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The effects of substrate compositions on adhesion of diamond films deposited on Fe-base alloys
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Surface & Coatings Technology 202 (2007) 280 – 287 www.elsevier.com/locate/surfcoat
The effects of substrate compositions on adhesion of diamond films deposited on Fe-base alloys Y.S. Li ⁎, A. Hirose Plasma Physics Laboratory, University of Saskatchewan, 116 Science Place Saskatoon, SK, Canada S7N 5E2 Received 1 December 2006; accepted in revised form 13 May 2007 Available online 24 May 2007
Abstract Diamond deposition on a series of ferrous alloy substrates has been examined in a microwave plasma enhanced chemical vapor deposition reactor. The results show that the nucleation, growth, and adhesion properties of the diamond are strongly dependent on the types and relative concentrations of the alloy elements in the substrates. Along with high nucleation densities, continuous and adherent diamond films have been successfully fabricated on the Al-modified alloy substrates, even in the absence of inter-layers or nucleation enhancing surface pre-treatment. The mechanism is preliminarily clarified regarding the delicate competition and balance between the base metal and the alloying elements in terms of their activation and deactivation ability on catalyzing graphite formation during deposition process. © 2007 Elsevier B.V. All rights reserved. Keywords: Alloy; Diamond; Plasma CVD; Adhesion; Raman scattering spectroscopy
1. Introduction Alloy steels are among the most commonly used and costeffective structural materials in modern industry and their properties can be widely adjusted by designing the initial composition and controlling the microstructure through thermomechanical treatment. However, when steel is used as critical components in harsh (wear, corrosive and erosive) environments, accelerated damage usually occurs. As the early failure is usually initiated at the outer-most surface, it is extremely important to obtain strengthened high performance surface for longevity of service. Diamond films are characterized by super hardness, high thermal conductivity, high chemical inertness and remarkable wear resistance, which make them one of the most promising coating materials as heat sink, optical devices, electronic devices and especially wear resistant coatings [1,2]. Coating diamond films on steel substrate will combine the unique surface properties of diamond with the superior toughness and strength of the core steel substrates, yielding integrated properties superior to their individual ones [3]. ⁎ Corresponding author. Tel.: +1 306 966 6433; fax: +1 306 966 6400. E-mail address: [email protected] (Y.S. Li). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.037
Chemical vapor deposition (CVD) diamond coatings on transition metal substrates (iron, cobalt, nickel) and their alloys is still a great challenge due to low nucleation density and poor adhesion. These obstacles are associated with large thermal expansion coefficient mismatch between diamond and steel, and the base metal induced catalytic formation of graphite on steel surface. Furthermore, high diffusivity of carbon in steel substrates leads to a long incubation time for diamond nucleation. Imposing additional inter-layers, along with extensive surface pre-treatments, has been a conventional method so far to suppress these detrimental effects [4–6]. Nevertheless, preparing the intermediate layers involves complex and costly multi-step procedures and the interfacial adhesion at both substrate/interlayer and interlayer/diamond film has to be guaranteed. Alternatively, direct coating of continuous and adherent diamond films on the steel substrates would be much simpler and cost-saving, despite slow progress toward this goal has been made so far [7–11]. Previous studies on the nucleation, growth and adhesion ability of diamond films deposited on various hetero-substrates, particularly on transition metals, reveal that the deposition behavior is affected not only by gas-phase chemistry and deposition parameters, but also by the chemical compositions of
Y.S. Li, A. Hirose / Surface & Coatings Technology 202 (2007) 280–287
the substrates [8–16]. For instance, elements Al and Si in some binary compounds have shown beneficial to reduce the catalytic reactivity of base metals for graphite formation and result in enhanced diamond adhesion. Recent studies on high temperature carburization behaviors of metals and alloys indicate that adding a small fraction of Si and/or Al additions to high temperature alloys also significantly increase the intrinsic resistance to carbon diffusion and carbide formation [17,18]. Stimulated by these previous findings, we are interested to explore the feasibility of modifying Fe-base alloys through certain types of alloying elements to improve the quality of diamond deposited on them. The results have demonstrated that adherent diamond coatings can be directly deposited on the modified alloy substrates, and this will widely expand the practical applications of both the alloy steels and the diamond materials. 2. Experimental The substrate materials used for diamond film deposition are a series of Fe-based model alloys and their nominal chemical compositions are listed in Table 1. They contain different fractions of chromium, aluminium and silicon, and have been prepared by melting the desired amounts of the component metals using an arc-furnace with Ar protection. The as-cast Fe– Cr, Fe–Al and Fe–Cr–Al alloys have a single phase microstructure. The Fe–Ni–Al and Fe–Ni–Cr–Al alloys are further subjected to solid solution treatment at 1200 °C for 24 h in vacuum and followed by water quenching, then finally aged at 700 °C for 1000 h in air. Spinodal microstructures were formed on them. Fe–Ni–Al alloy exhibits a two phase structure consisting of α-Fe and NiAl-type precipitates, while three phases are formed on Fe–Ni–Cr–Al alloy, including α-Fe and FeAl, NiAl phases, with dissolved Cr in each phase [19,20]. The substrates were machined into specimens with dimension of 10 mm × 10 mm × 1 mm and polished with 600 grit SiC paper, cleaned in acetone and finally dried in a nitrogen flow. For comparison, some Fe–Ni–Al alloy samples were also preoxidized at 900 °C in air to form an Al2O3 barrier. Diamond deposition was performed in 2.45 GHz microwave plasma assisted CVD apparatus (Plasmionique) using a gas mixture of H2 and 1 vol. % CH4 with a total flow rate of 100 sccm. The base pressure was 10– 7 Torr and working pressure was maintained at 30 Torr. Microwave power was 800 W and substrate temperature was kept at 670 °C as measured by a thermocouple mounted underneath the stainless steel substrate holder. The morphology observation and thickness measurement from metallographic cross-sections were performed by scanning electron microscope. The film purity and structures were characterized by micro-Raman (Renishaw 2000, Ar laser wavelength 514 nm) and X-ray diffraction (Rotaflex Ru-200, Cu Kα radiation, λ = 0.15418 nm). In the case of a thin film mode, the vertically standing film was rotated on the center axis, with the incident angle of the X-ray beam to the film deposited on steels at 1°. The surface hardness and the adhesion property of diamond films are preliminarily evaluated by micro-hardness indentation and scratch test.
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3. Results and discussion Fig. 1 shows typical SEM images and Raman spectra of diamond films grown for 10 h on Fe–10Al substrate. The film has a porous and severely cracking surface structure containing poorly faceted diamond in cauliflower shape (a, b). Consequently, the film is only weakly bonded to the substrate and patches of diamond films have spontaneously peeled off the substrate after cooling, leaving a dark interlayer of loosely packed fine particles (c). Fig. 1d shows the Raman spectra for the top surface of the diamond film (b) and the exposed substrate surface (c), respectively. The spectrum for the film shows characteristic 1332 cm− 1 peak of diamond, with low intensity and significantly broadened shape. Two additional peaks emerge around 1140 cm− 1 and 1470 cm− 1 and they are attributed to either grain size effect of nanocrystalline diamond or the trans-polyacetylene in nanocrystalline diamond films [21]. The Raman spectrum from the exposed substrate surface demonstrates two sharp peaks at 1354 cm− 1 and 1585 cm− 1 associated with D and G bands of graphite and/or amorphous carbon. These results indicate that an intermediate layer composed of sp2 carbon has formed on the substrate surface and is primarily responsible for the film adhesion failure. Fig. 2 shows SEM images and Raman spectra of diamond films grown for 10 h on Fe–25Al substrate. Despite the film spalls locally after CVD process (a), the substrate is still mostly covered with well adherent and closely packed diamond films (b). It is important to emphasize that the exposed substrate surface shows fresh and bright metallic luster (c), indicating the absence of any contamination. The Raman data corresponding to the top surface of the diamond film is similar to that on Fe– 10Al, but the spectrum for the exposed substrate surface is completely different, which is basically flat and no apparent peaks related to graphite or other carbon species are present. In addition, the Raman spectrum measured on the substrate side of the delaminated diamond film fragment indicates that the relative ratio of sp2/sp3 carbon is comparable to that on the diamond top surface. This excludes an enrichment of sp2 carbon on the backside of the diamond film. Consequently, it is reasonable to conclude that a higher Al content in Fe–Al alloys has effectively eliminated the formation of non-diamond phase Table 1 Diamond film growth rate (DGR, μm/h) and adhesion properties (AP) dependent upon the chemical composition of the materials (in wt.%)
Fe–15Cr Fe–35Cr Fe–10Al Fe–25Al Fe–Cr–Al Fe–Ni–Al Fe–Ni–Al–Cr Fe–Cr–Al–Si [8]
Fe
Cr
Al
Ni
Si
DGR⁎
AP⁎
85 65 90 75 80 70 56 86
15 35 – – 15 – 10 9
– – 10 25 5 10 12 2.2
– – – – – 20 22 –
– – – – – – – 2.2
– – – 0.3 0.26 – 0.22 0.24
Weak Weak Weak Strong Strong Weak Strong Strong
“Weak” assigned to the adhesion properties corresponds to a spontaneous delamination, primarily caused by the formation of sp2-bonded intermediate layer; “Strong ” is related to the continuous and non-cracking films under a critical thickness, adhesion failure due to thermal stress, see descriptions in the text.
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Fig. 1. SEM images (a,b,c) and Raman spectra (d) of diamond films formed on Fe–10Al substrate after 10 h deposition. (a) a general view and (b) a magnified view of diamond film; (c) a magnified view of the exposed substrate surface after removal of the top diamond layer; (d) the Raman spectra measured from (b) and (c), respectively.
on the substrate surface and thereby greatly improves the adhesion ability. Fig. 3a shows the diamond film grown on Fe–Ni–Al substrate. The film is loose with low purity and poor adhesion to the substrate, even can be easily scratched away from the substrate by a tweezer. As this alloy is an alumina-forming material upon high temperature oxidation, a surface pre-oxidation
at 900 °C for 2 h in air has produced an external α-Al2O3 barrier separating the substrate from the ambient atmosphere. This thin oxide scale is found to have effectively improved the quality of diamond subsequently grown on it (Fig. 3b). Although the diamond film is still discontinuous, the nucleation density and purity are remarkably improved while non-diamond phase formation has been successfully hindered, as confirmed by
Fig. 2. SEM images (a,b,c) and Raman spectra (d) of diamond films deposited for 10 h on Fe–25Al substrate. (a) a general view showing local film spalling; (b) a magnified view of residual diamond film still adhering to the substrate; (c) exposed substrate surface after diamond spallation; (d) Raman spectra from (b) and (c), respectively.
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Fig. 3. SEM images (a,b) and Raman spectra (c) of diamond films deposited for 10 h on (a) as-cast and (b) pre-oxidized Fe–Ni–Al substrate.
Raman analysis (Fig. 3c). This implies that a pre-oxidation treatment may be applicable for this kind of material to improve the diamond quality deposited on it. Fig. 4a shows that continuous and dense diamond film has formed on the Fe–Cr–Al ternary alloy. An enlarged view in the inset reveals the film is closely packed and has well-faceted crystalline structure with submicron-scaled grain size. A maximum thickness of 3 μm is measured from the cross section
(b). Raman spectrum (c) of this film displays an obvious line splitting at 1344 and 1351 cm− 1, respectively. The significant upward-shift of the diamond peak from the standard 1332 cm− 1 is related to high compressive stress in the film. Even so, the adhesion of the diamond film to the substrate appears to be strong enough to withstand the huge load. Surface microhardness of this film-coated Fe–Cr–Al substrate has increased more than ten times (2883 kg/mm2) over the value for the
Fig. 4. SEM images of diamond film deposited for 12 h on Fe–Cr–Al substrate. (a) general and magnified (inset) views of the diamond film; (b) a cross sectional view; (c) Raman spectrum showing the exact peak position (inset); (d) a top view of the film after indentation test at a load of 9.8 N.
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Fig. 5. XRD pattern of diamond films deposited for 12 h on Fe–Cr–Al substrate. Single phase diamond film is exclusively identified from thin film XRD mode as shown in the lower section.
uncoated substrate (260 kg/mm2) with a load of 4.9 N. Fig. 4d shows an indentation crater under a load of 9.8 N. No spalling or cracking around the film is observed, implying a strong adhesion ability of the diamond film to the substrate. Fig. 5 shows the XRD patterns of this film-substrate duplex system and only diamond and steel substrate phases are determined. Because XRD peaks of the steel substrate are very close to that of polycrystalline diamond, in order to confirm the exact structure of the film, a thin film XRD mode is further employed to exclude the interference from the substrate. A single diamond phase has been obtained as shown in the figure. For Fe–Cr binary alloys, however, a Cr concentration as high as 35 wt. % is still insufficient to produce well adherent diamond films (results are not shown), indicating the unique function of the alloying element Al plays in promoting the diamond adhesion on steel substrates. Fig. 6 shows the surface morphology evolution of diamond films deposited on Fe–Ni–Al–Cr substrate for different CVD
periods. After 2 h deposition (Fig. 6a), a nearly continuous diamond film with a high nucleation density about 108 cm− 2 has formed on the whole substrate surface. After 4 h deposition, the diamond film becomes dense, thickened and continuous, and no film spalling is observed (Fig. 6b). The film is composed of fine grain size in several tens nanometers resulting in a very smooth surface. The root mean square surface roughness of the film measured with AFM is about 270 nm over a 20 μm × 20 μm AFM image area. When the deposition time exceeds 10 h, due to huge thermal stress produced inside the thickened films, severe fracture and spalling of diamond films occur (Fig. 6c). Similar thickness-dependent adhesion phenomenon has been previously observed for diamond films coated on steel substrates with interlayer systems [22]. Nevertheless, a large fraction of residual diamond patches are still adhesively bonded to the substrate. The exposed substrate surface (inset) is very clear and shows no evidence of soot formation. Grain size of the diamond film after longer time growth has increased to a submicron scale (Fig. 6d). The Raman spectra of the diamond films are shown in Fig. 7. The intensity of non-diamond peaks gradually decrease with prolonged deposition time, while all the diamond peaks are of relatively low intensity and significantly broadened shape (Fig. 7a). They are related to combined effects of the nanoscaled grain features and the presence of internal stress in the films. Special attention is paid to the Raman peak shifts since they are closely linked to the residual stress affecting the adhesion properties. Raman spectrum I corresponding to diamond films after 2 h deposition reveals clear diamond characteristic peak along with signatures of graphite phase. Since the sp2-bonded carbon (graphite) has 50 times higher Raman scattering efficiency than the sp 3 -bonded carbon (diamond), the relative intensity of these peaks indicate that
Fig. 6. SEM images of diamond films deposited on Fe–Ni–Al–Cr substrate for (a) 2 h, (b) 4 h and (c,d) 10 h. Inset in (c) corresponds to the exposed substrate surface after film detachment and (d) is a magnified view of the residual adherent film.
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Fig. 7. Raman spectra (a) of diamond films deposited on Fe–Ni–Al–Cr substrate for (I) 2 h, (II) 4 h. After 10 h, thickened film is partially detached from the substrate, and spectrum (III) corresponds to the diamond film still adhesively remained on substrate and (IV) the spalled free-standing diamond film and (V) the exposed substrate surface after film detachment, respectively. The two Raman spectra in (b) derive from (a) and clearly show the different peak positions associated with varied internal stress.
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this film has a fairly high diamond fraction even at the early stage of deposition. Raman spectrum II is measured from the continuous and adherent diamond film after 4 h deposition. A Raman peak upward-shift from standard diamond position 1332 cm− 1 to 1341 cm− 1 is noticed. The spectrum III arising from the film that is still adherent to substrate and has the biggest surface area after 10 h deposition, shows a maximum Raman shift of 17 cm− 1 (Fig. 7b). However, no peak shift is observed in spectrum IV, corresponding to spalled free-standing diamond film, probably due to complete stress relief due to the delamination. Apparently, the residual stress in the film increases with the thickness and may cause film failure once critical thickness is reached. In certain low thickness range below 2 μm, the film can be well adherent to the steel substrate, as reflected by the intermediate levels of residual stress. Finally, spectrum V is determined from the exposed substrate surface after film detachment for 10 h deposition. Graphitic or amorphous carbon species has been completely eliminated due to the alloying effects of Al and Cr, which plays a key role in the adhesion enhancement. The adhesion ability of the diamond film after 4 h deposition on Fe–Ni–Al–Cr substrate is examined. The thickness of this adhesive diamond film is approximately 1 μm as determined from a SEM cross section (Fig. 8a), and no gap between the film-substrate interface is produced after metallographic preparation. Fig. 8b shows a film surface that has been partly manually polished. The surface becomes much smoother but remains continuous and undetached, implying the film is well adherent to the steel substrate. Scratch tests were performed using a progressively loaded scratch tester from 300 g to 4000 g. Fig. 8c–d shows surface morphologies of the scratched films. At lower loads, parts of the diamond film were embedded into
Fig. 8. SEM images of (a) cross section of as coated film; (b) partly mechanically polished film surface; (c,d) scratched surface of films deposited for 4 h on Fe–Ni–Al– Cr substrate.
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the softer substrate due to plastic deformation, whereas at higher loads the film is completely removed from the substrate. In both cases, neither crack nor delamination of diamond films is observed along the scratch tracks. These features demonstrate an excellent adhesion of the diamond films to the steel substrate. The results clearly indicate that strongly adhesive diamond films can be directly deposited on steel substrates with the aid of the alloying elements. In comparison with the diamond films formed on bulk metals Fe, Ni, carbon steels, Fe–Cr, Fe–Ni–Cr stainless steels, which usually spontaneously peel off the substrate after cooling, the adhesion improvement of diamond films on Fe–Al, Fe–Cr–Al and Fe–Ni–Al–Cr type alloys are closely dependent on the using of the alloying elements Al. Recent studies have speculated that the 3d shell electronic structures of the transition metal atoms have strong influence on their catalytic ability for the graphite formation [12,13]. The 3d orbital (3d6) of Fe and 3d shell configuration (3d8) of nickel are only partially filled and they are catalytically reactive with hydrocarbon species. On the surface of the conventional steel substrates, Fe and Ni facilitate breaking of the C–H bonds in the CxHy precursors, releasing excessive free carbon atoms which subsequently enhance the overgrowth of non-diamond carbon phases. In such a case, a long incubation time is required for diamond to nucleate and grow from the surface of this loose interlayer, rather than an adherent contact with the steel substrate. On the Al-alloyed substrate, Al donates electrons and gradually fills up the 3d shell of iron and nickel as new 3d10 structures, which are less reactive to catalyze graphitization. The single alloying by Cr in the Fe–Cr alloy shows an inferior ability to improve the diamond quality, and it may be attributed to the time-consuming formation of intermediate chromium carbides prior to diamond nucleation. Nevertheless, it demonstrates a beneficial effect to decrease the Al content needed to obtain high quality diamond when they are simultaneously presented in the alloys, and this is particularly important for alloy design in order to guarantee the substrate mechanical properties. Element Si also shows positive effects as Fe and Si can share electrons by forming covalent bonds and 3d shell structure of Fe becomes inert. This usually requires a high Si/Fe ratio to ensure a sufficient inhibition of the catalytic activity. Our recent study reveals that by using a combination of Cr, Al and Si, adherent nanocrystalline diamond films are obtainable at much lower fractions of the total alloying elements [8]. As the roles of these alloying elements played are also interactive, more reasonable explanations on the complex mechanism need to be explored in the future. The nucleation density of diamond on the alloyed substrates like Fe–Cr–Al, Fe–Ni–Al–Cr and Fe–Cr–Al-Si [8] have been significantly enhanced even at an early stage of deposition, normally exceeding 108 cm− 2 after 1 h deposition. On the one hand, catalytic effect of the base metal (Fe) is beneficial to adsorb more carbon source on substrate surface. On the other hand, this effect is partially deactivated by the alloying elements Al, and a large amount of carbon consumption through graphite formation is prevented. Furthermore, the penetration of carbon into alloy substrate is significantly suppressed by the barrier effect imposed by the alloying elements Al or Si [17]. Instead,
carbonaceous species is primarily restricted on substrate surface to facilitate rapid establishment of carbon-saturated condition which is an important prerequisite for diamond nucleation. As a direct result, this delicate competition and balance between the base metal and alloying elements have given rise to high nucleation density and improved purity of diamond on these modified substrates. Such phenomenon is in good agreement with previous observations that nucleation density of diamond on Si substrate can be significantly enhanced by alloying Si with small fraction of Fe or by imposing an Fe thin film of several nanometers thick [23,24], while an excessive Fe like on iron substrate will adversely deteriorate the nucleation density and require prolonged incubation period. The high initial nucleation density has facilitated the formation of fine-grained diamond films, which are especially desirable for triboligical applications because of their low friction coefficient and high surface smoothness. In particular, diamond films with fine-grained structures may lead to adhesion improvement by providing more contact points and bonding strength with the substrate. Finally, if metal carbides such as Fe3C and CrxCy and others can form in the near-surface zones of the substrate, as have been observed on the conventional stainless steel substrates or CrN-interlayered steels [22,25], they will help decrease the surface activity of the base metals and act as pegs or glue to increase the bonding force between the film and substrate. In current study, such carbide phases are not detected from preliminary XRD analysis, and more details on the fine structural and compositional changes at the substrate near surface should be thoroughly examined in the continued research. 4. Conclusions The diamond films grown on Fe-base alloys show distinctive features depending on the types and relative concentrations of the alloying elements in the substrates. Adherent diamond films can be directly deposited on certain alloy steels with the aid of alloying elements Al, Cr, and Si. The following observations have been made: 1. Due to strong catalytic effects of Fe and Ni on the preferential formation of non-diamond phases over diamond, deposition of adherent diamond films on Fe–Cr and Fe–Ni– Cr type alloys is restricted. A higher Al ratio in binary Fe–Al alloys is beneficial to enhance the adhesion. Well adherent diamond films are also obtained on ternary Fe–Cr–Al alloy substrates. 2. A pre-oxidation treatment of Fe–Ni–Al alloy to form an Al2O3 barrier on the substrate improves the quality of diamond subsequently formed on it. A further alloying with Cr to form Fe–Ni–Al–Cr alloys has a similar effect. In the case of Fe–Cr–Al–Si alloy, much smaller total fractions of Al, Si and Cr are required to obtain adherent diamond film. 3. The promoted debonding resistance of the diamond films on the alloyed substrates is closely associated with an effective removal of non-diamond phases from the film/substrate interface, whereas the local spallation occurred on thickened
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diamond films is primarily attributed to increased thermal stress. Acknowledgements This research has been sponsored by the Canada Research Chair Program and by the Natural Sciences and Engineering Research Council of Canada. References [1] P.W. May, Philos. Trans. R. Soc. Lond., A 358 (2000) 473. [2] A.V. Sumant, A.R. Krauss, D.M. Gruen, O. Auciello, A. Erdemir, M. Williams, A.F. Artiles, W. Adams, Tribol. Trans. 48 (2005) 24. [3] V.S. Sundaram, Surf. Coat. Technol. 201 (2006) 2707. [4] R.J. Narayan, J. Adhes. Sci. Technol. 18 (2004) 1339. [5] R. Polini, F.P. Mantini, M. Braic, M. Amar, W. Ahmed, H. Taylor, Thin solid Films 494 (2006) 116. [6] J.C. Bareiβ, G. Hackl, N. Popovska, S.M. Rosiwal, R.F. Singer, Surf. Coat. Technol. 201 (2006) 718. [7] F. Davanloo, H. Park, C.B. Collins, J. Mater. Res. 11 (1996) 2042. [8] Y.S. Li, A. Hirose, Chem. Phys. Lett. 433 (2006) 150. [9] S. Kumar, M. Malhotra, Diamonds Relat. Mater 7 (1998) 1043.
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[10] N.G. Shang, F.Y. Meng, C.Y. Chan, Q. Li, C.S. Lee, S.T. Lee, I. Bello, Int. J. Mod. Phys. B 16 (2002) 881. [11] Y.S. Li, A. Hirose, J. Appl. Phys. 101 (073503) (2007). [12] X. Chen, J. Narayan, J. Appl. Phys. 74 (1993) 4168. [13] D.F. Mihec, S.H. Baek, J.B. Metson, Int. J. Mod. Phys. B 17 (2003) 1170. [14] H.G. Jentsch, G. Rosenbauer, S.M. Rosiwal, R.F. Singer, Adv. Eng. Mater. 2 (2000) 369. [15] J. Narayan, M. Nelson, S. Oktyabrsky, K. Jagannadham, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 38 (1996) 46. [16] A. Köpf, M. Sommer, R. Haubner, B. Lux, Diamods Relat. Mater. 10 (2001) 790. [17] S.K. Roy, H.J. Grabke, W. Weppner, Arch. Eisenhuttenwes. 51 (1980) 91. [18] M.A. Harper, J.P. Cotner, Oxid. Met. 53 (2000) 427. [19] C. Stallybrass, A. Schneider, G. Sauthoff, Intermetallics 13 (2005) 1263. [20] G. Ghosh, G.B. Olson, T.J. Kinkus, M.E. Fine, in: W.C. Johnson, J.M. Howe, D.E. Laughlin, W.A. Soffa (Eds.), Solid to Solid Phase Transformation, The Minerals, Metals & Materials Society, 1994, p. 359. [21] H. Kuzmany, R. Pfeiffer, N. Salk, B. Gunther, Carbon 42 (2004) 911. [22] R. Polini, G. Mattei, R. Valle, F. Casadei, Thin Solid Films 515 (2006) 1011. [23] J.E. Yehoda, R.I. Fuentes, J.C. Tsang, S.J. Whitehair, C.R. Guarnieri, J.J. Cuomo, Appl. Phys. Lett. 60 (1992) 2865. [24] V.P. Godbole, J. Narayan, J. Appl. Phys. 71 (1992) 4944. [25] J.G. Buijnsters, P. Shankar, J.J. Ter Meulen, Surf. Coat. Technol., doi:10.1016/j.surfcoat.2007.04.012.
Surface & Coatings Technology 202 (2007) 280 – 287 www.elsevier.com/locate/surfcoat
The effects of substrate compositions on adhesion of diamond films deposited on Fe-base alloys Y.S. Li ⁎, A. Hirose Plasma Physics Laboratory, University of Saskatchewan, 116 Science Place Saskatoon, SK, Canada S7N 5E2 Received 1 December 2006; accepted in revised form 13 May 2007 Available online 24 May 2007
Abstract Diamond deposition on a series of ferrous alloy substrates has been examined in a microwave plasma enhanced chemical vapor deposition reactor. The results show that the nucleation, growth, and adhesion properties of the diamond are strongly dependent on the types and relative concentrations of the alloy elements in the substrates. Along with high nucleation densities, continuous and adherent diamond films have been successfully fabricated on the Al-modified alloy substrates, even in the absence of inter-layers or nucleation enhancing surface pre-treatment. The mechanism is preliminarily clarified regarding the delicate competition and balance between the base metal and the alloying elements in terms of their activation and deactivation ability on catalyzing graphite formation during deposition process. © 2007 Elsevier B.V. All rights reserved. Keywords: Alloy; Diamond; Plasma CVD; Adhesion; Raman scattering spectroscopy
1. Introduction Alloy steels are among the most commonly used and costeffective structural materials in modern industry and their properties can be widely adjusted by designing the initial composition and controlling the microstructure through thermomechanical treatment. However, when steel is used as critical components in harsh (wear, corrosive and erosive) environments, accelerated damage usually occurs. As the early failure is usually initiated at the outer-most surface, it is extremely important to obtain strengthened high performance surface for longevity of service. Diamond films are characterized by super hardness, high thermal conductivity, high chemical inertness and remarkable wear resistance, which make them one of the most promising coating materials as heat sink, optical devices, electronic devices and especially wear resistant coatings [1,2]. Coating diamond films on steel substrate will combine the unique surface properties of diamond with the superior toughness and strength of the core steel substrates, yielding integrated properties superior to their individual ones [3]. ⁎ Corresponding author. Tel.: +1 306 966 6433; fax: +1 306 966 6400. E-mail address: [email protected] (Y.S. Li). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.037
Chemical vapor deposition (CVD) diamond coatings on transition metal substrates (iron, cobalt, nickel) and their alloys is still a great challenge due to low nucleation density and poor adhesion. These obstacles are associated with large thermal expansion coefficient mismatch between diamond and steel, and the base metal induced catalytic formation of graphite on steel surface. Furthermore, high diffusivity of carbon in steel substrates leads to a long incubation time for diamond nucleation. Imposing additional inter-layers, along with extensive surface pre-treatments, has been a conventional method so far to suppress these detrimental effects [4–6]. Nevertheless, preparing the intermediate layers involves complex and costly multi-step procedures and the interfacial adhesion at both substrate/interlayer and interlayer/diamond film has to be guaranteed. Alternatively, direct coating of continuous and adherent diamond films on the steel substrates would be much simpler and cost-saving, despite slow progress toward this goal has been made so far [7–11]. Previous studies on the nucleation, growth and adhesion ability of diamond films deposited on various hetero-substrates, particularly on transition metals, reveal that the deposition behavior is affected not only by gas-phase chemistry and deposition parameters, but also by the chemical compositions of
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the substrates [8–16]. For instance, elements Al and Si in some binary compounds have shown beneficial to reduce the catalytic reactivity of base metals for graphite formation and result in enhanced diamond adhesion. Recent studies on high temperature carburization behaviors of metals and alloys indicate that adding a small fraction of Si and/or Al additions to high temperature alloys also significantly increase the intrinsic resistance to carbon diffusion and carbide formation [17,18]. Stimulated by these previous findings, we are interested to explore the feasibility of modifying Fe-base alloys through certain types of alloying elements to improve the quality of diamond deposited on them. The results have demonstrated that adherent diamond coatings can be directly deposited on the modified alloy substrates, and this will widely expand the practical applications of both the alloy steels and the diamond materials. 2. Experimental The substrate materials used for diamond film deposition are a series of Fe-based model alloys and their nominal chemical compositions are listed in Table 1. They contain different fractions of chromium, aluminium and silicon, and have been prepared by melting the desired amounts of the component metals using an arc-furnace with Ar protection. The as-cast Fe– Cr, Fe–Al and Fe–Cr–Al alloys have a single phase microstructure. The Fe–Ni–Al and Fe–Ni–Cr–Al alloys are further subjected to solid solution treatment at 1200 °C for 24 h in vacuum and followed by water quenching, then finally aged at 700 °C for 1000 h in air. Spinodal microstructures were formed on them. Fe–Ni–Al alloy exhibits a two phase structure consisting of α-Fe and NiAl-type precipitates, while three phases are formed on Fe–Ni–Cr–Al alloy, including α-Fe and FeAl, NiAl phases, with dissolved Cr in each phase [19,20]. The substrates were machined into specimens with dimension of 10 mm × 10 mm × 1 mm and polished with 600 grit SiC paper, cleaned in acetone and finally dried in a nitrogen flow. For comparison, some Fe–Ni–Al alloy samples were also preoxidized at 900 °C in air to form an Al2O3 barrier. Diamond deposition was performed in 2.45 GHz microwave plasma assisted CVD apparatus (Plasmionique) using a gas mixture of H2 and 1 vol. % CH4 with a total flow rate of 100 sccm. The base pressure was 10– 7 Torr and working pressure was maintained at 30 Torr. Microwave power was 800 W and substrate temperature was kept at 670 °C as measured by a thermocouple mounted underneath the stainless steel substrate holder. The morphology observation and thickness measurement from metallographic cross-sections were performed by scanning electron microscope. The film purity and structures were characterized by micro-Raman (Renishaw 2000, Ar laser wavelength 514 nm) and X-ray diffraction (Rotaflex Ru-200, Cu Kα radiation, λ = 0.15418 nm). In the case of a thin film mode, the vertically standing film was rotated on the center axis, with the incident angle of the X-ray beam to the film deposited on steels at 1°. The surface hardness and the adhesion property of diamond films are preliminarily evaluated by micro-hardness indentation and scratch test.
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3. Results and discussion Fig. 1 shows typical SEM images and Raman spectra of diamond films grown for 10 h on Fe–10Al substrate. The film has a porous and severely cracking surface structure containing poorly faceted diamond in cauliflower shape (a, b). Consequently, the film is only weakly bonded to the substrate and patches of diamond films have spontaneously peeled off the substrate after cooling, leaving a dark interlayer of loosely packed fine particles (c). Fig. 1d shows the Raman spectra for the top surface of the diamond film (b) and the exposed substrate surface (c), respectively. The spectrum for the film shows characteristic 1332 cm− 1 peak of diamond, with low intensity and significantly broadened shape. Two additional peaks emerge around 1140 cm− 1 and 1470 cm− 1 and they are attributed to either grain size effect of nanocrystalline diamond or the trans-polyacetylene in nanocrystalline diamond films [21]. The Raman spectrum from the exposed substrate surface demonstrates two sharp peaks at 1354 cm− 1 and 1585 cm− 1 associated with D and G bands of graphite and/or amorphous carbon. These results indicate that an intermediate layer composed of sp2 carbon has formed on the substrate surface and is primarily responsible for the film adhesion failure. Fig. 2 shows SEM images and Raman spectra of diamond films grown for 10 h on Fe–25Al substrate. Despite the film spalls locally after CVD process (a), the substrate is still mostly covered with well adherent and closely packed diamond films (b). It is important to emphasize that the exposed substrate surface shows fresh and bright metallic luster (c), indicating the absence of any contamination. The Raman data corresponding to the top surface of the diamond film is similar to that on Fe– 10Al, but the spectrum for the exposed substrate surface is completely different, which is basically flat and no apparent peaks related to graphite or other carbon species are present. In addition, the Raman spectrum measured on the substrate side of the delaminated diamond film fragment indicates that the relative ratio of sp2/sp3 carbon is comparable to that on the diamond top surface. This excludes an enrichment of sp2 carbon on the backside of the diamond film. Consequently, it is reasonable to conclude that a higher Al content in Fe–Al alloys has effectively eliminated the formation of non-diamond phase Table 1 Diamond film growth rate (DGR, μm/h) and adhesion properties (AP) dependent upon the chemical composition of the materials (in wt.%)
Fe–15Cr Fe–35Cr Fe–10Al Fe–25Al Fe–Cr–Al Fe–Ni–Al Fe–Ni–Al–Cr Fe–Cr–Al–Si [8]
Fe
Cr
Al
Ni
Si
DGR⁎
AP⁎
85 65 90 75 80 70 56 86
15 35 – – 15 – 10 9
– – 10 25 5 10 12 2.2
– – – – – 20 22 –
– – – – – – – 2.2
– – – 0.3 0.26 – 0.22 0.24
Weak Weak Weak Strong Strong Weak Strong Strong
“Weak” assigned to the adhesion properties corresponds to a spontaneous delamination, primarily caused by the formation of sp2-bonded intermediate layer; “Strong ” is related to the continuous and non-cracking films under a critical thickness, adhesion failure due to thermal stress, see descriptions in the text.
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Fig. 1. SEM images (a,b,c) and Raman spectra (d) of diamond films formed on Fe–10Al substrate after 10 h deposition. (a) a general view and (b) a magnified view of diamond film; (c) a magnified view of the exposed substrate surface after removal of the top diamond layer; (d) the Raman spectra measured from (b) and (c), respectively.
on the substrate surface and thereby greatly improves the adhesion ability. Fig. 3a shows the diamond film grown on Fe–Ni–Al substrate. The film is loose with low purity and poor adhesion to the substrate, even can be easily scratched away from the substrate by a tweezer. As this alloy is an alumina-forming material upon high temperature oxidation, a surface pre-oxidation
at 900 °C for 2 h in air has produced an external α-Al2O3 barrier separating the substrate from the ambient atmosphere. This thin oxide scale is found to have effectively improved the quality of diamond subsequently grown on it (Fig. 3b). Although the diamond film is still discontinuous, the nucleation density and purity are remarkably improved while non-diamond phase formation has been successfully hindered, as confirmed by
Fig. 2. SEM images (a,b,c) and Raman spectra (d) of diamond films deposited for 10 h on Fe–25Al substrate. (a) a general view showing local film spalling; (b) a magnified view of residual diamond film still adhering to the substrate; (c) exposed substrate surface after diamond spallation; (d) Raman spectra from (b) and (c), respectively.
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Fig. 3. SEM images (a,b) and Raman spectra (c) of diamond films deposited for 10 h on (a) as-cast and (b) pre-oxidized Fe–Ni–Al substrate.
Raman analysis (Fig. 3c). This implies that a pre-oxidation treatment may be applicable for this kind of material to improve the diamond quality deposited on it. Fig. 4a shows that continuous and dense diamond film has formed on the Fe–Cr–Al ternary alloy. An enlarged view in the inset reveals the film is closely packed and has well-faceted crystalline structure with submicron-scaled grain size. A maximum thickness of 3 μm is measured from the cross section
(b). Raman spectrum (c) of this film displays an obvious line splitting at 1344 and 1351 cm− 1, respectively. The significant upward-shift of the diamond peak from the standard 1332 cm− 1 is related to high compressive stress in the film. Even so, the adhesion of the diamond film to the substrate appears to be strong enough to withstand the huge load. Surface microhardness of this film-coated Fe–Cr–Al substrate has increased more than ten times (2883 kg/mm2) over the value for the
Fig. 4. SEM images of diamond film deposited for 12 h on Fe–Cr–Al substrate. (a) general and magnified (inset) views of the diamond film; (b) a cross sectional view; (c) Raman spectrum showing the exact peak position (inset); (d) a top view of the film after indentation test at a load of 9.8 N.
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Fig. 5. XRD pattern of diamond films deposited for 12 h on Fe–Cr–Al substrate. Single phase diamond film is exclusively identified from thin film XRD mode as shown in the lower section.
uncoated substrate (260 kg/mm2) with a load of 4.9 N. Fig. 4d shows an indentation crater under a load of 9.8 N. No spalling or cracking around the film is observed, implying a strong adhesion ability of the diamond film to the substrate. Fig. 5 shows the XRD patterns of this film-substrate duplex system and only diamond and steel substrate phases are determined. Because XRD peaks of the steel substrate are very close to that of polycrystalline diamond, in order to confirm the exact structure of the film, a thin film XRD mode is further employed to exclude the interference from the substrate. A single diamond phase has been obtained as shown in the figure. For Fe–Cr binary alloys, however, a Cr concentration as high as 35 wt. % is still insufficient to produce well adherent diamond films (results are not shown), indicating the unique function of the alloying element Al plays in promoting the diamond adhesion on steel substrates. Fig. 6 shows the surface morphology evolution of diamond films deposited on Fe–Ni–Al–Cr substrate for different CVD
periods. After 2 h deposition (Fig. 6a), a nearly continuous diamond film with a high nucleation density about 108 cm− 2 has formed on the whole substrate surface. After 4 h deposition, the diamond film becomes dense, thickened and continuous, and no film spalling is observed (Fig. 6b). The film is composed of fine grain size in several tens nanometers resulting in a very smooth surface. The root mean square surface roughness of the film measured with AFM is about 270 nm over a 20 μm × 20 μm AFM image area. When the deposition time exceeds 10 h, due to huge thermal stress produced inside the thickened films, severe fracture and spalling of diamond films occur (Fig. 6c). Similar thickness-dependent adhesion phenomenon has been previously observed for diamond films coated on steel substrates with interlayer systems [22]. Nevertheless, a large fraction of residual diamond patches are still adhesively bonded to the substrate. The exposed substrate surface (inset) is very clear and shows no evidence of soot formation. Grain size of the diamond film after longer time growth has increased to a submicron scale (Fig. 6d). The Raman spectra of the diamond films are shown in Fig. 7. The intensity of non-diamond peaks gradually decrease with prolonged deposition time, while all the diamond peaks are of relatively low intensity and significantly broadened shape (Fig. 7a). They are related to combined effects of the nanoscaled grain features and the presence of internal stress in the films. Special attention is paid to the Raman peak shifts since they are closely linked to the residual stress affecting the adhesion properties. Raman spectrum I corresponding to diamond films after 2 h deposition reveals clear diamond characteristic peak along with signatures of graphite phase. Since the sp2-bonded carbon (graphite) has 50 times higher Raman scattering efficiency than the sp 3 -bonded carbon (diamond), the relative intensity of these peaks indicate that
Fig. 6. SEM images of diamond films deposited on Fe–Ni–Al–Cr substrate for (a) 2 h, (b) 4 h and (c,d) 10 h. Inset in (c) corresponds to the exposed substrate surface after film detachment and (d) is a magnified view of the residual adherent film.
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Fig. 7. Raman spectra (a) of diamond films deposited on Fe–Ni–Al–Cr substrate for (I) 2 h, (II) 4 h. After 10 h, thickened film is partially detached from the substrate, and spectrum (III) corresponds to the diamond film still adhesively remained on substrate and (IV) the spalled free-standing diamond film and (V) the exposed substrate surface after film detachment, respectively. The two Raman spectra in (b) derive from (a) and clearly show the different peak positions associated with varied internal stress.
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this film has a fairly high diamond fraction even at the early stage of deposition. Raman spectrum II is measured from the continuous and adherent diamond film after 4 h deposition. A Raman peak upward-shift from standard diamond position 1332 cm− 1 to 1341 cm− 1 is noticed. The spectrum III arising from the film that is still adherent to substrate and has the biggest surface area after 10 h deposition, shows a maximum Raman shift of 17 cm− 1 (Fig. 7b). However, no peak shift is observed in spectrum IV, corresponding to spalled free-standing diamond film, probably due to complete stress relief due to the delamination. Apparently, the residual stress in the film increases with the thickness and may cause film failure once critical thickness is reached. In certain low thickness range below 2 μm, the film can be well adherent to the steel substrate, as reflected by the intermediate levels of residual stress. Finally, spectrum V is determined from the exposed substrate surface after film detachment for 10 h deposition. Graphitic or amorphous carbon species has been completely eliminated due to the alloying effects of Al and Cr, which plays a key role in the adhesion enhancement. The adhesion ability of the diamond film after 4 h deposition on Fe–Ni–Al–Cr substrate is examined. The thickness of this adhesive diamond film is approximately 1 μm as determined from a SEM cross section (Fig. 8a), and no gap between the film-substrate interface is produced after metallographic preparation. Fig. 8b shows a film surface that has been partly manually polished. The surface becomes much smoother but remains continuous and undetached, implying the film is well adherent to the steel substrate. Scratch tests were performed using a progressively loaded scratch tester from 300 g to 4000 g. Fig. 8c–d shows surface morphologies of the scratched films. At lower loads, parts of the diamond film were embedded into
Fig. 8. SEM images of (a) cross section of as coated film; (b) partly mechanically polished film surface; (c,d) scratched surface of films deposited for 4 h on Fe–Ni–Al– Cr substrate.
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the softer substrate due to plastic deformation, whereas at higher loads the film is completely removed from the substrate. In both cases, neither crack nor delamination of diamond films is observed along the scratch tracks. These features demonstrate an excellent adhesion of the diamond films to the steel substrate. The results clearly indicate that strongly adhesive diamond films can be directly deposited on steel substrates with the aid of the alloying elements. In comparison with the diamond films formed on bulk metals Fe, Ni, carbon steels, Fe–Cr, Fe–Ni–Cr stainless steels, which usually spontaneously peel off the substrate after cooling, the adhesion improvement of diamond films on Fe–Al, Fe–Cr–Al and Fe–Ni–Al–Cr type alloys are closely dependent on the using of the alloying elements Al. Recent studies have speculated that the 3d shell electronic structures of the transition metal atoms have strong influence on their catalytic ability for the graphite formation [12,13]. The 3d orbital (3d6) of Fe and 3d shell configuration (3d8) of nickel are only partially filled and they are catalytically reactive with hydrocarbon species. On the surface of the conventional steel substrates, Fe and Ni facilitate breaking of the C–H bonds in the CxHy precursors, releasing excessive free carbon atoms which subsequently enhance the overgrowth of non-diamond carbon phases. In such a case, a long incubation time is required for diamond to nucleate and grow from the surface of this loose interlayer, rather than an adherent contact with the steel substrate. On the Al-alloyed substrate, Al donates electrons and gradually fills up the 3d shell of iron and nickel as new 3d10 structures, which are less reactive to catalyze graphitization. The single alloying by Cr in the Fe–Cr alloy shows an inferior ability to improve the diamond quality, and it may be attributed to the time-consuming formation of intermediate chromium carbides prior to diamond nucleation. Nevertheless, it demonstrates a beneficial effect to decrease the Al content needed to obtain high quality diamond when they are simultaneously presented in the alloys, and this is particularly important for alloy design in order to guarantee the substrate mechanical properties. Element Si also shows positive effects as Fe and Si can share electrons by forming covalent bonds and 3d shell structure of Fe becomes inert. This usually requires a high Si/Fe ratio to ensure a sufficient inhibition of the catalytic activity. Our recent study reveals that by using a combination of Cr, Al and Si, adherent nanocrystalline diamond films are obtainable at much lower fractions of the total alloying elements [8]. As the roles of these alloying elements played are also interactive, more reasonable explanations on the complex mechanism need to be explored in the future. The nucleation density of diamond on the alloyed substrates like Fe–Cr–Al, Fe–Ni–Al–Cr and Fe–Cr–Al-Si [8] have been significantly enhanced even at an early stage of deposition, normally exceeding 108 cm− 2 after 1 h deposition. On the one hand, catalytic effect of the base metal (Fe) is beneficial to adsorb more carbon source on substrate surface. On the other hand, this effect is partially deactivated by the alloying elements Al, and a large amount of carbon consumption through graphite formation is prevented. Furthermore, the penetration of carbon into alloy substrate is significantly suppressed by the barrier effect imposed by the alloying elements Al or Si [17]. Instead,
carbonaceous species is primarily restricted on substrate surface to facilitate rapid establishment of carbon-saturated condition which is an important prerequisite for diamond nucleation. As a direct result, this delicate competition and balance between the base metal and alloying elements have given rise to high nucleation density and improved purity of diamond on these modified substrates. Such phenomenon is in good agreement with previous observations that nucleation density of diamond on Si substrate can be significantly enhanced by alloying Si with small fraction of Fe or by imposing an Fe thin film of several nanometers thick [23,24], while an excessive Fe like on iron substrate will adversely deteriorate the nucleation density and require prolonged incubation period. The high initial nucleation density has facilitated the formation of fine-grained diamond films, which are especially desirable for triboligical applications because of their low friction coefficient and high surface smoothness. In particular, diamond films with fine-grained structures may lead to adhesion improvement by providing more contact points and bonding strength with the substrate. Finally, if metal carbides such as Fe3C and CrxCy and others can form in the near-surface zones of the substrate, as have been observed on the conventional stainless steel substrates or CrN-interlayered steels [22,25], they will help decrease the surface activity of the base metals and act as pegs or glue to increase the bonding force between the film and substrate. In current study, such carbide phases are not detected from preliminary XRD analysis, and more details on the fine structural and compositional changes at the substrate near surface should be thoroughly examined in the continued research. 4. Conclusions The diamond films grown on Fe-base alloys show distinctive features depending on the types and relative concentrations of the alloying elements in the substrates. Adherent diamond films can be directly deposited on certain alloy steels with the aid of alloying elements Al, Cr, and Si. The following observations have been made: 1. Due to strong catalytic effects of Fe and Ni on the preferential formation of non-diamond phases over diamond, deposition of adherent diamond films on Fe–Cr and Fe–Ni– Cr type alloys is restricted. A higher Al ratio in binary Fe–Al alloys is beneficial to enhance the adhesion. Well adherent diamond films are also obtained on ternary Fe–Cr–Al alloy substrates. 2. A pre-oxidation treatment of Fe–Ni–Al alloy to form an Al2O3 barrier on the substrate improves the quality of diamond subsequently formed on it. A further alloying with Cr to form Fe–Ni–Al–Cr alloys has a similar effect. In the case of Fe–Cr–Al–Si alloy, much smaller total fractions of Al, Si and Cr are required to obtain adherent diamond film. 3. The promoted debonding resistance of the diamond films on the alloyed substrates is closely associated with an effective removal of non-diamond phases from the film/substrate interface, whereas the local spallation occurred on thickened
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diamond films is primarily attributed to increased thermal stress. Acknowledgements This research has been sponsored by the Canada Research Chair Program and by the Natural Sciences and Engineering Research Council of Canada. References [1] P.W. May, Philos. Trans. R. Soc. Lond., A 358 (2000) 473. [2] A.V. Sumant, A.R. Krauss, D.M. Gruen, O. Auciello, A. Erdemir, M. Williams, A.F. Artiles, W. Adams, Tribol. Trans. 48 (2005) 24. [3] V.S. Sundaram, Surf. Coat. Technol. 201 (2006) 2707. [4] R.J. Narayan, J. Adhes. Sci. Technol. 18 (2004) 1339. [5] R. Polini, F.P. Mantini, M. Braic, M. Amar, W. Ahmed, H. Taylor, Thin solid Films 494 (2006) 116. [6] J.C. Bareiβ, G. Hackl, N. Popovska, S.M. Rosiwal, R.F. Singer, Surf. Coat. Technol. 201 (2006) 718. [7] F. Davanloo, H. Park, C.B. Collins, J. Mater. Res. 11 (1996) 2042. [8] Y.S. Li, A. Hirose, Chem. Phys. Lett. 433 (2006) 150. [9] S. Kumar, M. Malhotra, Diamonds Relat. Mater 7 (1998) 1043.
287
[10] N.G. Shang, F.Y. Meng, C.Y. Chan, Q. Li, C.S. Lee, S.T. Lee, I. Bello, Int. J. Mod. Phys. B 16 (2002) 881. [11] Y.S. Li, A. Hirose, J. Appl. Phys. 101 (073503) (2007). [12] X. Chen, J. Narayan, J. Appl. Phys. 74 (1993) 4168. [13] D.F. Mihec, S.H. Baek, J.B. Metson, Int. J. Mod. Phys. B 17 (2003) 1170. [14] H.G. Jentsch, G. Rosenbauer, S.M. Rosiwal, R.F. Singer, Adv. Eng. Mater. 2 (2000) 369. [15] J. Narayan, M. Nelson, S. Oktyabrsky, K. Jagannadham, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 38 (1996) 46. [16] A. Köpf, M. Sommer, R. Haubner, B. Lux, Diamods Relat. Mater. 10 (2001) 790. [17] S.K. Roy, H.J. Grabke, W. Weppner, Arch. Eisenhuttenwes. 51 (1980) 91. [18] M.A. Harper, J.P. Cotner, Oxid. Met. 53 (2000) 427. [19] C. Stallybrass, A. Schneider, G. Sauthoff, Intermetallics 13 (2005) 1263. [20] G. Ghosh, G.B. Olson, T.J. Kinkus, M.E. Fine, in: W.C. Johnson, J.M. Howe, D.E. Laughlin, W.A. Soffa (Eds.), Solid to Solid Phase Transformation, The Minerals, Metals & Materials Society, 1994, p. 359. [21] H. Kuzmany, R. Pfeiffer, N. Salk, B. Gunther, Carbon 42 (2004) 911. [22] R. Polini, G. Mattei, R. Valle, F. Casadei, Thin Solid Films 515 (2006) 1011. [23] J.E. Yehoda, R.I. Fuentes, J.C. Tsang, S.J. Whitehair, C.R. Guarnieri, J.J. Cuomo, Appl. Phys. Lett. 60 (1992) 2865. [24] V.P. Godbole, J. Narayan, J. Appl. Phys. 71 (1992) 4944. [25] J.G. Buijnsters, P. Shankar, J.J. Ter Meulen, Surf. Coat. Technol., doi:10.1016/j.surfcoat.2007.04.012.