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Effect of substrate temperature on SiC interlayers for diamond coatings deposition on WC-Co substrates
Vacuum 109 (2014) 15e20
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Effect of substrate temperature on SiC interlayers for diamond coatings deposition on WC-Co substrates Hongjun Hei a, Yanyan Shen a, Jing Ma a, Xiaojing Li b, Shengwang Yu a, Bin Tang a, *, Weizhong Tang c a b c
Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China The Ningbo Branch of Ordnance Science Institute of China, Ningbo 315103, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2014 Received in revised form 11 May 2014 Accepted 2 June 2014 Available online 12 June 2014
SiC films were synthesized on WC-Co substrate as interlayers for improving the adhesion of diamond coatings. The influence of the substrate temperature on the SiC films was investigated. The results showed that when the temperature was 600 C, the film was formed of loose agglomerates piled up by SiC nanoparticles. As the temperature increased to 850 C, shell structure composed of SiC particles and graphite replaced the agglomerates. When the temperature exceeded 950 C, flower-like SiC clusters and graphite spherical particles coexisted in the surface, and Co2Si particles embedded at the SiC/substrate interface. SiC films deposited at 600 C, 850 C and 950 C were typically used as interlayer for diamond deposition. The coatings exhibited the similar nano-diamond structure. However, the diamond coating on the interlayer produced at 850 C possessed the best adhesion; and the diffusion of Co was effectively inhibited by SiC interlayer which would not interfere with the process of diamond deposition. © 2014 Elsevier Ltd. All rights reserved.
Keywords: SiC films Interlayer Adhesion Cemented carbide Diamond coatings
Chemical Vapor Deposition (CVD) diamond coated cemented carbide (WC-Co) cutting tool acts as a most promising candidate for machining application [1]. However, it exhibits several limitations regarding the poor adhesion of diamond coating due to the chemical interaction between the binder phase cobalt (Co) in the substrate and the diamond CVD environment [2]. The traditional approach to overcome the problem, namely removal cobalt from the surface, will deteriorate significantly the fracture strength of the WC-Co substrates [3]. Therefore, the approach of introducing an interlayer has become the hotspot for solving the problem of adhesion. Among all the interlayer materials, the effectiveness of silicon carbide (SiC), especially cubic SiC (b-SiC), has been confirmed recently [4]. SiC interlayer can reduce the thermal stress in the diamond coating caused by the Coefficient of Thermal Expansion (CTE) mismatch between the WC-Co and diamond [5]. SiC will react with Co to form C and cobalt silicides (CoSi and Co2Si), which do not affect the diamond deposition process and deteriorate the adhesion of diamond coatings [6,7].
* Corresponding author. Tel.: þ86 351 6010540. E-mail address: [email protected] (B. Tang). http://dx.doi.org/10.1016/j.vacuum.2014.06.001 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
In fact, the properties of SiC interlayer are crucial in determining the performance of diamond coating. However, up to the time of writing, little information on the deposition process of the SiC interlayer has been systematically reported. In the present work, SiC films were fabricated on WC-Co substrates using high current extended DC arc plasma CVD. The influence of the substrate temperature on the deposition rate, morphology, phase composition, and adhesion property of the SiC films was investigated. Furthermore, the SiC films with typical structures were used as interlayers for diamond deposition. The WC-6wt.% Co commercial inserts with the dimensions of 10 mm 5 mm 10 mm were used as substrates. Both the depositions of SiC films and diamond coatings were carried out by high current extended DC arc plasma CVD apparatus, reported in the previous works [8]. An arc plasma column was produced by a DC electrical discharge and maintained between a tungsten cathode and a copper anode. Before SiC deposition, the substrates were ground using 40 mm diamond powders, and then washed by ultrasonic bath in alcohol. Argon (Ar, purity of 99.99%), hydrogen (H2, purity of 99.999%) and tetramethylsiline (TMS, purity of 99.9%) were chosen as the precursors for SiC deposition. Before diamond deposition, the samples were ultrasonically abrade for 30 min in alcohol suspension containing diamond powders (<1 mm), with the aim of enhancing the diamond nucleation density. The precursors
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for diamond deposition were Ar, H2 and methane (CH4, purity of 99.995%). The detailed experimental parameters for the SiC deposition and the diamond deposition were listed in Table 1. As the environment of the CVD chamber was considered to be uniform, the substrate temperature was changed systematically by adjusting the distance of the sample to the plasma column. The substrate temperature and the deposition pressure were measured by optical pyrometers and piezoresistive diaphragm manometers, respectively. The morphologies of the surface and the cross-section were investigated by field emission scanning electron microscopy (FESEM, ZEISS-SUPRA™ 55), and the compositions of the SiC films were examined by energy dispersive X-ray spectrometer (EDS). The phase composition of SiC films was analyzed by X-ray diffractometer (XRD, D/max2550HBþ/PC) with a grazing angle of 2 . The adhesion of SiC films was tested by WS-2005 scratching tester, under a maximum load of 120 N, the loading speed of 100 N/min and the total length of 1 mm. The qualities of the diamond coatings were evaluated by Raman spectroscope (Renishaw RM2000) with a laser wavelength of 512 nm and laser power of 100 mW. The adhesion of the diamond coatings was evaluated by indentation test using a HBRV -187.5 hardness tester with a load of 1500 N. From the XRD patterns (Fig. 1(a)), b-SiC (220) broad peak appeared at 2q ¼ 60.1 in the film deposited at 600 C, in addition bSiC (111) and (222) peaks overlapped with most intense peaks of WC at 2q ¼ 35.7 and 75.4 [5]. No significant change in the peaks was detected in the pattern of the film produced at 700 C. For the film deposited at 850 C, the graphite peak appeared at 2q ¼ 26.5 . When the temperature reached at 950 C, three peaks at 2q ¼ 28.9 , 44.2 and 45.4 presented corresponding to CoSi and Co2Si [6]. Additionally, the intensity of the graphite peak was increased and the width was narrowed compared with that in the pattern of the film obtained at 850 C. It indicated that the grain size and the number of the graphite particles increased. As the temperature increased, the intensities of the graphite peak, the CoSi peak and the Co2Si peaks enhanced. As shown in Fig. 1(b), with the increase of the temperature, the peak intensity of b-SiC at 2q ¼ 60.1 enhanced, indicating the increase of its content. From Fig. 1, it could be seen that the films were mainly formed of b-SiC particles with very small size. The experimental results showed that three different types of the morphologies could roughly be identified in the SiC films deposited at different temperatures. Fig. 2 showed the SEM morphologies of the films with three typical structures deposited at 600 C, 850 C and 950 C, respectively. From Fig. 2(a), the film obtained at 600 C was formed of loose cauliflower-like agglomerates which were piled up by SiC nanoparticles, and voids were clearly seen between the aggregations. The film deposited at 850 C (in Fig. 2(b)) possessed a compact and continuous shell structure composed of nano particles and the particle size increased. The EDS spectrum (not given in Fig. 2(b)) and the XRD patterns (in Fig. 1) showed that the particles were SiC crystals. When the temperature
Table 1 Deposition parameters of SiC interlayers and diamond coatings. Process parameters
SiC interlayers
Diamond coatings
Flow rate of Ar (FAr/sccm) Flow rate of H2 (FH/sccm) Flow rate of TMS (FTMS/sccm) Flow rate of CH4 (FCH/sccm) Substrate temperature(ST/ C) Deposition pressure(P/Pa) Deposition time (T/h)
1800 100 5.0 e ~600e1100 500 3
1800 100 e 10.0 ~900 500 6
Fig. 1. XRD patterns of the deposits prepared at different substrate temperatures shows the whole patterns; (b) shows the partial enlarged patterns at the range of 2q ¼ 55.1 69.9 .
reached at 950 C, the film was composed of spherical particles denoted by letter C (in Fig. 2(c)) and flower-like clusters (in Fig. 2(d)). The EDS spectrum (not given in Fig. 2(c)) and XRD patterns (in Fig. 1) showed that the spherical particles were formed of graphite, and the flower-like clusters were composed of SiC nanoplatelets. Fig. 3 showed the surface and the cross-sectional morphologies of the SiC films varying with the substrate temperature, as well as the corresponding scratches. As shown in Fig. 3(a), the film produced at 600 C was formed of the cauliflower-like agglomerates. From the cross-sectional image, two layers were observed, i.e. the SiC film with a thickness of about 0.5 mm and the WC-Co substrate. The EDS spectrum showed that Si, C, W and Co was detected in the surface, which demonstrated that Co could not completely be stopped in the substrate by the film. From the scratches images made on the SiC film in Fig. 3(a), it was obviously seen that pieces fall off from the surface around the edge of the scratch, which suggested that the adhesion of the film was poor. When the temperature increased to 700 C (in Fig. 3(b)), the uniformity and the continuous improved, and the content of Co reduced. The thickness of the film increased to about 0.8 mm. Additionally, the amount of the pieces around the edge of the scratch reduced. It illustrated that the capability of the SiC film for resisting Co enhanced, and the adhesion improved. As the temperature reached 850 C, no single of Co was detected in the surface, which suggested that Co was completely stopped by the film. The film deposited at 850 C showed in Fig. 3(c) had the similar
H. Hei et al. / Vacuum 109 (2014) 15e20
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Fig. 2. SEM morphologies of the films deposited at different substrate temperatures.
cross-section structure with that of the film produced at 700 C, except that the thickness increased from about 0.8 mm to 1.0 mm. The pieces around the edge of the scratch were barely appeared. But when the temperature was up to 950 C, the flower-like SiC clusters and the graphite particles coexisted in the surface. The thickness of the film was about 1.3 mm. Between the clusters and the particles, voids could be clearly seen. The signal of Co was detected in the surface again. In addition, the loose structure appeared at the SiC/substrate interface with some particles denoted in the cross-sectional image. The EDS spectrum (not given in Fig. 3) indicated that the particles had a composition of about 67% Co and 33% Si, which had been proved to be Co2Si (analyzed by Cabral G et al. [9]). It was obvious that the loose structure would deteriorate the adhesion. Furthermore, the pieces reappeared around the edges of the scratch, suggesting that the adhesion of the SiC film deposited at 950 C reduced. With the substrate temperature reached at 1100 C (as shown in Fig. 3(e)), the amount of graphite particles increases significantly and the uniformity and compactness of the film decreased. Further, the content of Co and C in the surface increased obviously. The thickness of the film increased to about 1.6 mm. More pieces broken and missing from the surface indicated that the adhesion worsened further. The reason for the experimental results about SiC film could be explained as following. The chemical species produced by the TMS pyrolysis reaction [10] diffused to the substrate surface together with argon, atomic hydrogen. The carbonehydrogen (CeH) species and the siliconehydrogen (SieH) species reacted with atomic hydrogen to form SiC which deposited on the substrate surface. It was generally appreciated that the reaction would be take place with a certain phase transformation undercooling (DT ¼ Tg T > 0; where Tg was the balance temperature of the reaction). When the temperature was about 600 C, the high phase transformation undercooling and the high supersaturation caused the increase of nucleation. But the surface activation energy of new SiC nucleus was too low to merge with each other. And hence the SiC particles piled up to form loose agglomerates. With the increase of the temperature, the undercooling decreased. At the same time, owing that the reaction was determined by the thermal activation of the chemical species [11], the higher temperature caused the higher activity and the higher
mobility of the species on the substrate surface. More quantities of the species diffused to the surface and involved in the process of diffusion, which caused the decrease of the supersaturation. Thus, the reaction rate increased leading to the increase of the deposition rate. when the temperature reached 850 C, Co diffused to the surface and catalyzed the reaction of the CeH species to form non-diamond sp2-bonded carbon structures (such as graphite) [12]. When the temperature exceeded 950 C, more Co diffused from the interior to the surface. The effects of Co on the SiC deposition process should be considered in two ways as follow. One was that the Co atom would function as a catalyst facilitating the formation of SiC nanoplatelets or even clusters composed of SiC nanoplatelets [5]. The other is that Co would react with SiC to form cobalt silicides (CoSi and Co2Si) and graphite [13,14]. Therefore, the film was formed of SiC clusters, graphite spherical particles, and cobalt silicides (Co2Si) which exited at the SiC/substrate interface. For validating the effectiveness of SiC films with different structures, the SiC films produced at 600 C, 850 C and 950 C were chosen as interlayers for diamond deposition. Fig. 4 showed the surface morphologies and Raman spectrum of the diamond coatings deposited on different SiC interlayers, together with the indentations under a load of 1500 N. The diamond coatings deposited on the SiC interlayers exhibited a similar uniform nanoscale structure. Four peaks at 1140 cm1, 1332 cm1, 1480 cm1 and 1550 cm1 were detected in the Raman spectrums of all the three samples. The broadened peak at 1332 cm1 was due to the small crystallite diamond [15]; the peaks at 1480 cm1 and 1550 cm1 were attributed to sp2-bonded carbon at the grain boundaries [16]; and the peak located at about 1140 cm1 was the characteristic peak of nanocrystalline diamond [17]. However, the adhesion of the coatings showed different performances. The circumferential diameters of the indentations Fig. 4(d) and (e) could be comparable, and the interfacial cracks of the coatings were similar. But several tiny spalls peeled off at the edge of the cracks on the coating in Fig. 4(d), while no such spalls appeared on the coating in Fig. 4(e). It suggested that the adhesion of the diamond coating with the SiC interlayer produced at 850 C was better than that of the coating with the SiC interlayer produced at 600 C. But in Fig. 4(f), the indentation showed catastrophic delamination,
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Fig. 3. Surface and cross-sectional morphologies of SiC films deposited at different substrate temperatures, as well as SEM images of the corresponding scratches made on them.
which indicated that the adhesion was disastrous. The results of the indentation tests demonstrated that the SiC interlayer deposited at 850 C was the most appropriate interlayer for the deposition of diamond coating with excellent adhesion. Fig. 5 showed the cross-sectional morphology of the diamond coating on the SiC interlayers obtained at 850 C, as well as the element distributions of C, W, Co and Si on this cross-section. Three different layers presented clearly, i.e. diamond coating, SiC interlayer and WC-Co substrate. The thicknesses of the SiC interlayer and the diamond coating were about 1.0 mm and 3.0 mm, respectively. No obvious cracks existed either at the diamond/SiC interface or at the SiC/substrate interface, clarifying the excellent adhesion of both the SiC interlayer and the diamond coating. C distributed over the sample, and the signal in the diamond coating was stronger than that in the interlayer and WC-Co substrate. Co appeared almost in the substrate whose distribution was overlapping with
that of W, and it was barely existed in the interlayer and the coating. Si distributed almost in the interlayer. The results proved that the diffusion of Co was effectively inhibited in the substrate by the SiC interlayer, thus no Co element would interfere with the diamond deposition. In summary, the SiC films with different microstructure and adhesive property were synthesized at different temperature. Three types of morphologies were roughly identified that: the film deposited at 600 C was formed of SiC cauliflower-like agglomerates; the film obtained at 850 C showed shell structures consisted of SiC and graphite; the film produced at 950 C manifested as coexistence of SiC flower-like clusters, graphite spherical particles and Co2Si particles which embedded in the SiC/substrate interlayer. Diamond coatings deposited on three types of SiC interlayer exhibited the similar uniform nano structure. However, the coating
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19
Fig. 4. SEM morphologies and Raman spectrum of diamond coatings on the SiC interlayer produced at 600 C (a), 850 C (b) and 950 C (c); together with the indentations made on them, under a load of 1500 N. In these photographs, (d), (e) and (f) were corresponding to the morphologies of (a), (b) and (c), respectively.
Fig. 5. Cross-sectional morphology (a) of diamond coating and corresponding distributions of C (b), W (c), Co (d) and Si (e), respectively.
on the interlayer with shell structures possessed the best adhesion; and the diffusion of Co was effectively inhibited which would not interfere with the process of diamond deposition. Acknowledgments This work was financially supported by the Project Supported by Shanxi Provincial Natural Science Foundation of China (Grant No. 2013011012-4), National Natural Science Foundation of China (Grant No. 51171125) and Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ13F010002).
References [1] Lu P, Gomez H, Xiao X, Lukitsch M, Durham D, Sachdeve A, et al. Coating thickness and interlayer effects on CVD-diamond film adhesion to cobaltcemented tungsten carbides. Surf Coat Technol 2013;215:272e9. [2] Lu FX, Tang WZ, Tong YM, Miao JQ, He LF, Li CM, et al. Novel pretreatment of hard metal substrate for better performance of diamond coated cutting tools. Diam Relat Mater 2006;15(11):2039e45. [3] Geng CL, Tang WZ, Hei LF, Liu ST, Lu FX. Fracture strength of two-step pretreated and diamond coated cemented carbide microdrills. Int J Refract Met Hard Mater 2007;25(2):159e65. [4] Yu X, Liu Y, Ma L, Yang G, Wang C. Investigation of thick CVD diamond film with SiC interlayer on tungsten carbide for possible usage in geologic explorations. Vacuum 2013;94:53e6.
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[5] Yu SW, Fan PW, Tang WZ, Li XJ, Hu HL, Hei HJ, et al. Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates. Thin Solid Films 2011;520(2):828e32. [6] Wang T, Xiang L, Shi W, Jiang X. Deposition of diamond/b-SiC/cobalt silicide composite interlayers to improve adhesion of diamond coating on WC-Co substrates by DC-plasma assisted HFCVD. Surf Coat Technol 2011;205(8):3027e34. [7] Park SW, Kim YI, Kwak JS, Baik HK. Investigation of Co/SiC interface reaction. J Electron Mater 1997;26(3):172e7. [8] Meng XM, Askari SJ, Tang WZ, Hei LF, Wang FY, Jiang CS, et al. Nano-crystalline CVD diamond films deposited on cemented carbide using high current extended DC arc plasma process. Vacuum 2008;82(5):543e6. cio J, Polini R. A study of diamond film [9] Cabral G, G€ abler J, Lindner J, Gra deposition on WC-Co inserts for graphite machining: effectiveness of SiC interlayers prepared by HFCVD. Diam Relat Mater 2008;17(6):1008e14. [10] Veintemillas S, Madigou V, Rodríguez-Clemente R, Figueras A. Thermodynamic analysis of metalorganic chemical vapour deposition of SiC using tetramethylsilane as precursor. I. Identification of the main reactions. J Cryst Growth 1995;148(4):383e9.
[11] Shen CH. Performance and applications of CVD thin film diamond coated tools. Key Eng Mater 1997;138:25e56. [12] Hojman E, Akhvlediani R, Layyous A, Hoffman A. Diamond CVD film formation onto WCeCo substrates using a thermally nitrided Cr diffusion-barrier. Diam Relat Mater 2013;39:65e72. [13] Lim CS, Ha JS, Ryu JH, Auh KH, Bae IT, Ishimaru M, et al. Interface formation and phase distribution induced by Co/SiC solid state reactions [J]. Mater Trans 2002;43(5):1225e9. [14] Lim CS. Interface chemistry and kinetics induced by SiC/Co solid state reactions. J Ceram Process Res 2011;(1):46e51. [15] Cicala G, Magaletti V, Senesi GS, Carbone G, Altamura D, Giannini C, et al. Superior hardness and young's modulus of low temperature nanocrystalline diamond coatings. Mater Chem Phys 2014;144(3):105e11. [16] Chandran M, Kumaran CR, Gowthama S, Shanmugam P, Natarajan R, Bhattacharya SS, et al. Chemical vapor deposition of diamond coatings on tungsten carbide (WC-Co) riveting inserts. Int J Refract Met Hard Mater 2013;37:117e20. [17] Kuzmany H, Pfeiffer R, Salk N, Günther B. The mystery of the 1140 cm-1 Raman line in nanocrystalline diamond films. Carbon 2004;42(5):911e7.
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Rapid communication
Effect of substrate temperature on SiC interlayers for diamond coatings deposition on WC-Co substrates Hongjun Hei a, Yanyan Shen a, Jing Ma a, Xiaojing Li b, Shengwang Yu a, Bin Tang a, *, Weizhong Tang c a b c
Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China The Ningbo Branch of Ordnance Science Institute of China, Ningbo 315103, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2014 Received in revised form 11 May 2014 Accepted 2 June 2014 Available online 12 June 2014
SiC films were synthesized on WC-Co substrate as interlayers for improving the adhesion of diamond coatings. The influence of the substrate temperature on the SiC films was investigated. The results showed that when the temperature was 600 C, the film was formed of loose agglomerates piled up by SiC nanoparticles. As the temperature increased to 850 C, shell structure composed of SiC particles and graphite replaced the agglomerates. When the temperature exceeded 950 C, flower-like SiC clusters and graphite spherical particles coexisted in the surface, and Co2Si particles embedded at the SiC/substrate interface. SiC films deposited at 600 C, 850 C and 950 C were typically used as interlayer for diamond deposition. The coatings exhibited the similar nano-diamond structure. However, the diamond coating on the interlayer produced at 850 C possessed the best adhesion; and the diffusion of Co was effectively inhibited by SiC interlayer which would not interfere with the process of diamond deposition. © 2014 Elsevier Ltd. All rights reserved.
Keywords: SiC films Interlayer Adhesion Cemented carbide Diamond coatings
Chemical Vapor Deposition (CVD) diamond coated cemented carbide (WC-Co) cutting tool acts as a most promising candidate for machining application [1]. However, it exhibits several limitations regarding the poor adhesion of diamond coating due to the chemical interaction between the binder phase cobalt (Co) in the substrate and the diamond CVD environment [2]. The traditional approach to overcome the problem, namely removal cobalt from the surface, will deteriorate significantly the fracture strength of the WC-Co substrates [3]. Therefore, the approach of introducing an interlayer has become the hotspot for solving the problem of adhesion. Among all the interlayer materials, the effectiveness of silicon carbide (SiC), especially cubic SiC (b-SiC), has been confirmed recently [4]. SiC interlayer can reduce the thermal stress in the diamond coating caused by the Coefficient of Thermal Expansion (CTE) mismatch between the WC-Co and diamond [5]. SiC will react with Co to form C and cobalt silicides (CoSi and Co2Si), which do not affect the diamond deposition process and deteriorate the adhesion of diamond coatings [6,7].
* Corresponding author. Tel.: þ86 351 6010540. E-mail address: [email protected] (B. Tang). http://dx.doi.org/10.1016/j.vacuum.2014.06.001 0042-207X/© 2014 Elsevier Ltd. All rights reserved.
In fact, the properties of SiC interlayer are crucial in determining the performance of diamond coating. However, up to the time of writing, little information on the deposition process of the SiC interlayer has been systematically reported. In the present work, SiC films were fabricated on WC-Co substrates using high current extended DC arc plasma CVD. The influence of the substrate temperature on the deposition rate, morphology, phase composition, and adhesion property of the SiC films was investigated. Furthermore, the SiC films with typical structures were used as interlayers for diamond deposition. The WC-6wt.% Co commercial inserts with the dimensions of 10 mm 5 mm 10 mm were used as substrates. Both the depositions of SiC films and diamond coatings were carried out by high current extended DC arc plasma CVD apparatus, reported in the previous works [8]. An arc plasma column was produced by a DC electrical discharge and maintained between a tungsten cathode and a copper anode. Before SiC deposition, the substrates were ground using 40 mm diamond powders, and then washed by ultrasonic bath in alcohol. Argon (Ar, purity of 99.99%), hydrogen (H2, purity of 99.999%) and tetramethylsiline (TMS, purity of 99.9%) were chosen as the precursors for SiC deposition. Before diamond deposition, the samples were ultrasonically abrade for 30 min in alcohol suspension containing diamond powders (<1 mm), with the aim of enhancing the diamond nucleation density. The precursors
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for diamond deposition were Ar, H2 and methane (CH4, purity of 99.995%). The detailed experimental parameters for the SiC deposition and the diamond deposition were listed in Table 1. As the environment of the CVD chamber was considered to be uniform, the substrate temperature was changed systematically by adjusting the distance of the sample to the plasma column. The substrate temperature and the deposition pressure were measured by optical pyrometers and piezoresistive diaphragm manometers, respectively. The morphologies of the surface and the cross-section were investigated by field emission scanning electron microscopy (FESEM, ZEISS-SUPRA™ 55), and the compositions of the SiC films were examined by energy dispersive X-ray spectrometer (EDS). The phase composition of SiC films was analyzed by X-ray diffractometer (XRD, D/max2550HBþ/PC) with a grazing angle of 2 . The adhesion of SiC films was tested by WS-2005 scratching tester, under a maximum load of 120 N, the loading speed of 100 N/min and the total length of 1 mm. The qualities of the diamond coatings were evaluated by Raman spectroscope (Renishaw RM2000) with a laser wavelength of 512 nm and laser power of 100 mW. The adhesion of the diamond coatings was evaluated by indentation test using a HBRV -187.5 hardness tester with a load of 1500 N. From the XRD patterns (Fig. 1(a)), b-SiC (220) broad peak appeared at 2q ¼ 60.1 in the film deposited at 600 C, in addition bSiC (111) and (222) peaks overlapped with most intense peaks of WC at 2q ¼ 35.7 and 75.4 [5]. No significant change in the peaks was detected in the pattern of the film produced at 700 C. For the film deposited at 850 C, the graphite peak appeared at 2q ¼ 26.5 . When the temperature reached at 950 C, three peaks at 2q ¼ 28.9 , 44.2 and 45.4 presented corresponding to CoSi and Co2Si [6]. Additionally, the intensity of the graphite peak was increased and the width was narrowed compared with that in the pattern of the film obtained at 850 C. It indicated that the grain size and the number of the graphite particles increased. As the temperature increased, the intensities of the graphite peak, the CoSi peak and the Co2Si peaks enhanced. As shown in Fig. 1(b), with the increase of the temperature, the peak intensity of b-SiC at 2q ¼ 60.1 enhanced, indicating the increase of its content. From Fig. 1, it could be seen that the films were mainly formed of b-SiC particles with very small size. The experimental results showed that three different types of the morphologies could roughly be identified in the SiC films deposited at different temperatures. Fig. 2 showed the SEM morphologies of the films with three typical structures deposited at 600 C, 850 C and 950 C, respectively. From Fig. 2(a), the film obtained at 600 C was formed of loose cauliflower-like agglomerates which were piled up by SiC nanoparticles, and voids were clearly seen between the aggregations. The film deposited at 850 C (in Fig. 2(b)) possessed a compact and continuous shell structure composed of nano particles and the particle size increased. The EDS spectrum (not given in Fig. 2(b)) and the XRD patterns (in Fig. 1) showed that the particles were SiC crystals. When the temperature
Table 1 Deposition parameters of SiC interlayers and diamond coatings. Process parameters
SiC interlayers
Diamond coatings
Flow rate of Ar (FAr/sccm) Flow rate of H2 (FH/sccm) Flow rate of TMS (FTMS/sccm) Flow rate of CH4 (FCH/sccm) Substrate temperature(ST/ C) Deposition pressure(P/Pa) Deposition time (T/h)
1800 100 5.0 e ~600e1100 500 3
1800 100 e 10.0 ~900 500 6
Fig. 1. XRD patterns of the deposits prepared at different substrate temperatures shows the whole patterns; (b) shows the partial enlarged patterns at the range of 2q ¼ 55.1 69.9 .
reached at 950 C, the film was composed of spherical particles denoted by letter C (in Fig. 2(c)) and flower-like clusters (in Fig. 2(d)). The EDS spectrum (not given in Fig. 2(c)) and XRD patterns (in Fig. 1) showed that the spherical particles were formed of graphite, and the flower-like clusters were composed of SiC nanoplatelets. Fig. 3 showed the surface and the cross-sectional morphologies of the SiC films varying with the substrate temperature, as well as the corresponding scratches. As shown in Fig. 3(a), the film produced at 600 C was formed of the cauliflower-like agglomerates. From the cross-sectional image, two layers were observed, i.e. the SiC film with a thickness of about 0.5 mm and the WC-Co substrate. The EDS spectrum showed that Si, C, W and Co was detected in the surface, which demonstrated that Co could not completely be stopped in the substrate by the film. From the scratches images made on the SiC film in Fig. 3(a), it was obviously seen that pieces fall off from the surface around the edge of the scratch, which suggested that the adhesion of the film was poor. When the temperature increased to 700 C (in Fig. 3(b)), the uniformity and the continuous improved, and the content of Co reduced. The thickness of the film increased to about 0.8 mm. Additionally, the amount of the pieces around the edge of the scratch reduced. It illustrated that the capability of the SiC film for resisting Co enhanced, and the adhesion improved. As the temperature reached 850 C, no single of Co was detected in the surface, which suggested that Co was completely stopped by the film. The film deposited at 850 C showed in Fig. 3(c) had the similar
H. Hei et al. / Vacuum 109 (2014) 15e20
17
Fig. 2. SEM morphologies of the films deposited at different substrate temperatures.
cross-section structure with that of the film produced at 700 C, except that the thickness increased from about 0.8 mm to 1.0 mm. The pieces around the edge of the scratch were barely appeared. But when the temperature was up to 950 C, the flower-like SiC clusters and the graphite particles coexisted in the surface. The thickness of the film was about 1.3 mm. Between the clusters and the particles, voids could be clearly seen. The signal of Co was detected in the surface again. In addition, the loose structure appeared at the SiC/substrate interface with some particles denoted in the cross-sectional image. The EDS spectrum (not given in Fig. 3) indicated that the particles had a composition of about 67% Co and 33% Si, which had been proved to be Co2Si (analyzed by Cabral G et al. [9]). It was obvious that the loose structure would deteriorate the adhesion. Furthermore, the pieces reappeared around the edges of the scratch, suggesting that the adhesion of the SiC film deposited at 950 C reduced. With the substrate temperature reached at 1100 C (as shown in Fig. 3(e)), the amount of graphite particles increases significantly and the uniformity and compactness of the film decreased. Further, the content of Co and C in the surface increased obviously. The thickness of the film increased to about 1.6 mm. More pieces broken and missing from the surface indicated that the adhesion worsened further. The reason for the experimental results about SiC film could be explained as following. The chemical species produced by the TMS pyrolysis reaction [10] diffused to the substrate surface together with argon, atomic hydrogen. The carbonehydrogen (CeH) species and the siliconehydrogen (SieH) species reacted with atomic hydrogen to form SiC which deposited on the substrate surface. It was generally appreciated that the reaction would be take place with a certain phase transformation undercooling (DT ¼ Tg T > 0; where Tg was the balance temperature of the reaction). When the temperature was about 600 C, the high phase transformation undercooling and the high supersaturation caused the increase of nucleation. But the surface activation energy of new SiC nucleus was too low to merge with each other. And hence the SiC particles piled up to form loose agglomerates. With the increase of the temperature, the undercooling decreased. At the same time, owing that the reaction was determined by the thermal activation of the chemical species [11], the higher temperature caused the higher activity and the higher
mobility of the species on the substrate surface. More quantities of the species diffused to the surface and involved in the process of diffusion, which caused the decrease of the supersaturation. Thus, the reaction rate increased leading to the increase of the deposition rate. when the temperature reached 850 C, Co diffused to the surface and catalyzed the reaction of the CeH species to form non-diamond sp2-bonded carbon structures (such as graphite) [12]. When the temperature exceeded 950 C, more Co diffused from the interior to the surface. The effects of Co on the SiC deposition process should be considered in two ways as follow. One was that the Co atom would function as a catalyst facilitating the formation of SiC nanoplatelets or even clusters composed of SiC nanoplatelets [5]. The other is that Co would react with SiC to form cobalt silicides (CoSi and Co2Si) and graphite [13,14]. Therefore, the film was formed of SiC clusters, graphite spherical particles, and cobalt silicides (Co2Si) which exited at the SiC/substrate interface. For validating the effectiveness of SiC films with different structures, the SiC films produced at 600 C, 850 C and 950 C were chosen as interlayers for diamond deposition. Fig. 4 showed the surface morphologies and Raman spectrum of the diamond coatings deposited on different SiC interlayers, together with the indentations under a load of 1500 N. The diamond coatings deposited on the SiC interlayers exhibited a similar uniform nanoscale structure. Four peaks at 1140 cm1, 1332 cm1, 1480 cm1 and 1550 cm1 were detected in the Raman spectrums of all the three samples. The broadened peak at 1332 cm1 was due to the small crystallite diamond [15]; the peaks at 1480 cm1 and 1550 cm1 were attributed to sp2-bonded carbon at the grain boundaries [16]; and the peak located at about 1140 cm1 was the characteristic peak of nanocrystalline diamond [17]. However, the adhesion of the coatings showed different performances. The circumferential diameters of the indentations Fig. 4(d) and (e) could be comparable, and the interfacial cracks of the coatings were similar. But several tiny spalls peeled off at the edge of the cracks on the coating in Fig. 4(d), while no such spalls appeared on the coating in Fig. 4(e). It suggested that the adhesion of the diamond coating with the SiC interlayer produced at 850 C was better than that of the coating with the SiC interlayer produced at 600 C. But in Fig. 4(f), the indentation showed catastrophic delamination,
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Fig. 3. Surface and cross-sectional morphologies of SiC films deposited at different substrate temperatures, as well as SEM images of the corresponding scratches made on them.
which indicated that the adhesion was disastrous. The results of the indentation tests demonstrated that the SiC interlayer deposited at 850 C was the most appropriate interlayer for the deposition of diamond coating with excellent adhesion. Fig. 5 showed the cross-sectional morphology of the diamond coating on the SiC interlayers obtained at 850 C, as well as the element distributions of C, W, Co and Si on this cross-section. Three different layers presented clearly, i.e. diamond coating, SiC interlayer and WC-Co substrate. The thicknesses of the SiC interlayer and the diamond coating were about 1.0 mm and 3.0 mm, respectively. No obvious cracks existed either at the diamond/SiC interface or at the SiC/substrate interface, clarifying the excellent adhesion of both the SiC interlayer and the diamond coating. C distributed over the sample, and the signal in the diamond coating was stronger than that in the interlayer and WC-Co substrate. Co appeared almost in the substrate whose distribution was overlapping with
that of W, and it was barely existed in the interlayer and the coating. Si distributed almost in the interlayer. The results proved that the diffusion of Co was effectively inhibited in the substrate by the SiC interlayer, thus no Co element would interfere with the diamond deposition. In summary, the SiC films with different microstructure and adhesive property were synthesized at different temperature. Three types of morphologies were roughly identified that: the film deposited at 600 C was formed of SiC cauliflower-like agglomerates; the film obtained at 850 C showed shell structures consisted of SiC and graphite; the film produced at 950 C manifested as coexistence of SiC flower-like clusters, graphite spherical particles and Co2Si particles which embedded in the SiC/substrate interlayer. Diamond coatings deposited on three types of SiC interlayer exhibited the similar uniform nano structure. However, the coating
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Fig. 4. SEM morphologies and Raman spectrum of diamond coatings on the SiC interlayer produced at 600 C (a), 850 C (b) and 950 C (c); together with the indentations made on them, under a load of 1500 N. In these photographs, (d), (e) and (f) were corresponding to the morphologies of (a), (b) and (c), respectively.
Fig. 5. Cross-sectional morphology (a) of diamond coating and corresponding distributions of C (b), W (c), Co (d) and Si (e), respectively.
on the interlayer with shell structures possessed the best adhesion; and the diffusion of Co was effectively inhibited which would not interfere with the process of diamond deposition. Acknowledgments This work was financially supported by the Project Supported by Shanxi Provincial Natural Science Foundation of China (Grant No. 2013011012-4), National Natural Science Foundation of China (Grant No. 51171125) and Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ13F010002).
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