Deposition of cubic boron nitride films on diamond-coated WC:Co inserts

Diamond & Related Materials 18 (2009) 1387–1392

Contents lists available at ScienceDirect

Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d

Deposition of cubic boron nitride films on diamond-coated WC:Co inserts Y.M. Chong, W.J. Zhang ⁎, Y. Yang, Q. Ye, I. Bello ⁎, S.T. Lee Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, People's Republic of China

a r t i c l e

i n f o

Article history: Received 29 January 2009 Received in revised form 14 July 2009 Accepted 24 August 2009 Available online 1 September 2009 Keywords: Cubic boron nitride Chemical vapor deposition Cutting inserts Mechanical properties

a b s t r a c t Cubic boron nitride (cBN) thin films were deposited on diamond-coated tungsten carbide (WC) cutting inserts using electron cyclotron resonance (ECR) microwave plasma chemical vapor deposition (MPCVD). The effects of gas flow rate and substrate bias on the phase composition and structure of the BN films deposited on diamond surfaces were studied. It was revealed that both the cubic phase formation and the selective etching of hexagonal phase were controlled by modulating the hydrogen and boron trifluoride flow rate ratio. By the trial and error method the gas flow rate ratio and substrate bias voltage were optimized. Moreover the phase composition of the BN film was found to be affected by the thickness of diamond buffer layer and interrelated to the effective substrate bias. The hardness of the resulting cBN films reached the value of 70 GPa. In the synthesized coatings, the diamond beneath renders the best mechanical supporting capacity while the top cBN provides the superior chemical resistance and extreme hardness. The cBN/ diamond bilayers deposited on WC inserts may serve as universal tool coatings for machining steels and other ferrous metals. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Cubic boron nitride (cBN) is a synthetic material exhibiting both high hardness and high thermal conductivity which are nearly comparable to diamond [1,2]. Particularly, chemical inertness and thermal stability of cBN are superior to diamond [3,4]. While diamond burns at 600 °C and dissolves in iron at elevated temperature, cBN is chemically stable and does not react with ferrous materials at temperatures up to 1200 °C [5]. These properties along with maintaining the extreme hardness up to temperatures above 1000 °C make cBN the best candidate for machining steels and other ferrous materials. Thus far, cBN powders synthesized by high-pressure high-temperature (HPHT) methods are commercially available as abrasives, and they are also molded and cemented by metal binders to produce cutting tools with different shapes [6]. However, the technological difficulty in cementing cBN powder into complex shapes and the cost of these tools require the development of new methods for cBN synthesis. Various physical and chemical vapor deposition (PVD and CVD) techniques have been exploited to synthesize cBN films [7–9]. The common characteristic of the deposition of cBN films by either PVD or CVD methods is the application of ion bombardment with an energy ranging from 50 to 1000 eV during both nucleation and growth. The growth of cBN films is dominated by physical phase transfer from sp2 to sp3 BN at the subsurface regions. The ion bombardment, however, is ⁎ Corresponding authors. W. Zhang is to be contacted at Fax: +852 2788 7830. Bello, Fax: +852 2788 7830. E-mail addresses: [email protected] (W.J. Zhang), [email protected] (I. Bello). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.08.010

inevitably accompanied with significant build-up of compressive stress (5–20 GPa). Cubic BN films usually delaminate above a thickness larger than 200 nm due also to high internal stress. The characteristic of cBN growth also is that cBN films normally grow via amorphous (aBN) and turbostratic boron nitride (tBN) interfacial layers. These non-cubic phases are soft and contain many defects including boron dangling bonds which are highly reactive particularly at exposure to humid environment. These reactions lead to the formation of chemically unstable oxyboron hydrides and as a result the mechanical destabilization of the films [10]. Cubic BN films have been deposited on tungsten carbide (WC) inserts for tooling applications. Ikeda et al. [11] synthesized BN films with 50% cubic phase at 400 °C using an ion plating technique. Yu et al. [12] grew cBN films directly on the WC inserts at 1050 °C by DC jet plasma. However, the cobalt binder of the WC reacted with the nitrogen to form cobalt nitride, promoting the formation of hexagonal BN (hBN) and in turn inhibiting the growth of cBN. Since cBN deposited directly on WC was mechanically unstable, transition layers were introduced to improve the adhesion and mechanical stability of cBN films. Okamoto et al. [13] deposited 150 nm thick cBN films on mirror-polished WC substrates via a thin boron buffer layer and using diborane precursor in an electron cyclotron resonance (ECR) plasma enhanced CVD (PECVD) system. However as discussed above, boron is highly reactive and tends to form oxyboron hydrides upon the exposure to ambient atmosphere. Setsuhara et al. [14] deposited cBN coatings with thickness of 0.7 µm on WC substrates at 400 °C by inserting a 1.7 µm-thick boron-rich BN buffer layer using ion beam assisted deposition (IBAD). It can be expected that the boron-rich layers suffer from the same reactive problems. Keunecke et al. [15]

1388

Y.M. Chong et al. / Diamond & Related Materials 18 (2009) 1387–1392

reported 1.2 µm-thick cBN films deposited on WC inserts with TiN/ BCN transition layers by radio-frequency diode sputtering. Their turning test shows that the cutting life of the cBN coated inserts is prolonged by a factor of 2–3 when compared to the TiN and TiAlN coatings [16]. Thus far, the deposition of thick (N2 µm), adherent, and mechanically stable cBN films with low internal stress on WC inserts remains a challenge for tooling applications. In this study, cBN films were deposited on WC cutting inserts by ECR microwave plasma CVD (MPCVD) with assistance of fluorine chemistry. This approach allowed the deposition of cBN films at very low ion energies. As a consequence, the residual stress in the cBN films decreased to 1 ~ 2 GPa, and cBN films with a thickness of 20 µm were prepared [17]. In this work the substrates were precoated with transition CVD diamond layers prior to growing cBN films. The match of cBN and diamond in lattice structures, lattice parameters, and surface energies enables direct or even heteroepitaxial growth of cBN on diamond without soft non-cubic BN interfacial layers [18–20]. Diamond coatings on drill bits and cutting tools have already been demonstrated. The diamond coating technology is well established, and diamond-coated cutting tools are commercially available. Deposition of the cBN on diamond-coated cutting inserts can furnish the excellent cutting performance of the tools used in machining hard steels. The top cBN provides the chemical resistance of the cutting insert while the diamond beneath renders the best mechanical supporting capacity. Moreover, combination of two of the most thermally conductive materials yields the effective heat dissipation which can reduce the temperature of the cutting inserts in the contact areas of tools and workpieces. The cBN deposition conditions were optimized by varying the processing gas flow rate ratio, the bias voltage and thickness of the diamond coating.

(SEM, Philips XL 30). The deposited BN films were characterized by Fourier transform infrared spectroscopy (FTIR, Perkin Elmer PC 16) operating in a reflection mode. The IR beam was irradiated to the sample surfaces at a fix incident angle of 35°. The reflection spectra obtained were converted to absorption spectra by using Kramers–Kronig transformation. The phase composition was further examined by Raman spectroscopy operating with an excitation UV wavelength of 244 nm. The film thickness of the cBN films was measured by SEM. Thickness measurement required dissection of the cBN–diamondcoated cutting inserts with diamond saw. The dissected inserts were mounted in a conductive epoxy and mirror-polished with diamond lapping paper before the measurement. The hardness of the cBN– diamond composite films was evaluated using nano-indentation (MTS) in continuous stiffness measurement mode. 3. Results and discussion

2. Experimental

Fig. 1(a) shows the SEM surface morphology of the polycrystalline diamond film coated on a WC insert. It is revealed that the film is mainly composed of faceted diamond grains. The Raman spectrum in Fig. 1(b) depicts a sharp diamond peak at 1337 cm− 1 and a broad graphite characteristic peak centered at 1530 cm− 1, which denotes the high quality of diamond grains and the inclusion of a small amount of sp2 carbon. The phase composition and structure of cBN films are very sensitive to deposition condition. Among all the parameters, the partial pressures of gas mixture and bias voltage are the most crucial. Temperature is also an important parameter, and the effect of temperature on the growth of cBN has been investigated over a wide temperature range previously [22]. It has been shown that only the deposition at relatively high temperature enables to prepare thick films which are mechanically stable and have microstructures giving rise to Raman spectra

Cubic BN films were deposited on the rake face of diamond-coated WC:Co (K10-K20 SEKN 1203 AFFN) inserts by ECR MPCVD plasma running in a gas mixture of BF3 + N2 + Ar + He+ H2. The gas flow rates of BF3:N2:Ar:He were maintained at 1:50:10:140 in sccm, respectively. The flow rate of hydrogen was varied in different depositions. Diamond films with high phase purity were prepared on chemically etched WC: Co cutting inserts by hot filament CVD using methane and hydrogen as reactant gases. The chemical etching was conducted by immersing the top surface region of WC:Co inserts in a 10% HNO3 solution for 10– 15 min at room temperature, as a result Co in surface region was depleted to eliminate catalytic effect of cobalt on diamond–graphite conversion. The thickness of the diamond coatings was 4 ± 1 µm unless specified otherwise. The configuration of the cBN deposition system was reported elsewhere [21]. First the deposition reactor was pumped down to ~10− 6 Torr. Then the substrate temperature was gradually increased to 900 °C and kept at this value during cBN growth. The total flow rate of gases and effective pumping speed maintained the total deposition pressure of 2 × 10− 3 Torr, while flow rate ratios of gases were optimized to yield the highest cBN quality. The ECR plasma was induced by the 1400 W microwave power generator (2.45 GHz) with the assistance of a magnetic field of 875 G. Prior to the cBN deposition, the substrates were in-situ cleaned and etched by ECR plasma at a positive substrate bias of +30 V for 5 min in a gas mixture the same as that used for cBN growth. The reactive negative fluorine ions attracted by the positive bias effectively removed the residual impurities and prepared the surface for subsequent cBN nucleation and growth. Switching the positive to negative substrate bias converted the etching to nucleation and growth process. The negative direct current (D.C.) bias varied from −25 to −55 V in different deposition runs. The deposition time was 4 h in all cases unless specified otherwise. The phase composition and the surface morphology of the diamond coatings on inserts were investigated by Raman spectroscopy (Renishaw InVia, λ = 514.5 nm) and scanning electron microscopy

Fig. 1. (a) The SEM micrograph taken from the surface of a diamond-coated WC and (b) the corresponding Raman spectrum.

Y.M. Chong et al. / Diamond & Related Materials 18 (2009) 1387–1392

characteristic to the cBN phase. For the deposition on WC inserts all experiments were conducted at a constant temperature of 900 °C to minimize the diffusion of internal Co to the surface and catalytic phase conversion that might mechanically destabilize the deposited films. The details of these processes are not further discussed here. The partial pressures of gas phase constituents and thus individual flow rates of all gases (N2, BF3, H2, Ar and He) supplied into the reactor and their ratios are key factors directly related to the cBN phase formation. Gas flow rates and their ratios along with other deposition parameters had to be optimized by trial and error methods in numerous experiments. Obviously, N2 and BF3 molecular gases are precursors for deposition of BN. It was experimentally found that nitrogen has to be supplied in an excess amount with respect to the boron precursor to provide stoichiometric BN [23]. However, the function of BF3 is not only to serve as a boron source by providing fluorinated species, but also to form fluorine that assists to selectively etch non-cubic BN phases [24]. Furthermore, the thermodynamic calculations show that BN films can hardly be grown from Ar–N2–BF3 plasma [24,25] unless H2 is supplied into the reaction gas mixture [26]. As a result, the H2 to BF3 ratio is the key factor determining the phase composition of the deposited BN films. The chemical compositions of the BN films prepared at different of H2 to BF3 ratios are depicted in Fig. 2. All films were deposited at a constant bias voltage of −35 V. The FTIR absorbance spectra indicate a characteristic peak located at 1076 cm− 1 for the ratio H2/BF3 = 2. This peak is assigned to the transverse optical (TO) phonon mode of cBN. This phonon mode also emerges in the spectra acquired from the cemented cBN inserts. As shown in the figure, cBN is detected in the films prepared at H2/BF3 ratios of 2.5, 3 and 4 too. However, at the ratio of 4.5, a less dense phase hBN is primarily formed instead of the expected cBN. Addition of H2 leads to the formation of BN composite which can be described by the following reaction [24]: 2BF3 þ N2 þ 3H2 →2BN þ 6HF: In this work, BN films with high cubic content were prepared in the window of H2/BF3 ratios ranging from 2 to 4. Further increase of the H2/BF3 ratio led to the formation of hBN because the reactive fluorine radicals were excessively neutralized by activated hydrogen to form HF which is unreactive in respect to the BN structures. Hence the selective etching of the hBN phase was ineffective. On the other hand, when the H2/BF3 was reduced to 1.5, no BN product was formed and the diamond film was removed by severe reactive etching. Similar etching phenomenon was reported at a D.C. jet deposition of cBN on silicon substrates [26,27]. A precise control of the H2/BF3 flow rate ratio is essential to balance the formation of cBN and etching of hBN. The H2/BF3 ratio determines the growth rate and phase composition

Fig. 2. The FTIR spectra acquired from BN films deposited at different H2 to BF3 ratio.

1389

of cBN films deposited at a given bias voltage. Therefore the H2/BF3 may also affect the crystallinity of the cBN films. Nevertheless, as discussed below, the bias-induced ion bombardment was revealed to be a predominant factor to determine the crystallinity of the cBN films. The substrate bias voltage is another imperative parameter that determines the energy of ions impinging the growth surface and thus the phase formation of BN films. Energetic bombardment has been demonstrated to be indispensible for growing cBN. Investigation of the effect of the ion energy on the cBN nucleation and growth led to several proposed growing mechanisms [28–32]. Each of them explains the bombardment as a primary cause of cBN nucleation and growth. These models refer to PVD techniques. In CVD depositions employing fluorine chemistry, the ion bombardment mainly contributes to the breakage of the B–F bonds on the boron surfaces which also requires energy of ~ 20 V [33]. However the threshold values of energetic ion bombardment needed for cBN nucleation and growth very much depend on the deposition techniques and conditions. Knowledge on the importance of bias voltage for cBN formation and induced stress leads to searching for the lower nucleation and growth thresholds. Fig. 3 shows the FTIR spectra measured from the BN films prepared at various bias voltages whereas a constant BF3/H2 ratio of 2.5:1 was employed for all depositions. An absorption peak at ~1013 cm− 1 appeared in all tested samples. The origin of this peak is not clearly known, however, it is observed to be originated from pristine WC:Co inserts before diamond and cBN depositions. For the films prepared at − 45 and −35 V, the IR signal of cBN is shifted up to 1090 cm− 1 and the films are stable at the ambient conditions. A shoulder peak at 1290 cm− 1 appeared in the sample deposited at −35 V may be due to the interference of IR signals from diamond surface and interface. Increasing the bias voltage to −55 V causes downshift of the cBN to 1056 cm− 1. The cBN film deposited at −55 V spontaneously peels off upon exposure to ambient which is caused by the high residual stress accumulated by the relatively high energy ion bombardment during deposition. The difference in peak position at −55 V is due to the stress relaxation after the film delamination. On the other hand reducing the bias voltage to −25 V yields a BN film with a small amount of cBN (~ 20%). Therefore in this study, −35 V is taken as the lower threshold for cBN formation. It should be noted that the effective bias voltage is the sum of external bias and potential difference between the plasma potential and biased electrode. Since the plasma potential may vary with changing other electric discharge

Fig. 3. The FTIR spectra of boron nitride films deposited at different bias voltage.

1390

Y.M. Chong et al. / Diamond & Related Materials 18 (2009) 1387–1392

Fig. 4. The UV Rama spectra (244 nm) of the BN films deposited at different bias voltage.

parameters in principle from few volts up to ~ 100 V, it can significantly affect the energy of ions impinging the growth surface. Hence two systems should not be compared based on the external bias, but on the effective bias which however requires the measurement of plasma potentials. The cBN films prepared at the bias voltages of −35, −45, and −55 V were further examined by UV Raman spectroscopy, and the collected spectra are shown in Fig. 4. Two peaks center at 1337 and 1560 cm− 1, being associated with diamond and graphite, are observed in the three spectra. Two minor peaks of the spectrum acquired from the film prepared at −35 V are located at wavenumbers of 1045 and 1294 cm− 1. The peak at 1045 and 1294 cm− 1 are assigned to the TO and longitudinal optical (LO) modes of cBN, respectively. It is reported that the full width

at half maximum (FWHM) of the Raman peaks depends on the crystallinity of cBN films. [34,35] The crystallinity here refers to the degree of structural perfection, which is associated with the existence of grain boundaries (crystallite size) and other defects. Poor crystallinity cause peak broadening and downshift of the peak position. The cBN Raman peaks do not emerge when the films are nanocrystalline. As a consequence, the Raman signatures of cBN peaks are not resolved for the films deposited at −45 and −55 V. In the film deposited under substrate bias of −55 V, an intensive hBN peak centered at 1370 cm− 1 is detected. However the peak corresponding to the hBN structure is not evident in the FTIR spectrum. This phenomenon is related to the bandgap of hBN (~5.5 eV) which is closer to the energy of UV excitation source (5.1 eV). The intensity of hBN peak is thus enhanced by resonance and the cBN spectral characteristics are discriminated as previously reported. [36,37] In this work, cBN Raman peaks were observed in the films deposited at −35 V. However, they were not resolved for the films deposited at −45 and −55 V, suggesting a reduced crystallinity of films deposited at elevated bias voltages. Fig. 5(a), (b), and (c) shows the surface morphologies of the cBN films deposited at − 35, − 45 and − 55 V, respectively. The diamond surface is totally covered by the cBN at film deposited at −35 and −45 V. However, as described above, the cBN film deposited at −55 V is discontinuous partially due to its peeling off upon the exposure to ambient. The detailed analysis of FTIR and Raman spectra as well as morphology shown in the SEM images suggest that the bias voltage of −35 V is optimal for growing mechanically stable cBN films with high phase purity. Fig. 5(d) shows the cross-sectional SEM image of the polished cBN–diamond-coated tungsten carbide cutting inserts. The 2.8 µm-thick cBN were grown on WC cutting inserts precoated with a diamond layer of 3 µm for 10 h at a bias voltage of −35 V. The BF3/H2 ratio was maintained at 2.5:1 throughout the growth process. No cracks and delamination were observed at the cBN and diamond interface. The average deposition rate was 200–300 nm/h which is

Fig. 5. The SEM micrograph of boron nitride films prepared at bias voltage of (a) − 35, (b) − 45 (c) − 55 V. (d) The cross-sectional SEM micrograph of the polished BN film deposited on diamond-coated WC inserts.

Y.M. Chong et al. / Diamond & Related Materials 18 (2009) 1387–1392

practically similar to that measured for cBN grown on Si substrates. The prepared cBN films are already within the range of thicknesses used for coating cutting tools (2–4 µm). It is known that much thinner cBN films deposited by PVD methods delaminate spontaneously upon the air exposure or over a short time period. The thick cBN coatings reported in this work do not show any signs of delamination when exposed to ambient condition. Our cBN coatings (grown via a diamond buffer layer) show enhanced mechanical stability in contrast to those grown directly on WC inserts without transition layers. As reported in Ref [12], cBN films were synthesized on WC inserts by DC jet CVD employing fluorine chemistry. However, the cBN films delaminated easily and spontaneously in the ambient. In our case, the cBN and diamond films do not delaminate even after being cut with a diamond blade for SEM and nano-indentation measurements, which indicates progress in deposition of cBN films. Besides the parameters discussed above, the nature of substrate also appeared to affect the phase composition and structure of the films deposited. Using the buffer layers of CVD diamond enables growing cBN films right on the diamond surface without any transition layer which contributes to the mechanical stability of the cBN films. Polishing of the CVD diamond films is not needed when cBN films are prepared by ECR MPCVD, which contrasts growing cBN films by PVD techniques [38]. However, we noted the thickness of the buffer layer influences the formation of cBN phase. The thickness effect illustrates the examples of cBN films grown on WC coated by CVD diamond films with thickness of 3, 6 and 12 µm. Growing BN films at identical conditions gives the structures characteristic with FTIR spectra seen in Fig. 6(a). The spectral peak at ~1072 cm− 1 being attributed to cBN, is collected from the BN structure prepared on the diamond layers with thickness of 3 and 6 µm. The pattern of periodic

1391

Fig. 7. The hardness of the polished cBN–diamond—WC inserts in reference to CVD diamond and bare WC as measured using indentation depth of 200 nm.

wave oscillation is induced by the interference of IR signals from diamond surface and interface with WC. The oscillation pattern depends on the thickness of the diamond coating as illustrated in Fig. 6(b). However, the formation of cBN is not evident on the WC substrate coated with the 12 µm because of the moderate substrate bias. The large diamond film thickness represents resistance which restricts the electric direct current flow to the earth, causing the substrate charging and lowering the effective bias. As a result only hBN can be formed. The hardness of the cutting inserts was evaluated using a nanoindenter as shown in Fig. 7. The hardness measurements were performed on the cross-section of the cBN sample shown in Fig. 5(d). The sample was mechanically polished prior to the indentation. The indentation depth was limited to 200 nm to ensure the spatial resolution of hardness measurements. The maximum loading used in the hardness measurements was 22 mN. Three independent measurements were performed, and the hardness obtained showed sound reproducibility. The hardness of 71 and 91 GPa were determined for cBN and diamond films respectively. These values are comparable to the hardness of cBN deposited on silicon substrates in our previous reports [2]. The measured hardness values are referenced to calibration standard of fused silica (~10 GPa) and the hardness of ~37 GPa being recorded for bare WC substrates. The combination of the two hardest materials in bilayer structures will certainly enhance the cutting performance of the cutting tool. The top cBN layer with the second highest hardness is chemically resistant to the ferrous materials, while the underlying diamond, the hardest material, provides the best compatibility for growing cBN films and the best mechanical supporting capacity for cutting application. 4. Conclusions

Fig. 6. FTIR spectra of (a) cBN films grown on WC cutting inserts coated by CVD diamond with different thickness; (b) The WC cutting inserts with diamond coatings of different thickness.

Thick cBN films were deposited on WC:Co cutting inserts by using ECR MPCVD with assistance of fluorine chemistry. The influence of growth parameters on the structure and phase composition of cBN films was studied systematically. The optimal windows for the gas flow rate ratio of the H2 and BF3 and the substrate bias were determined. The 2.8 µm-thick cBN films with high phase purity were prepared as evidenced by the FTIR and Raman spectroscopies. These thick coatings do not show any signs of delamination when exposed to ambient condition. An enhanced mechanical stability of cBN films was achieved due to the structural compatibility of interfacing diamond and cBN layers. However, the growing BN structure was affected by the thickness of diamond layers. Growing BN on thick CVD diamond does not provide cubic phase due to the reduction of effective DC bias. The hardness of the cBN and diamond is 70 and

1392

Y.M. Chong et al. / Diamond & Related Materials 18 (2009) 1387–1392

90 GPa, respectively. The combined super hardness and chemical inertness of cBN/diamond bilayers on WC inserts suggest potential use of cBN/diamond coatings in industrial practice. Acknowledgements This work was financially supported by the Research Grants Council (Project No. CityU 123806) and the Innovation and Technology Commission (No. ITS/077/06) of the Hong Kong Special Administrative Region, China. References [1] X. Jiang, J. Philip, W.J. Zhang, P. Hess, S. Matsumoto, J. Appl. Phys. 93 (2003) 1515. [2] C.Y. Chan, W.J. Zhang, S. Matsumoto, I. Bello, S.T. Lee, J. Cryst. Growth 247 (2003) 438. [3] T. Komarsu, Y. Kakudate, S. Fujiwara, J. Chem. Soc., Faraday Trans. 92 (1996) 5067. [4] V.L. Solozhenko, V.Z. Turkevich, W.B. Holzapfel, J. Phys. Chem. B 103 (1999) 2903. [5] R.C. DeVries, Cubic Boron Nitride: Handbook of Properties, in Rep. 72 CRD 178 1972 (General Electric Company). [6] T. Taniguchi, K. Watanabe, S. Koizumi, Phys. Stat. Sol. A 201 (2004) 2573. [7] W.J. Zhang, Y.M. Chong, I. Bello, S.T. Lee, J. Phys. D-Appl. Phys. 40 (2007) 6159. [8] S. Matsumoto, W.J. Zhang, New Diam. Front. Carbon Technol. 11 (2001) 1. [9] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, Mater. Sci. Eng. R 21 (1997) 47. [10] K.M. Leung, C.Y. Chan, Y.M. Chong, Y. Yao, K.L. Ma, I. Bello, W.J. Zhang, S.T. Lee, J. Phys. Chem. B 109 (2005) 16272. [11] T. Ikeda, Y. Kawate, Y. Hirai, J. Vac. Sci. Technol. A 8 (1990) 3168. [12] J. Yu, S. Matsumoto, J. Mater. Res. 19 (2004) 1408. [13] M. Okamoato, Y. Utsumi, Y. Osaka, Jpn. J. Appl. Phys. 31 (1992) 3455. [14] Y. Setsuhara, M. Kumagai, M. Suzuki, T. Suzuki, S. Miyake, Surf. Coat. Technol. 116– 119 (1999) 100. [15] M. Keunecke, K. Yamamoto, K. Bewilogua, Thin Solid Films, 399 (2201) 142.

[16] M. Keunecke, E. Wiemann, K. Weigel, S.T. Park, K. Bewilogua, Thin Solid Films 515 (2006) 967. [17] W.J. Zhang, S. Matsumoto, Appl. Phys. A 71 (2000) 469. [18] X.W. Zhang, H.-G. Boyen, N. Deyneka, P. Ziemann, F. Banhart, M. Schreck, Nat. Mater. 2 (2003) 312. [19] W.J. Zhang, I. Bello, Y. Lifshitz, K.M. Chan, X.M. Meng, Y. Wu, C.Y. Chan, S.T. Lee, Adv. Mater. 16 (2004) 1405. [20] T.S. Yang, Y.P. Cheng, C.L. Cheng, M.S. Wong, Thin Solid Films 447–448 (2004) 136. [21] W.J. Zhang, C.Y. Chan, K.M. Chan, I. Bello, Y. Lifshitz, S.T. Lee, Appl. Phys. A 76 (2003) 953. [22] Y.M. Chong, K.M. Leung, K.L. Ma, W.J. Zhang, I. Bello, S.T. Lee, Diamond Relat. Mater. 15 (2006) 1155. [23] C.Y. Chan, W.J. Zhang, K.M. Leung, I. Bello, S.T. Lee, Chem. Vapor Depos. 9 (2003) 181. [24] W. Kalss, R. Haubner, B. Lux, Diamond Relat. Mater. 7 (1998) 369. [25] S. Matsumoto, M. Nishida, K. Akashi, K. Sugai, J. Mater. Sci. 31 (1996) 713. [26] W.J. Zhang, S. Matsumoto, Chem. Phys. Lett. 330 (2000) 243. [27] J. Yu, S. Matsumoto, K. Okada, Appl. Phys. A 80 (2005) 777. [28] S. Amagi, D. Takahashi, T. Yoshida, Appl, Phys. Lett. 70 (1997) 946. [29] H. Hofsäss, H. Feldermann, M. Sebastian, C. Ronning, Phys, Rev. B 55 (1997) 13230. [30] C. Weissmantel, K. Bewilogua, D. Dietrich, H.-J. Hinneberg, S. Klose, W. Nowick, G. Reisse, Thin Solid Films 72 (1980) 19. [31] D.R. McKenzie, W.D. McFall, W.G. Sainty, C.A. Davis, R.E. Collins, Diamond Relat. Mater. 2 (1993) 970. [32] J. Robertson, Diamond Relat. Mater. 2 (1993) 984. [33] W.J. Zhang, C.Y. Chan, X.M. Meng, M.K. Fung, I. Bello, Y. Lifshitz, S.T. Lee, X. Jiang, Angew. Chem. Int. Ed. 44 (2005) 4749. [34] T. Werninghaus, J. Hahn, F. Richter, D.R.T. Zahn, Appl. Phys. Lett. 70 (1997) 958. [35] W.J Zhang, S. Matsumoto, Phys. Rev. B 63 (2001) 073201. [36] S. Reich, A.C. Ferraro, R. Arenal, A. Loiseau, I. Bello, J. Robertson, Phys. Rev. B 71 (2005) 205201. [37] K.M. Leung, H.Q. Li, Y.S. Zou, K.L. Ma, Y.M. Chong, Q. Ye, W.J. Zhang, S.T. Lee, Appl. Phys. Lett. 88 (2006) 241922. [38] C.Y. Chan, S. Eyhusen, X.M. Meng, I. Bello, S.T. Lee, C. Ronning, H. Hofsäss, Diamond Relat. Mater. 15 (2006) 55.