Growth and adhesion failure of diamond thin films deposited on stainless steel with ultra-thin dual metal interlayers

Applied Surface Science 256 (2010) 7653–7657

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Growth and adhesion failure of diamond thin films deposited on stainless steel with ultra-thin dual metal interlayers Y.S. Li a,∗ , Y. Tang a,b , Q. Yang b , C. Xiao a , A. Hirose a a b

Plasma Physics Laboratory, University of Saskatchewan, Saskatoon, SK, Canada Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, Canada

a r t i c l e

i n f o

Article history: Received 12 January 2010 Received in revised form 4 June 2010 Accepted 9 June 2010 Available online 15 June 2010 Keywords: Diamond thin film Chemical vapor deposition Steel Ultra-thin interlayer Adhesion

a b s t r a c t The nucleation and growth of diamond on ultra-thin Cr/Al and Ti/Al interlayered stainless steel substrates were investigated. The metal interlayers were produced by ion beam sputtering deposition and consisted of an outer layer of 20 nm Cr or Ti and an inner layer of 30 nm Al, respectively. During the microwave plasma-enhanced chemical vapor deposition process, the inner Al diffuses into steel surface and forms Fe–Al compounds, while the outer Cr or Ti is carburized and transformed into corresponding carbides. These two ultra-thin dual metal interlayers are effective in suppressing the graphite soot formation on steel surfaces. Nevertheless, continuous diamond thin film is difficult to be obtained due to severe buckling and fragmentation deformation induced by residual stress. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Coating of diamond thin films on low cost steel substrates is a promising method to modify the wear/corrosion resistance of steel surfaces and to offer novel functionality. However, direct synthesis of high quality diamond thin films on steels has been technically restricted due to the difficulties in achieving high diamond nucleation density and sufficient coating-substrate interfacial bonding strength. This is primarily caused by the transition metal-catalyzed preferential formation of loose graphite soot prior to diamond nucleation and growth [1–5]. A common method to solve the adhesion problems is to modify or to coat the substrate surface to form intermediate layers to prevent a direct contact of the carbonaceous precursor with the transition metals [6–10]. An ideal interlayer material should serve as good diffusion barriers between carbon and the base metals, and enable high diamond nucleation and interfacial bonding [11–15]. Conventionally interlayers as thick as a few micrometers are required to be an effective diffusion barrier, but such thick interlayers may alter the dimensional accuracy of certain precision components. A series of recent work has revealed aluminum is effective in suppressing graphite formation on the surface of transition metals [4,16–18] and that the carbide-forming metals such as titanium, chromium, tungsten, molybdenum, and their corresponding carbides are promising substrate or interme-

∗ Corresponding author. Tel.: +1 306 966 6433; fax: +1 306 966 6400. E-mail address: [email protected] (Y.S. Li). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.06.022

diate layer materials for diamond nucleation and growth [19–24]. A combination of Al and these carbide-forming metals is thereby considered excellent intermediate layer materials to promote diamond growth on steels. This paper reports on the nucleation and growth of diamond on 304 stainless steel coated with ultra-thin Cr/Al and Ti/Al dual metal interlayers. 2. Experimental Commercial 304 stainless steel (SS304) was used as substrate materials for diamond deposition. The steel was cut into specimens of 10 mm × 10 mm × 1 mm and mechanically polished with 600 # SiC sandpapers, and finally ultrasonically cleaned in acetone and dried by flowing N2 . Ti/Al and Cr/Al dual metal interlayers were deposited on SS304 using bias targeted ion beam sputtering. An Al target of 99.99% purity was first sputtered by an Ar ion beam for 1 h, then shifted to a Ti or Cr target of 99.99% purity, respectively, for another 0.5 h sputtering. The entire thickness of the dual metal interlayer was approximately 50 nm, as determined by a profile meter. Some of the interlayered steel samples were scratched for 90 s in 1 ␮m diamond powder suspension to enhance diamond nucleation. Deposition of diamond was conducted in a 2.45 GHz microwave plasma-enhanced CVD system (Plasmionique) using H2 and 1 vol.% CH4 at a total feed rate of 100 sccm, a working pressure of 30 Torr, and substrate temperature approximately 680 ◦ C [5]. The morphology, composition and structure of the deposits and interfaces were characterized by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), micro-Raman, glanc-

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Fig. 1. SEM images of diamond grown on (a) as-polished SS304, (b and c) Ti–Al as-interlayered SS304, and (d and e) Ti–Al interlayered and diamond suspension pre-scratched SS304; (a, b and d) 2 h, (c) 10 h, (e) 4 h.

ing incidence X-ray diffraction (GIXRD), and a synchrotron near edge extended X-ray absorption fine structure spectroscopy (NEXAFS). 3. Results and discussion Diamond nucleation directly on the as-polished SS304 substrate is subjected to a long incubation period because of rapid carbon diffusion into the substrate and the preferential formation of an intermediate graphite layer on steel surface catalyzed by iron and nickel in the substrate. Fig. 1a shows the loose graphite soot formed on the blank steel surface after 2 h deposition. This graphitic intermediate layer has been clearly verified by synchrotron XAS and Raman spectra as will be discussed in Fig. 2. The following growth process of diamond thin film actually occurs on the top of the graphite soot which has separated the diamond from the steel substrate caused severely deteriorated film-substrate interfacial adhesion. With a 50 nm thick Ti/Al or Cr/Al dual metal interlayer,

diamond growth on the steels has markedly changed. Fig. 1b–e shows the surface morphology evolutions of the Ti/Al interlayered SS304 after CVD processes. Although the diamond nucleation density/rate on the Ti/Al coated SS304 are very low (Fig. 1b and c), no porous graphitic soot was observed on the top surface, even after 10-h CVD processing, indicating that the ultra-thin interlayer is effective in preventing the interdiffusion of carbon and the metals in the substrate. A diamond pre-scratching treatment of the interlayer prior to diamond deposition is very crucial to largely enhance the diamond nucleation density (Fig. 1d), and the diamond nanocrystallites can coalesce quickly to form diamond thin films (Fig. 1e). Fig. 2a shows the soft X-ray absorption (XAS) spectra of carbon K-edge measured on the samples prepared on SS304 with various surface conditions. The XAS spectrum of carbon collected from the as-polished SS304 after 2 h deposition, shows a prominent absorption peak at 286 eV that is related to graphite-like * bonding (Fig. 2a, curve I). The carbon XAS spectrum measured from the samples after 2 h deposition on the steel surface that has

Fig. 2. XAS (a) spectra of the 2 h CVD carbon deposits formed on SS304 with different surface conditions. (I) As-polished steel, (II) Ti–Al interlayered and diamond pre-scratched steel, (III) Ti–Al as-interlayered steel. Raman spectra (b) of the carbon deposits formed on the blank and as-interlayered steel substrate, respectively.

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Fig. 3. SEM surface images of SS304 with pre-scratched Ti–Al interlayer and after 6 h CVD deposition. (a) A general view, (b) a magnified view of the exposed sub-layer and substrate after diamond local spallation. (c) The corresponding Raman spectra of top diamond (I), intermediate carbide layer (II) and exposed steel substrate (III).

been pre-coated with a Ti/Al interlayer then followed by a diamond scratching treatment, shows a sharp absorption edge at 290 eV and a dip at 303 eV (Fig. 2a, curve II), and these features are attributed to sp3 * bonding diamond. Comparatively, the non-diamond carbon peak at 285.8 eV associated with sp2 structured  bonding is very weak in the spectrum. For the samples prepared on SS304 covered with an as-coated Ti/Al interlayer, there are no prominent characteristic peaks of either diamond or graphite on the XAS spectrum (Fig. 2a, curve III). Fig. 2b shows the Raman spectra of samples after deposition on SS304 with different surface states. Sharp graphitic

peaks centered at 1600 and 1350 cm−1 are recorded for the samples prepared on the as-polished steel surface, but these graphitic peaks do not present when the dual metal interlayer is applied. This confirms that the ultra-thin interlayer has effectively inhibited graphitization occurring on the steel surface. The results have demonstrated that the ultra-thin interlayer can enhance diamond nucleation density/rate and inhibit the formation of graphite intermediate layer on steel surface. Nevertheless, continuous and adheresive diamond thin films are found difficult to obtain because of severe buckling and fragmentation of the

Fig. 4. SEM images of diamond grown on SS304 with (a) 20 nm thick Cr, and (b and c) 50 nm Cr–Al interlayer. (b) A general view, (c) a magnified view of free-standing diamond fragment. (d) Raman spectra of the exposed steel surface (I) and top diamond film (II).

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Fig. 5. GIXRD patterns of SS304 surfaces with (a) Ti–Al, and (b) Cr–Al interlayer after 6 h deposition. : Diamond, : Fe3 Al5 , &: Fe3 Al, : Cr3 C2 , *: TiC.

diamond film formed. Fig. 3 shows the surface morphologies of Ti/Al interlayered SS304 after 6 h deposition. The diamond films have broken into small pieces (white in Fig. 3a) and they can be very easily removed from the substrate. The delaminated diamond films also offer an opportunity to examine the underlying intermediate layer. This intermediate layer beneath diamond film is determined to be titanium carbide by electron probe microanalysis. It is basically continuous except for some cracking defects and local spalling, from which Al and the base metals Fe have been detected (Fig. 3b). Fig. 3c shows the Raman spectra taken from different surfaces including the diamond (curve I), the carbide sub-layer (curve II) and the exposed substrate (curve III), respectively. The diamond spectrum demonstrates typical nanocrystalline features, with sharp diamond characteristic peak at 1332 cm−1 and two broad peaks around 1140 and 1470 cm−1 , respectively. No peaks related to carbon species are recorded from the carbide layer, while graphitic carbon species are detected on the exposed substrate where the metal carbide sub-layer spalls locally. This indicates that if local spalling in the carbide layer occurs during CVD process, the ultra-thin Al inner layer alone may not completely suppress graphite formation. This will largely deteriorate the uniformity and adhesion properties of diamond thin films produced. Similarly, a single ultra-thin Cr interlayer (20 nm thick) produces no continuous diamond film, but overgrowth of graphitic carbon and diamond fragments are clearly observed (Fig. 4a). Fig. 4b and c shows the diamond grown on SS304 with an ultra-thin Cr/Al interlayer. Again, no continuous diamond films at a larger scale are obtained. In the Raman spectrum (Fig. 4d), characteristic diamond peak centered at 1332 cm−1 is observed, due to its free-standing state on the steel surface. Otherwise, an upward shift of the diamond peak position is normally noticed if the diamond film formed still sticks tightly to the substrate [18]. Fig. 5 shows the GIXRD identified structural evolutions of the two ultra-thin dual metal interlayers after CVD process. Besides the diamond phase and Fe–Al compounds, the carbide phases of Ti, Cr are formed. This confirms that during the CVD process, the Al interlayer has diffused into the steel surface and formed Fe–Al compounds [25], and the carbide-forming metals are transformed into their respective carbides. These results show that an ultra-thin Ti/Al or Cr/Al dual metal interlayer on SS304 substrate modified the surface conditions for diamond nucleation and growth. Inward diffusion of Al favors a local Al-rich layer on the steel surface, and it greatly suppresses the formation of graphite soot on the steel surface due to the catalytic effect of iron [4,5,17]. The effective inhibition of the non-diamond phase formation on steel surface provides abundant carbon source for diamond nucleation and would also enhance the interfacial adhesion ability of diamond films. On the other hand, the carburization of Ti or Cr layer at the early stage of deposition consumes

carbon and would delay the diamond nucleation. A pre-scratching treatment of the interlayer, however, embeds abundant diamond particles in the softer metal layer to act as diamond nucleation sites, enhancing diamond nucleation and deposition [15,20,26]. Formation of the metal carbide on the interface between diamond and steel would also provide a helpful chemical bonding to enhance film adhesion. Nevertheless, the diamond thin films formed still demonstrate unexpected de-bonding due to significant buckling and fragmentation. The diamond adhesion failure mechanism with the ultra-thin interlayer is different from that directly formed on bare steel. In the latter case, the diamond thin films grow on the top of loose graphite soot and are actually in a free-standing state. They can even remain continuous after spontaneous delamination [5,27]. With the aid of ultra-thin interlayers, the diamond thin films formed can stick more tightly to the substrate. However, due to the mismatch of thermal expansion coefficients among the diamond film, intermediate layer and the substrate, huge thermal stresses are produced in the films and interfaces. Buckling and fragmentation are initiated in the diamond films when the plastic deformation of diamond film is insufficient to accommodate the de-bonding strength induced by residual stress relief. Further optimization of the ultra-thin interlayer structures and the deposition conditions are required to address the aroused adhesion problems. 4. Conclusions Diamond coating on SS304 with two kinds of Ti/Al and Cr/Al dual metal interlayers of 50 nm thickness has been investigated. The results have demonstrated that the ultra-thin dual metal interlayers are able to effectively suppress the graphite formation on steel surface. Nevertheless, the diamond films produced still fail to remain continuous and adherent to the substrate. Further optimization of the ultra-thin interlayer structures and the deposition conditions are required in order to achieve adhesive diamond thin films on the steels. Acknowledgements This study is sponsored by the Canada Research Chair Program and by the Natural Sciences and Engineering Research Council of Canada. The NEXAFS was measured at the Canadian Light Source, University of Saskatchewan, which is supported by NSERC, NRC, CIHR, and the University of Saskatchewan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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