium give different slopes and forms to the curves, making it difficult to extract a single number for improvement. However, from Fig. 5 it can be seen that an empirical model in the form of eqn. (1), namely F(Ge) cc (6.1)"'
(2)
provides a reasonable if conservative fit for low levels of damage. Shown on the horizontal bar in Fig. 5 are
1.0
Ge t"
0.1
f
/
001
0 I
14/
/
../'
r/
Amorphlc Dlomond
~6.1) m
X---~-,~-- - - I " ' ' /
1
I
I
10
100
6. Conclusions T h e a d h e s i o n a n d m e c h a n i c a l p r o p e r t i e s of a m o r p l d i a m o n d films o n g e r m a n i u m s u b s t r a t e s h a v e be e x a m i n e d . T h e k i l o e l e c t r o n v o l t energies c a r r i e d b y t ions e n c o u r a g e the f o r m a t i o n of a n i n t e r r a c i a l la3 where G e a n d C a t o m s a r e mixed. T h e s i g n i f i c a n t le' of interdiffusion clearly d e m o n s t r a t e s the r e a c t i v e n a t t o f the a m o r p h i c d i a m o n d - g e r m a n i u m i n t e r f a c e a n d tl seems to e x p l a i n the g o o d a d h e r e n c e b e t w e e n films a g e r m a n i u m substrates. C o n s i d e r a b l e r e s i s t a n c e to t c h i p p i n g of high speed particles with d i a m e t e r s lar~ t h a n the thicknesses of t h e films has b e e n d e m o n s t r a t and it is s h o w n t h a t a c o a t i n g of a m o r p h i c d i a m o can p r o t e c t s u b s t r a t e s a n d increase t h e lifetime agai~ a b r a s i v e w e a r from p a r t i c u l a t e i m p a c t .
Acknowledgments
ix /'
Ix
the intercepts calculated from eqn. (2) at a modest lc of damage of about 10% of the m a x i m u m observ, Lower levels of scattering seem to s u p p o r t the moc but the data there are less reliable. Since catastropl failure occurs more rapidly once the coating is broach in a significant number of places, the curves tend converge at the highest levels of scattering and eqn. becomes inapplicable. However, it is the onset of damr that is usually of greatest interest in practical appli~ tions and there eqn. (2) should be most reliable. Un( the highly erosive conditions of this experiment a coati of amorphic diamond 1.4 p.m thick increased the lifetil of germanium samples by a factor of greater than 12
I
I
1000 10000
TIME ( sec ) Fig. 5. Plot of the damage observed on uncoated and coated germanium samples as a function of the time for which they were exposed to the high velocity impact of a flux of glass beads 20-40 gm in diameter. Damage is plotted in units of the ratio of the relative scattering of laser radiation incident at 20~ to the surface. Data describe measurements on a sample of 1.4 gm of amorphic diamond bonded to a germanium substrate in comparison with a similar uncoated substrate. Dotted curves guide the eye between data for the same coating thickness.
T h e a u t h o r s gratefully a c k n o w l e d g e t h e c o n t r i b u t i o o f J . C. Pivin of the C N R S L a b o r a t o r y at O r s a y , F r a n , for his m e a s u r e m e n t s a n d s i m u l a t i o n s o f t h e RBS spect a n d for the T E M e x a m i n a t i o n s , a n d o f A. M. Jon, C. J. Bedell and G. D e a r n a l e y of A E A Technolo~ Harwell, U K, for the d e t e r m i n a t i o n o f the Vickc h a r d n e s s a n d friction coefficient. We e x p r e s s o u r a p p r e , a t i o n to R. K. K r a u s e for a r r a n g i n g t h e depositi~ system. This w o r k was s u p p o r t e d by R e s e a r c h A p p l i c tions, Inc. a n d by the Texas A d v a n c e d T e c h n o l o P r o g r a m u n d e r G r a n t N o . 9741-011.
References I C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, H. Park a J. H. You, J. Appl. Phys., 19 ([991) 7862. 2 J. C. Angus, Y. Wang and R. W. Hoffman, in R. Messier, J. Glass, J. E. Butler and R. Roy (eds.), New Diamond Science c, Technology, MRS Proc. ICNDST-2 CotE, Washington DC, 19. MRS, Pittsburgh, PA, 1991, p. I I.
T. J. Lee et al. [ Protec~ioe coatings of amorphic diamond 3 C. B. Collins, F. Davanloo, E. M. Juengerman, W. R. Osborn and D. R. Jander, AppL Phys. Lett.. 54 (1989) 216. 4 F. Davanloo, E. M. Juengerman, D. R. Jander, T. J. Lee and C. B. Collins, J. Appl. Phys., 67 ([990) 2081. 5 J. Stevefelt and C. B. Collins, J. Phys. D." Appl. Phys., 24 (1991) 2149. r C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, J. H. You, H. Park, J. C. Pivin, K, Glejbol and A. R. Th61~n, ./. Appl. Phys., 72 (1992) 239. 7 F. Davanloo, T. J. Lee, D. R. Jander, H. Park, J. H. You and C. B. Collins, J. Appl. Phys.. 71 (1992) I446. 8 C. B. Collins, F. Davanloo, T. J. Lee, D. R. Jander, J. H. You, H. Park and J. C. Pivin, J. AppL Phys., 71 (1992) 3260.
585
9 C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, J. H. You and H. Park, Diamond Fihns Technol., in the press. 10 S, J. Bull and D. S. Riekerby, Surf. Coat. Technol., 42 (1990) 149. 11 A. M. Jones, C. B. Bedell, G. Dearnaley, C. B. Collins and F. Davanloo, in Y. Tzeng, M. Yoskikawa, M. Murakawa and A. Feldman (eds.), Application of Diamond Films and Related Materials, Material Science Monographs, Vol. 73, Elsevier, Amsterdam, 1991, p. 749. 12 W. G. Moffatt, The Handbook of Binary' Phase Diagrams, General Electric Co., Schenectady, NY, 1986. 13 M. Wittmer, D. Ugolini and P. Oelhafen, Y. Electrochem. Soc., 137 (1990) 1256.
581
Protective coatings of amorphic diamond on fragile and sensitive substrates T. J. Lee, H. Park, J. H. You, F. D a v a n l o o a n d C. B. C o l l i n s * Centerfor Quantum Electronics, University of Texas at Dallas, PO Box 830688. Richardson. TX 75083-0688 (USA)
Abstract Amorphic diamond filmscan be grown with a laser plasma dischargesourcein an ultrahigh vacuumenvironmentwithoutthe use of any catalyst. This technique produces fihns that adhere more readily to substrates. Describedhere is a study of the bonding and properties realized in one such example, the deposition of amorphic diamond on germanium. Measurements with Rutherfordbackscattering spectrometryand transmission electronmicroscopyshowed that the diamondcoatingsdepositedfromlaser plasmas were bonded to tile germanium substrates through an interracial layer 200-300 A thick. Resistanceto wear was estimated with a modified sandblaster and it was shown that a coating of only 1.4 ~.tmof amorphicdiamond can increasethe lifetimeof samples by a factor of better than 12. Results of other mechanicalmeasurementssuch as hardness and frictiontests are also described.
1. Introduction Recent microstructural studies of amorphic diamond films showed them to be composed of hard, dense nodules. Grain sizes were on the order of 200-1000 A and the diamond character was attested by the agreement of morphology, high density, optical properties, soft X-ray spectroscopy and lack of appreciable hydrogen [-1]. A range of properties was found that correlated with the packing density of the diamond nodules. The better the packing, the closer the mechanical properties approached those of crystalline diamond. The principal conclusion was that material prepared with a laser plasma source had a structure which agreed closely with the theoretical predictions I-2.]. Amorphic diamond film may be the unique product of the laser ablation of carbon. It has been produced by the acceleration and quenching of a laser-produced plasma of C 3+ and C 4+ on a cold substrate [1, 3--5]. No other source has been reported for this material and to date the quenching of C + beams on a substrate has been shown to produce only i-C or defected graphite 1-1, 6]. The importance of this amorphic diamond material has been suggested by recent reports of its unique mechanical properties I-7, 8.]. In those studies a beam-bending method was used to measure the internal stress of nominally produced amorphic diamond film and a relatively low value of compressive stress was found. The dependence of stress on the laser intensity at the graphite ablation target indicated *Author to whom correspondenceshould be addressed.
0257-8972/92/$5.00
that the stress was largely due to the graphitic content and that it could be reduced further by increasing the sp 3 fraction of the films. Analyses of the interfaces of amorphic diamond film on several substrates showed significant interracial layers I-7, 8]. This was encouraged by the highly localized but intense levels of energy density created by impact of the C 3+ and C 4+ ions. The mechanical properties of hardness, Young's modulus and stiffness were obtained and it was shown that a coating of laser plasma diamond can protect substrates and increase the lifetime against abrasive wear from particulate or rain impacts [7, 9]. Amorphic diamond has the unique properties of being hard but resilient, extremely resistant to chemical attacks and transparent in the IR region of the spectrum. These properties, together with the room temperature growth environment, make this material suitable for abrasion and corrosion protection and for the antireflective coating of sensitive and fragile optical materials. In this paper we report details of the adhesion and mechanical properties of amorphic diamond deposited on germanium substrates. Analyses of the interface by Rutherford-backscattering spectrometry (RBS) and transmission electron microscopy (TEM) indicated significant intermixing of the substrate and the coating. Encouraged by the high kinetic energies of the laser plasma ions, these enhanced diffusions of substrate and coating were concluded to be the cause of the good adherence between the amorphic diamond films and the germanium substrates. The results of this study showed that both the mechanical performances and the lifetimes against abrasion were strongly enhanced by the bonding
~_~1992- ElsevierSequoia.All rights reserved
582
T, J. Lee et al. / Protective coatings of amorphic diamond
of a few microns' thickness of amorphic diamond to g e r m a n i u m surfaces.
2. Deposition method The preparation of amorphic diamond by a laser plasma discharge has been realized with a Q-switched N d : Y A G laser [3, 4]. Figure 1 shows a schematic representation of the deposition system reported earlier and reproduced here for convenience. In our deposition system the laser delivers 250-1400 m] to a graphite feed stock in an ultrahigh vacuum system at a repetition rate of 10 Hz. For the production of films with nominal qualities, the beam is focused to a diameter chosen to keep the intensity on the target near 5 x 10 t~ W cm -2 a n d the graphite target is moved so that each ablation occurs from a new surface. A high current discharge confined to the path of the laser-ignited plasma is used to heat and process the ion flux further. Discharge current densities typically reach 106 A c m - 2 through the area of the laser focus. As seen in Fig. 1, a planetary drive system for rotating substrates within the core of the plasma, where they are exposed only to ions, ensures the simultaneous deposition of uniform layers of amorphic diamond over several substrate disks 30 mm in diameter. The combination of laser ablation and confined electric discharge has been shown to produce unhydrogenated diamond films with unique nodular structure and with more diamond-like properties than achieved with a laser ablation source of traditional design [1-1. With this system we have produced over 2000 different films of thicknesses varying fror0 0.1 to 6.0 pm. Films arc grown over areas of 100 cm 2 on a variety of substrates, including fragile IR substrates such as germanium and ZnS.
UHVEhamber
5ubslrate ...... Planelary Drive Discharge ...... Electrode
....
The hardness and Young's modulus of amorphic diamond film on a silicon substrate have been measured by a nano-indenter system at Oak Ridge National Laboratory [7]. The average values of hardness and Young's modulus from 20 separate naeasurements were 37 G P a and 369 G P a respectively. This seemed to be consistent with a composition o f 75% d i a m o n d reported for nominally produced a m o r p h i c d i a m o n d films [ I ] . Independent hardness measurements were made at Harwell Laboratory on an amorphic diamond film 0.97 I,tm thick deposited on a polished germanium singlecrystal substrata. The hardnesses of both coating and substrate were measured by a Vickers indenter and results are shown in Fig. 2. To obtain the true coating material hardness, a computer-modeling code was used to account for the indentation size effect and to deconvolute the coating hardness from that of the substrata [I0, I l l . Values for Vickers hardness in the range 3500-4000 k g f m m -2 were extracted. These values were consistent and agreed well with hanD-indentation measurements [7]. Scratch testing with loads varying between 100 gf and 5 kgf was performed at Harwell L a b o r a t o r y as well on the same sample of amorphic d i a m o n d on germanium. No evidence of adhesive failure was detected a n d the coefficient of friction remained relatively constant with load until the substrate was crushed at around 40 GPa.
4. lnterfacial layer formation The protective ability of any thin film depends upon the strength of the bond joining the film to the substrata material. In "bulk", amorphic d i a m o n d is in compressive stress to an extent that correlates with the a m o u n t of
2500
.............High Refleclien ........ Turning Mirror ....
.....'
Focusing ...... Optics
3. Hardness and friction tests
,
_]
................Window
o 6e
r
.g
. ~ oa .
qJ
1000 500 0
Plasma ...... Plume '""i
............. Rotatable 5raphile Target
Fig, I. Schematic representation of the laser plasma discharge source used to deposit thin film amorphic diamond.
9 Coating /
1500
H lASER
C 3+ and C 4+ ions ............. ol 1 KeY
2000
Substrate
t 100
t 200
1 300
l 400
I 500
600
Loud (gins.) Fig. 2. Vickers hardness as a function of load for substrate and coating-substrate composite. (Measurements courtesy of A. M. Jones, C. J. Bedell and G. Dearnaley of Harwell Laboratory, UK.)
T. J. Lee et al. / Protectice coatings o f amorphic diamond
sp a content between nodules [7]. The effect of compressive stress in a tilm is to generate a force tending to "pop" it off the substrate to which it is applied. To produce a successful coating, a greater opposing force must be provided to hold the film on the substrate. Our method of preparing amorphic diamond is capable of producing such a force by naturally forming an interlayer between the film and ahnost any substrate. Recently we reported [7, 8] that the impact of C 3+ and C 4+ ions carrying kiloelectronvolt energies from the laser plasma did form interracial layers of disturbed compositions of 100-300 ~ thickness on several materials. RBS was an important diagnostic tool in that work and a profile of elemental composition vs. depth below the original surface could be obtained [7, 8]. Figure 3 shows comparable data for an amorphic diamond film deposited on a germanium substrate. While an effective interlayer of about 2.5 x 10 t7 cm -2 can be readily seen, the mixing of the C and Ge atoms has continued to much greater depths. Figure 4 shows a tapered cross-section through an interface on germanium obtained by thinning a sample from the back side through the substrate and then imaging it with TEM. Although the sample suffered
583
some distortion during the thinning, the interfacial layer is clearly apparent. This seems to be in good agreement with what was inferred from RBS measurements. Our earlier studies of interlayer formation of amorphic diamond on several substrates such as silicon and titanium have shown that the adhesion is strongly promoted by chemical bonding and atomic interdiffusion [7, 8]. Formation of carbide has been confirmed and microcrystals of TiC precipitated in the interracial layer have been identified. However, in the case of germanium the situation may be different. No germanium carbide compound is found in Ge-C binary alloys [-12]. Recently the existence of metastable compounds has been reported at the a-C: H - G e interface [13-1. This was attributed to the impingement of C,~H,, ions with energies in the range 200-400 eV. The interlayer thicknesses formed were much smaller than the intermixing observed in the case of amorphic diamond as shown in Figs. 3 and 4. The fact that amorphic diamond was condensed from plasmas of multiply charged ions carrying kiloelectronvolts of kinetic energies, together with superior adhesion to the germanium substrates, seemed to indicate that metastable phases were formed in the boundaries of amorphic diamond-germanium as well.
Energy ( MeV ) Z.O
I0
5. Protective coatings of amorphie diamond on germanium substrates
7_5
ZO
1oo ~0 60 40 ZO 0
200
250
350 Chonnel
300
400
450
i
i
-... i
r
200 400 600 000 fO00 Oepth Below Diamond L o v e r ( xfO 15 cm -2 )
Fig. 3. (Left) RBS measurements of amorphic diamond film on germanium substrate. The solid curve plots the computer-generated fit obtained by considering interracial Ge-C layers. The edges in the data correspond to the different elements as indicated. (Right) Plot of the concentration of the C atom component as a I'unction of depth into the interracial layer used to generate the solid curve fit to the RBS data. (Measurements courtesy of J. C. Pivin of CNRS Laboratory, Orsay.)
Fig. 4. Tapered cross-section through an interracial layer thinned and then imaged by TEM.
It can be seen from the results of previous sections that amorphic diamond is only half as hard as natural diamond. However, there is another aspect of importance in mechanical applications in which it is better. Natural diamond, while hard, is also very brittle and so is not particularly resistant to the impact of high speed particles. The problem is that a rigid coating with a high Young's modulus of elasticity (YME) will pass the shock of particulate impact through to the substrate largely unattenuated. To dissipate such shocks, protective coatings with lower YME are necessary. Clearly, amorphic diamond should be much better than natural diamond in its ability to dissipate the shock of impact. Recent reports [7, 8] have described the use of a convenient system to quantitatively estimate the increase in lifetime against particulate erosion of silicon and titanium afforded by different thicknesses of amorphic diamond films. In that work a jet of compressed gas carried glass beads 20-40 I-tin in diameter thro~lgh a venturi to impact upon a l cm z target region. Test samples were prepared from films of amorphic diamond a few microns thick which were chemically bonded to silicon and titanium substrates. It was found that the increase in lifetime against a given level of erosion could be expressed by a factor F: F(Si) oc (35)', F(Yi)cc (8.7)"
(1)
T. d. Lee et al. / Protective coatings of amorphic diamond
584
where m (I.Lm) is the thickness of the amorphic diamond film. It is difficult to quantify the resistance to damage imparted by a thin film coating. In this work we again [7, 8] used 1og[(I-Io)/I~], where I is the scattered laser intensity from the damaged film, Io is the same quantity measured before damage and I~ is the intensity from a standard scattering object for calibration. Of the possible angles of incidence, 20 ~ has been found to work best. At this low angle of incidence the scattering from the rims of the pits chipped by impact of the larger particles provided a better contrast between damaged and undamaged regions of film. Figure 5 shows a plot o f the relative scattering from a bare germanium substrate and from one with 1.4 lain of amorphic diamond as a function of the time of exposure to a flux of 20-40 I.tm glass beads driven from the venturi at 40 Ibf in-2. Differences in the details of the damage mechanisms between uncoated germanium and amorphic diamond-germanium give different slopes and forms to the curves, making it difficult to extract a single number for improvement. However, from Fig. 5 it can be seen that an empirical model in the form of eqn. (1), namely F(Ge) cc (6.1)"'
(2)
provides a reasonable if conservative fit for low levels of damage. Shown on the horizontal bar in Fig. 5 are
1.0
Ge t"
0.1
f
/
001
0 I
14/
/
../'
r/
Amorphlc Dlomond
~6.1) m
X---~-,~-- - - I " ' ' /
1
I
I
10
100
6. Conclusions T h e a d h e s i o n a n d m e c h a n i c a l p r o p e r t i e s of a m o r p l d i a m o n d films o n g e r m a n i u m s u b s t r a t e s h a v e be e x a m i n e d . T h e k i l o e l e c t r o n v o l t energies c a r r i e d b y t ions e n c o u r a g e the f o r m a t i o n of a n i n t e r r a c i a l la3 where G e a n d C a t o m s a r e mixed. T h e s i g n i f i c a n t le' of interdiffusion clearly d e m o n s t r a t e s the r e a c t i v e n a t t o f the a m o r p h i c d i a m o n d - g e r m a n i u m i n t e r f a c e a n d tl seems to e x p l a i n the g o o d a d h e r e n c e b e t w e e n films a g e r m a n i u m substrates. C o n s i d e r a b l e r e s i s t a n c e to t c h i p p i n g of high speed particles with d i a m e t e r s lar~ t h a n the thicknesses of t h e films has b e e n d e m o n s t r a t and it is s h o w n t h a t a c o a t i n g of a m o r p h i c d i a m o can p r o t e c t s u b s t r a t e s a n d increase t h e lifetime agai~ a b r a s i v e w e a r from p a r t i c u l a t e i m p a c t .
Acknowledgments
ix /'
Ix
the intercepts calculated from eqn. (2) at a modest lc of damage of about 10% of the m a x i m u m observ, Lower levels of scattering seem to s u p p o r t the moc but the data there are less reliable. Since catastropl failure occurs more rapidly once the coating is broach in a significant number of places, the curves tend converge at the highest levels of scattering and eqn. becomes inapplicable. However, it is the onset of damr that is usually of greatest interest in practical appli~ tions and there eqn. (2) should be most reliable. Un( the highly erosive conditions of this experiment a coati of amorphic diamond 1.4 p.m thick increased the lifetil of germanium samples by a factor of greater than 12
I
I
1000 10000
TIME ( sec ) Fig. 5. Plot of the damage observed on uncoated and coated germanium samples as a function of the time for which they were exposed to the high velocity impact of a flux of glass beads 20-40 gm in diameter. Damage is plotted in units of the ratio of the relative scattering of laser radiation incident at 20~ to the surface. Data describe measurements on a sample of 1.4 gm of amorphic diamond bonded to a germanium substrate in comparison with a similar uncoated substrate. Dotted curves guide the eye between data for the same coating thickness.
T h e a u t h o r s gratefully a c k n o w l e d g e t h e c o n t r i b u t i o o f J . C. Pivin of the C N R S L a b o r a t o r y at O r s a y , F r a n , for his m e a s u r e m e n t s a n d s i m u l a t i o n s o f t h e RBS spect a n d for the T E M e x a m i n a t i o n s , a n d o f A. M. Jon, C. J. Bedell and G. D e a r n a l e y of A E A Technolo~ Harwell, U K, for the d e t e r m i n a t i o n o f the Vickc h a r d n e s s a n d friction coefficient. We e x p r e s s o u r a p p r e , a t i o n to R. K. K r a u s e for a r r a n g i n g t h e depositi~ system. This w o r k was s u p p o r t e d by R e s e a r c h A p p l i c tions, Inc. a n d by the Texas A d v a n c e d T e c h n o l o P r o g r a m u n d e r G r a n t N o . 9741-011.
References I C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, H. Park a J. H. You, J. Appl. Phys., 19 ([991) 7862. 2 J. C. Angus, Y. Wang and R. W. Hoffman, in R. Messier, J. Glass, J. E. Butler and R. Roy (eds.), New Diamond Science c, Technology, MRS Proc. ICNDST-2 CotE, Washington DC, 19. MRS, Pittsburgh, PA, 1991, p. I I.
T. J. Lee et al. [ Protec~ioe coatings of amorphic diamond 3 C. B. Collins, F. Davanloo, E. M. Juengerman, W. R. Osborn and D. R. Jander, AppL Phys. Lett.. 54 (1989) 216. 4 F. Davanloo, E. M. Juengerman, D. R. Jander, T. J. Lee and C. B. Collins, J. Appl. Phys., 67 ([990) 2081. 5 J. Stevefelt and C. B. Collins, J. Phys. D." Appl. Phys., 24 (1991) 2149. r C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, J. H. You, H. Park, J. C. Pivin, K, Glejbol and A. R. Th61~n, ./. Appl. Phys., 72 (1992) 239. 7 F. Davanloo, T. J. Lee, D. R. Jander, H. Park, J. H. You and C. B. Collins, J. Appl. Phys.. 71 (1992) I446. 8 C. B. Collins, F. Davanloo, T. J. Lee, D. R. Jander, J. H. You, H. Park and J. C. Pivin, J. AppL Phys., 71 (1992) 3260.
585
9 C. B. Collins, F. Davanloo, D. R. Jander, T. J. Lee, J. H. You and H. Park, Diamond Fihns Technol., in the press. 10 S, J. Bull and D. S. Riekerby, Surf. Coat. Technol., 42 (1990) 149. 11 A. M. Jones, C. B. Bedell, G. Dearnaley, C. B. Collins and F. Davanloo, in Y. Tzeng, M. Yoskikawa, M. Murakawa and A. Feldman (eds.), Application of Diamond Films and Related Materials, Material Science Monographs, Vol. 73, Elsevier, Amsterdam, 1991, p. 749. 12 W. G. Moffatt, The Handbook of Binary' Phase Diagrams, General Electric Co., Schenectady, NY, 1986. 13 M. Wittmer, D. Ugolini and P. Oelhafen, Y. Electrochem. Soc., 137 (1990) 1256.