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Effects of SiH4 flow rate on microstructure and mechanical properties of TiSiN nanocomposite coatings by cathodic arc ion plating
Vacuum 117 (2015) 12e16
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of SiH4 flow rate on microstructure and mechanical properties of TiSiN nanocomposite coatings by cathodic arc ion plating C.X. Tian a, b, B. Yang a, b, *, Q. Wan a, H. Ding a, M.Q. Hong b, R.Y. Wang a, H.D. Liu a, Y.M. Chen a, S.J. Yan b, F. Ren b, V.O. Pelenovich a, c, D.J. Fu a, ** a b c
School of Power & Mechanical Engineering, Wuhan University, 430072 Wuhan, China Accelerator Laboratory, Department of Physics, Wuhan University, 430072 Wuhan, China Institute of Ion-Plasma and Laser Technologies, Academy of Sciences of Uzbekistan, 700135 Tashkent, Uzbekistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2014 Received in revised form 30 March 2015 Accepted 1 April 2015 Available online 11 April 2015
TiSiN nanocomposite coatings were deposited on Si and cemented carbide substrates by cathodic arc ion plating in SiH4 ambient. The effects of SiH4 flow rate on microstructure, mechanical and tribological properties of the coatings were investigated systemically. Transmission electron microscopy and X-ray diffraction were employed to probe the crystalline microstructure and X-ray photoelectron spectroscopy was used to investigate the chemical bonding states. Microhardness testers and tribometer were used to evaluate the mechanical and tribological properties of TiSiN nanocomposite coatings. The results showed that the structure of TiSiN coating changed from columnar grain to nanocrystalline with increasing SiH4 flow rate. The hardness increased up to 4100 HV0.025 at the SiH4 flow rate of 42 sccm (9.5 at.% Si in the coating), but it decreases with further addition of SiH4 gases. Friction coefficients of TiSiN composite coatings in the range of 0.6e0.7 were obtained when tested against Si3N4 balls. © 2015 Elsevier Ltd. All rights reserved.
Keywords: TiSiN coating Microstructure Hardness Tribological property
1. Introduction TiSiN composite coatings have attracted much attention due to its high hardness [1e3], excellent oxidization resistance [4,5], good chemical inertness [6] and tribological performance [7,8] since the publication of Veprek's report on the successful fabrication of nanocomposite TiSiN coatings in 1995. Various deposition techniques, such as chemical vapor deposition (CVD) [9], metal-organic chemical vapor deposition (MOCVD) [10], plasma enhanced chemical vapor deposition (PECVD) [11], hypersonic plasma particle deposition (HPPD) [12], inductively coupled plasma (ICP) [13], magnetron sputteringandcathodic arc ion plating [14e17], have been utilized to deposit TiSiN composite coatings. Cathodic arc ion plating (AIP)is one of the most popular PVD techniques useful for industrial production of the TiSiN coatings. In AIPTiSiN coatings, silicon is usually generated from TieSi alloys. However, Si concentration and doping uniformity of the composite coating on industrialscale are still a challenging issue. Previous
* Corresponding author. School of Power & Mechanical Engineering, Wuhan University, 430072 Wuhan, China. Tel./fax: þ86 27 6877 2253. ** Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (D.J. Fu). http://dx.doi.org/10.1016/j.vacuum.2015.04.002 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
researches showed that SiH4 is the best silicon source for deposition of TiSiN coatings in high temperature CVD [9,18], and the Si content exhibits great influences on the structure and mechanical properties of the TiSiN coatings [17,19]. Few works has been devoted to utilization of silane to synthesize TiSiN coatings in AIP process. In this work, TiSiN nanocomposite coatings were deposited by AIP under silane ambient using 99.99% high purity Ti targets. The effects of silane flow rate on structure and mechanical properties of the coatings were systematically investigated. The results may provide scientific base for application of silane to TiSiN deposition in AIP process. 2. Experiment details TiSiN coatings were deposited onto cemented carbide and silicon wafer by a home-made notfiltered vacuum cathodic arc plating system using Ti metal targets. The chamber had inner dimensions of 55 30 40 cm3. The substrates were mounted on the sample holder rotated at 3 rpm. The minimum distance between sample and targets was 22.5 cm. Before deposition the chamber was evacuated to a base pressure of 103 Pa. The cemented carbide substrates with dimensions of 15 15 5 mm3 were mechanically ground and polished to mirror finishing. After being ultrasonically
C.X. Tian et al. / Vacuum 117 (2015) 12e16
cleaned in acetone, ethanol and deionized water, the substrates were mounted on the substrate holder and etched in Ar glow discharge. The ion bombarding was carried out to remove possible contaminations on the substrate surfaces biased with 800 V at 0.02 Pa in Ar ambient. After bombardment etching, two Ti targets were ignited to deposit TiSiN composite coatings. The SiH4 gas carried by nitrogen with a volume ratio N2:SiH4~9:1 was fed into the chamber with its flow rate being varied between 3 and 68 sccm for deposition of coatings with different Si contents. In addition, another flow of N2 gas was fed into the chamber from separate mass flow controller to balance the total deposition pressure at a fixed value of 0.7 Pa. More details of the deposition parameters were listed in Table 1. The microstructure of the coatings was observed by transmission electron microscopy (TEM, JEOL JEM 2010). The crystal structure of the coatings were identified by X-ray diffraction (XRD, Bruker-axs D8 advanced), where the Cu Ka line at 0.15405 nm was used as the excitation source. The chemical bonding and composition were investigated by X-ray photoelectron spectroscopy (Kratos2AXIS2HS XPS)using the Mg Ka (1253.6 eV) radiation. The morphology of the coatings were determined by field emission scanning electron microscope (SEM) equipped with the EDAX 7000 system. The hardness was measured by using an HX-1000 microhardness tester with a load of 25 g and taking the average of 10 random values. The wear behavior was evaluated through sliding tests using a conventional ball-on-disc wear apparatus (MS-T3000). The Si3N4 balls (3 mm in diameter) were used as counterpart materials. The sliding tests were conducted with a sliding speed of 0.02 m/s under a load of 4.0 N at ambient air (30 C) and 70e75% relative humidity. The wear time was 60 min and the total sliding distance of each sample was 28 m.
3. Results and discussion Fig. 1 shows the Si contents of TiSiN coatings as a function of SiH4 gas flow rate. The thicknesses of coatingson cemented carbide substrates used for the XPS measurement are about 1.5 mm. Before the measurements, sputter cleaning procedure with Arþ ion beams (EX06 ion gun, 500 eV, 5 min) was performed. The Si contents increases from 1.0 at% to 15.1 at% when SiH4 flow rate increases from 3.0 sccm to 68 sccm, respectively. The increasing Si contents are attributed to Si atoms or ions generated from decomposition of SiH4. The symmetrical geometry X-ray diffraction was used to probe the crystallinity and orientation of TiSiN coatings deposited on Si substrates with various SiH4 flow rate, as shown in Fig. 2. The diffraction peaks were observed at 2q ¼ 36.3 and 61.8 corresponding to TiN(111) and TiN(220), respectively. The small peak observed at 34.7 is the artifact of W La1 radiation from cathode material contamination on the anode surface. When the SiH4 flow rate is lower than 42 sccm, the samples exhibit a strong TiN (111) diffraction peak with the peak width of 0.5 deg. Increasing SiH4
Fig. 1. Si contents and deposition rate of TiSiN composite coatings as a function of SiH4 flow rate.
flow rates change the width of TiN (111) peak and it is continuously increased with feeding of SiH4 gases. At SiH4 flow rate of 42 sccm and 68 sccm, the TiN (111) and TiN (220) peaks become very weak with a full width more than 2 deg. Using Debye-Scherrer formula one can estimate the crystallite size of the TiN phase: 17 nm for the 8 sccm sample and less than 5 nm for the 68 sccm sample. However, there is no obvious peak shift, which means that the varied Si contents have little influence on crystal cell parameters of TiN. No signal related to crystalline Si3N4 or TiSi compounds is observed, suggesting they are amorphous or subnanocrystalline [20e22]. The present results are in agreement with reports on nc-TiN/a-Si3N4 nanocomposite films prepared by either CVD or PVD [23,24]. Fig. 3 shows cross-sectional TEM bright field images of TiSiN coatings deposited on Si substrates at different SiH4 flow rates. It reveals that the coatings are typical columnar grain structure when SiH4 flow rate is lower than 42 sccm. The columnar grain width of the TiSiN coatings is 20~50 nm. The SAED patterns on the righthand side shows that the coatings are polycrystalline with TiN(111), (200) and (220) orientations. At the lower SiH4 flow rates of 3 and 30 sccm, the incorporation of Si cannot suppress the columnar grain growth of TiSiN, as shown in Fig. 3aeb. With increasing SiH4 flow rates up to 42 sccm, no columnar grain growth could be observed in the TEM bright field image and the coating
Table 1 Deposition parameters of TiSiN composite coatings. Parameters value
Value
Target material (Ti) Bias voltage (V) Cleaning and reactive gases Working pressure(Pa) SiH4 (sccm) Target current (A) Substrate temperature ( C) Rotation speed (rpm) Deposition time (min)
99.99% Etching: 800, deposition: 100 Ar, N2 and SiH4 Etching: 0.02, deposition: 0.7 3, 8, 18, 30, 42, 68 70 300 3 30
13
Fig. 2. X-ray diffraction of TiSiN coatings deposited at various SiH4 flow rates.
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Fig. 3. Cross-sectional TEM bright-field image of TiSiN coatings deposited at SiH4 flow rates of 3 sccm, 30 sccm, and 42 sccm and the corresponding diffraction patterns. The arrows indicate growth direction.
exhibit amorphous or subnanocrystalline morphology and diffraction pattern. To investigate more details of TiSiN composite coatings deposited at 42 and 68 sccm, high resolution transmission electron micrograph (HRTEM) analysis was performed,as shown in Fig. 4. The insert shows fast Fourier transformed patterns and inverse fast Fourier transformed images of the selected areas A and B. It is clearly seen that there exist TiN nanocrystallites embedded into amorphous matrix. The grain size of TiN nanocrystallites is about 5e8 nm, which is in agreement with the XRD data. This reveals that larger amount of Si in the coatings can suppress the growth of TiN grain and makes TiSiN composite coating more amorphous-like.
Fig. 5 shows core level spectra of N1s, Si2p and Ti2p of the asreceived TiSiN coatings deposited on Si substrates at different SiH4 flow rates. The spectra were fitted by using XPSPEAK software. The Gaussian fitting of the Ti2p spectra gives three peaks situated at 457.6 eV, 456.5 eV and 454.8 eV, which correspond to TiO2, TiNO and TiN, respectively [25]. As shown in Fig. 5a, Gaussian deconvolution of N1s spectra gives rise to two peaks at 396.1 eV and 397 eV; the high energy peak attributed to TiN is in good agreement with literature data [18,25]. The origin of the second peak which was also observed in Ref. [25] remains uncertain. But comparing the XPS spectra of N and Ti one notes their whole spectra intensity correlation, which suggests that the N peak at 396.1 eV belongs to some Ti bearing phase. Fitting of the Si2p spectra by Gaussian deconvolution shows two peaks at 101.9 eV and 100.4 eV, corresponding to silicon oxide and silicon nitride, respectively (as shown in Fig. 5b) [27]. Since no crystalline Si3N4 phase was detected in the samples using XRD (Fig. 2) and TEM analysis (Figs. 3 and 4), it can be concluded that Si3N4 exists in an amorphous or subnanocrystalline state in the coatings [24,26]. The intensity of Si peaks increases as a result of increasing SiH4 flow rate. With the SiH4 flow rate rising up to 68 sccm, the peak at 99.2 eV corresponding to the Si can be seen, which is from the excessive SiH4 decomposition. Fig. 6 shows the relationship between the hardness and the SiH4 flowrate. The penetration depth in the measurements is ~0.3 mm, whereas the thickness of all of the coatings is 1.5 mm, therefore the substrate contribution in hardness cannot be negligible. However, it is believed that though the substrate can change the values of hardness, its trend remains. The hardness increases from 2700 HV0.025 to 4100 HV0.025 with increasing SiH4 flow rate from 3 sccm to 42 sccm, which corresponds to 9.5 at% Si in the coating. With further addition of SiH4 gases up to 68 sccm (15.1 at% Si), the hardness declines quickly and then reaches a lower value of 1500 HV0.025. The increase of the hardness can be explained by decrease of the grain size [28]. Indeed from the XRD patterns (Fig. 2), it is seen that the width of the TiN (111) peaks increases along with increase of silane flow from 3 to 30 sccm and further to 42 and 68 sccm. The hardness increases with the flow rate up to 42 sccm. With even more SiH4 feeding, excessive amorphous or subnanocrystalline Si3N4prevented the crystallization of TiN [29]. As a result, the hardness of the coatings decreases. The tendency of the hardness was similar to the TiSiN
Fig. 4. High-resolution cross-sectional TEM image of TiSiN coating deposited at different SiH4 flow rate, the insert are the corresponding fast Fourier transformed patterns and inverse fast Fourier transform image of the selected area A and B. (a) 42 sccm; (b) 68 sccm.
C.X. Tian et al. / Vacuum 117 (2015) 12e16
15
Fig. 5. XPS spectra of N1s, Si 2p, and Ti2p core levels of the TiSiN coatings deposited at different SiH4 flow rates.
films prepared by CVD [18] and PVD [19]. In the literature [9,30], the maximum hardness value of TiSiN film reached 40e50 GPa as the Si content approached 6e9 at%. Fig. 7 shows the average friction coefficient and root mean square roughness of the TiSiN coatings deposited on cemented carbide substrates as a function of SiH4 flow rate. It is found that the friction coefficient keeps at a relative stable value in the range of 0.6e0.7when SiH4 flow rate increases from 8 sccm to 68 sccm. Whereas the roughness gradually increases along with SiH4 flow rate. This is mostly due to the fact that number of micro-droplets from the cathodes increases.
Fig. 6. Micro-hardness of TiSiN coatings as a function of SiH4 flow rate.
4. Conclusions TiSiN nanocomposite coatings have been prepared by using a cathodic arc system in SiH4 ambient. The SiH4 flow rate exhibited significant influence on the structure and the mechanical performance of TiSiN coatings. With increasing SiH4 flow rate, the TiSiN coating structure varied from columnar grain to nanocrystalline composite. The highest microhardness value of 4100 HV0.025was achieved at 42 sccm of SiH4 and then it decreased down to 1500 HV0.025 with further addition of SiH4 gases. The friction coefficient kept at a stable value in the range of 0.6e0.7when the SiH4 flow rate increases from 8 sccm to 68 sccm. The results are useful for
Fig. 7. Average friction coefficient and root mean square roughness of TiSiN composite coatings against Si3N4 balls.
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applications of nanocomposite TiSiN coatings for cutting tools or other wear-resistant components because of the relatively simple deposition system and easy process control. Acknowledgments This work was supported by The National Natural Science Foundation of China under grants 11275141, 11405117and 50905130, The Fundamental Research Funds for Central Universities under grants 2014208020202 and 2042014gf015 and The Wuhan University Experimental Technology Projects. References [1] Veprek S, Reiprich S. Thin Solid Films 1995;268:64. [2] Li SP, Deng JX, Yan GY, Cheng HW. J Refract Metals Hard Mater 2014;42: 108e15. [3] Zhang YJ, Yang YZ, Zhai YH, Zhang PY. Appl Surf Sci 2012;258:6897e901. ge-Revil A, Steyer P, Cardinal S, Thollet G, Esnouf C, Jacquot P, et al. Thin [4] Me Solid Films 2010;518:5932e7. [5] Steyer P, Pilloud D, Pierson JF, Millet JP, Charnay M, Stauder B, et al. Surf Coat Technol 2006;201:4158e62. [6] Chang CL, Lin CT, Tsai PC, Ho WY, Liu WJ, Wang DY. Surf Coat Technol 2008;202:5516e20. [7] Chawl V, Jayaganthan R, Chandra R. Surf Coat Technol 2010;204:1582e9. ~ ones-Galva n JG, Flores M. Surf Coat Technol [8] Camps I, Muhl S, Camps E, Quin 2014;255:74e8. € hn M, Schmidt J, Scholz S, Herrmann M, Knaut M. Surf Coat [9] Endler I, Ho Technol 2013;215:133e40.
[10] Varesi E, Pavia G, Zenkevich A, Lebedinskii Yu, Besana P, Giussani A, et al. J Phys Chem Solids 2007;68:1046e51. [11] Ma DY, Ma SL, Xu KW. Surf Coat Technol 2004;184:182e7. [12] Hafiz J, Mukherjee R, Wang X, Marshall M, Twesten RD, Cullinan M, et al. Surf Coat Technol 2005;200:1524e9. [13] Ee YC, Chen Z, Wang WD, Chi DZ, Xuc S, Law SB. Surf Coat Technol 2005;198: 291e5. [14] Yazdi M, Lomello F, Wang J, Sanchette F, Dong Z, White T, et al. Vacuum 2014;109:43e51. [15] Wang Z, Xu S, Zhang D, Liu X, Ke P, Wang A. Acta Metall Sin 2014;50:540e6. [16] Komarov FF, Konstantinov SV, Pilko VV. J Frict Wear 2014;35:215e23. [17] Bouzakis KD, Bouzakis E, Kombogiannis S, Paraskevopoulou R, Skordaris G, Makrimallakis S, et al. Surf Coat Technol 2013;237:379e89. [18] Perez-Mariano J, Lau KH, Sanjurjo A, Caro J, Casellas D, Colominas C. Surf Coat Technol 2006;201:2217e25. [19] Zhang P, Cai ZH, Xiong WQ. Surf Coat Technol 2007;201:6819e23. [20] Shizhi L, Yulong S, Hongrui P. Plasma Chem Plasma Process 1992;12:287. [21] Veprek S, Reiprich S, Shizhi L. Appl Phys Lett 1995;66:2640. [22] Diserens M, Patscheider J, Levy F. Surf Coat Technol 1998;108e109:241. [23] Guo CT, Lee D, Chen PC. Appl Surf Sci 2008;254:3130e6. [24] Zhang RF, Veprek S. Mater Sci Eng A 2006;424:128e37. [25] Rodriguez RJ, Garcia JA, Medrano A, Rico M, Sanchez R, Martinez R, et al. Vacuum 2002;67:559e66. ndez A. [26] Philippon D, Godinho V, Nagy PM, Delplancke-Ogletree MP, Ferna Wear 2011;270:541e9. [27] Nose M, Deguchi Y, Mae T, Honbo E, Nagae T, Nogi K. Surf Coat Technol 2003;174e175:261e5. [28] Carvalho S, Rebouta L, Cavaleiro A, Rocha LA, Gomes J, Alves E. Thin Solid Films 2001;398e399:391e6. [29] Yang ZT, Yang B, Guo LP, Fu DJ. Appl Surf Sci 2009;255:4720e4. [30] Chang CL, Lin CT, Tsai PC, Ho WY, Wang DY. Thin Solid Films 2008;516: 5324e9.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of SiH4 flow rate on microstructure and mechanical properties of TiSiN nanocomposite coatings by cathodic arc ion plating C.X. Tian a, b, B. Yang a, b, *, Q. Wan a, H. Ding a, M.Q. Hong b, R.Y. Wang a, H.D. Liu a, Y.M. Chen a, S.J. Yan b, F. Ren b, V.O. Pelenovich a, c, D.J. Fu a, ** a b c
School of Power & Mechanical Engineering, Wuhan University, 430072 Wuhan, China Accelerator Laboratory, Department of Physics, Wuhan University, 430072 Wuhan, China Institute of Ion-Plasma and Laser Technologies, Academy of Sciences of Uzbekistan, 700135 Tashkent, Uzbekistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2014 Received in revised form 30 March 2015 Accepted 1 April 2015 Available online 11 April 2015
TiSiN nanocomposite coatings were deposited on Si and cemented carbide substrates by cathodic arc ion plating in SiH4 ambient. The effects of SiH4 flow rate on microstructure, mechanical and tribological properties of the coatings were investigated systemically. Transmission electron microscopy and X-ray diffraction were employed to probe the crystalline microstructure and X-ray photoelectron spectroscopy was used to investigate the chemical bonding states. Microhardness testers and tribometer were used to evaluate the mechanical and tribological properties of TiSiN nanocomposite coatings. The results showed that the structure of TiSiN coating changed from columnar grain to nanocrystalline with increasing SiH4 flow rate. The hardness increased up to 4100 HV0.025 at the SiH4 flow rate of 42 sccm (9.5 at.% Si in the coating), but it decreases with further addition of SiH4 gases. Friction coefficients of TiSiN composite coatings in the range of 0.6e0.7 were obtained when tested against Si3N4 balls. © 2015 Elsevier Ltd. All rights reserved.
Keywords: TiSiN coating Microstructure Hardness Tribological property
1. Introduction TiSiN composite coatings have attracted much attention due to its high hardness [1e3], excellent oxidization resistance [4,5], good chemical inertness [6] and tribological performance [7,8] since the publication of Veprek's report on the successful fabrication of nanocomposite TiSiN coatings in 1995. Various deposition techniques, such as chemical vapor deposition (CVD) [9], metal-organic chemical vapor deposition (MOCVD) [10], plasma enhanced chemical vapor deposition (PECVD) [11], hypersonic plasma particle deposition (HPPD) [12], inductively coupled plasma (ICP) [13], magnetron sputteringandcathodic arc ion plating [14e17], have been utilized to deposit TiSiN composite coatings. Cathodic arc ion plating (AIP)is one of the most popular PVD techniques useful for industrial production of the TiSiN coatings. In AIPTiSiN coatings, silicon is usually generated from TieSi alloys. However, Si concentration and doping uniformity of the composite coating on industrialscale are still a challenging issue. Previous
* Corresponding author. School of Power & Mechanical Engineering, Wuhan University, 430072 Wuhan, China. Tel./fax: þ86 27 6877 2253. ** Corresponding author. E-mail addresses: [email protected] (B. Yang), [email protected] (D.J. Fu). http://dx.doi.org/10.1016/j.vacuum.2015.04.002 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
researches showed that SiH4 is the best silicon source for deposition of TiSiN coatings in high temperature CVD [9,18], and the Si content exhibits great influences on the structure and mechanical properties of the TiSiN coatings [17,19]. Few works has been devoted to utilization of silane to synthesize TiSiN coatings in AIP process. In this work, TiSiN nanocomposite coatings were deposited by AIP under silane ambient using 99.99% high purity Ti targets. The effects of silane flow rate on structure and mechanical properties of the coatings were systematically investigated. The results may provide scientific base for application of silane to TiSiN deposition in AIP process. 2. Experiment details TiSiN coatings were deposited onto cemented carbide and silicon wafer by a home-made notfiltered vacuum cathodic arc plating system using Ti metal targets. The chamber had inner dimensions of 55 30 40 cm3. The substrates were mounted on the sample holder rotated at 3 rpm. The minimum distance between sample and targets was 22.5 cm. Before deposition the chamber was evacuated to a base pressure of 103 Pa. The cemented carbide substrates with dimensions of 15 15 5 mm3 were mechanically ground and polished to mirror finishing. After being ultrasonically
C.X. Tian et al. / Vacuum 117 (2015) 12e16
cleaned in acetone, ethanol and deionized water, the substrates were mounted on the substrate holder and etched in Ar glow discharge. The ion bombarding was carried out to remove possible contaminations on the substrate surfaces biased with 800 V at 0.02 Pa in Ar ambient. After bombardment etching, two Ti targets were ignited to deposit TiSiN composite coatings. The SiH4 gas carried by nitrogen with a volume ratio N2:SiH4~9:1 was fed into the chamber with its flow rate being varied between 3 and 68 sccm for deposition of coatings with different Si contents. In addition, another flow of N2 gas was fed into the chamber from separate mass flow controller to balance the total deposition pressure at a fixed value of 0.7 Pa. More details of the deposition parameters were listed in Table 1. The microstructure of the coatings was observed by transmission electron microscopy (TEM, JEOL JEM 2010). The crystal structure of the coatings were identified by X-ray diffraction (XRD, Bruker-axs D8 advanced), where the Cu Ka line at 0.15405 nm was used as the excitation source. The chemical bonding and composition were investigated by X-ray photoelectron spectroscopy (Kratos2AXIS2HS XPS)using the Mg Ka (1253.6 eV) radiation. The morphology of the coatings were determined by field emission scanning electron microscope (SEM) equipped with the EDAX 7000 system. The hardness was measured by using an HX-1000 microhardness tester with a load of 25 g and taking the average of 10 random values. The wear behavior was evaluated through sliding tests using a conventional ball-on-disc wear apparatus (MS-T3000). The Si3N4 balls (3 mm in diameter) were used as counterpart materials. The sliding tests were conducted with a sliding speed of 0.02 m/s under a load of 4.0 N at ambient air (30 C) and 70e75% relative humidity. The wear time was 60 min and the total sliding distance of each sample was 28 m.
3. Results and discussion Fig. 1 shows the Si contents of TiSiN coatings as a function of SiH4 gas flow rate. The thicknesses of coatingson cemented carbide substrates used for the XPS measurement are about 1.5 mm. Before the measurements, sputter cleaning procedure with Arþ ion beams (EX06 ion gun, 500 eV, 5 min) was performed. The Si contents increases from 1.0 at% to 15.1 at% when SiH4 flow rate increases from 3.0 sccm to 68 sccm, respectively. The increasing Si contents are attributed to Si atoms or ions generated from decomposition of SiH4. The symmetrical geometry X-ray diffraction was used to probe the crystallinity and orientation of TiSiN coatings deposited on Si substrates with various SiH4 flow rate, as shown in Fig. 2. The diffraction peaks were observed at 2q ¼ 36.3 and 61.8 corresponding to TiN(111) and TiN(220), respectively. The small peak observed at 34.7 is the artifact of W La1 radiation from cathode material contamination on the anode surface. When the SiH4 flow rate is lower than 42 sccm, the samples exhibit a strong TiN (111) diffraction peak with the peak width of 0.5 deg. Increasing SiH4
Fig. 1. Si contents and deposition rate of TiSiN composite coatings as a function of SiH4 flow rate.
flow rates change the width of TiN (111) peak and it is continuously increased with feeding of SiH4 gases. At SiH4 flow rate of 42 sccm and 68 sccm, the TiN (111) and TiN (220) peaks become very weak with a full width more than 2 deg. Using Debye-Scherrer formula one can estimate the crystallite size of the TiN phase: 17 nm for the 8 sccm sample and less than 5 nm for the 68 sccm sample. However, there is no obvious peak shift, which means that the varied Si contents have little influence on crystal cell parameters of TiN. No signal related to crystalline Si3N4 or TiSi compounds is observed, suggesting they are amorphous or subnanocrystalline [20e22]. The present results are in agreement with reports on nc-TiN/a-Si3N4 nanocomposite films prepared by either CVD or PVD [23,24]. Fig. 3 shows cross-sectional TEM bright field images of TiSiN coatings deposited on Si substrates at different SiH4 flow rates. It reveals that the coatings are typical columnar grain structure when SiH4 flow rate is lower than 42 sccm. The columnar grain width of the TiSiN coatings is 20~50 nm. The SAED patterns on the righthand side shows that the coatings are polycrystalline with TiN(111), (200) and (220) orientations. At the lower SiH4 flow rates of 3 and 30 sccm, the incorporation of Si cannot suppress the columnar grain growth of TiSiN, as shown in Fig. 3aeb. With increasing SiH4 flow rates up to 42 sccm, no columnar grain growth could be observed in the TEM bright field image and the coating
Table 1 Deposition parameters of TiSiN composite coatings. Parameters value
Value
Target material (Ti) Bias voltage (V) Cleaning and reactive gases Working pressure(Pa) SiH4 (sccm) Target current (A) Substrate temperature ( C) Rotation speed (rpm) Deposition time (min)
99.99% Etching: 800, deposition: 100 Ar, N2 and SiH4 Etching: 0.02, deposition: 0.7 3, 8, 18, 30, 42, 68 70 300 3 30
13
Fig. 2. X-ray diffraction of TiSiN coatings deposited at various SiH4 flow rates.
14
C.X. Tian et al. / Vacuum 117 (2015) 12e16
Fig. 3. Cross-sectional TEM bright-field image of TiSiN coatings deposited at SiH4 flow rates of 3 sccm, 30 sccm, and 42 sccm and the corresponding diffraction patterns. The arrows indicate growth direction.
exhibit amorphous or subnanocrystalline morphology and diffraction pattern. To investigate more details of TiSiN composite coatings deposited at 42 and 68 sccm, high resolution transmission electron micrograph (HRTEM) analysis was performed,as shown in Fig. 4. The insert shows fast Fourier transformed patterns and inverse fast Fourier transformed images of the selected areas A and B. It is clearly seen that there exist TiN nanocrystallites embedded into amorphous matrix. The grain size of TiN nanocrystallites is about 5e8 nm, which is in agreement with the XRD data. This reveals that larger amount of Si in the coatings can suppress the growth of TiN grain and makes TiSiN composite coating more amorphous-like.
Fig. 5 shows core level spectra of N1s, Si2p and Ti2p of the asreceived TiSiN coatings deposited on Si substrates at different SiH4 flow rates. The spectra were fitted by using XPSPEAK software. The Gaussian fitting of the Ti2p spectra gives three peaks situated at 457.6 eV, 456.5 eV and 454.8 eV, which correspond to TiO2, TiNO and TiN, respectively [25]. As shown in Fig. 5a, Gaussian deconvolution of N1s spectra gives rise to two peaks at 396.1 eV and 397 eV; the high energy peak attributed to TiN is in good agreement with literature data [18,25]. The origin of the second peak which was also observed in Ref. [25] remains uncertain. But comparing the XPS spectra of N and Ti one notes their whole spectra intensity correlation, which suggests that the N peak at 396.1 eV belongs to some Ti bearing phase. Fitting of the Si2p spectra by Gaussian deconvolution shows two peaks at 101.9 eV and 100.4 eV, corresponding to silicon oxide and silicon nitride, respectively (as shown in Fig. 5b) [27]. Since no crystalline Si3N4 phase was detected in the samples using XRD (Fig. 2) and TEM analysis (Figs. 3 and 4), it can be concluded that Si3N4 exists in an amorphous or subnanocrystalline state in the coatings [24,26]. The intensity of Si peaks increases as a result of increasing SiH4 flow rate. With the SiH4 flow rate rising up to 68 sccm, the peak at 99.2 eV corresponding to the Si can be seen, which is from the excessive SiH4 decomposition. Fig. 6 shows the relationship between the hardness and the SiH4 flowrate. The penetration depth in the measurements is ~0.3 mm, whereas the thickness of all of the coatings is 1.5 mm, therefore the substrate contribution in hardness cannot be negligible. However, it is believed that though the substrate can change the values of hardness, its trend remains. The hardness increases from 2700 HV0.025 to 4100 HV0.025 with increasing SiH4 flow rate from 3 sccm to 42 sccm, which corresponds to 9.5 at% Si in the coating. With further addition of SiH4 gases up to 68 sccm (15.1 at% Si), the hardness declines quickly and then reaches a lower value of 1500 HV0.025. The increase of the hardness can be explained by decrease of the grain size [28]. Indeed from the XRD patterns (Fig. 2), it is seen that the width of the TiN (111) peaks increases along with increase of silane flow from 3 to 30 sccm and further to 42 and 68 sccm. The hardness increases with the flow rate up to 42 sccm. With even more SiH4 feeding, excessive amorphous or subnanocrystalline Si3N4prevented the crystallization of TiN [29]. As a result, the hardness of the coatings decreases. The tendency of the hardness was similar to the TiSiN
Fig. 4. High-resolution cross-sectional TEM image of TiSiN coating deposited at different SiH4 flow rate, the insert are the corresponding fast Fourier transformed patterns and inverse fast Fourier transform image of the selected area A and B. (a) 42 sccm; (b) 68 sccm.
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Fig. 5. XPS spectra of N1s, Si 2p, and Ti2p core levels of the TiSiN coatings deposited at different SiH4 flow rates.
films prepared by CVD [18] and PVD [19]. In the literature [9,30], the maximum hardness value of TiSiN film reached 40e50 GPa as the Si content approached 6e9 at%. Fig. 7 shows the average friction coefficient and root mean square roughness of the TiSiN coatings deposited on cemented carbide substrates as a function of SiH4 flow rate. It is found that the friction coefficient keeps at a relative stable value in the range of 0.6e0.7when SiH4 flow rate increases from 8 sccm to 68 sccm. Whereas the roughness gradually increases along with SiH4 flow rate. This is mostly due to the fact that number of micro-droplets from the cathodes increases.
Fig. 6. Micro-hardness of TiSiN coatings as a function of SiH4 flow rate.
4. Conclusions TiSiN nanocomposite coatings have been prepared by using a cathodic arc system in SiH4 ambient. The SiH4 flow rate exhibited significant influence on the structure and the mechanical performance of TiSiN coatings. With increasing SiH4 flow rate, the TiSiN coating structure varied from columnar grain to nanocrystalline composite. The highest microhardness value of 4100 HV0.025was achieved at 42 sccm of SiH4 and then it decreased down to 1500 HV0.025 with further addition of SiH4 gases. The friction coefficient kept at a stable value in the range of 0.6e0.7when the SiH4 flow rate increases from 8 sccm to 68 sccm. The results are useful for
Fig. 7. Average friction coefficient and root mean square roughness of TiSiN composite coatings against Si3N4 balls.
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applications of nanocomposite TiSiN coatings for cutting tools or other wear-resistant components because of the relatively simple deposition system and easy process control. Acknowledgments This work was supported by The National Natural Science Foundation of China under grants 11275141, 11405117and 50905130, The Fundamental Research Funds for Central Universities under grants 2014208020202 and 2042014gf015 and The Wuhan University Experimental Technology Projects. References [1] Veprek S, Reiprich S. Thin Solid Films 1995;268:64. [2] Li SP, Deng JX, Yan GY, Cheng HW. J Refract Metals Hard Mater 2014;42: 108e15. [3] Zhang YJ, Yang YZ, Zhai YH, Zhang PY. Appl Surf Sci 2012;258:6897e901. ge-Revil A, Steyer P, Cardinal S, Thollet G, Esnouf C, Jacquot P, et al. Thin [4] Me Solid Films 2010;518:5932e7. [5] Steyer P, Pilloud D, Pierson JF, Millet JP, Charnay M, Stauder B, et al. Surf Coat Technol 2006;201:4158e62. [6] Chang CL, Lin CT, Tsai PC, Ho WY, Liu WJ, Wang DY. Surf Coat Technol 2008;202:5516e20. [7] Chawl V, Jayaganthan R, Chandra R. Surf Coat Technol 2010;204:1582e9. ~ ones-Galva n JG, Flores M. Surf Coat Technol [8] Camps I, Muhl S, Camps E, Quin 2014;255:74e8. € hn M, Schmidt J, Scholz S, Herrmann M, Knaut M. Surf Coat [9] Endler I, Ho Technol 2013;215:133e40.
[10] Varesi E, Pavia G, Zenkevich A, Lebedinskii Yu, Besana P, Giussani A, et al. J Phys Chem Solids 2007;68:1046e51. [11] Ma DY, Ma SL, Xu KW. Surf Coat Technol 2004;184:182e7. [12] Hafiz J, Mukherjee R, Wang X, Marshall M, Twesten RD, Cullinan M, et al. Surf Coat Technol 2005;200:1524e9. [13] Ee YC, Chen Z, Wang WD, Chi DZ, Xuc S, Law SB. Surf Coat Technol 2005;198: 291e5. [14] Yazdi M, Lomello F, Wang J, Sanchette F, Dong Z, White T, et al. Vacuum 2014;109:43e51. [15] Wang Z, Xu S, Zhang D, Liu X, Ke P, Wang A. Acta Metall Sin 2014;50:540e6. [16] Komarov FF, Konstantinov SV, Pilko VV. J Frict Wear 2014;35:215e23. [17] Bouzakis KD, Bouzakis E, Kombogiannis S, Paraskevopoulou R, Skordaris G, Makrimallakis S, et al. Surf Coat Technol 2013;237:379e89. [18] Perez-Mariano J, Lau KH, Sanjurjo A, Caro J, Casellas D, Colominas C. Surf Coat Technol 2006;201:2217e25. [19] Zhang P, Cai ZH, Xiong WQ. Surf Coat Technol 2007;201:6819e23. [20] Shizhi L, Yulong S, Hongrui P. Plasma Chem Plasma Process 1992;12:287. [21] Veprek S, Reiprich S, Shizhi L. Appl Phys Lett 1995;66:2640. [22] Diserens M, Patscheider J, Levy F. Surf Coat Technol 1998;108e109:241. [23] Guo CT, Lee D, Chen PC. Appl Surf Sci 2008;254:3130e6. [24] Zhang RF, Veprek S. Mater Sci Eng A 2006;424:128e37. [25] Rodriguez RJ, Garcia JA, Medrano A, Rico M, Sanchez R, Martinez R, et al. Vacuum 2002;67:559e66. ndez A. [26] Philippon D, Godinho V, Nagy PM, Delplancke-Ogletree MP, Ferna Wear 2011;270:541e9. [27] Nose M, Deguchi Y, Mae T, Honbo E, Nagae T, Nogi K. Surf Coat Technol 2003;174e175:261e5. [28] Carvalho S, Rebouta L, Cavaleiro A, Rocha LA, Gomes J, Alves E. Thin Solid Films 2001;398e399:391e6. [29] Yang ZT, Yang B, Guo LP, Fu DJ. Appl Surf Sci 2009;255:4720e4. [30] Chang CL, Lin CT, Tsai PC, Ho WY, Wang DY. Thin Solid Films 2008;516: 5324e9.