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Growth and mechanical properties of diamond films on cemented carbide with buffer layers
Thin Solid Films 584 (2015) 165–169
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Growth and mechanical properties of diamond films on cemented carbide with buffer layers M.N. Liu a, Y.B. Bian a, S.J. Zheng a, T. Zhu a, Y.G. Chen a,⁎, Y.L. Chen b, J.S. Wang b a b
Department of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Shanghai Well-Sun Precision Tool Co., Ltd, 868 Zhenchen Rd., Baoshan Urban Industrial Park, Shanghai, China
a r t i c l e
i n f o
Available online 21 January 2015 Keywords: Diamond Coatings Hot filament chemical vapor deposition Buffer layer Vickers hardness
a b s t r a c t The main hindrance to applications of diamond coatings on cemented carbide cutting tools is poor adhesion between diamond films and the substrates, caused mainly by Co acting as a catalyst for the formation of graphite during the growth of the diamond films. The buffer-layer technique is commonly used to reduce the elemental diffusion or residual stress in the interface between the film and substrate. In this paper, four kinks of buffer layers, namely TiN, CrN, TiC, and SiN, were applied to (111)-textured diamond films grown on mirror-polished cemented carbide substrates using the hot-filament chemical vapor deposition method. The adhesion strengths of different samples are measured by Vickers hardness tests and compared to show the different effects of the buffer layers. The sample with an 800-nm-thick CrN layer and processed by 10-min chemical etching has the best adhesion among the four buffer layers. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Diamond films are applied on cemented carbide cutting tools due to their outstanding physical and chemical properties, such as the highest hardness, low friction coefficient, chemical inertness, and wear resistance [1,2]. However, these applications have been hindered since the binder cobalt, which tends to diffuse from the cemented carbide during the growth of the diamond films, promotes the formation of interfacial graphite layer that remarkably reduces the adhesion between the films and substrates [3]. Therefore, several kinds of pretreatment procedures have been proposed to eliminate or reduce the negative effects of Co, including one-step chemical etching, two-step chemical etching, formation of stable Co compounds, and deposition of buffer layers. The procedures of chemical etching can dramatically remove Co binder at surfaces of cemented carbide tools. In one-step chemical etching, the cemented carbide tools are ultrasonic washed in single or mixed inorganic acid, such as HCl and HNO3, for different time. In two-step chemical etching, Murakami solution (K3Fe(CN)6:KOH:H2O = 1:1:10) is used first to erode WC grains on micron level, and then aqua regia (HNO3:HCl = 1:3) is used to dissolve the remained Co. Despite of one-step or two-step, a fragile layer with low concentrations of Co could not be avoided on the surface of substrate owing to the outflow of Co, thus is subject to fracture or delamination [4] when the diamond-coated tools are in use. Improvements on adhesion could also be achieved with substrate pretreatments to form stable Co compounds (such as borides or silicides), ⁎ Corresponding author. Tel./fax: +86 21 66132807. E-mail address: [email protected] (Y.G. Chen).
http://dx.doi.org/10.1016/j.tsf.2015.01.021 0040-6090/© 2015 Elsevier B.V. All rights reserved.
which decrease the Co vapor pressure and mitigate other influences originated from the mobile Co phase [5]. Another pretreatment for improvements on adhesion is using various intermediate layers, ranging from metals to superhard ceramics, for example, tungsten [6] and aluminum [7], and carbide and nitride layers such as TiN, Ti(C,N) and CrN [8,9], whose purpose is to reduce the diffusion of binder cobalt. For the buffer-layer methods the adhesion strength of the buffer layers to substrates and to diamond is an important factor of the performance of the diamond coatings. In this paper, we have synthesized diamond-coated cemented carbide inserts with and without buffer layers, and compared the effects of the buffer layers on the performance of the diamond coatings. TiN, CrN, TiC and SiN are selected for the buffer layers. 2. Experimental details Φ10 mm × 2 mm WC–6 wt.% Co inserts (obtained from a sales agent of Sandvik Coromant) were used as the substrates. Before deposition of the buffer layers, the inserts were successively polished by diamond powder suspension with different particle sizes until a mirror surface was formed. To reduce the catalytic effect of Co binder and to improve the nucleation rate of diamond films, each sample was pretreated with chemical etching before the introduction of buffer layers and subsequent ultrasonic seeding. The Co binder was ultrasonically etched by mixed acids (HCl(36%):HNO3(68%):H2O = 1:1:1) for 10 min firstly. The samples were then cleaned in deionized water and ethanol. Afterwards, the buffer layers (TiN, CrN, TiC SiN) were deposited with direct current (DC) or radio frequency (RF) magnetron sputtering. All the substrates were
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heated to 300 °C. After the depositions, the thicknesses of the buffer layers were about 800 nm as measured with a probe profilometer. Table I shows the pretreatments of each sample. Before diamond coating, all the samples were ultrasonically treated in diamond powder suspension for 1 h to improve the nucleation rate of diamond films. The deposition of diamond films was performed with a hot filament chemical vapor deposition (HFCVD) system (sp3 Diamond Technologies Model 655) with CH4 (2000 sccm) and H2 (80 sccm). The substrate's temperature is about 750 °C ± 30 °C and the filament's temperature is up to 2250 °C. After 10-h deposition, the thickness of diamond films was about 3 μm. Grazing incidence X-ray diffraction (XRD, D/MAX2500, Cu Kα radiation, 40 kV and 25 mA) was adopted to identify the crystal structure of the buffer layers and the diamond films. XRD patterns were acquired in the 2θ range of 20–80°, using a grazing incidence angle geometry with ω = 0.5° (scan step size = 0.02°, time per step = 1.2 s). The phase particularity of the diamond films was characterized by a confocal Raman spectroscopy (INVIA, Renishaw plc, 633 nm). The surface morphology of the films was observed by field-emission scanning electron microscopy (SEM, Quanta FEG-250, 10 kV). The hardness and elastic modulus of the diamond films were obtained from in-situ nanomechanical testing system (Tribo indenter, Hysitror. Co. Ltd, nano-indentation mode). The adhesion of the diamond films on the substrates was determined by Vickers hardness indentation test (load = 9.8 N, holding time = 10 s). 3. Results and discussions 3.1. Grazing incidence XRD test Fig. 1 shows the XRD patterns of the four buffer layers (TiN, CrN, TiC, and SiN), and the diamond films deposited on the four buffer layers, respectively. The figure shows that the films deposited on these four buffer layers are of diamond phases and, from the relative intensities of the diffraction peaks, the diamond films contain well-grown (111) plane, which corresponds to the scattering angle 2θ = 45°. In Fig. 1(a), the (111), (200) and (220) diffraction peaks of TiN are clearly detected at 2θ = 36.78°, 42.6°, 62.0°, respectively, and (111) is the preferred orientation. After the diamond film was deposited, a new peak was found at 39.7° and is confirmed to be for the (101) plane of TiN0.3 (PDF no. 41-1352) [10]. During the deposition of the diamond films, the temperature of the substrate increased to nearly 750 °C and the binder cobalt might migrate to the interface between the substrate and TiN layer, or even enter the TiN layer. The diffused Co could form CoN with N, which causes elemental segregation in the TiN layer and the appearance of non-stoichiometric compounds like TiN0.3. Similarly, a tiny mismatch between the XRD pattern of the TiC in this experiment and the standard card of TiC (PDF no. 32-1383), also appears in the XRD pattern of the TiC films with and without a diamond layer. From Fig. 1(c), the diffraction peak of the “TiC” without diamond at 2θ = 36.21° corresponds to the (111) plane of TiC. This scattering angle is located between 35.9° (the (111) plane of TiC (PDF no. 32-1383)) and 36.85° (the (111) plane of (W,Ti)C1 − x (PDF no.20 − 1309)). This tiny mismatch may be resulted from thermal reactions between TiC and WC as the deposition temperature of the TiC is 300 °C, high enough to activate atomic diffusion and chemical reactions. Table I Substrate pretreatment before diamond deposition. Sample Pretreatment
Deposition conditions of buffer layers
1 2 3 4
Chemical etching (CE) 10 min CE 10 min + 800 nm TiN layer CE 10 min + 800 nm CrN layer CE 10 min + 800 nm TiC layer
5
CE 10 min + 800 nm SiN layer
– Ti target (DC), Ar + N2 (0.5 Pa), sputtering Cr target (DC), Ar + N2 (0.8 Pa), sputtering Ti target (DC), C target (RF), Ar (0.6 Pa), co-sputtering Si target (RF), Ar + N2 (0.5 Pa), sputtering
As distinct from the TiN and TiC buffer layers, the CrN buffer layer might have an apparent reaction with C during the diamond deposition process since additional peaks of Cr3C2 [11] appear in the XRD pattern, as shown in Fig. 1(b). In Fig. 1(d), apart from the diffraction peaks of WC, no peak of SiN exists, neither before nor after diamond deposition, suggesting that the SiN buffer layer fabricated in this experiment is amorphous instead of crystalline. Because the chemical activity of Si is inferior to Ti and Cr, no reactions should arise between SiN and Co, WC, as well as C.
3.2. SEM and Raman test The surface morphology of Samples 2, 3, 4 and 5 are shown in Fig. 2(a, b, c, d), which indicates that the diamond deposition has nucleated effectively on the four kinds of buffer layers. Unlike typical pyramid-shaped diamond grains mentioned in other articles [11,12], rice-shaped diamond grains were observed with extremely small grain sizes, about 50–100 nm. The small grains should lead to a large content of grain boundaries in the diamond films. The Raman shift of Sample 2 is shown in Fig. 2(e). In order to obtain more information about the quality of the diamond films from the Raman spectra, a data reduction process was implemented. However, the first-order diamond Raman band at 1332 cm− 1 is absent in the spectra. This may be originated from that the diamond Raman band is covered by other non-diamond Raman band [13], like the broad band at 1340 cm−1. According to the rules of the reduction procedure, i.e. Gaussian fitting, a base line including luminescence contribution was removed and four main bands contribute to the Raman spectra of diamond films [14,15]. The two broad bands at around 1340 cm− 1 and 1560 cm−1 are considered as the disorder (D) and graphite (G) bands of graphite, respectively. Besides, two relatively narrow peaks also exist at around 1200 cm−1 and 1450 cm−1. The Raman shifts of Samples 3, 4 and 5 are also analyzed using the same method and are shown in the inserts of Fig. 2(b, c, d). Due to the existence of a large quantity of grain boundaries in the nanocrystalline diamond, these two bands always appeared simultaneously, which are assigned to trans-poly-acetylene lying in grain boundaries in low-quality CVD diamond film [16]. The origin of such trans-poly-acetylene must be related to the deposition mechanism. Firstly, the well-grown and dense buffer layers (nitride and carbide) could significantly prevent the diffusion of Co from the substrates to the diamond films during the diamond deposition [17,18]. The effective reduction of the diffusion of Co could eliminate the negative influence of Co. Secondly, the location of the G-band is around 1560 cm−1 and ID/IG, the intensity ratio of D-band and G-band, is 1.274, suggesting that the state of graphite is NC-graphite [19], which is the graphite phase existing in low-quality nano-diamond films and is not induced by Co. The ID/IG of Samples 3, 4 and 5 are 1.278, 1467 and 1.487 respectively. For the diamond films with different buffer layers, the relationship of ID/IG is as follows: ID =I G ðTiNÞ≈ID =I G ðCrNÞbID =IG ðTiCÞbID =IG ðSiNÞ:
According to the literature [19], while the state of graphite is NCgraphite in the diamond films, the larger the value of ID/IG is, the smaller the content of sp3 phase is and the larger the content of sp2 phase is. Since NC-graphite is the main sp2 phase and diamond is the sp3 phase, the relationship of the values of ID/IG for these four buffer layers implies that the content of diamond phase may be the highest in the diamond films with TiN and CrN buffer layers, less with TiC, and the lowest with SiN. Diamond films containing different sp2 and sp3 contents with different buffer layers may have different behaviors during mechanical tests and service.
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Fig. 1. Grazing incidence XRD patterns of the four buffer layers and the diamond films. (a) TiN and diamond with TiN; (b) CrN and diamond with CrN; (c) TiC and diamond with TiC; (d) SiN and diamond with SiN.
Fig. 2. Surface morphologies of Samples 2 (a), 3 (b), 4 (c), and 5 (d); Raman spectra of Samples 2 (e), 3 (insert of b), 4 (insert of c), and 5 (insert of d).
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Table II Nanomechanical results of substrate, buffer layers and diamond film. Samples WC–Co (6 wt.%) substrate TiN buffer layer CrN buffer layer TiC buffer layer SiN buffer layer Diamond film a
Microhardness (GPa)
Young's modulus (GPa)
Thermal expansion (×106 K−1)a
29.8 ± 1.63
310 ± 14.5
4.9
10.3 ± 0.07 17.7 ± 0.96 17.5 ± 0.38 25.2 ± 0.69 38.9 ± 0.36
255 ± 6.40 273 ± 5.66 277 ± 7.57 236 ± 2.55 380 ± 26.0
6.8–7 23 7.4 3 2
Thermal expansion data are obtained from the references.
3.3. Nanomechanical test Mechanical properties of the buffer layers are known to have remarkable effects on the overall performance of the diamond-coated tools. The hardness and Young's modulus of the diamond films and the buffer layers were investigated by in situ nanomechanical testing. The maximum load was 5000 μN and the holding time was 45 s in the nanoindentation tests. The results are listed in Table II.
3.4. Vickers hardness indentation test Scratch methods, which utilize diamond indenters to slide on samples' surfaces under certain loads, are relatively common for measuring interfacial adhesion strengths [20]. In our experiments, the thickness of the films is only several microns and the hardness is too high, unsuitable for scratch methods. Therefore, the test was implemented with the indentation method as in a Rockwell or Vickers hardness test [1,21], which employs a diamond indenter depressing on the surface of the samples with a certain load and certain holding time until the films split off. In some articles [1,21], the adhesion strength was only characterized qualitatively by the size of indentation marks and the breakage modes. In this paper, three parameters are introduced to characterize the
Table III Vickers hardness indentation results of samples. Sample
Square of separation (μm2)
Vickers hardness (HV1.0)
1 2 3 4 5
447.86 ± 43.6 377.68 ± 5.35 164.39 ± 1.89 669.39 ± 18.8 6259.70
2959 ± 363.0 2430 ± 34.5 2555 ± 29.4 2407 ± 67.7 2818
adhesion strength quantitatively, i.e. the square of pit, the square of indentation, and the square of separation, as shown in Fig. 3(a). The square of pit is the original size of a pit caused by the indentation. The square of indentation is the real size of the indentation mark created directly by the indenter. The square of separation is the difference between the square of pit and the square of indentation, and is conducted as the standard of adhesion strengths in this paper. The values of the Vickers hardness and square of separation are listed in Table III, and the indentation marks of all the samples are shown in Fig. 3. From Table III for Sample 1, which corresponds to the diamond film deposited on the substrate only with 10-min chemical etching and without a buffer layer, the square of separation is about 447.86 μm2, which could be an evaluation criterion to characterize the enhancement to the interfacial adhesion strengths of the diamond coatings. Through comparing the breakage of the diamond films with these four buffer layers, as shown in Fig. 3(b–e), and carbide and nitride layers such as TiN, in Table III and also in Fig. 4, the square of separation of Sample 3 (diamond with CrN buffer layer) is the smallest (164.39 μm2) and no crack emerges in this sample (see Fig. 3c). In contrast, for Sample 5 (diamond with SiN) a large area of the film was damaged seriously and flakes were formed when the diamond indenter was holding on this sample. Different from Sample 3 and Sample 5, only slight breakage and a few tiny cracks appeared in Sample 4 (diamond with TiC) and Sample 2 (diamond with TiN), as shown in Figs. 3(d) and (b). Accordingly, the squares of separation of the samples are very different, as summarized in Fig. 4.
Fig. 3. Optical micrograph of the indentation mark of Samples 1 (a), 2 (b), 3 (c), 4 (d), and 5 (e).
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buffer layer are 38.9 GPa and 380 GPa, respectively. The adhesion strengths of the samples are investigated using an indentation method implemented in Vickers hardness tests. The diamond sample with an 800-nm-thick CrN buffer layer and preprocessed by 10-min chemical etching has a relatively good adhesion among the four samples with buffer layers. Reactions between a buffer layer and substrate/diamond could obviously improve the adhesion strength of diamond films. Acknowledgments This study is primarily supported by Shanghai Eastern-scholar program and Shanghai Pujiang Program, China (no. 11PJ1403400). The authors thank the support from the Instrumental Analysis and Research Center of Shanghai University and GE (China) Research and Development Center Co., Ltd. Fig. 4. Relationship between squares of separation of the samples and buffer-layer materials.
According to the thermal expansion data in Table II, residual stresses may exist in the diamond films with CrN, TiN and TiC buffer layers, especially CrN whose thermal mismatch with respect to diamond is the largest among the four materials. While from the results of the Vickers hardness tests, huge residual stresses emerge in the diamond films with the SiN buffer layer, not in the diamond films with CrN, TiN and TiC buffer layers. From the results of the grazing incidence XRD tests, the adhesion behaviors of the four buffer layers could be generalized to three types, i.e. nonreaction with diamond and substrate, like SiN; reaction with diamond, like CrN; and reaction with substrate, like TiN and TiC. By analyzing the results of the Vickers hardness tests, we found that the buffer layer of nonreaction with diamond and substrate may have negative influence to the adhesion of diamond films, like SiN. The amorphous nature, which would result in lattice mismatch and thermal expansion mismatch, and the high hardness may be other reasons to this negative influence. For the type of reaction with substrate, the buffer layers would have positive influence on the adhesion of diamond films to the substrates, like TiC and TiN, even though this positive influence is not strongly obvious as shown in Table III and in Fig. 4. The most obviously positive influence on the adhesion of diamond films is produced by the buffer layers of the type of reaction with diamond. The chemical etching generates pits on the substrate surface. When the buffer-layer material fills in the pits, mechanical chelation may form at the interface between the buffer layer and substrate. This mechanical chelation could appear in all the four buffer layers, while chemical reactions with diamond only occur in the CrN buffer layer. That is why the adhesion of the diamond film with CrN is the best among these four buffer layers. According to the analysis of XRD and Vickers hardness tests, we may surmise that relative to the buffer layers without reaction, reactions between a buffer layer and substrate/diamond could improve the adhesion strength of diamond films. 4. Conclusion In this paper, diamond films have been grown by HFCVD on mirrorpolished and chemical-etched cemented carbide substrates with four kinds of buffer layers (TiN, CrN, TiC, SiN). The crystal structures of the buffer layers and diamond films are characterized by grazing angle XRD analysis and the preferred orientation of the diamond films is (111). The surface morphology of the diamond films is characterized by rice-shaped grains, and the grain size is in the range of 50–100 nm. The Vickers hardness and elastic modulus of the diamond films on the
References [1] F. Xu, J.H. Xu, M.F. Yuen, L. Zheng, W.Z. Lu, D.W. Zuo, Adhesion improvement of diamond coatings on cemented carbide with high cobalt content using PVD interlayer, Diamond Relat. Mater. 34 (2013) 70. [2] G. Straffelini, P. Scardi, A. Molinari, R. Polini, Characterization and sliding behavior of HFCVD diamond coatings on WC–Co wear, 252 (2002) 1020. [3] F.M. Pan, Interface studies of the diamond film grown on the cobalt cemented tungsten carbide, J. Vac. Sci. Technol. A 12 (1994) 1519. [4] R. Polini, F. Bravi, F. Casadei, P. D'Antonio, E. Traversa, Effect of substrate grain size and surface treatments on the cutting properties of diamond coated Co-cemented tungsten carbide tools, Diamond Relat. Mater. 11 (2002) 726. [5] S. Kubelka, R. Haubner, B. Lux, R. Steiner, G. Stingeder, M. Grasserbauer, Influences of WC–Co hard metal substrate pre-treatments with boron and silicon on low pressure diamond deposition, Diamond Relat. Mater. 3 (1994) 1360. [6] Y. Tang, Y.S. Li, Q. Yang, A. Hirose, Deposition and characterization of diamond coatings on WC–Co cutting tools with W/Al interlayer, Diamond Relat. Mater. 19 (2010) 496. [7] Y.S. Li, Y. Tang, Q. Yang, S. Shimada, R. Wei, K.Y. Lee, A. Hirose, Al-enhanced nucleation and adhesion of diamond films on WC–Co substrates, Int. J. Refract. Met. Hard Mater. 26 (2008) 465. [8] M. Gowri, H. Li, T. Kacsich, J.J. Schermer, W.J.P. van Enckevort, J.J. ter Meulen, Critical parameters in hot filament chemical vapor deposition of diamond films on tool steel substrates with CrN interlayers, Surf. Coat. Technol. 201 (2007) 4601. [9] J.P. Manaud, A. Poulon, S. Gomez, Y. Le Petitcorps, A comparative study of CrN, ZrN, NbN and TaN layers as cobalt diffusion barriers for CVD diamond deposition on WC– Co tools, Surf. Coat. Technol. 202 (2007) 222. [10] A.L. Kameneva, Features of the formation and nanostructure of the film with the basic hexagonal phase TiN0.3 by arc evaporation, J. Biol. Chem. 02 (2011) 26. [11] R. Polini, Chemically vapour deposited diamond coatings on cemented tungsten carbides: substrate pretreatments, adhesion and cutting performance, Thin Solid Films 515 (2006) 4. [12] R. Polini, F.P. Mantini, M. Barletta, R. Valle, F. Casadei, Hot filament chemical vapour deposition and wear resistance of diamond films on WC–Co substrates coated using PVD-arc deposition technique, Diamond Relat. Mater. 15 (2006) 1284. [13] P.M. Fauchet, I.H. Campbell, Raman spectroscopy of low-dimensional semiconductors, Crit. Rev. Solid State Mater. Sci. 14 (1988) s79. [14] P.K. Bachmann, D.U. Wiechert, Optical characterization of diamond, Diamond Relat. Mater. 1 (1992) 422. [15] R. Polini, E. Traversa, A. Marucci, G. Mattei, G. Marcheselli, A Raman study of diamond film growth on co-cemented tungsten carbide, J. Electrochem. Soc. 144 (1997) 1371. [16] A.C. Ferrari, J. Robertson, Origin of the 1150 cm−1 Raman mode in nanocrystalline diamond, Phys. Rev. B 63 (2001). [17] K. Petrikowski, M. Fenker, J. Gabler, A. Hagemann, S. Pleger, L. Schafer, Study of CrNx and NbC interlayers for HFCVD diamond deposition onto WC–Co substrates, Diamond Relat. Mater. 33 (2013) 38. [18] E. Hojman, R. Akhvlediani, A. Layyous, A. Hoffman, Diamond CVD film formation onto WC–Co substrates using a thermally nitrided Cr diffusion-barrier (vol 39, pg 65, 2013), Diamond Relat. Mater. 40 (2013) 123-123. [19] A.C. Ferrari, J. Robertson, Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond, Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 362 (2004) 2477. [20] R. Polini, M. Barletta, G. Rubino, S. Vesco, Recent advances in the deposition of diamond coatings on Co-cemented tungsten carbides, Adv. Mater. Sci. Eng. 1–14 (2012). [21] v W. Tang, Q. Wang, S. Wang, R. Lu, A comparison in performance of diamond coated cemented carbide cutting tools with and without a boride interlayer, Surf. Coat. Technol. 153 (2002) 298.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Growth and mechanical properties of diamond films on cemented carbide with buffer layers M.N. Liu a, Y.B. Bian a, S.J. Zheng a, T. Zhu a, Y.G. Chen a,⁎, Y.L. Chen b, J.S. Wang b a b
Department of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Shanghai Well-Sun Precision Tool Co., Ltd, 868 Zhenchen Rd., Baoshan Urban Industrial Park, Shanghai, China
a r t i c l e
i n f o
Available online 21 January 2015 Keywords: Diamond Coatings Hot filament chemical vapor deposition Buffer layer Vickers hardness
a b s t r a c t The main hindrance to applications of diamond coatings on cemented carbide cutting tools is poor adhesion between diamond films and the substrates, caused mainly by Co acting as a catalyst for the formation of graphite during the growth of the diamond films. The buffer-layer technique is commonly used to reduce the elemental diffusion or residual stress in the interface between the film and substrate. In this paper, four kinks of buffer layers, namely TiN, CrN, TiC, and SiN, were applied to (111)-textured diamond films grown on mirror-polished cemented carbide substrates using the hot-filament chemical vapor deposition method. The adhesion strengths of different samples are measured by Vickers hardness tests and compared to show the different effects of the buffer layers. The sample with an 800-nm-thick CrN layer and processed by 10-min chemical etching has the best adhesion among the four buffer layers. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Diamond films are applied on cemented carbide cutting tools due to their outstanding physical and chemical properties, such as the highest hardness, low friction coefficient, chemical inertness, and wear resistance [1,2]. However, these applications have been hindered since the binder cobalt, which tends to diffuse from the cemented carbide during the growth of the diamond films, promotes the formation of interfacial graphite layer that remarkably reduces the adhesion between the films and substrates [3]. Therefore, several kinds of pretreatment procedures have been proposed to eliminate or reduce the negative effects of Co, including one-step chemical etching, two-step chemical etching, formation of stable Co compounds, and deposition of buffer layers. The procedures of chemical etching can dramatically remove Co binder at surfaces of cemented carbide tools. In one-step chemical etching, the cemented carbide tools are ultrasonic washed in single or mixed inorganic acid, such as HCl and HNO3, for different time. In two-step chemical etching, Murakami solution (K3Fe(CN)6:KOH:H2O = 1:1:10) is used first to erode WC grains on micron level, and then aqua regia (HNO3:HCl = 1:3) is used to dissolve the remained Co. Despite of one-step or two-step, a fragile layer with low concentrations of Co could not be avoided on the surface of substrate owing to the outflow of Co, thus is subject to fracture or delamination [4] when the diamond-coated tools are in use. Improvements on adhesion could also be achieved with substrate pretreatments to form stable Co compounds (such as borides or silicides), ⁎ Corresponding author. Tel./fax: +86 21 66132807. E-mail address: [email protected] (Y.G. Chen).
http://dx.doi.org/10.1016/j.tsf.2015.01.021 0040-6090/© 2015 Elsevier B.V. All rights reserved.
which decrease the Co vapor pressure and mitigate other influences originated from the mobile Co phase [5]. Another pretreatment for improvements on adhesion is using various intermediate layers, ranging from metals to superhard ceramics, for example, tungsten [6] and aluminum [7], and carbide and nitride layers such as TiN, Ti(C,N) and CrN [8,9], whose purpose is to reduce the diffusion of binder cobalt. For the buffer-layer methods the adhesion strength of the buffer layers to substrates and to diamond is an important factor of the performance of the diamond coatings. In this paper, we have synthesized diamond-coated cemented carbide inserts with and without buffer layers, and compared the effects of the buffer layers on the performance of the diamond coatings. TiN, CrN, TiC and SiN are selected for the buffer layers. 2. Experimental details Φ10 mm × 2 mm WC–6 wt.% Co inserts (obtained from a sales agent of Sandvik Coromant) were used as the substrates. Before deposition of the buffer layers, the inserts were successively polished by diamond powder suspension with different particle sizes until a mirror surface was formed. To reduce the catalytic effect of Co binder and to improve the nucleation rate of diamond films, each sample was pretreated with chemical etching before the introduction of buffer layers and subsequent ultrasonic seeding. The Co binder was ultrasonically etched by mixed acids (HCl(36%):HNO3(68%):H2O = 1:1:1) for 10 min firstly. The samples were then cleaned in deionized water and ethanol. Afterwards, the buffer layers (TiN, CrN, TiC SiN) were deposited with direct current (DC) or radio frequency (RF) magnetron sputtering. All the substrates were
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heated to 300 °C. After the depositions, the thicknesses of the buffer layers were about 800 nm as measured with a probe profilometer. Table I shows the pretreatments of each sample. Before diamond coating, all the samples were ultrasonically treated in diamond powder suspension for 1 h to improve the nucleation rate of diamond films. The deposition of diamond films was performed with a hot filament chemical vapor deposition (HFCVD) system (sp3 Diamond Technologies Model 655) with CH4 (2000 sccm) and H2 (80 sccm). The substrate's temperature is about 750 °C ± 30 °C and the filament's temperature is up to 2250 °C. After 10-h deposition, the thickness of diamond films was about 3 μm. Grazing incidence X-ray diffraction (XRD, D/MAX2500, Cu Kα radiation, 40 kV and 25 mA) was adopted to identify the crystal structure of the buffer layers and the diamond films. XRD patterns were acquired in the 2θ range of 20–80°, using a grazing incidence angle geometry with ω = 0.5° (scan step size = 0.02°, time per step = 1.2 s). The phase particularity of the diamond films was characterized by a confocal Raman spectroscopy (INVIA, Renishaw plc, 633 nm). The surface morphology of the films was observed by field-emission scanning electron microscopy (SEM, Quanta FEG-250, 10 kV). The hardness and elastic modulus of the diamond films were obtained from in-situ nanomechanical testing system (Tribo indenter, Hysitror. Co. Ltd, nano-indentation mode). The adhesion of the diamond films on the substrates was determined by Vickers hardness indentation test (load = 9.8 N, holding time = 10 s). 3. Results and discussions 3.1. Grazing incidence XRD test Fig. 1 shows the XRD patterns of the four buffer layers (TiN, CrN, TiC, and SiN), and the diamond films deposited on the four buffer layers, respectively. The figure shows that the films deposited on these four buffer layers are of diamond phases and, from the relative intensities of the diffraction peaks, the diamond films contain well-grown (111) plane, which corresponds to the scattering angle 2θ = 45°. In Fig. 1(a), the (111), (200) and (220) diffraction peaks of TiN are clearly detected at 2θ = 36.78°, 42.6°, 62.0°, respectively, and (111) is the preferred orientation. After the diamond film was deposited, a new peak was found at 39.7° and is confirmed to be for the (101) plane of TiN0.3 (PDF no. 41-1352) [10]. During the deposition of the diamond films, the temperature of the substrate increased to nearly 750 °C and the binder cobalt might migrate to the interface between the substrate and TiN layer, or even enter the TiN layer. The diffused Co could form CoN with N, which causes elemental segregation in the TiN layer and the appearance of non-stoichiometric compounds like TiN0.3. Similarly, a tiny mismatch between the XRD pattern of the TiC in this experiment and the standard card of TiC (PDF no. 32-1383), also appears in the XRD pattern of the TiC films with and without a diamond layer. From Fig. 1(c), the diffraction peak of the “TiC” without diamond at 2θ = 36.21° corresponds to the (111) plane of TiC. This scattering angle is located between 35.9° (the (111) plane of TiC (PDF no. 32-1383)) and 36.85° (the (111) plane of (W,Ti)C1 − x (PDF no.20 − 1309)). This tiny mismatch may be resulted from thermal reactions between TiC and WC as the deposition temperature of the TiC is 300 °C, high enough to activate atomic diffusion and chemical reactions. Table I Substrate pretreatment before diamond deposition. Sample Pretreatment
Deposition conditions of buffer layers
1 2 3 4
Chemical etching (CE) 10 min CE 10 min + 800 nm TiN layer CE 10 min + 800 nm CrN layer CE 10 min + 800 nm TiC layer
5
CE 10 min + 800 nm SiN layer
– Ti target (DC), Ar + N2 (0.5 Pa), sputtering Cr target (DC), Ar + N2 (0.8 Pa), sputtering Ti target (DC), C target (RF), Ar (0.6 Pa), co-sputtering Si target (RF), Ar + N2 (0.5 Pa), sputtering
As distinct from the TiN and TiC buffer layers, the CrN buffer layer might have an apparent reaction with C during the diamond deposition process since additional peaks of Cr3C2 [11] appear in the XRD pattern, as shown in Fig. 1(b). In Fig. 1(d), apart from the diffraction peaks of WC, no peak of SiN exists, neither before nor after diamond deposition, suggesting that the SiN buffer layer fabricated in this experiment is amorphous instead of crystalline. Because the chemical activity of Si is inferior to Ti and Cr, no reactions should arise between SiN and Co, WC, as well as C.
3.2. SEM and Raman test The surface morphology of Samples 2, 3, 4 and 5 are shown in Fig. 2(a, b, c, d), which indicates that the diamond deposition has nucleated effectively on the four kinds of buffer layers. Unlike typical pyramid-shaped diamond grains mentioned in other articles [11,12], rice-shaped diamond grains were observed with extremely small grain sizes, about 50–100 nm. The small grains should lead to a large content of grain boundaries in the diamond films. The Raman shift of Sample 2 is shown in Fig. 2(e). In order to obtain more information about the quality of the diamond films from the Raman spectra, a data reduction process was implemented. However, the first-order diamond Raman band at 1332 cm− 1 is absent in the spectra. This may be originated from that the diamond Raman band is covered by other non-diamond Raman band [13], like the broad band at 1340 cm−1. According to the rules of the reduction procedure, i.e. Gaussian fitting, a base line including luminescence contribution was removed and four main bands contribute to the Raman spectra of diamond films [14,15]. The two broad bands at around 1340 cm− 1 and 1560 cm−1 are considered as the disorder (D) and graphite (G) bands of graphite, respectively. Besides, two relatively narrow peaks also exist at around 1200 cm−1 and 1450 cm−1. The Raman shifts of Samples 3, 4 and 5 are also analyzed using the same method and are shown in the inserts of Fig. 2(b, c, d). Due to the existence of a large quantity of grain boundaries in the nanocrystalline diamond, these two bands always appeared simultaneously, which are assigned to trans-poly-acetylene lying in grain boundaries in low-quality CVD diamond film [16]. The origin of such trans-poly-acetylene must be related to the deposition mechanism. Firstly, the well-grown and dense buffer layers (nitride and carbide) could significantly prevent the diffusion of Co from the substrates to the diamond films during the diamond deposition [17,18]. The effective reduction of the diffusion of Co could eliminate the negative influence of Co. Secondly, the location of the G-band is around 1560 cm−1 and ID/IG, the intensity ratio of D-band and G-band, is 1.274, suggesting that the state of graphite is NC-graphite [19], which is the graphite phase existing in low-quality nano-diamond films and is not induced by Co. The ID/IG of Samples 3, 4 and 5 are 1.278, 1467 and 1.487 respectively. For the diamond films with different buffer layers, the relationship of ID/IG is as follows: ID =I G ðTiNÞ≈ID =I G ðCrNÞbID =IG ðTiCÞbID =IG ðSiNÞ:
According to the literature [19], while the state of graphite is NCgraphite in the diamond films, the larger the value of ID/IG is, the smaller the content of sp3 phase is and the larger the content of sp2 phase is. Since NC-graphite is the main sp2 phase and diamond is the sp3 phase, the relationship of the values of ID/IG for these four buffer layers implies that the content of diamond phase may be the highest in the diamond films with TiN and CrN buffer layers, less with TiC, and the lowest with SiN. Diamond films containing different sp2 and sp3 contents with different buffer layers may have different behaviors during mechanical tests and service.
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Fig. 1. Grazing incidence XRD patterns of the four buffer layers and the diamond films. (a) TiN and diamond with TiN; (b) CrN and diamond with CrN; (c) TiC and diamond with TiC; (d) SiN and diamond with SiN.
Fig. 2. Surface morphologies of Samples 2 (a), 3 (b), 4 (c), and 5 (d); Raman spectra of Samples 2 (e), 3 (insert of b), 4 (insert of c), and 5 (insert of d).
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Table II Nanomechanical results of substrate, buffer layers and diamond film. Samples WC–Co (6 wt.%) substrate TiN buffer layer CrN buffer layer TiC buffer layer SiN buffer layer Diamond film a
Microhardness (GPa)
Young's modulus (GPa)
Thermal expansion (×106 K−1)a
29.8 ± 1.63
310 ± 14.5
4.9
10.3 ± 0.07 17.7 ± 0.96 17.5 ± 0.38 25.2 ± 0.69 38.9 ± 0.36
255 ± 6.40 273 ± 5.66 277 ± 7.57 236 ± 2.55 380 ± 26.0
6.8–7 23 7.4 3 2
Thermal expansion data are obtained from the references.
3.3. Nanomechanical test Mechanical properties of the buffer layers are known to have remarkable effects on the overall performance of the diamond-coated tools. The hardness and Young's modulus of the diamond films and the buffer layers were investigated by in situ nanomechanical testing. The maximum load was 5000 μN and the holding time was 45 s in the nanoindentation tests. The results are listed in Table II.
3.4. Vickers hardness indentation test Scratch methods, which utilize diamond indenters to slide on samples' surfaces under certain loads, are relatively common for measuring interfacial adhesion strengths [20]. In our experiments, the thickness of the films is only several microns and the hardness is too high, unsuitable for scratch methods. Therefore, the test was implemented with the indentation method as in a Rockwell or Vickers hardness test [1,21], which employs a diamond indenter depressing on the surface of the samples with a certain load and certain holding time until the films split off. In some articles [1,21], the adhesion strength was only characterized qualitatively by the size of indentation marks and the breakage modes. In this paper, three parameters are introduced to characterize the
Table III Vickers hardness indentation results of samples. Sample
Square of separation (μm2)
Vickers hardness (HV1.0)
1 2 3 4 5
447.86 ± 43.6 377.68 ± 5.35 164.39 ± 1.89 669.39 ± 18.8 6259.70
2959 ± 363.0 2430 ± 34.5 2555 ± 29.4 2407 ± 67.7 2818
adhesion strength quantitatively, i.e. the square of pit, the square of indentation, and the square of separation, as shown in Fig. 3(a). The square of pit is the original size of a pit caused by the indentation. The square of indentation is the real size of the indentation mark created directly by the indenter. The square of separation is the difference between the square of pit and the square of indentation, and is conducted as the standard of adhesion strengths in this paper. The values of the Vickers hardness and square of separation are listed in Table III, and the indentation marks of all the samples are shown in Fig. 3. From Table III for Sample 1, which corresponds to the diamond film deposited on the substrate only with 10-min chemical etching and without a buffer layer, the square of separation is about 447.86 μm2, which could be an evaluation criterion to characterize the enhancement to the interfacial adhesion strengths of the diamond coatings. Through comparing the breakage of the diamond films with these four buffer layers, as shown in Fig. 3(b–e), and carbide and nitride layers such as TiN, in Table III and also in Fig. 4, the square of separation of Sample 3 (diamond with CrN buffer layer) is the smallest (164.39 μm2) and no crack emerges in this sample (see Fig. 3c). In contrast, for Sample 5 (diamond with SiN) a large area of the film was damaged seriously and flakes were formed when the diamond indenter was holding on this sample. Different from Sample 3 and Sample 5, only slight breakage and a few tiny cracks appeared in Sample 4 (diamond with TiC) and Sample 2 (diamond with TiN), as shown in Figs. 3(d) and (b). Accordingly, the squares of separation of the samples are very different, as summarized in Fig. 4.
Fig. 3. Optical micrograph of the indentation mark of Samples 1 (a), 2 (b), 3 (c), 4 (d), and 5 (e).
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buffer layer are 38.9 GPa and 380 GPa, respectively. The adhesion strengths of the samples are investigated using an indentation method implemented in Vickers hardness tests. The diamond sample with an 800-nm-thick CrN buffer layer and preprocessed by 10-min chemical etching has a relatively good adhesion among the four samples with buffer layers. Reactions between a buffer layer and substrate/diamond could obviously improve the adhesion strength of diamond films. Acknowledgments This study is primarily supported by Shanghai Eastern-scholar program and Shanghai Pujiang Program, China (no. 11PJ1403400). The authors thank the support from the Instrumental Analysis and Research Center of Shanghai University and GE (China) Research and Development Center Co., Ltd. Fig. 4. Relationship between squares of separation of the samples and buffer-layer materials.
According to the thermal expansion data in Table II, residual stresses may exist in the diamond films with CrN, TiN and TiC buffer layers, especially CrN whose thermal mismatch with respect to diamond is the largest among the four materials. While from the results of the Vickers hardness tests, huge residual stresses emerge in the diamond films with the SiN buffer layer, not in the diamond films with CrN, TiN and TiC buffer layers. From the results of the grazing incidence XRD tests, the adhesion behaviors of the four buffer layers could be generalized to three types, i.e. nonreaction with diamond and substrate, like SiN; reaction with diamond, like CrN; and reaction with substrate, like TiN and TiC. By analyzing the results of the Vickers hardness tests, we found that the buffer layer of nonreaction with diamond and substrate may have negative influence to the adhesion of diamond films, like SiN. The amorphous nature, which would result in lattice mismatch and thermal expansion mismatch, and the high hardness may be other reasons to this negative influence. For the type of reaction with substrate, the buffer layers would have positive influence on the adhesion of diamond films to the substrates, like TiC and TiN, even though this positive influence is not strongly obvious as shown in Table III and in Fig. 4. The most obviously positive influence on the adhesion of diamond films is produced by the buffer layers of the type of reaction with diamond. The chemical etching generates pits on the substrate surface. When the buffer-layer material fills in the pits, mechanical chelation may form at the interface between the buffer layer and substrate. This mechanical chelation could appear in all the four buffer layers, while chemical reactions with diamond only occur in the CrN buffer layer. That is why the adhesion of the diamond film with CrN is the best among these four buffer layers. According to the analysis of XRD and Vickers hardness tests, we may surmise that relative to the buffer layers without reaction, reactions between a buffer layer and substrate/diamond could improve the adhesion strength of diamond films. 4. Conclusion In this paper, diamond films have been grown by HFCVD on mirrorpolished and chemical-etched cemented carbide substrates with four kinds of buffer layers (TiN, CrN, TiC, SiN). The crystal structures of the buffer layers and diamond films are characterized by grazing angle XRD analysis and the preferred orientation of the diamond films is (111). The surface morphology of the diamond films is characterized by rice-shaped grains, and the grain size is in the range of 50–100 nm. The Vickers hardness and elastic modulus of the diamond films on the
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