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Effects of titanium-implanted pre-treatments on the residual stress of TiN coatings on high-speed steel substrates
Surface & Coatings Technology 201 (2007) 6702 – 6706 www.elsevier.com/locate/surfcoat
Effects of titanium-implanted pre-treatments on the residual stress of TiN coatings on high-speed steel substrates Chi-Lung Chang a,⁎, Jui-Yun Jao b , Wei-Yu Ho a , Da-Yung Wang c a
Department of Materials Science and Engineering, MingDao University, 369 Wen-Hua Rd., Peetow, Changhua 523, Taiwan, ROC b Department of Materials Engineering, National Chung-Hsing University, Taiwan, ROC c Institute of Materials and System Engineering, MingDao University, Taiwan, ROC Available online 24 October 2006
Abstract This investigation examined how titanium ion implantation pre-treatment affects the residual stress of TiN coatings on M2 high-speed steel. Ions were implanted by metal plasma ion implantation. The adhesion strength of the TiN coatings was enhanced by pre-treatment that implanted Ti into the M2 tool steel substrate. The implanted substrate functioned as a buffer layer between the deposited TiN and the tool steel substrate, resulting in variations of the residual stress. The residual stress determined by glancing-angle XRD demonstrates that the deposited TiN films on ion-implanted substrates exhibited reduced compressive stress, from −3.95 to −2.41 GPa, which corresponded to a decrease in the grain size of the TiN films. The texture of the TiN film was clearly transformed from the preferred orientation of (220) to (111), subsequently enhancing wear resistance against a tungsten ball. © 2006 Elsevier B.V. All rights reserved. Keywords: Residual stress; Metal plasma ion implantation; TiN coating; Adhesion strength
1. Introduction Plasma source ion implantation (PSII), also referred to as plasma-based ion implantation (PBII), is a surface modification technique that was developed in the 1980s by Conrad and Castagna [1]. Metal plasma ion implantation (MPII), developed in the early 1990s by Brown et al. [2–4], is a plasma-based technique that combines PSII and cathodic arc evaporation, which applies to modify the surface characteristics of materials. The MPII is a metal ion source and can make new intermetallic compounds or crystalline phases form on the surface of the material [4–6]. The particular advantage of MPII lies in the wide range of metal ions, with adjustable ion flux and energy, that can be implanted to improve the surface characteristics of metals and alloys without altering their bulk properties, such as hardness, corrosion, oxidation and wear resistance [7–13]. A. Anders [4–6] reported that ion implantation could significantly increase the hardness of some steels, Al and Ti alloys, and could even be used to form hard coatings. The increase in surface hardness is caused by the considerable change in the surface ⁎ Corresponding author. Tel.: +886 4 8876660x8303; fax: +886 4 8879050. E-mail address: [email protected] (C.-L. Chang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.035
composition of the materials, resulting in solid solution strengthening, dispersion strengthening, grain boundary strengthening (Hall–Petch hardening) and an increase in dislocation density. Ward et al. [14] investigated the effects of the post-treatment of TiN films by carbon ion implantation to increase the wear resistance of coatings used for cutting tools. C-implanted samples exhibited increased in nano-hardness near the surface of the TiN coatings caused by the formation of the TiC phase. Kim et al. [15] reported that hard stoichiometric TiN coatings were generated by pre-treatment by plasma immersion ion implantation (PI3) and reactive magnetron sputtering. The films thus formed had dense columnar-to-equiaxed structures and a (111) preferred orientation. D. Y. Wang et al. [16–18] further studied the adhesive strength of PVD hard coatings to the metallic substrate, and developed a hybrid technology that combined cathodic arc evaporation (CAE) and metal plasma ion implantation (MPII). Advantages of the hybrid process, which combines implantation pre-treatment and cathodic arc evaporation, are that it is economically attractive and easy to integrate in the production process. Direct implantation pre-treatment therefore leads to the formation of an intermixed layer, improving the adhesion of the deposited films. However, the hard coatings
C.-L. Chang et al. / Surface & Coatings Technology 201 (2007) 6702–6706
deposited by combining MPII pre-treatment and CAE deposition exhibit varying residual stress, unlike those deposited by CAE alone. This work investigates the effect of pre-treatment by the implantation of Ti ions into tool steel on the residual stress of TiN films deposited by cathodic arc evaporation. The adhesive strength, the phase transition and the residual stress of TiN films are examined by performing a scratch test and by Xray diffraction pattern. 2. Experimental details A hybrid system, with a metal plasma ion implantation (MPII) source and a cathodic arc evaporator, is applied [19]. Flat coupons of M2 tool steel with a diameter of 30 mm and a thickness of 5 mm were polished with SiC paper with decreasing grit size. Final polishing was conducted using a 0.05μm liquid Al2O3 suspension. Then, the specimen was fixed on a substrate holder and placed approximately 70 cm under the MPII source, while ensuring that the ion beam profile uniformly covered the specimen. Before the process was begun, the chamber was evacuated using a diffusion pump to 1.0 × 10− 3 Pa. During the implantation stage, Ti ions emitted from the MPII source were directly implanted into the surface of the M2 tool steel substrate without substrate bias. The MPII module was operated in pulse mode at 10 Hz, with an accelerating voltage from 30 to 60 kV. The implantation doses were 0.5 and 1.0 × 10− 17 ions/cm2. Following Ti ion implantation, the specimen faced a cathodic arc evaporation source and the TiN films were then deposited onto the M2 tool steel substrate. Table 1 presents the details of the process of combining ion implantation and deposition process parameters. The hardness of the coatings was measured using a Vicker's tester under a 25-g load. The adhesive strength was measured using a scratch tester with increasing loads of up to 100 N. The wear resistance of each coating was studied using a pin-on-disk tribometer, manufactured by CSEM. WC balls with a diameter of 6 mm were used as the counter-wear under an applied load of 10 N at a sliding speed of 30 cm s− 1. Cross-sections of the structure of the TiN coatings following Ti-implantation preTable 1 Process parameters of Ti-implantation and TiN coating by the hybrid PVD system Parameters MPII process MPII target Implantation dose (ions/cm2) Accelerated voltage (kV) Partial pressure (Pa) Ion beam current (mA) CAE process CAE target Reactive gas Partial pressure (Pa) Substrate bias (−V) Temperature (°C) Deposition times (min) Substrate material
Values Ti 5 × 1016, 1 × 1017 30, 45, 60 b2 × 10− 3 2 Ti N2 2 × 10− 1 150 200–220 30 M2 tool steel with ϕ 30 mm × 6 mm t
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treatment were observed by scanning electron microscopy (SEM: JOEL JSM-5400). The crystal structure of the TiN films was identified by glancing-angle X-ray diffraction (XRD: PHILIPS X'PERT PRO 1857). The Cu Kα line at a wavelength of 0.15405 nm was the source for the diffraction pattern analysis. The residual stress of TiN films was determined using a modified XRD sin2Ψ method [20,21] and a four-circle diffractometer with psigoniometer geometry. The glancing-angle X-ray was incident at an angle of 0.5° to maximize the diffraction volume of the thinfilm specimen. The (220) peak had the weakest oscillation, in the sin2Ψ method but was sufficiently intense to enable the peak position to be determined when the thickness of the films was under 600 nm. Therefore, the (220) diffraction was adopted to derive the variation of the lattice parameter with the Ψ angle. In all tests, the residual stress of the TiN film was determined using the following equation [22,23]. daw −d0 1 þ m 1 þ m 2 2m rcos2 asin2 w þ sin a− r ¼ E E E d0 where σ is the residual stress; (dαΨ − d0) / d0 is the strain at tilt angle y; E is Young's modulus; dαΨ is the lattice constant obtained by XRD analysis; d0 is the lattice constant indicated in JCPDS files; θ is the referenced Bragg angle at y = 0°; α is θ — the grazing incident angle; Ψ is the tilt angle of the specimen surface and ν is Poisson's ratio. 3. Results and discussion 3.1. Effect of ion dosage on hardness The hardness of the specimens was determined using a Vicker's hardness tester under a load of 25 g. Five measurements were made to yield average values with a ± 10% error. The hardness of the as-received tool steel substrate was obtained at Hv 630 for comparison. The results showed that the hardness of the Ti-implanted tool steels was Hv 1125, at an ion dosage of 0.5 or 1.0 × 1017/cm2. Interestingly, no significant difference existed between the hardness of the two treated specimens, indicating that a low dosage of 0.5 × 1017/cm2 was sufficed to cause the desired hardening. The increase in hardness is attributable to the microstructural changes caused by Ti ion implantation. Fig. 1(a) and (b) reveals in detail the formation of titanium carbides, based on the patterns of the XRD spectrum. The figures indicate the presence of new, dispersed titanium carbide phases in the implanted substrate. Therefore, the formation of carbides significantly increases the hardness of tool steels after the MPII process, with dosages of 0.5 and 1.0 × 1017/cm2, respectively. Fig. 1(c) and (d) also compares the glancing incidence Xray diffraction patterns of the TiN on the as-received samples, and those of a specific sample of TiN (S3) that has been pretreated by implantation of 0.5 × 10 − 17 ions/cm 2 of titanium. The figures indicate that the texture of the TiN film with a strongly preferred (220) orientation is transformed into one with a preferred (111) orientation after ion implantation pre-
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Fig. 1. X-ray diffraction patterns of different specific samples: (a) M2 tool steel, (b) as-received tool steel with Ti-implantation dose of 1 × 10− 17 ions/cm2, (c) TiN film on the tool steel with Ti-implantation of 0.5 × 10− 17 ions/cm2 (S3) and (d) TiN film on the as-received tool steel.
treatment. The following sections of this study discuss the transformation of the texture of TiN coatings, changing the residual stress and wear behavior.
Comparing the effects of various ion energies and ion dosages on the hardness of TiN coatings revealed that S4 with the implantation of 1.0 × 1017 ions/cm2 at low ion energy (30 kV) was the hardest of all TiN-coated specimens. Implantation at low energy (30 kV) causes the formation of a carbide phase near the top surface with significant titanium concentrations, resulting in a (111) preferred orientation with a high atomic packing factor. However, increasing the implantation energy from 45 to 60 kV resulted in a buried titanium carbide phase in the sub-surface area, reducing the hardness of specimens that had undergone combined pre-treatment and TiN coating. The scratch test indicated that titanium implantations at various ion energies and ion dosages similarly affect the adhesive strengths of TiN coatings on implanted substrates. They also have higher critical loads than the TiN coating on the asreceived substrate, as presented in Table 2. The implanted substrates exhibited greater resistance against plastic deformation during the scratch test since the substrate hardness was increased by the formation of a carbide layer at the substrate surface. 3.3. Residual stress of TiN coatings
3.2. Effect of ion implantation of substrate on TiN coating The measured hardness values of the TiN coatings on implanted substrates with various ion dosages (marked S1 to S6) were similar, as presented in Table 2. The hardness of the TiN coating on the as-received substrate is less than that of the treated specimens. The Vicker's test results under a load of 25 g reveal that the substrate increases the hardness of the specimens that have undergone ion implantation pre-treatment and TiN coating. Table 2 also summarizes the adhesive strength of the TiN coatings on the specimens with and without ion implantation. Scratch test results indicate a similar coating failure of the treated specimens at ion dosages of 0.5 and 1.0 × 1017/ cm2. However, the adhesive strength of the TiN coating on the as-received specimen is markedly lower than that on the ionimplanted specimens. The coating failure depended strongly on the deformation of the substrate. The increased hardness of the substrate following ion implantation provided increased substrate resistance against deformation during scratching. Therefore, a larger load was required to cause the same degree of deformation.
In all of the tests, the residual stress of the TiN films was determined using the equation reported by C.H. Ma et al. [22,23]. The calculated compressive stress of the TiN films that had undergone Ti-implantation pre-treatment is between − 2.41 and − 3.05 GPa, which is less than the value of − 3.95 GPa of the sample of the TiN film only deposited on the M2 tool steel, as presented in Table 2. The drop in residual stress associated with the TiN coating due to pre-treatment by Ti-implantation on the M2 tool steel substrate is evident. The grain size in the TiN films that had undergone pre-treatment by Ti-implantation was lower than that of the TiN film deposited on the M2 tool steel substrate by cathodic arc deposition, as presented in Table 2. The residual stress of the TiN film is smaller because the grains are smaller. A finer columnar structure was observed near the interlayer between the TiN film and the M2 tool steel substrate, with a strongly preferred (111) orientation near the surface (Figs. 1 and 2). SEM showed that the finer microstructure of the TiN films on the implanted substrate was similar to that of the TiN films on the as-received specimen.
Table 2 Summary of results for pre-treatment with different Ti-implanted on the M2 tool steel Sample no. M2 Ti-implanted M2 tool steel TiN S1 S2 S3 S4 S5 S6
Acc. voltage (kV)
Ion dose (×1017/cm2)
30–60
0.1–1.0
30 45 60 30 45 60
0.5 0.5 0.5 1.0 1.0 1.0
Hardness (Hv, 25 g)
Adhesion strength (N)
Residual stress (GPa)
Grain size (nm)
630 1125 ± 150 1893 2178 2182 2338 2519 2174 2240
56 78 80 74 78 85 80
− 3.95 − 2.41 − 2.86 − 2.98 − 2.51 − 2.68 − 3.05
30.84 24.50 24.52 27.11 24.08 24.04 23.41
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Fig. 3. Coefficient of friction of TiN coated on the M2 tool steel substrate with (a) un-implanted and (b) a typical pre-treatment of Ti-implanted (S3).
on the M2 tool steel that had been pre-treated by Tiimplantation has a clear fine columnar structure. Above results indicate that a TiN film with a preferred (111) orientation, higher hardness, adhesive strength and wear resistance can be obtained by ion implantation pre-treatment of the as-received substrate before TiN coating.
Fig. 2. Cross-section microstructure observed by SEM, (a) TiN film on asreceived tool steel and (b) TiN film on tool steel with Ti-implantation of 0.5 × 10− 17 ions/cm2 (S3).
3.4. Tribological test Fig. 3 plots the friction coefficients of TiN films with/ without Ti-implantation pre-treatment of the M2 tool steel substrate. The figure shows that, without Ti-implantation pretreatment, the TiN film has a frictional coefficient similar to that of the films that had undergone Ti-implantation pretreatment. However, TiN film without any implantation treatment has a shorter run-in stage. SEM images of the wear track of the Ti-implanted specimen (S3) at the end of the 2 km slide, indicated that it had a better wear resistance than the TiN film on the as-received substrate, as shown in Fig. 4. The specimens without pre-treatment by implantation on the M2 tool steel had wider tracks. Fig. 1 presents the glancing incidence X-ray diffraction patterns of the TiN on the asreceived samples and on the specific sample of TiN (S3) that had been pre-treated by titanium implantation at a dose of 0.5 × 10− 17 ions/cm2 . Additionally, Fig. 2 shows typical crosssectional images of the TiN coatings with or without pretreatment of Ti-implanted on the M2 tool steel substrate. The SEM observations showed that the microstructure of the TiN films on the as-received samples differed from that of the implanted pre-treatment substrate. They also clearly showed that the textures of grain growth changed. The TiN deposited
Fig. 4. SEM viewgraphs of the wear track of TiN film grown on the M2 tool steel substrate with (a) un-implanted and (b) a typical pre-treatment of Ti-implanted (S3).
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4. Conclusions This study examined the characteristics of the TiN deposited on the M2 tool steel with pre-treated Ti-implantation. Based on the results of this study, we conclude the following: 1. The calculated compressive stress of the TiN films with Tiimplanted pre-treatment ranges between − 2.41 and − 3.05 GPa, which is less than that, − 3.95 GPa, of the TiN on the as-received M2 tool steel. The drop in the residual stress of the TiN film is caused by the drop in the grain size and the transition of the texture from (220) to (111) preferred orientation. 2. Pre-treatment by Ti ion implantation of the M2 tool steel substrate improves the hardness and adhesive strength of the specimens over those of the TiN film on the as-received substrate. 3. TiN films with and without ion implantation have similar friction coefficients. However, wear resistance of the TiN films on the implanted substrate was higher than that of those on the non-implanted substrate, because they had a higher atomic backing factor, hardness and adhesive strength. Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 93-2216-E-451-002. References [1] J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 (1986) 1479. [2] I.G. Brown, R.A. MacGill, J.E. Galvin, U.S. patent 5,013,578 (1989).
[3] I.G. Brown, X. Godechot, K.M. Yu, Appl. Phys, Lett. 58 (1991) 1392. [4] A. Anders, Surf. Coat. Technol. 93 (1997) 157. [5] A. Wenzel, C. Hammeh, A. Koniger, B. Rauschenbach, Nucl. Instrum. Methods B 129 (1997) 369. [6] S. Yan, W.J. Zhao, D.M. Ruck, J.M. Xue, Y.G. Wang, Surf. Coat. Technol. 103–104 (1998) 348. [7] U. Bernabai, M. Cavallini, G. Bombara, G. Dearnaley, M.A. Wilkins, Corros. Sci. 20 (1980) 19. [8] M.J. Bennett, M.R. Houlton, G. Dearnaley, Corros. Sci. 20 (1980) 69. [9] F.H. Stott, Z. Peide, W.A. Grant, R.P.M. Procter, Corros. Sci. 22 (1982) 305. [10] M. Pons, M. Caillet, A. Galerie, Nucl. Instrum. Methods 209 (1983) 1011. [11] P.J. Smith, R.M. Beauprie, W.W. Smeltzer, D.V. Stevanovic, D.A. Thompson, Oxid. Met. 28 (1987) 259. [12] P.Y. Hou, J. Stringer, Oxid. Met. 34 (1990) 299. [13] A. Mitsuo, T. Tanaki, T. Shinozaki, M. Iwaki, Surf. Coat. Technol. 66 (1994) 260. [14] R.R. Manory, S. Mollica, L. Ward, K.P. Purushotham, P. Evans, J. Noorman, A.J. Perry, Surf. Coat. Technol. 155 (2002) 136. [15] S.A. Nikiforov, K.W. Urm, G.H. Kim, G.H. Rim, S.H. Lee, Surf. Coat. Technol. 171 (2003) 106. [16] D.Y. Wang, M.C. Chiu, Surf. Coat. Technol. 156 (2002) 201. [17] K.W. Wong, C.L. Chang, D.Y. Wang, Diamond Relat. Mater. 11 (2002) 1447. [18] C.L. Chang, D.Y. Wang, Nucl. Instrum. Methods B 194 (4) (2002) 463. [19] D.Y. Wang, K.W. Weng, Surf. Coat. Technol. 156 (2002) 195. [20] L. Hultman, W.-D. Munz, J. Musil, K. Kadlec, I. Petrov, J.E. Greene, J. Vac.Sci. Technol. A 9 (1991) 434. [21] G. Knuyt, C. Quaeyhaegens, J. D'Haen, L.M. Stals, Thin Solid Films 258 (1995) 159. [22] C.H. Ma, J.H. Huang, Haydn Chen, Thin Solid Films 418 (2002) 73. [23] C.I. Noyan, B.J. Cohen, Residual Stress Measurement by Diffraction and Interpolation, Springer-Verlag, New York, 1987.
Effects of titanium-implanted pre-treatments on the residual stress of TiN coatings on high-speed steel substrates Chi-Lung Chang a,⁎, Jui-Yun Jao b , Wei-Yu Ho a , Da-Yung Wang c a
Department of Materials Science and Engineering, MingDao University, 369 Wen-Hua Rd., Peetow, Changhua 523, Taiwan, ROC b Department of Materials Engineering, National Chung-Hsing University, Taiwan, ROC c Institute of Materials and System Engineering, MingDao University, Taiwan, ROC Available online 24 October 2006
Abstract This investigation examined how titanium ion implantation pre-treatment affects the residual stress of TiN coatings on M2 high-speed steel. Ions were implanted by metal plasma ion implantation. The adhesion strength of the TiN coatings was enhanced by pre-treatment that implanted Ti into the M2 tool steel substrate. The implanted substrate functioned as a buffer layer between the deposited TiN and the tool steel substrate, resulting in variations of the residual stress. The residual stress determined by glancing-angle XRD demonstrates that the deposited TiN films on ion-implanted substrates exhibited reduced compressive stress, from −3.95 to −2.41 GPa, which corresponded to a decrease in the grain size of the TiN films. The texture of the TiN film was clearly transformed from the preferred orientation of (220) to (111), subsequently enhancing wear resistance against a tungsten ball. © 2006 Elsevier B.V. All rights reserved. Keywords: Residual stress; Metal plasma ion implantation; TiN coating; Adhesion strength
1. Introduction Plasma source ion implantation (PSII), also referred to as plasma-based ion implantation (PBII), is a surface modification technique that was developed in the 1980s by Conrad and Castagna [1]. Metal plasma ion implantation (MPII), developed in the early 1990s by Brown et al. [2–4], is a plasma-based technique that combines PSII and cathodic arc evaporation, which applies to modify the surface characteristics of materials. The MPII is a metal ion source and can make new intermetallic compounds or crystalline phases form on the surface of the material [4–6]. The particular advantage of MPII lies in the wide range of metal ions, with adjustable ion flux and energy, that can be implanted to improve the surface characteristics of metals and alloys without altering their bulk properties, such as hardness, corrosion, oxidation and wear resistance [7–13]. A. Anders [4–6] reported that ion implantation could significantly increase the hardness of some steels, Al and Ti alloys, and could even be used to form hard coatings. The increase in surface hardness is caused by the considerable change in the surface ⁎ Corresponding author. Tel.: +886 4 8876660x8303; fax: +886 4 8879050. E-mail address: [email protected] (C.-L. Chang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.035
composition of the materials, resulting in solid solution strengthening, dispersion strengthening, grain boundary strengthening (Hall–Petch hardening) and an increase in dislocation density. Ward et al. [14] investigated the effects of the post-treatment of TiN films by carbon ion implantation to increase the wear resistance of coatings used for cutting tools. C-implanted samples exhibited increased in nano-hardness near the surface of the TiN coatings caused by the formation of the TiC phase. Kim et al. [15] reported that hard stoichiometric TiN coatings were generated by pre-treatment by plasma immersion ion implantation (PI3) and reactive magnetron sputtering. The films thus formed had dense columnar-to-equiaxed structures and a (111) preferred orientation. D. Y. Wang et al. [16–18] further studied the adhesive strength of PVD hard coatings to the metallic substrate, and developed a hybrid technology that combined cathodic arc evaporation (CAE) and metal plasma ion implantation (MPII). Advantages of the hybrid process, which combines implantation pre-treatment and cathodic arc evaporation, are that it is economically attractive and easy to integrate in the production process. Direct implantation pre-treatment therefore leads to the formation of an intermixed layer, improving the adhesion of the deposited films. However, the hard coatings
C.-L. Chang et al. / Surface & Coatings Technology 201 (2007) 6702–6706
deposited by combining MPII pre-treatment and CAE deposition exhibit varying residual stress, unlike those deposited by CAE alone. This work investigates the effect of pre-treatment by the implantation of Ti ions into tool steel on the residual stress of TiN films deposited by cathodic arc evaporation. The adhesive strength, the phase transition and the residual stress of TiN films are examined by performing a scratch test and by Xray diffraction pattern. 2. Experimental details A hybrid system, with a metal plasma ion implantation (MPII) source and a cathodic arc evaporator, is applied [19]. Flat coupons of M2 tool steel with a diameter of 30 mm and a thickness of 5 mm were polished with SiC paper with decreasing grit size. Final polishing was conducted using a 0.05μm liquid Al2O3 suspension. Then, the specimen was fixed on a substrate holder and placed approximately 70 cm under the MPII source, while ensuring that the ion beam profile uniformly covered the specimen. Before the process was begun, the chamber was evacuated using a diffusion pump to 1.0 × 10− 3 Pa. During the implantation stage, Ti ions emitted from the MPII source were directly implanted into the surface of the M2 tool steel substrate without substrate bias. The MPII module was operated in pulse mode at 10 Hz, with an accelerating voltage from 30 to 60 kV. The implantation doses were 0.5 and 1.0 × 10− 17 ions/cm2. Following Ti ion implantation, the specimen faced a cathodic arc evaporation source and the TiN films were then deposited onto the M2 tool steel substrate. Table 1 presents the details of the process of combining ion implantation and deposition process parameters. The hardness of the coatings was measured using a Vicker's tester under a 25-g load. The adhesive strength was measured using a scratch tester with increasing loads of up to 100 N. The wear resistance of each coating was studied using a pin-on-disk tribometer, manufactured by CSEM. WC balls with a diameter of 6 mm were used as the counter-wear under an applied load of 10 N at a sliding speed of 30 cm s− 1. Cross-sections of the structure of the TiN coatings following Ti-implantation preTable 1 Process parameters of Ti-implantation and TiN coating by the hybrid PVD system Parameters MPII process MPII target Implantation dose (ions/cm2) Accelerated voltage (kV) Partial pressure (Pa) Ion beam current (mA) CAE process CAE target Reactive gas Partial pressure (Pa) Substrate bias (−V) Temperature (°C) Deposition times (min) Substrate material
Values Ti 5 × 1016, 1 × 1017 30, 45, 60 b2 × 10− 3 2 Ti N2 2 × 10− 1 150 200–220 30 M2 tool steel with ϕ 30 mm × 6 mm t
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treatment were observed by scanning electron microscopy (SEM: JOEL JSM-5400). The crystal structure of the TiN films was identified by glancing-angle X-ray diffraction (XRD: PHILIPS X'PERT PRO 1857). The Cu Kα line at a wavelength of 0.15405 nm was the source for the diffraction pattern analysis. The residual stress of TiN films was determined using a modified XRD sin2Ψ method [20,21] and a four-circle diffractometer with psigoniometer geometry. The glancing-angle X-ray was incident at an angle of 0.5° to maximize the diffraction volume of the thinfilm specimen. The (220) peak had the weakest oscillation, in the sin2Ψ method but was sufficiently intense to enable the peak position to be determined when the thickness of the films was under 600 nm. Therefore, the (220) diffraction was adopted to derive the variation of the lattice parameter with the Ψ angle. In all tests, the residual stress of the TiN film was determined using the following equation [22,23]. daw −d0 1 þ m 1 þ m 2 2m rcos2 asin2 w þ sin a− r ¼ E E E d0 where σ is the residual stress; (dαΨ − d0) / d0 is the strain at tilt angle y; E is Young's modulus; dαΨ is the lattice constant obtained by XRD analysis; d0 is the lattice constant indicated in JCPDS files; θ is the referenced Bragg angle at y = 0°; α is θ — the grazing incident angle; Ψ is the tilt angle of the specimen surface and ν is Poisson's ratio. 3. Results and discussion 3.1. Effect of ion dosage on hardness The hardness of the specimens was determined using a Vicker's hardness tester under a load of 25 g. Five measurements were made to yield average values with a ± 10% error. The hardness of the as-received tool steel substrate was obtained at Hv 630 for comparison. The results showed that the hardness of the Ti-implanted tool steels was Hv 1125, at an ion dosage of 0.5 or 1.0 × 1017/cm2. Interestingly, no significant difference existed between the hardness of the two treated specimens, indicating that a low dosage of 0.5 × 1017/cm2 was sufficed to cause the desired hardening. The increase in hardness is attributable to the microstructural changes caused by Ti ion implantation. Fig. 1(a) and (b) reveals in detail the formation of titanium carbides, based on the patterns of the XRD spectrum. The figures indicate the presence of new, dispersed titanium carbide phases in the implanted substrate. Therefore, the formation of carbides significantly increases the hardness of tool steels after the MPII process, with dosages of 0.5 and 1.0 × 1017/cm2, respectively. Fig. 1(c) and (d) also compares the glancing incidence Xray diffraction patterns of the TiN on the as-received samples, and those of a specific sample of TiN (S3) that has been pretreated by implantation of 0.5 × 10 − 17 ions/cm 2 of titanium. The figures indicate that the texture of the TiN film with a strongly preferred (220) orientation is transformed into one with a preferred (111) orientation after ion implantation pre-
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Fig. 1. X-ray diffraction patterns of different specific samples: (a) M2 tool steel, (b) as-received tool steel with Ti-implantation dose of 1 × 10− 17 ions/cm2, (c) TiN film on the tool steel with Ti-implantation of 0.5 × 10− 17 ions/cm2 (S3) and (d) TiN film on the as-received tool steel.
treatment. The following sections of this study discuss the transformation of the texture of TiN coatings, changing the residual stress and wear behavior.
Comparing the effects of various ion energies and ion dosages on the hardness of TiN coatings revealed that S4 with the implantation of 1.0 × 1017 ions/cm2 at low ion energy (30 kV) was the hardest of all TiN-coated specimens. Implantation at low energy (30 kV) causes the formation of a carbide phase near the top surface with significant titanium concentrations, resulting in a (111) preferred orientation with a high atomic packing factor. However, increasing the implantation energy from 45 to 60 kV resulted in a buried titanium carbide phase in the sub-surface area, reducing the hardness of specimens that had undergone combined pre-treatment and TiN coating. The scratch test indicated that titanium implantations at various ion energies and ion dosages similarly affect the adhesive strengths of TiN coatings on implanted substrates. They also have higher critical loads than the TiN coating on the asreceived substrate, as presented in Table 2. The implanted substrates exhibited greater resistance against plastic deformation during the scratch test since the substrate hardness was increased by the formation of a carbide layer at the substrate surface. 3.3. Residual stress of TiN coatings
3.2. Effect of ion implantation of substrate on TiN coating The measured hardness values of the TiN coatings on implanted substrates with various ion dosages (marked S1 to S6) were similar, as presented in Table 2. The hardness of the TiN coating on the as-received substrate is less than that of the treated specimens. The Vicker's test results under a load of 25 g reveal that the substrate increases the hardness of the specimens that have undergone ion implantation pre-treatment and TiN coating. Table 2 also summarizes the adhesive strength of the TiN coatings on the specimens with and without ion implantation. Scratch test results indicate a similar coating failure of the treated specimens at ion dosages of 0.5 and 1.0 × 1017/ cm2. However, the adhesive strength of the TiN coating on the as-received specimen is markedly lower than that on the ionimplanted specimens. The coating failure depended strongly on the deformation of the substrate. The increased hardness of the substrate following ion implantation provided increased substrate resistance against deformation during scratching. Therefore, a larger load was required to cause the same degree of deformation.
In all of the tests, the residual stress of the TiN films was determined using the equation reported by C.H. Ma et al. [22,23]. The calculated compressive stress of the TiN films that had undergone Ti-implantation pre-treatment is between − 2.41 and − 3.05 GPa, which is less than the value of − 3.95 GPa of the sample of the TiN film only deposited on the M2 tool steel, as presented in Table 2. The drop in residual stress associated with the TiN coating due to pre-treatment by Ti-implantation on the M2 tool steel substrate is evident. The grain size in the TiN films that had undergone pre-treatment by Ti-implantation was lower than that of the TiN film deposited on the M2 tool steel substrate by cathodic arc deposition, as presented in Table 2. The residual stress of the TiN film is smaller because the grains are smaller. A finer columnar structure was observed near the interlayer between the TiN film and the M2 tool steel substrate, with a strongly preferred (111) orientation near the surface (Figs. 1 and 2). SEM showed that the finer microstructure of the TiN films on the implanted substrate was similar to that of the TiN films on the as-received specimen.
Table 2 Summary of results for pre-treatment with different Ti-implanted on the M2 tool steel Sample no. M2 Ti-implanted M2 tool steel TiN S1 S2 S3 S4 S5 S6
Acc. voltage (kV)
Ion dose (×1017/cm2)
30–60
0.1–1.0
30 45 60 30 45 60
0.5 0.5 0.5 1.0 1.0 1.0
Hardness (Hv, 25 g)
Adhesion strength (N)
Residual stress (GPa)
Grain size (nm)
630 1125 ± 150 1893 2178 2182 2338 2519 2174 2240
56 78 80 74 78 85 80
− 3.95 − 2.41 − 2.86 − 2.98 − 2.51 − 2.68 − 3.05
30.84 24.50 24.52 27.11 24.08 24.04 23.41
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Fig. 3. Coefficient of friction of TiN coated on the M2 tool steel substrate with (a) un-implanted and (b) a typical pre-treatment of Ti-implanted (S3).
on the M2 tool steel that had been pre-treated by Tiimplantation has a clear fine columnar structure. Above results indicate that a TiN film with a preferred (111) orientation, higher hardness, adhesive strength and wear resistance can be obtained by ion implantation pre-treatment of the as-received substrate before TiN coating.
Fig. 2. Cross-section microstructure observed by SEM, (a) TiN film on asreceived tool steel and (b) TiN film on tool steel with Ti-implantation of 0.5 × 10− 17 ions/cm2 (S3).
3.4. Tribological test Fig. 3 plots the friction coefficients of TiN films with/ without Ti-implantation pre-treatment of the M2 tool steel substrate. The figure shows that, without Ti-implantation pretreatment, the TiN film has a frictional coefficient similar to that of the films that had undergone Ti-implantation pretreatment. However, TiN film without any implantation treatment has a shorter run-in stage. SEM images of the wear track of the Ti-implanted specimen (S3) at the end of the 2 km slide, indicated that it had a better wear resistance than the TiN film on the as-received substrate, as shown in Fig. 4. The specimens without pre-treatment by implantation on the M2 tool steel had wider tracks. Fig. 1 presents the glancing incidence X-ray diffraction patterns of the TiN on the asreceived samples and on the specific sample of TiN (S3) that had been pre-treated by titanium implantation at a dose of 0.5 × 10− 17 ions/cm2 . Additionally, Fig. 2 shows typical crosssectional images of the TiN coatings with or without pretreatment of Ti-implanted on the M2 tool steel substrate. The SEM observations showed that the microstructure of the TiN films on the as-received samples differed from that of the implanted pre-treatment substrate. They also clearly showed that the textures of grain growth changed. The TiN deposited
Fig. 4. SEM viewgraphs of the wear track of TiN film grown on the M2 tool steel substrate with (a) un-implanted and (b) a typical pre-treatment of Ti-implanted (S3).
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4. Conclusions This study examined the characteristics of the TiN deposited on the M2 tool steel with pre-treated Ti-implantation. Based on the results of this study, we conclude the following: 1. The calculated compressive stress of the TiN films with Tiimplanted pre-treatment ranges between − 2.41 and − 3.05 GPa, which is less than that, − 3.95 GPa, of the TiN on the as-received M2 tool steel. The drop in the residual stress of the TiN film is caused by the drop in the grain size and the transition of the texture from (220) to (111) preferred orientation. 2. Pre-treatment by Ti ion implantation of the M2 tool steel substrate improves the hardness and adhesive strength of the specimens over those of the TiN film on the as-received substrate. 3. TiN films with and without ion implantation have similar friction coefficients. However, wear resistance of the TiN films on the implanted substrate was higher than that of those on the non-implanted substrate, because they had a higher atomic backing factor, hardness and adhesive strength. Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 93-2216-E-451-002. References [1] J.R. Conrad, T. Castagna, Bull. Am. Phys. Soc. 31 (1986) 1479. [2] I.G. Brown, R.A. MacGill, J.E. Galvin, U.S. patent 5,013,578 (1989).
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