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Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers
Diamond & Related Materials 49 (2014) 9–13
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers F. Xu a,b, M.F. Yuen b, B. He b, C.D. Wang b, X.R. Zhao a,c, X.L. Tang a, D.W. Zuo a,⁎, W.J. Zhang b,⁎⁎ a b c
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, People's Republic of China Center of Super-Diamond and Advanced Films, City University of Hong Kong, Hong Kong Special Administrative Region Zhejiang Provincial Key Laboratory for Cutting Tools, Taizhou University, Taizhou 318000, People's Republic of China
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Article history: Received 20 June 2014 Received in revised form 24 July 2014 Accepted 30 July 2014 Available online 7 August 2014 Keywords: Cubic boron nitride film Silicon nitride insert Boron doped diamond Tribological properties
a b s t r a c t Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si3N4) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si3N4 as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10−7 mm3/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si3N4 inserts may have great potentials for advanced materials machining. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cubic boron nitride (cBN) is structurally analogous to diamond. Due to the strong covalent bonds and high atom density, both of them have outstanding physical and chemical properties, such as extreme hardness, excellent thermal conductivity, high chemical stability and low friction coefficient [1–3]. Compared with diamond, cBN has an even higher thermal stability, and oxidation and graphitization resistances [4,5]. While diamond burns at 600 °C and reacts with iron at elevated temperatures, cBN is chemically stable and does not react with ferrous materials at temperatures up to 1200 °C [6]. These properties along with maintaining its hardness up to 1000 °C pose cBN as an ideal material 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 polycrystalline cBN (PcBN) cutting inserts [7–9]. Such PcBN tools have shown significantly improved processing efficiency as compared with the traditional cemented carbide (WC:Co) and high speed steel (HSS) tools. However, such cBN tools still suffer from the technical difficulties in cementing cBN powder into complex shapes and the high cost of the cBN powder and
⁎ Corresponding author. Tel.: +86 25 84890249 (office). ⁎⁎ Corresponding author. Tel.: +86 852 3442 7433 (office). E-mail addresses: [email protected] (D.W. Zuo), [email protected] (W.J. Zhang).
http://dx.doi.org/10.1016/j.diamond.2014.07.014 0925-9635/© 2014 Elsevier B.V. All rights reserved.
cementing, which motivate ion for thin film synthesis of cBN for tooling applications. Various physical and chemical vapor deposition (PVD and CVD) techniques have been exploited to synthesize cBN coatings [1–3], and cBN coatings synthesized on cemented tungsten carbide (WC:Co) substrates for tooling applications have been reported [10,11]. It has been demonstrated that cBN deposited directly on WC was mechanically unstable, and transition layers, such as boron and boron-rich BN, were introduced to improve the adhesion and mechanical stability of cBN coatings [12,13]. Keunecke et al. reported that deposition of 1.2 μmthick cBN films on WC inserts with TiN/BCN transition layers by radiofrequency diode sputtering, and the turning test showed that the cutting lifetime of the cBN coated inserts was prolonged as compared to the TiN and TiAlN coatings [14,15]. Due to the high compressive stress induced by intense ion bombardment in cBN deposition, the deposition of thick (N 2 μm), adherent, and mechanically stable cBN coatings on cutting inserts remains a challenge. The development of fluorineassisted CVD enabled the growth of high-quality, thick cBN films with significantly reduced residual stress [16,17]; and the cBN coatings were found to grow directly and epitaxially on diamond substrates, skipping the soft sp2-BN incubation layer usually needed for cBN nucleation and improving the adhesion of cBN films greatly [18,19]. 2.8 μmthick cBN films with high phase purity were synthesize on diamondcoated WC:Co inserts by using ECR microwave plasma CVD (MPCVD) with the assistance of fluorine chemistry [20]. Surface pretreatment of the WC:Co inserts by well-controlled chemical etching was required to remove the Co binder in the surface region of cutting inserts, so
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that the adhesion of diamond films could be enhanced. However, the high substrate temperatures during cBN deposition led to diffusion of Co from the bulk of insert to the diamond/insert interface, which resulted in the catalytic formation of graphite and deterioration of diamond adhesion [20,21]. The machining performance tests showed that the diamond–WC interface was the weak link in cBN/diamond coatings on WC inserts. We report in this paper synthesis of cBN films on Si3N4 substrates through boron-doped diamond (BDD) buffer layers by fluorineassisted ECR MPCVD. Si3N4 is one of the major ceramics for cutting tools due to its high hardness, outstanding thermo-mechanical and tribological properties and low fabricating cost [22]. In comparison with WC:Co substrates, Si3N4 has a lower mismatch of thermal expansion coefficient with that of diamond (αSi3N4 = 2.11 × 10−6 K−1 and αWC = 4.11 × 10−6 K−1 versus αDiamond = 0.8 × 10−6 K−1) [23], and the degradation of diamond/substrate interface strength caused by Co diffusion can be avoided, which makes Si3N4 a remarkable substrate for highquality diamond growth [24,25]. In this work, the effects of doping level of BDD films on the microstructure and composition of cBN/BDD coatings were investigated. The friction and wear performance of cBN/ BDD coatings on Si3N4 (cBN/BDD/Si3N4) and the tribological behaviors of the reference TiAlN/WC were also evaluated. 2. Experimental BDD films were deposited on Si3N4 cutting inserts (CN2000, ZCCCT, Co., Ltd, 12 × 12 mm) in a gas mixture of trimethylboron (TMB) in an ASTeX MPCVD system. The substrates were pretreated ultrasonically in diamond powder (~50 nm) suspension for 60 min to enhance the nucleation. TMB was diluted by H2 to 0.1 vol.%, and the total gas flow rate was 300 sccm. The carbon to hydrogen ratio was maintained at 1%, and the boron to carbon ratio (B/C) was varied from 3000 to 12,000 ppm. For all samples, the microwave power was 1200 W, the substrate temperature was kept at 760 °C, the reactive pressure was 30 Torr, and the growth duration was 12 h. Cubic BN films were deposited on the BDD-coated Si3N4 inserts in the ASTeX ECR MPCVD system in a gas mixture of BF3 + N2 + Ar + He + H 2 . The cBN coatings on BDD films with B/C ratios of 3000, 7000 and 12,000 ppm were named as Samples 1, 2 and 3, respectively, in this paper. The gas flow rates of BF3, N2, Ar, He and H2 were maintained at 10, 50, 10, 131 and 2.5 sccm respectively. In the deposition chamber, the Si3N4 insert was mounted on a Mo holder. A pyrolytic BN was used to cover the Mo holder and expose only the insert to plasma during deposition. The deposition pressure and substrate temperature were kept at 2 × 10−3 Torr and 900 °C, respectively. A DC bias of −40 V was applied to the substrate during deposition, and the growth time was 8 h. The deposition was performed under the optimized conditions based on our previous works [18–20]. A microwave power of 1400 W was employed to generate ECR plasma; and the high microwave power could enhance the plasma density and ionization degree, enabling cBN deposition at reduced bias voltages. Moreover, the growth rate and crystallinity of cBN could also be improved, similar to the deposition of diamond/β-SiC composite films by conventional MPCVD [26,27]. The surface and cross-section morphologies of BDD and cBN coatings were observed by scanning electron microscopy (SEM, PHILIPS XL30), and the elemental compositions of cBN films were studied by using energy dispersive X-ray spectroscopy (EDX) attached to the SEM. The phase compositions of BDD and cBN films were characterized by visible and UV Raman spectroscopy (Renishaw inVia) with excitation wavelength of 514 and 244 nm. Small-angle X-ray diffraction (XRD, Rigaku SmartLab) was carried out to investigate the microstructure, grain size, and stress of cBN films. The resistivity of the BDD films was detected by a 4-point probe method. The mechanical and tribological properties of cBN coatings were investigated in comparison with those of commercial TiAlN coatings on
WC:Co substrates. The hardness and elastic modulus of the cBN coating were measured by nanoindentation (Nano Indenter XP). The tribological behaviors of the coatings were investigated using a ball-on-disc (BOD) tribometer (HT1000). Si3N4 balls (HV 1600) with a radius of 3 mm were used as the friction counterpart. The temperature and relative humidity were kept between 21–25 °C and 38–55%, respectively. The sliding speed of 0.2 m/s was preset at a load of 5 N. The Si3N4 ball slid on a circular track with a radius of 2 mm, and the total sliding distance was 2000 m. The cross-section area of wear tracks on the coating was determined with a non-contact optical profilometer (Nanomap 500LS). 3. Result and discussion 3.1. Microstructure of cBN coatings The SEM surface morphologies of the BDD films deposited with B/C ratios of 3000, 7000, and 12,000 ppm are shown in Fig. 1(a)–(c), respectively. It can be seen that all films are polycrystalline with the facets of diamond grains well resolved. More small diamond grains of multiple twinning structures were formed on the surfaces of BDD films grown with an elevated B/C ratio. The Raman spectra of the corresponding BDD films are presented in Fig. 1(d). A sharp diamond peak located at about 1339 cm−1 was revealed for the film prepared with the B/C ratio of 3000 ppm, indicating a high crystallinity of the film. The diamond peak shifted from 1339 to 1329 cm−1 as the B/C ratio increased from 3000 to 12,000 ppm, and the downshift could be attributed to the Fano effect caused by boron doping [28,29]. With the increase of B/C ratio, the intensity of G peak at 1580 cm−1 increased, which suggested an increased sp2 carbon phase in the BDD films of higher boron doping levels. Moreover, for the film deposited with the B/C ratio of 12,000 ppm, broad peaks at 500 and 1220 cm−1 appeared, which are also due to the Fano effect as reported previously [28,29]. The electrical resistivity of the BDD films was revealed to decrease with the increase of B/C ratios. At the B/C ratio of 3000, 7000 and 12,000 ppm, the resistivity of BDD films was measured to be about 0.75, 0.16 and 0.09 Ω cm, respectively. The boron doping of diamond buffer layer results in the improvement of electrical conductivity, which is critical for the application of substrate bias for cBN deposition. Fig. 2(a)–(c) depicts the SEM surface morphologies of the samples after cBN deposition on the BDD films prepared with the B/C ratios of 3000 (Sample 1), 7000 (Sample 2) and 12,000 ppm (Sample 3), respectively, and the inserts show the EDX spectra of the corresponding films. For Sample 1, the facets of diamond grains could still be clearly distinguished, and the EDX spectrum revealed only a carbon signal, which indicated that no cBN film was deposited atop the BDD layer. For Sample 2, a thin film was formed on the BDD film. The EDX spectrum showed boron and nitrogen signals in addition to carbon, which suggested that the cBN film partly covered the underlying BDD layer. In contrast to the above two samples, only boron and nitrogen were detected in EDX spectrum of Sample 3, implying a full coverage of the BDD layer by the cBN coating. The SEM morphology also confirmed the uniform deposition of a continuous BN film, as shown in Fig. 2(c). The crosssectional SEM image in Fig. 2(d) shows that a 2.2 μm-thick cBN film was obtained on Si3N4 cutting inserts precoated with a 4 μm-thick BDD layer. Fig. 3(a) shows the UV Raman spectra of Samples 1–3. Only diamond and graphitic carbon peaks located at 1332 and 1580 cm−1 respectively were observed for Samples 1 and 2, and these two spectra were almost the same as those acquired from the corresponding BDD substrates as shown in Fig. 1(d). Nevertheless, two additional peaks located at 1045 and 1305 cm−1 were revealed for Sample 3, which were assigned to the transverse (TO) and longitudinal optical (LO) modes of cBN [30]. Small-angle XRD was carried out to further verify the growth of cBN film in Sample 3, as shown in Fig. 3(b). The incident X-ray angle α was set at 0.3° and diffraction pattern was taken by 2θ continuous mode
F. Xu et al. / Diamond & Related Materials 49 (2014) 9–13
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Fig. 1. SEM surface morphologies of BDD films grown with B/C ratios of (a) 3000 ppm, (b) 7000 ppm, and (c) 12,000 ppm. (d) Visible Raman spectra of the corresponding BDD films prepared with different B/C ratios. The spot size of laser beam was 1 μm.
with a step width of 0.02°. Two sets of diffraction peaks for diamond and cBN were clearly revealed, and a diffraction peak for Si3N4 (302) was also observed, as denoted in the figure. Due to the small lattice mismatch between diamond and cBN (1.34%), their strongest (111) diffractions overlap with each other. The deconvolution in the insert, however, indeed resolved two components for diamond (111) and cBN (111). The Raman and XRD measurements confirmed successful deposition of cBN films in Sample 3. For the fluorine-assisted CVD of cBN films, bombardment of the growing cBN surfaces by energetic ions has been illustrated to be critical for formation of cBN phase, though the ion bombardment plays a different role from the subsurface driving force for sp 2 to
sp3 phase transition [17]. The effective ion bombardment is strongly influenced by the substrate bias and conductivity of substrate. Insulator substrates will cause positive charge accumulation on its surface, and ion bombardment cannot be continuously proceeded. In this work, Si 3N 4 inserts are insulative, thus the BDD film on Si3N 4 not only functions as a buffer layer for cBN nucleation but also provides the necessary conduction path for charge transport. Therefore the conductivity of the BDD layer is an important factor which determines the effective ion bombarding energy and ion flux during cBN deposition. It was found in our experiments that the substrate bias current increased from 0.04 A to 0.08 A when the resistivity of BDD films decreased from 0.75 (Sample 1) to 0.09 Ω cm (Sample 3). The
Fig. 2. SEM surface morphologies of the samples after cBN coating on the BDD films grown with B/C ratios of (a) 3000 ppm, (b) 7000 ppm, and (c) 12,000 ppm. The EDX spectra of the corresponding samples are shown as inserts. (d) SEM cross-sectional micrograph of the sample shown in (c).
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732 GPa, respectively. As a reference, the H and E of TiAlN coatings were reported to be 20 GPa and 200 GPa [32]. Moreover, the resistance to plastic deformation of a coating is proportional to the H3/E2 ratio, and it is closely correlated to the relative wear resistance [33]. The H3/E2 ratio of cBN film is 0.9, which is significantly larger than that of TiAlN (0.2) and indicates a much greater wear resistance of the cBN coating. From the elastic modulus and XRD pattern of cBN coating, the biaxial stress neglecting shear components in the cBN film can be roughly estimated by using Hooke's law: σ¼
Fig. 3. (a) UV Raman spectra of the BN coatings on Si3N4 inserts precoated with BDD layers deposited with different B/C ratios. (b) Small-angle XRD pattern of Sample 3.
εψ E 1−γ
ð1Þ
where elastic modulus of the film E = 732 GPa, Poisson's ratio γ = 0.112 [34,35]. The strain εψ can be calculated from lattice distortion (d0 − d) / d0, where d and d0 are the lattice spacing of measured cBN reflection and the corresponding value from JCPDS 35–1365. A compressive stress of 1.5–1.9 GPa was obtained by using the two strongest XRD peaks of cBN, i.e., (111) and (220). The existence of a considerable compressive stress in the film may also enhance its hardness [36]. Fig. 5(a) illustrates the coefficients of friction (COFs) of the cBN and TiAlN coatings by sliding Si3N4 balls over their surfaces at a constant load of 5 N. The initial calculated contact pressure of the cBN coating and Si3N4 ball is about 1.7 GPa, which is high enough to simulate the machining conditions according to Hertzian's ball-on-disc contact model. As shown in Fig. 5, the COF of the cBN coating dropped from 0.4 to 0.16 in the first 400 m, and then almost maintained this value until the sliding distance of 2000 m. The initial drop of the COF is believed to be due to the rough surface of the as-deposited cBN coating. The comparative study revealed that the COF of the TiAlN coating was about 0.3 over the whole sliding distance of 2000 m.
highly conductive BDD layer in Sample 3 enables growth of continuous thick cBN top films with high purity for mechanical applications. In the following sections, only Sample 3 was selected for detailed mechanical and tribological characterizations. 3.2. Mechanical and tribological properties of the cBN coatings The hardness (H) and elastic modulus (E) of Sample 3 were measured by using nanoindentation based on the standard Oliver and Pharr approach [31]. The displacement-dependent E and H are depicted in Fig. 4. Because a rough cBN surface may result in inaccuracy in E and H measurements, the cBN film was mechanically smoothed with fine diamond lapping paper (average grain size ~ 100 nm). The E and H measurements show plateaus upon the indenter displacement; and based on them, H and E of the cBN coating were estimated to be 78 GPa and
Fig. 4. Hardness and elastic modulus of a polished cBN film obtained at variable nanoindenter displacement.
Fig. 5. (a) Sliding distance dependent COFs of cBN and TiAlN coatings. (b) The crosssectional wear profiles of the TiAlN and cBN coatings after sliding with a Si3N4 ball for 2000 m.
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The wear rate of cBN coating was evaluated by sliding Si3N4 ball on cBN surface with a sliding distance of 2000 m. The wear rate (Ws) of the cBN was estimated from the cross-sectional area of the wear track measured by a surface profilometer: Ws ¼
AC Fl
ð2Þ
where A is the mean of wear area, C is the wear scar length, F is the normal load, and l is the sliding distance. Three scans were performed to get the mean wear area. Characteristic cross-sectional profiles of the wear scars of the cBN and TiAlN coatings after the sliding distance of 2000 m are shown in Fig. 5(b). It can be seen that a much broader and deeper wear scar was formed on the TiAlN coating than that on the cBN coating. According to the measurements, the wear rates of the cBN and TiAlN coatings were derived to be about 4.1 × 10−7 mm3/N m and 73 × 10−7 mm3/N m, respectively, indicating a much superior wear resistance of the cBN coating. As a reference, the wear rate obtained here is comparable to that of cBN/diamond composite films on silicon substrates measured by sliding Si3N4 balls over prolonged distances of 12.56 km (about 1.0 × 10−7 mm3/N·m) [37]. 4. Conclusion Cubic BN coatings with thickness over 2 μm were synthesized on BDD-coated Si3N4 inserts by using ECR MPCVD. BDD films were demonstrated to not only function as a buffer layer for cBN nucleation but also provide the necessary charge transport path for performing effective ion bombardment during cBN growth, thus a high conductivity of BDD was obligatory for the deposition of cBN phase. The deposited cBN coatings showed ultrahigh hardness and elastic modulus of 78 GPa and 732 GPa, respectively. The COF of cBN coatings was revealed to drop quickly from 0.4 to 0.17 after smoothing of the cBN surfaces by sliding Si3N4 balls over the first 400 m, which is smaller than that of the commercial TiAlN coatings (0.3). The wear rate of the cBN coatings against sliding of Si3N4 balls at ambient condition was estimated to be about 4.1 × 10−7 mm3/N m, which is almost 1/20 of that obtained on TiAlN coatings. For mechanical applications, the top cBN could furnish hardness and chemical inertness, and the diamond and Si3N4 beneath will provide the mechanical supporting capacity. Prime novelty statement Cubic BN coatings with thickness over 2 μm were synthesized for the first time on Si3N4 inserts through boron-doped diamond buffer layer. The cubic BN coatings showed ultrahigh hardness and elastic modulus, and excellent tribological properties. Acknowledgment The authors acknowledge the financial supports of National Natural Science Foundation of China (61176007 and 51005117), Zhejiang Provincial Key Laboratory for Cutting Tools (ZD201305), and CityU Applied Research Grant (ARG 9667089). References [1] W.J. Zhang, Y.M. Chong, B. He, I. Bello, S.T. Lee, Cubic boron nitride films: properties and applications, in: V.K. Sarin (Editor-in-Chief) & C.E. Nebel (Vol. Ed.), Comprehensive Hard Materials, Elsevier, 2014, pp. 607–639. [2] W.J. Zhang, Y.M. Chong, I. Bello, S.T. Lee, Nucleation, growth and characterization of cubic boron nitride (cBN) films, J. Phys. D Appl. Phys. 40 (2007) 6159. [3] P.B. Mirkarimi, K.F. Mecarty, D.L. Medlin, Review of advances in cubic boron nitride film synthesis, Mater. Sci. Eng. R 21 (1997) 47. [4] T. Komarsu, Y. Kakudate, S. Fujiwara, Heat resistance of a shock-synthesized B–C–N heterodiamond, J. Chem. Soc. Faraday Trans. 92 (1996) 5067. [5] V.L. Solozhenko, V.Z. Turkevich, W.B. Holzapfel, Refined phase diagram of boron nitride, J. Phys. Chem. B 103 (1999) 2903.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers F. Xu a,b, M.F. Yuen b, B. He b, C.D. Wang b, X.R. Zhao a,c, X.L. Tang a, D.W. Zuo a,⁎, W.J. Zhang b,⁎⁎ a b c
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, People's Republic of China Center of Super-Diamond and Advanced Films, City University of Hong Kong, Hong Kong Special Administrative Region Zhejiang Provincial Key Laboratory for Cutting Tools, Taizhou University, Taizhou 318000, People's Republic of China
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Article history: Received 20 June 2014 Received in revised form 24 July 2014 Accepted 30 July 2014 Available online 7 August 2014 Keywords: Cubic boron nitride film Silicon nitride insert Boron doped diamond Tribological properties
a b s t r a c t Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si3N4) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si3N4 as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10−7 mm3/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si3N4 inserts may have great potentials for advanced materials machining. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cubic boron nitride (cBN) is structurally analogous to diamond. Due to the strong covalent bonds and high atom density, both of them have outstanding physical and chemical properties, such as extreme hardness, excellent thermal conductivity, high chemical stability and low friction coefficient [1–3]. Compared with diamond, cBN has an even higher thermal stability, and oxidation and graphitization resistances [4,5]. While diamond burns at 600 °C and reacts with iron at elevated temperatures, cBN is chemically stable and does not react with ferrous materials at temperatures up to 1200 °C [6]. These properties along with maintaining its hardness up to 1000 °C pose cBN as an ideal material 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 polycrystalline cBN (PcBN) cutting inserts [7–9]. Such PcBN tools have shown significantly improved processing efficiency as compared with the traditional cemented carbide (WC:Co) and high speed steel (HSS) tools. However, such cBN tools still suffer from the technical difficulties in cementing cBN powder into complex shapes and the high cost of the cBN powder and
⁎ Corresponding author. Tel.: +86 25 84890249 (office). ⁎⁎ Corresponding author. Tel.: +86 852 3442 7433 (office). E-mail addresses: [email protected] (D.W. Zuo), [email protected] (W.J. Zhang).
http://dx.doi.org/10.1016/j.diamond.2014.07.014 0925-9635/© 2014 Elsevier B.V. All rights reserved.
cementing, which motivate ion for thin film synthesis of cBN for tooling applications. Various physical and chemical vapor deposition (PVD and CVD) techniques have been exploited to synthesize cBN coatings [1–3], and cBN coatings synthesized on cemented tungsten carbide (WC:Co) substrates for tooling applications have been reported [10,11]. It has been demonstrated that cBN deposited directly on WC was mechanically unstable, and transition layers, such as boron and boron-rich BN, were introduced to improve the adhesion and mechanical stability of cBN coatings [12,13]. Keunecke et al. reported that deposition of 1.2 μmthick cBN films on WC inserts with TiN/BCN transition layers by radiofrequency diode sputtering, and the turning test showed that the cutting lifetime of the cBN coated inserts was prolonged as compared to the TiN and TiAlN coatings [14,15]. Due to the high compressive stress induced by intense ion bombardment in cBN deposition, the deposition of thick (N 2 μm), adherent, and mechanically stable cBN coatings on cutting inserts remains a challenge. The development of fluorineassisted CVD enabled the growth of high-quality, thick cBN films with significantly reduced residual stress [16,17]; and the cBN coatings were found to grow directly and epitaxially on diamond substrates, skipping the soft sp2-BN incubation layer usually needed for cBN nucleation and improving the adhesion of cBN films greatly [18,19]. 2.8 μmthick cBN films with high phase purity were synthesize on diamondcoated WC:Co inserts by using ECR microwave plasma CVD (MPCVD) with the assistance of fluorine chemistry [20]. Surface pretreatment of the WC:Co inserts by well-controlled chemical etching was required to remove the Co binder in the surface region of cutting inserts, so
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that the adhesion of diamond films could be enhanced. However, the high substrate temperatures during cBN deposition led to diffusion of Co from the bulk of insert to the diamond/insert interface, which resulted in the catalytic formation of graphite and deterioration of diamond adhesion [20,21]. The machining performance tests showed that the diamond–WC interface was the weak link in cBN/diamond coatings on WC inserts. We report in this paper synthesis of cBN films on Si3N4 substrates through boron-doped diamond (BDD) buffer layers by fluorineassisted ECR MPCVD. Si3N4 is one of the major ceramics for cutting tools due to its high hardness, outstanding thermo-mechanical and tribological properties and low fabricating cost [22]. In comparison with WC:Co substrates, Si3N4 has a lower mismatch of thermal expansion coefficient with that of diamond (αSi3N4 = 2.11 × 10−6 K−1 and αWC = 4.11 × 10−6 K−1 versus αDiamond = 0.8 × 10−6 K−1) [23], and the degradation of diamond/substrate interface strength caused by Co diffusion can be avoided, which makes Si3N4 a remarkable substrate for highquality diamond growth [24,25]. In this work, the effects of doping level of BDD films on the microstructure and composition of cBN/BDD coatings were investigated. The friction and wear performance of cBN/ BDD coatings on Si3N4 (cBN/BDD/Si3N4) and the tribological behaviors of the reference TiAlN/WC were also evaluated. 2. Experimental BDD films were deposited on Si3N4 cutting inserts (CN2000, ZCCCT, Co., Ltd, 12 × 12 mm) in a gas mixture of trimethylboron (TMB) in an ASTeX MPCVD system. The substrates were pretreated ultrasonically in diamond powder (~50 nm) suspension for 60 min to enhance the nucleation. TMB was diluted by H2 to 0.1 vol.%, and the total gas flow rate was 300 sccm. The carbon to hydrogen ratio was maintained at 1%, and the boron to carbon ratio (B/C) was varied from 3000 to 12,000 ppm. For all samples, the microwave power was 1200 W, the substrate temperature was kept at 760 °C, the reactive pressure was 30 Torr, and the growth duration was 12 h. Cubic BN films were deposited on the BDD-coated Si3N4 inserts in the ASTeX ECR MPCVD system in a gas mixture of BF3 + N2 + Ar + He + H 2 . The cBN coatings on BDD films with B/C ratios of 3000, 7000 and 12,000 ppm were named as Samples 1, 2 and 3, respectively, in this paper. The gas flow rates of BF3, N2, Ar, He and H2 were maintained at 10, 50, 10, 131 and 2.5 sccm respectively. In the deposition chamber, the Si3N4 insert was mounted on a Mo holder. A pyrolytic BN was used to cover the Mo holder and expose only the insert to plasma during deposition. The deposition pressure and substrate temperature were kept at 2 × 10−3 Torr and 900 °C, respectively. A DC bias of −40 V was applied to the substrate during deposition, and the growth time was 8 h. The deposition was performed under the optimized conditions based on our previous works [18–20]. A microwave power of 1400 W was employed to generate ECR plasma; and the high microwave power could enhance the plasma density and ionization degree, enabling cBN deposition at reduced bias voltages. Moreover, the growth rate and crystallinity of cBN could also be improved, similar to the deposition of diamond/β-SiC composite films by conventional MPCVD [26,27]. The surface and cross-section morphologies of BDD and cBN coatings were observed by scanning electron microscopy (SEM, PHILIPS XL30), and the elemental compositions of cBN films were studied by using energy dispersive X-ray spectroscopy (EDX) attached to the SEM. The phase compositions of BDD and cBN films were characterized by visible and UV Raman spectroscopy (Renishaw inVia) with excitation wavelength of 514 and 244 nm. Small-angle X-ray diffraction (XRD, Rigaku SmartLab) was carried out to investigate the microstructure, grain size, and stress of cBN films. The resistivity of the BDD films was detected by a 4-point probe method. The mechanical and tribological properties of cBN coatings were investigated in comparison with those of commercial TiAlN coatings on
WC:Co substrates. The hardness and elastic modulus of the cBN coating were measured by nanoindentation (Nano Indenter XP). The tribological behaviors of the coatings were investigated using a ball-on-disc (BOD) tribometer (HT1000). Si3N4 balls (HV 1600) with a radius of 3 mm were used as the friction counterpart. The temperature and relative humidity were kept between 21–25 °C and 38–55%, respectively. The sliding speed of 0.2 m/s was preset at a load of 5 N. The Si3N4 ball slid on a circular track with a radius of 2 mm, and the total sliding distance was 2000 m. The cross-section area of wear tracks on the coating was determined with a non-contact optical profilometer (Nanomap 500LS). 3. Result and discussion 3.1. Microstructure of cBN coatings The SEM surface morphologies of the BDD films deposited with B/C ratios of 3000, 7000, and 12,000 ppm are shown in Fig. 1(a)–(c), respectively. It can be seen that all films are polycrystalline with the facets of diamond grains well resolved. More small diamond grains of multiple twinning structures were formed on the surfaces of BDD films grown with an elevated B/C ratio. The Raman spectra of the corresponding BDD films are presented in Fig. 1(d). A sharp diamond peak located at about 1339 cm−1 was revealed for the film prepared with the B/C ratio of 3000 ppm, indicating a high crystallinity of the film. The diamond peak shifted from 1339 to 1329 cm−1 as the B/C ratio increased from 3000 to 12,000 ppm, and the downshift could be attributed to the Fano effect caused by boron doping [28,29]. With the increase of B/C ratio, the intensity of G peak at 1580 cm−1 increased, which suggested an increased sp2 carbon phase in the BDD films of higher boron doping levels. Moreover, for the film deposited with the B/C ratio of 12,000 ppm, broad peaks at 500 and 1220 cm−1 appeared, which are also due to the Fano effect as reported previously [28,29]. The electrical resistivity of the BDD films was revealed to decrease with the increase of B/C ratios. At the B/C ratio of 3000, 7000 and 12,000 ppm, the resistivity of BDD films was measured to be about 0.75, 0.16 and 0.09 Ω cm, respectively. The boron doping of diamond buffer layer results in the improvement of electrical conductivity, which is critical for the application of substrate bias for cBN deposition. Fig. 2(a)–(c) depicts the SEM surface morphologies of the samples after cBN deposition on the BDD films prepared with the B/C ratios of 3000 (Sample 1), 7000 (Sample 2) and 12,000 ppm (Sample 3), respectively, and the inserts show the EDX spectra of the corresponding films. For Sample 1, the facets of diamond grains could still be clearly distinguished, and the EDX spectrum revealed only a carbon signal, which indicated that no cBN film was deposited atop the BDD layer. For Sample 2, a thin film was formed on the BDD film. The EDX spectrum showed boron and nitrogen signals in addition to carbon, which suggested that the cBN film partly covered the underlying BDD layer. In contrast to the above two samples, only boron and nitrogen were detected in EDX spectrum of Sample 3, implying a full coverage of the BDD layer by the cBN coating. The SEM morphology also confirmed the uniform deposition of a continuous BN film, as shown in Fig. 2(c). The crosssectional SEM image in Fig. 2(d) shows that a 2.2 μm-thick cBN film was obtained on Si3N4 cutting inserts precoated with a 4 μm-thick BDD layer. Fig. 3(a) shows the UV Raman spectra of Samples 1–3. Only diamond and graphitic carbon peaks located at 1332 and 1580 cm−1 respectively were observed for Samples 1 and 2, and these two spectra were almost the same as those acquired from the corresponding BDD substrates as shown in Fig. 1(d). Nevertheless, two additional peaks located at 1045 and 1305 cm−1 were revealed for Sample 3, which were assigned to the transverse (TO) and longitudinal optical (LO) modes of cBN [30]. Small-angle XRD was carried out to further verify the growth of cBN film in Sample 3, as shown in Fig. 3(b). The incident X-ray angle α was set at 0.3° and diffraction pattern was taken by 2θ continuous mode
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Fig. 1. SEM surface morphologies of BDD films grown with B/C ratios of (a) 3000 ppm, (b) 7000 ppm, and (c) 12,000 ppm. (d) Visible Raman spectra of the corresponding BDD films prepared with different B/C ratios. The spot size of laser beam was 1 μm.
with a step width of 0.02°. Two sets of diffraction peaks for diamond and cBN were clearly revealed, and a diffraction peak for Si3N4 (302) was also observed, as denoted in the figure. Due to the small lattice mismatch between diamond and cBN (1.34%), their strongest (111) diffractions overlap with each other. The deconvolution in the insert, however, indeed resolved two components for diamond (111) and cBN (111). The Raman and XRD measurements confirmed successful deposition of cBN films in Sample 3. For the fluorine-assisted CVD of cBN films, bombardment of the growing cBN surfaces by energetic ions has been illustrated to be critical for formation of cBN phase, though the ion bombardment plays a different role from the subsurface driving force for sp 2 to
sp3 phase transition [17]. The effective ion bombardment is strongly influenced by the substrate bias and conductivity of substrate. Insulator substrates will cause positive charge accumulation on its surface, and ion bombardment cannot be continuously proceeded. In this work, Si 3N 4 inserts are insulative, thus the BDD film on Si3N 4 not only functions as a buffer layer for cBN nucleation but also provides the necessary conduction path for charge transport. Therefore the conductivity of the BDD layer is an important factor which determines the effective ion bombarding energy and ion flux during cBN deposition. It was found in our experiments that the substrate bias current increased from 0.04 A to 0.08 A when the resistivity of BDD films decreased from 0.75 (Sample 1) to 0.09 Ω cm (Sample 3). The
Fig. 2. SEM surface morphologies of the samples after cBN coating on the BDD films grown with B/C ratios of (a) 3000 ppm, (b) 7000 ppm, and (c) 12,000 ppm. The EDX spectra of the corresponding samples are shown as inserts. (d) SEM cross-sectional micrograph of the sample shown in (c).
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732 GPa, respectively. As a reference, the H and E of TiAlN coatings were reported to be 20 GPa and 200 GPa [32]. Moreover, the resistance to plastic deformation of a coating is proportional to the H3/E2 ratio, and it is closely correlated to the relative wear resistance [33]. The H3/E2 ratio of cBN film is 0.9, which is significantly larger than that of TiAlN (0.2) and indicates a much greater wear resistance of the cBN coating. From the elastic modulus and XRD pattern of cBN coating, the biaxial stress neglecting shear components in the cBN film can be roughly estimated by using Hooke's law: σ¼
Fig. 3. (a) UV Raman spectra of the BN coatings on Si3N4 inserts precoated with BDD layers deposited with different B/C ratios. (b) Small-angle XRD pattern of Sample 3.
εψ E 1−γ
ð1Þ
where elastic modulus of the film E = 732 GPa, Poisson's ratio γ = 0.112 [34,35]. The strain εψ can be calculated from lattice distortion (d0 − d) / d0, where d and d0 are the lattice spacing of measured cBN reflection and the corresponding value from JCPDS 35–1365. A compressive stress of 1.5–1.9 GPa was obtained by using the two strongest XRD peaks of cBN, i.e., (111) and (220). The existence of a considerable compressive stress in the film may also enhance its hardness [36]. Fig. 5(a) illustrates the coefficients of friction (COFs) of the cBN and TiAlN coatings by sliding Si3N4 balls over their surfaces at a constant load of 5 N. The initial calculated contact pressure of the cBN coating and Si3N4 ball is about 1.7 GPa, which is high enough to simulate the machining conditions according to Hertzian's ball-on-disc contact model. As shown in Fig. 5, the COF of the cBN coating dropped from 0.4 to 0.16 in the first 400 m, and then almost maintained this value until the sliding distance of 2000 m. The initial drop of the COF is believed to be due to the rough surface of the as-deposited cBN coating. The comparative study revealed that the COF of the TiAlN coating was about 0.3 over the whole sliding distance of 2000 m.
highly conductive BDD layer in Sample 3 enables growth of continuous thick cBN top films with high purity for mechanical applications. In the following sections, only Sample 3 was selected for detailed mechanical and tribological characterizations. 3.2. Mechanical and tribological properties of the cBN coatings The hardness (H) and elastic modulus (E) of Sample 3 were measured by using nanoindentation based on the standard Oliver and Pharr approach [31]. The displacement-dependent E and H are depicted in Fig. 4. Because a rough cBN surface may result in inaccuracy in E and H measurements, the cBN film was mechanically smoothed with fine diamond lapping paper (average grain size ~ 100 nm). The E and H measurements show plateaus upon the indenter displacement; and based on them, H and E of the cBN coating were estimated to be 78 GPa and
Fig. 4. Hardness and elastic modulus of a polished cBN film obtained at variable nanoindenter displacement.
Fig. 5. (a) Sliding distance dependent COFs of cBN and TiAlN coatings. (b) The crosssectional wear profiles of the TiAlN and cBN coatings after sliding with a Si3N4 ball for 2000 m.
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The wear rate of cBN coating was evaluated by sliding Si3N4 ball on cBN surface with a sliding distance of 2000 m. The wear rate (Ws) of the cBN was estimated from the cross-sectional area of the wear track measured by a surface profilometer: Ws ¼
AC Fl
ð2Þ
where A is the mean of wear area, C is the wear scar length, F is the normal load, and l is the sliding distance. Three scans were performed to get the mean wear area. Characteristic cross-sectional profiles of the wear scars of the cBN and TiAlN coatings after the sliding distance of 2000 m are shown in Fig. 5(b). It can be seen that a much broader and deeper wear scar was formed on the TiAlN coating than that on the cBN coating. According to the measurements, the wear rates of the cBN and TiAlN coatings were derived to be about 4.1 × 10−7 mm3/N m and 73 × 10−7 mm3/N m, respectively, indicating a much superior wear resistance of the cBN coating. As a reference, the wear rate obtained here is comparable to that of cBN/diamond composite films on silicon substrates measured by sliding Si3N4 balls over prolonged distances of 12.56 km (about 1.0 × 10−7 mm3/N·m) [37]. 4. Conclusion Cubic BN coatings with thickness over 2 μm were synthesized on BDD-coated Si3N4 inserts by using ECR MPCVD. BDD films were demonstrated to not only function as a buffer layer for cBN nucleation but also provide the necessary charge transport path for performing effective ion bombardment during cBN growth, thus a high conductivity of BDD was obligatory for the deposition of cBN phase. The deposited cBN coatings showed ultrahigh hardness and elastic modulus of 78 GPa and 732 GPa, respectively. The COF of cBN coatings was revealed to drop quickly from 0.4 to 0.17 after smoothing of the cBN surfaces by sliding Si3N4 balls over the first 400 m, which is smaller than that of the commercial TiAlN coatings (0.3). The wear rate of the cBN coatings against sliding of Si3N4 balls at ambient condition was estimated to be about 4.1 × 10−7 mm3/N m, which is almost 1/20 of that obtained on TiAlN coatings. For mechanical applications, the top cBN could furnish hardness and chemical inertness, and the diamond and Si3N4 beneath will provide the mechanical supporting capacity. Prime novelty statement Cubic BN coatings with thickness over 2 μm were synthesized for the first time on Si3N4 inserts through boron-doped diamond buffer layer. The cubic BN coatings showed ultrahigh hardness and elastic modulus, and excellent tribological properties. Acknowledgment The authors acknowledge the financial supports of National Natural Science Foundation of China (61176007 and 51005117), Zhejiang Provincial Key Laboratory for Cutting Tools (ZD201305), and CityU Applied Research Grant (ARG 9667089). References [1] W.J. Zhang, Y.M. Chong, B. He, I. Bello, S.T. Lee, Cubic boron nitride films: properties and applications, in: V.K. Sarin (Editor-in-Chief) & C.E. Nebel (Vol. Ed.), Comprehensive Hard Materials, Elsevier, 2014, pp. 607–639. [2] W.J. Zhang, Y.M. Chong, I. Bello, S.T. Lee, Nucleation, growth and characterization of cubic boron nitride (cBN) films, J. Phys. D Appl. Phys. 40 (2007) 6159. [3] P.B. Mirkarimi, K.F. Mecarty, D.L. Medlin, Review of advances in cubic boron nitride film synthesis, Mater. Sci. Eng. R 21 (1997) 47. [4] T. Komarsu, Y. Kakudate, S. Fujiwara, Heat resistance of a shock-synthesized B–C–N heterodiamond, J. Chem. Soc. Faraday Trans. 92 (1996) 5067. [5] V.L. Solozhenko, V.Z. Turkevich, W.B. Holzapfel, Refined phase diagram of boron nitride, J. Phys. Chem. B 103 (1999) 2903.
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