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Diamond & Related Materials 17 (2008) 294 – 299 www.elsevier.com/locate/diamond
Adherent and low friction nano-crystalline diamond film grown on titanium using microwave CVD plasma S.J. Askari a,b,⁎, G.C. Chen b , F. Akhtar b , F.X. Lu b a
b
Institute of Manufacturing Engineering, PNEC, National University of Sciences and Technology (NUST), Karachi-75350, Pakistan Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 12 December 2006; received in revised form 20 December 2007; accepted 31 December 2007 Available online 11 January 2008
Abstract The use of titanium alloys for aerospace and biomedical applications could increase if their tribological behavior was improved. The deposition of an adherent diamond coating can resolve this issue. However, due to the different thermal expansion coefficients of the two materials, it is difficult to grow adherent thin diamond layers on Ti and its metallic alloys. In the present work microwave plasma chemical vapor deposition (MWPCVD) was used to deposit smooth nano-crystalline diamond (NCD) film on pure titanium substrate using Ar, CH4 and H2 gases at moderate deposition temperatures. Of particular interest in this study was the exceptional adhesion of approximately 2 μm-thick diamond film to the metal substrate as observed by indentation testing up to 150 kg load. The friction coefficient, which was measured with a cemented carbide ball of 10 mm diameter with 20 N load, was estimated to be around 0.04 in dry air. Morphology, surface roughness, diamond crystal orientation and quality were obtained by characterizing the sample with field emission electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy, respectively. © 2008 Elsevier B.V. All rights reserved. Keywords: Pure titanium; Nano-crystalline diamond film; Adhesion; Low friction coefficient
1. Introduction The outstanding properties and useful applications of diamond films have aroused a great deal of interest in the synthesis of microcrystalline diamond (MCD) films by a variety of chemical vapor deposition (CVD) techniques with carbon containing gas mixtures such as CH4/H2. Generally, the thicker the film and the larger the grain size, the rougher the surface of the diamond films in CH4/H2 plasma. Consequently, it is necessary to polish the surface of the PCD film in order to improve its friction and wear characteristics. However, the polishing process is very tedious, time-consuming, and costly for large areas [1]. Polishing of MCD coatings by either chemical or mechanical methods seems
⁎ Corresponding author. Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail address:
[email protected] (S.J. Askari). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.045
impractical because of diamond's extreme hardness and chemical inertness. Therefore it is of importance to produce uniform, flat and smooth nano-crystalline diamond (NCD) films. NCD coating consisting of nanometer-sized diamond crystals leads to the growth of smooth, electrically conductive and still extremely hard diamond films. These smooth and thin NCD films are very desirable as an engineering material for tribological, electronic and biomedical applications [2]. Most tribological applications of CVD diamond coatings require smooth surfaces, which are not readily prepared from the CH4/H2 plasma [3]. Titanium alloys possess the highest specific strength of all metals up to temperatures of 600 °C and an excellent corrosion resistance [4]. These properties make titanium an important material in aerospace, biomedical and chemical plant applications [5,6]. A big disadvantage, however, is the comparably high susceptibility to wear and friction [7,8]. The deposition of a well adherent diamond coating can overcome this problem [9–12]. However, high quality diamond coatings are difficult to achieve on these metal substrates for a number of reasons. Poor film
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adhesion occurs for many systems, in part because of the large mismatch in thermal expansion coefficient between diamond and the Ti substrate. At the temperatures required for diamond growth, this leads to large residual stresses at the film/substrate interface and can cause delamination. Additionally, Ti and Ti alloys undergo chemical reactions during the CVD diamond deposition process (typical growth temperatures N 700 °C), adversely altering the properties of the substrate and preventing growth of the film [10–12]. Past studies have revealed that NCD thin films from microwave Ar/H2/CH4 at various concentrations of Ar can be deposited on silicon substrate as Ar gas is essential to produce NCD films since the concentration of Ar influences secondary formation of nuclei [2,3]. Astonishingly, there are few reports available in the published literature on the effects of the addition of argon in a CH4/H2 gas mixture for the deposition of NCD film on pure Ti substrate in which the issue of interfacial adhesion is relevant for tribological applications. The purpose of the present research investigation was to deposit a smooth, homogenous and fine-grained NCD film in the environment of Ar/H2/CH4 gas mixture on pure Ti at a moderate temperature. The surface morphologies, structure, adhesion and tribological characteristics of the obtained film were examined. 2. Experimental 2.1. Sample preparation Pure Ti wafers with dimensions 12 × 12 × 2 mm3 were mechanically ground by using emery papers of grit sizes 400#, 600#, 800#, 1000#, 1400# and 2000# respectively, to get mirror-like smoothness. Afterwards these samples were cleaned in de-ionized water and in acetone for 10 min. The final preparation step involved ultrasonic seeding for 60 min in a bath of ethanol and diamond powder (30–40 μm grain size), followed by ultrasonic cleaning and rinsing in acetone and drying in open air. 2.2. NCD film deposition parameters
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images that were obtained in the tapping mode. The variation on the whole surface was approximately ±10%. The quality and purity of diamond grains grown on NCD film were assessed by a micro-Raman spectrometer (Jobin Yvon HR-800) with approximately 1 μm spot size using a 532 nm diode laser source. The coating crystalline structure was obtained by X-ray diffraction (XRD) with (Cu Kα 40 kV/150 mA) radiation source. The adhesion testing was done with a Rockwell indentation method using a 0.2 mm tip radius diamond spheroconical indenter (cone angle of 120°). In particular, the friction and wear properties of NCD film were evaluated using a Schwingungs Reibung und Verschleiss (SRV) ball on disk tribometer with reciprocating sliding against a cemented carbide ball of 10 mm diameter at a high contact load of 20 N in dry air. 3. Results and discussion 3.1. NCD film growth mechanism, morphology and structure NCD films can be produced in two ways using MWPCVD process: the first one is to increase the diamond nucleation density during the initial growth stages; the other is to control the crystal growth into nanometer scale. High methane concentrations as well as argon addition both have significant effects on the formation of nano-crystallite. The argon addition not only reduces diamond grain size but also enhances diamond film's tribological properties in the MWPCVD system [2]. A high diamond nucleation density is initially obtained by ultrasonic scratching with diamond powders and using biasenhanced nucleation (BEN) from our earlier work [13,14]. Fig. 1 shows the FE-SEM image of as-deposited NCD film. It can be seen from the micrograph that the NCD film is composed of large diamond clusters (500 nm− 1 µm in size) that contain tiny diamond crystals. The average grain size of these crystals is between 20 and 30 nm. The r.m.s. surface roughness of this smooth and fine-grained film is approximately 25 nm obtained from AFM micrograph as shown in Fig. 2. The NCD film is approximately 2 μm thick. In NCD film growth conditions, smooth granular surfaces similar to ball-like particles and of
The NCD film was deposited by using bell-jar type MWPCVD equipment. The feed gases consisted of Ar, H2 and CH4 in the ratios 100:98:2, respectively. A total pressure of 7–8 kPa and a substrate temperature of 600–650 °C were maintained at an input microwave power of 800 W with total deposition time of 6 h. 2.3. Characterization of deposited film The investigations of surface morphology were examined with a (Zesis supra 55 with 1.0 nm resolution) field emission scanning electron microscopy (FE-SEM). The grain size was also measured by using this technique. The growth rates were obtained in units of μm h− 1 by calculating from the weight differences of the substrates before and after deposition, assuming a constant density of 3.5 g cm− 3 for diamond. The r.m.s surface roughness was evaluated with a (Shimadzu, SPM-9500J3) atomic force microscopy (AFM), by the analyses of several 2 × 2 µm2 AFM
Fig. 1. FE-SEM image illustrates the surface morphology of as-deposited NCD film.
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Fig. 2. AFM micrograph shows surface smoothness of as-deposited NCD film.
cauliflower-like morphology are known to be diamond clusters composed of nano-metric crystallites [15–17]. The NCD grains achieved in the film are not grown from the initial nuclei, which will lead to MCD films normally displaying columnar morphologies, but are the consequences of the steady re-nucleation or poly-nucleation processes. Before a diamond crystal can grow further, a new growth center is formed and the growth of the former crystal is inhibited. Thereby the nano-diamond crystals are obtained. Unlike MCD films, which are non-uniform over large areas, these coatings are relatively uniform over the entire surface. Previous studies have shown that NCD films with grain sizes of 10–50 nm may contain a significant proportion of amorphous or non-sp3-hybridized carbon atoms at grain boundaries. Gruen has suggested that the nano-crystalline phase results from the insertion of carbon dimers into carbon–carbon and carbon–hydrogen bonds, which leads to heterogeneous nucleation rates of the order of 1010 cm2 s− 1 [2]. The surface morphology of the diamond film is controlled by deposition and substrate pre-treatment parameters. For example, the diamond slurry pre-treatment reduces
the formation of ball-like clusters and allows for the formation of a smooth NCD film. The XRD pattern of NCD film specified in Fig. 3, displays an identical characteristic with peaks at a diffraction angle 2θ of 44.1°, 75.4° and 91.4° which corresponds to the (111), (220) and (311) reflections of the diamond, respectively. The diamond (111) peak became detectable after a 3-hour deposition, and its signal was extremely weak in contrast with the titanium peak. This indicates that, for a pure titanium substrate, a longer time is required for a continuous diamond film deposition with substantial thickness. This is possibly due to the high diffusion coefficient of carbon in pure titanium, and the formation of a thin TiC interlayer. No peak of titanium hydride can be seen. No graphite peak is discernable at about 26.4°, indicating the nonexistence of any substantial amount of crystalline graphite. However, apart from the easily recognizable diamond, Ti and TiC peaks, there are still few unknown (?) diffraction peaks in our XRD pattern. Regrettably, their detection is unclear because the diffraction patterns of structures that could be considered here, such as graphite or carbon and β-titanium, are considerably dissimilar. Furthermore, the formation of titanium hydrides by the in-diffusion of interstitial hydrogen, as well as the presence of titanium oxide in the structure [11,12] formed under plasma oxidation can be excluded. These similar unknown peaks have also been reported before during diamond deposition on Ti substrate [18]. X-ray diffraction patterns are less sensitive to the presence of amorphous carbons. In contrast, Raman scattering is approximately 50 times more sensitive to small grain size or amorphous diamond and graphite and, hence, it is frequently used to characterize CVD diamond films. Fig. 4 depicts the Raman spectra of the as-grown NCD film which clearly verifies the occurrence of the characteristic diamond and non-diamond carbon phases in the deposited film. The most significant feature in the Raman spectrum of the NCD film is the weak peak near 1140 cm− 1, whilst the diamond peak near 1340 cm− 1, an
Fig. 3. XRD patterns of the 6 h deposited NCD film.
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Fig. 4. Raman spectra of the 6 h deposited NCD film.
unambiguous signature of cubic crystalline diamond, is not very sharp. For some years now the 1140 cm− 1 peak has been attributed to either trans–para-acetylene [19] or trans-polyacetylene [20] or due to disordered sp2-bonded carbon at the grain boundaries of the NCD film [21], which could be evidence the presence of NCD [22]. In other words, the 1140 cm− 1 Raman peak does not originate from diamond. Absence of a sharp well defined diamond peak near 1332 cm− 1, in spite of having an intense NCD feature, could be due to a high density of defects incorporated in the films but could also be a sign of uniformly distributed short range sp3 crystallites in the films [23]. The Raman feature near 1550 cm− 1 may be related to sp2-bonded carbon residing at grain boundaries [24]. The high density of defects and a significant amount of graphitic carbon are, in fact, to be expected in the growth of uniformly distributed nano-crystals of diamond because of their large grain boundary area.
the indentations, revealing that the film has good adhesions to the substrate. The reasons of obtaining good adhesion were moderate deposition temperatures 600–650 °C were used in the present study. Due to the use of moderate deposition temperatures, it is expected to have much lower thermal stresses as compared to those deposited at higher deposition temperatures. A high nucleation density was initially obtained that could provide very large number of contact points with the Ti substrate. This high nucleation density also permitted a more rapid nucleation and growth of diamond particles at the moderate temperatures. This rapid diamond nucleation process offered an increased barrier to the diffusion of excited C and H ions reaching into the surface of Ti substrate and forming carbides and hydrides [10–12]. As a result a thin TiC interlayer was formed, as shown in Fig. 3, which could provide certain kinds of chemical bonding for the NCD film and Ti substrate and enhanced the interfacial adhesion drastically.
3.2. NCD film adhesion and low friction coefficient A primary requirement for the industrial use of a CVD diamond is its adhesion to the substrate. Good adhesion of coatings on pure Ti and its alloys' substrate materials is essential to their performance during friction processes and wear applications. It is necessary to characterize the bond strength between different coating layers and between the coating structures and the substrate by appropriate measurement techniques. In the adhesion testing the most widely applied indentation procedure is the Rockwell indentation method using a 0.2 mm tip radius diamond spheroconical indenter (cone angle of 120°). Fig. 5 illustrates the morphologies of indentations under four different loads of 30, 60, 100 and 150 kg. Each load was administered for a duration of approximately 15–20 s. The cone-shaped indenter penetrates through the diamond film into the substrate, causing the Ti to plastically deform. It is clear from the micrograph that no severe cracks were initiated around
Fig. 5. SEM image reveals good adhesion of NCD film under four different loads of 30, 60, 100 and 150 kg.
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Fig. 6. Friction coefficients of NCD film sliding against a cemented carbide ball in dry air. (Test conditions: relative humidity, 35%; sliding distance, 1000 m; ball diameter, 10 mm; speed, 0.2 m s− 1; contact load, 20 N).
The coefficient of friction (COF) was evaluated by using a ball on disk tribometer with reciprocating sliding against a cemented carbide ball of 10 mm diameter at a high contact load of 20 N in dry air. The other parameters were: relative humidity ~ 35 ± 2%; total sliding distance ~ 1000 m; temperature ~ 25 °C and sliding speed ~ 0.2 m s− 1. The COF was recorded on computer continuously during the whole experiment. Fig. 6 shows the COF of this NCD film, which reveals the friction coefficient of the cemented carbide ball against asdeposited NCD film, remains low, about 0.04. At the start COF was at approximately 0.24 and decreased to about 0.04 constant values after covering only 20 m distance. This was due to the small surface roughness of this film, which was approximately 25 nm r.m.s. and the average grain size, which was around 20–30 nm. There were no severe wear scars on the cemented carbide ball after sliding with this smooth and fine-grained NCD film. After the friction test, the NCD film was hardly worn out, and the wear rate of the counter face cemented carbide ball was approximately 1.3 × 10− 7 mm3/N m. The so-formed NCD film is highly wear resistant. The significant low COF is probably attributable to: (1) The nanocrystalline grain size, which yields a very smooth surface and consequently a low COF; and (2) a small fraction of amorphous carbon at the boundaries of diamond grains (see Raman spectra in Fig. 4), which has a lubricating effect. 4. Conclusions NCD film was successfully synthesized on Ti substrate using Ar/H2/CH4 microwave plasmas at moderate deposition temperatures. This was possible partly because the high secondary nucleation rates in Ar/H2/CH4 plasma favored its growth. The interfacial adhesion of this NCD film with Ti substrate was
exceptionally good. That is why this film did not peel off the substrate even at a high load of 150 kg. The friction coefficient, which was measured with a WC ball of 10 mm diameter with 20 N load, was estimated to be approximately 0.04 in dry air. The analyses of FE-SEM, AFM, Raman spectroscopy and XRD confirm that NCD film basically contains NCD and exhibits a fine-grained morphology with a low surface roughness. All these results reveal that smooth NCD films provide a wide range of possible technical applications for the use of Ti and its alloys in the field of tribology. Acknowledgements The authors wish to express their gratitude for the financial support to the NSFC (Natural Science Foundation of China) under contract No. 50572007, and the Foundation for Doctorial Stations of the Ministry of Education of China. References [1] A. Erdemir, G.R. Fenske, A.R. Krauss, D.M. Gruen, T. McCauley, R.T. Csencsits, Surf. Coat. Technol. 120–121 (1999) 565. [2] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [3] T.S. Yang, J.Y. Lai, C.L. Cheng, M.S. Wong, Diamond Relat. Mater. 10 (2001) 2161. [4] M.J. Donachie, Titanium: a Technical Guide, ASM, Metals Park, OH, 1988. [5] G. Heinrich, T. Grogler, S.M. Rosiwal, R.F. Singer, Surf. Coat. Technol. 94/95 (1997) 514. [6] T. Grogler, E. Zeiler, M. Dannenfeldt, S.M. Rosiwal, R.F. Singer, Diamond Relat. Mater. 6 (1997) 1658. [7] K.E. Budinski, Wear 151 (1991) 203. [8] K.G. Budinski, Conference Proceedings, Wear of Materials, ASME, Orlando, Florida, 1991, p. 289. [9] S.A. Catledge, Y.K. Vohra, J. Appl. Phys. 83 (1998) 198. [10] B. Yan, N.L. Loh, Y. Fu, C.Q. Sun, P. Hing, Surf. Coat. Technol. 115 (1999) 256.
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