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Effect of ion energy on microstructure and adhesion of diamond-like carbon on Ti6Al4V by ion beam deposition
Diamond & Related Materials 70 (2016) 12–17
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Effect of ion energy on microstructure and adhesion of diamond-like carbon on Ti6Al4V by ion beam deposition C.Z. Zhang a, S. Bhattacharya a, Y.S. Li a,b, S. Khatir b, Y.F. Hu c, S. Shiri a, Q. Yang a,⁎ a b c
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, S7N 5A9, Canada Plasma Physics Laboratory, University of Saskatchewan, 116 Science Place, Saskatoon, SK S7N 5E2, Canada Canadian Light Source, University of Saskatchewan, 44 Innovation Boulevard, Saskatoon, SK, S7N 2V3, Canada
a r t i c l e
i n f o
Article history: Received 13 May 2016 Received in revised form 12 September 2016 Accepted 15 September 2016 Available online 19 September 2016 Keywords: Diamond-like carbon Ti6Al4V Adhesion Ion energy Diamond nanoparticles
a b s t r a c t The microstructure and adhesion of diamond-like carbon (DLC) thin films on Ti6Al4V substrates were investigated using direct ion beam deposition with ion energy varying from 65 eV to 90 eV. The samples prepared were characterized by Raman spectroscopy, synchrotron near-edge X-ray absorption fine structure spectroscopy, scanning electron microscopy, and X-ray diffraction. Indentation testing by Vickers hardness tester was used for adhesion evaluation. The results show that the adhesion of DLC thin films on Ti6Al4V substrates mainly depends on ion energy used in the deposition process. Higher ion energy results in higher sp3 concentration in DLC thin films, which firstly increases then decreases the adhesion of DLC on Ti6Al4V substrate depending on the sp3 fractions and ion energy of 70 eV provides the highest adhesion on both the bare and diamond nanoparticle coated Ti6Al4V. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ti alloys are extensively used to make implantable medical devices [1]. A well-known example is the artificial hip joints that typically consist of a stem, a spherical femoral head articulating against an acetabular cup with a UHMWPE liner as the bearing surface. Wear of the articulating surfaces and corrosion at the Ti alloy/UHMWPE interfaces have been two major concerns in total hip replacement as they lead to early implant failure. Coating the implants with biocompatible inert hard materials is expected to significantly improve their biocompatibility and their tribological and corrosion performance, and thus their service lifetime. Diamond Like Carbon (DLC) possess many unique properties, including high hardness, low friction coefficient, high chemical stability, bio-inertness, high wear/corrosion resistance, and excellent biocompatibility, making them ideal for orthopedic implant applications [2]. Nevertheless, successful applications of DLC coatings on Ti alloys have been limited due to the high internal stress in DLC coatings, which together with insufficient adhesion induces early coating delamination or spallation [3,4]. Therefore, there is an increasing engineering demand for developing technologies that can achieve synthesis of well adherent DLC coatings on Ti alloys. A wide variety of methods, including optimization of deposition conditions, substrate pretreatment such as ion implantation and nitriding/ oxidizing, deposition of an interlayer, incorporation of a metal or metal compound layer to form alternative multilayered films, ⁎ Corresponding author. E-mail address: [email protected] (Q. Yang).
http://dx.doi.org/10.1016/j.diamond.2016.09.013 0925-9635/© 2016 Elsevier B.V. All rights reserved.
incorporation of foreign elements, have been developed to solve adhesion barriers of DLC thin films on Ti alloys. Although the adhesion has been improved, further enhancement of adhesion is still required for practical applications. Our previous work has demonstrated that diamond nanoparticle (DNP) incorporation is a promising method for adhesion enhancement of DLC on Ti6Al4V [5]. Nonetheless, DLC processing parameters plays important roles on the microstructure and adhesion properties of the coated Ti alloys. In order to optimize the performance of DNP incorporated DLC films on Ti alloys, we investigated the effect of ion energy on the adhesion of DLC thin films on Ti6Al4V substrate and the results show that ion energy has significant effect on the adhesion and the thin film exhibits the highest adhesion at an ion energy of 70 eV. 2. Experimental details Ti6Al4V sheets (Ra 926 ± 40 nm) with dimensions of 10 mm × 10 mm × 1 mm were used as substrate materials. The substrate surfaces were ground with silicon carbide paper (350 grit) and polished using 9 μm diamond slurry, and 3 μm diamond slurry sequentially. After polishing, the samples (Ra 24 ± 2 nm) were ultrasonically cleaned in ethanol for 15 min and dried in air. Nanodiamond deposition on Ti6Al4V was conducted in a Microwave Plasma Chemical Vapor Deposition (MPCVD) reactor at an excitation of 2.45 GHz, with a gas mixture of methane CH4 and hydrogen H2. Before deposition, the Ti6Al4V sheets were ultrasonically seeded for 1 h in a suspension of ethanol and nanodiamond powder to enhance diamond nucleation. The deposition parameters are listed in Table 1.
C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17 Table 1 Deposition condition of diamond nanoparticles (DNP) on Ti6Al4V by MPCVD. Parameters
First step (nucleation enhancement)
Second step (diamond growth)
H2 flow rate (sccm) CH4 flow rate (sccm) Total flow rate (sccm) Gas pressure (kPa) Microwave power (W) Temperature (°C) Deposition time (min)
40 10 50 4 600 350 10
199 1 200 4 600 350 60
Table 2 Deposition conditions and resulting thickness of DLC thin films on Ti6Al4V. Ion energy (eV)
CH4 flow rate (sccm)
Temperature (°C)
Deposition time (h)
Thickness (nm)
60 70 75 80
10 10 10 10
30 30 30 30
2 2 2 2
230 ± 22 217 ± 18 201 ± 25 192 ± 31
An ion beam deposition system with an End-Hall (EH) ion source (KRI EH-1000, manufactured by Kaufman & Robinson, Inc. USA) was used for DLC thin film deposition [6,7]. The sample stage was tilted 45° with respect to the ion beam, methane gas with a flow rate of 10 sccm was introduced into the ion source and the mean ion energy of the beam used was ranging from 60 eV to 90 eV, as listed in
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Table 2. The deposition was performed at room temperature and at a pressure of 1.2 × 10−1 Pa for 2 h. The thickness of the as-deposited DLC thin films as listed in Table 2 was measured by an optical surface profiler manufactured by Zygo Corporation. The bonding characteristics of the as prepared DLC thin films were characterized by Raman spectroscopy. The Raman spectrometer was operated at a laser wavelength of 514 nm generated by an argon laser. The adhesion evaluation by indentation testing was conducted using a Vickers hardness tester at a load of 1000 N. The surface morphology, microstructure, and chemical bonding of the as deposited DNP and DNP/DLC thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and synchrotron near-edge X-ray absorption fine structure spectroscopy (NEXAFS). 3. Results and discussion Fig. 1 shows the Gaussian-fitted Raman spectra of DLC thin films directly deposited on Ti6Al4V by different ion energy ranging from 60 eV to 80 eV. All the spectra show two broad peaks centered at around 1350 cm−1 (D band) and 1580 cm−1 (G band), a typical DLC feature [5]. The intensity ratio of D and G band, Id/Ig, obtained from Gaussian line fitting for all the samples are listed in Table 3. It can be seen that the Id/Ig value depends on ion energy, as shown in Fig. 2. The Id/Ig value decreases with the increase of the ion energy used for DLC deposition. According to Ferrari et al., the sp3 bond content in DLC thin films increases with the decrease of Id/Ig value [8]. So it can be concluded that the content of sp3 bonded carbon increases with the increase of the ion energy used for deposition. This is probably due to the subplantation of
Fig. 1. Gaussian-fitted Raman spectra (514 nm) of DLC coatings grown on Ti6Al4V by different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV.
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C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Table 3 Id and Ig ratio of DLC thin films on Ti6Al4V deposited with varying ion energy. Ion energy (eV)
60
70
75
80
Id/Ig
1.0289
0.9109
0.8256
0.7184
Fig. 2. Variation of the Id/Ig with ion energy of the DLC coatings grown on Ti6Al4V.
incident ions. The ions with higher energy would produce more quenched-in strain, higher local density and compressive stress, which give rise to a higher sp3 concentration [9]. Indentation testing was conducted at a load of 1000 N to evaluate the adhesion of DLC thin films directly deposited on Ti6Al4V with different ion energy. Fig. 3 shows the typical SEM images of DLC thin films after indentation testing (Fig. 3a–c) and without indentation testing (Fig. 3d). The light area corresponds to the exposed substrate, and the dark area corresponds to the film remaining onto the substrate. For the samples #a to #c with ion energies ranging from 60 eV to 75 eV, partial spallation or cracking of the films is observed in the areas around the imprint. For sample #a with an ion energy of 60 eV, the spallation area
accounts for about half of the imprint area, and small pieces of delaminated film can be found at the edge of the imprint, as shown in Fig. 3a. With the ion energy increased to 70 eV, much less spallation of the film is observed on sample #b, and cracking instead of spallation can be found at most of the area in the imprint, as shown in Fig. 3b, suggesting relatively good adhesion of DLC on Ti6Al4V. With the ion energy further increased to 75 eV, more spallation area can be observed, as shown in Fig. 3c, indicating poor adhesion. With further increasing the ion energy to 80 eV, the DLC thin film cracks and delaminates from Ti6Al4V substrate before indentation. The spallation area accounts for almost half of the total film area. And with the ion energy increased up to 90 eV, no DLC deposition was observed, probably because of the delamination. These results show that the DLC thin films deposited by EH ion beam of 70 eV possess the highest adhesion. As Robertson reported, a carbon ion flux at around 100 eV per carbon atom provides DLC films with the highest sp3 bonded carbon [1]. However, in our experiments, no film deposition was observed on Ti6Al4V substrate at ion energy of 90 eV or above. It has been confirmed that high compressive stress remains in DLC thin film because of ion bombardment during film deposition and the magnitude of the compressive stress depends on the film-substrate structure and the impact energy per atom [10]. Higher energy usually results in higher attractive (adhesion) strength between the substrate and the film but higher stress. The synergetic effect of these two factors would provide an optimum energy for the film with the highest load against delamination. In the present experiments, at the optimum ion energy is 70 eV. When the energy is higher than 70 eV, the measured adhesion decreases with the increase of ion energy, indicating that the compressive stress in the films increases with the increase of ion energy and the excessive stress level induces the delamination of the film [10], causing film failure at ion energy of 90 eV or above. When adhesion failure for a given film-substrate system cannot be avoided, application of an interlayer would be a good approach to address the issue. Our previous paper has reported that incorporation of DNP into DLC can significantly improve the adhesion of DLC on the Ti alloy due to diamond's ability to form strong bonds with both DLC and Ti alloy substrate, which can reduce the interfacial energy γi [11]. Nevertheless, the performance of DNP incorporation depends on the ion energy used during DLC deposition. Therefore, the effect of deposition ion energy on adhesion of DLC thin films has been investigated to further improve the adhesion.
Fig. 3. Typical SEM images of DLC coatings on Ti6Al4V after indentation testing deposited by different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV (before indentation).
C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Typical SEM image and XRD pattern of the as-deposited DNP on Ti6Al4V substrate are shown in Fig. 4. It can be seen that DNPs (light dots) cover most of the Ti6Al4V surface (see Fig. 4a), indicating high nucleation density, which might result in good adhesion as reported in our previous work [5]. In Fig. 4b, diamond peaks are denoted as D. It can be seen that there are diamond peaks at 2θ = 51°, and 91°, Ti peaks at 44°, 62°, 75°, 84°, and 99° (from the substrate), and TiC peaks at 41° and 92°, indicating the formation of a TiC intermediate layer on the substrate surface. The formation of a TiC interfacial layer is beneficial to the enhancement of adhesion of NDPs to the substrate and the reduction of substrate damage caused by hydrogenation, as revealed by our recent investigations [5,12]. To evaluate the bonding states of as-deposited DNP/DLC composite thin film, synchrotron NEXAFS spectroscopy was used. Fig. 5 shows the C K-edge NEXAFS spectrum of NDP/DLC thin films grown on Ti6Al4V substrate recorded in total electron yield (TEY). The peak located around 286.3 eV corresponds to the transition C 1s→ π⁎ for the sp2 C_C bond, and the peak located around 289.8 eV is the most interesting one as it is associated with the excitations from C 1s to σ⁎ in sp3-rich material, characteristic feature of DLC [13]. Indentation testing by Vickers hardness tester was conducted at a load of 1000 N to evaluate the adhesion of the DLC on NDP coated Ti6Al4V. Fig. 6 shows the SEM images of DNP/DLC thin films deposited with different ion energy ranging from 60 eV to 80 eV. It has found that all the DLC composite thin films show better adhesion on Ti6Al4V substrate comparing with the DLC thin films directly deposited on Ti6Al4V without interlayer. For DLC grown on DNP coated Ti6Al4V as shown in Fig. 6, much less cracking in the imprint and much less spallation around the edge of the imprint is observed comparing with DLC samples without interlayer.
Fig. 4. Typical (a) SEM image, and (b) XRD patterns of DNP grown on Ti6Al4V.
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It should be noted that among all the DLC composite thin films, DLC sample deposited at 70 eV shows the best adhesion, no spallation was observed within or around the imprint area as shown in Fig. 6b; instead, only fine cracking lines are observed inside the imprint. This is probably caused by the compressive stress inside DLC films. This compressive stress is induced by the energetic ion bombardment of the film surface during the DLC deposition, thus closely related to the deposition ion energy. This process causes carbon atoms to be implanted into the growing films by knock-on, which lead to an expansion of the films outwards from the substrate resulting in macroscopic compressive stress. The magnitude of the stress is strongly dependent on impact energy per atom. It is often found to increase with the square root of the ion energy for low normalized fluxes [14]. A certain value of compressive stress favor the formation of dense films, however, excessive compressive stress cause adhesion failure. In this work, we found that an ion energy of 70 eV yields the best adherent DLC films on both bare and DNP coated Ti6Al4V substrates. Those results have also demonstrated that DNP incorporation is effective in enhancing the adhesion of DLC on Ti6Al4V, which can be attributed to the reduced internal stress, increased interfacial bonding, and enhanced toughness due to the formation of composite film by the incorporation of DNP into DLC [5]. Fig. 7a shows a typical SEM image of DLC thin film deposited at ion energy of 90 eV on DNP coated Ti6Al4V, in which the light area is the Ti6Al4V substrate and the dark area corresponds to the residual DNP/ DLC thin film fragments. This significant delamination is probably due to the ion bombardment at relatively high energy of 90 eV, which produces too high compressive stress in the thin film and leads to adhesion failure. This can be also explained by the strain energy release rate
Fig. 5. Typical (a) SEM image and (b) NEXAFS spectrum of NDP/DLC composite thin film grown on Ti6Al4V.
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C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Fig. 6. Typical SEM images of NDP/DLC coated Ti6Al4V at different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV after indentation testing.
Fig. 7. Typical SEM images of DLC thin films deposited at deposition ion energy of 90 eV: (a) DNP/DLC on Ti6Al4V and (b) DLC on CrCoMo alloy after indentation testing. Þ 2 equation G ¼ ð1−ν 2E σ h where G is the strain energy release rate, υ is the Poisson's ratio of the film, E is the Young's modulus of the film, σ is the residual stress of the film, and h is the film thickness. With the ion energy of 90 eV, the high residual stress leads to a large strain energy release rate, easily exceeding the critical energy release rate or the fracture energy. On the contrary, at deposition ion energy of 90 eV, continuous DLC thin film can be deposited on CoCrMo with improved adhesion comparing to lower energy level as shown in Fig. 7b. This can be attributed to the much higher Young's Modulus of the CoCrMo substrate in comparison with the Ti alloy, which can result in lower compressive stress [14] and thus withstand ion bombardment with higher energy. These results indicate that the optimum ion energy for achieving adherent DLC films is dependent on substrate materials. 2
4. Conclusions The dependence of DLC properties upon deposition ion energy during direct ion beam deposition process has been investigated. The results show that the ion energy is a key factor affecting the microstructure and adhesion of DLC thin films on Ti6Al4V substrates. Higher ion energy usually results in a higher sp3 concentration in DLC thin films, but 70 eV is the optimum ion energy for DLC on Ti6Al4V substrate to achieve the highest adhesion. In addition to choosing appropriate ion energy, incorporation of DNP into DLC can further enhance the adhesion significantly confirming that DNP incorporation is an effective
approach to enhance adhesion of DLC on Ti6Al4V. Furthermore, this study reveals that the desirable ion energy used for adherent DLC deposition is dependent on the nature of the substrate materials, and case studies should be conducted for individual substrate material. Prime novelty statement This work first time studied the effect of ion energy on adhesion of DLC thin film on Ti6Al4V by ion beam deposition. An optimum ion energy of 70 eV is presented to achieve the best adhesion of DLC on Ti6Al4V by ion beam deposition. Acknowledgment This work is supported by Natural Science and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation and the China Scholarship Council. The authors are also very thankful to Prof. Akira Hirose, Plasma Physics Laboratory, University of Saskatchewan, for providing the MPCVD reactor. References [1] M. Allen, B. Myer, N. Rushton, In vitro and in vivo investigations into the biocompatibility of diamond-like carbon (DLC) coatings for orthopedic applications, J. Biome. Mater. Res. 58 (2001) 319–328. [2] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281.
C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17 [3] S.W. Jiang, B. Jiang, Y. Li, Y.R. Li, G.F. Yin, C.Q. Zheng, Friction and wear study of diamond-like carbon gradient coatings on Ti6Al4V substrate prepared by plasma source ion implant-ion beam enhanced deposition, Appl. Surf. Sci. 236 (2004) 285–291. [4] Z.X. Zhang, H. Dong, T. Bell, B.S. Xu, The effect of deep-case oxygen hardening on the tribological behaviour of a-C:H DLC coatings on Ti6Al4V alloy, J. Alloys Compd. 464 (2008) 519–525. [5] C.Z. Zhang, Y. Tang, Y.S. Li, Q. Yang, Adhesion enhancement of diamond-like carbon thin films on Ti alloys by incorporation of diamond nanoparticles, Thin Solid Films 528 (2013) 111–115. [6] H.R. Kaufman, R.S. Robinson, Operation of Broad-beam Sources, Commonwealth Scientific Corporation, Alexandria, 1987. [7] Y. Tang, Y.S. Li, C.Z. Zhang, J. Wang, Q. Yang, A. Hirose, Synthesis of cobalt/diamondlike carbon thin films by biased target ion beam deposition, Diam. Relat. Mater. 20 (2011) 538–541. [8] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 95–107.
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[9] P.J. Fallon, V.S. Veerasamy, C.A. Davis, J. Robertson, G.A.J. Amaratunga, W.I. Milne, J. Koskinen, Properties of filtered-ion-beam-deposited diamondlike carbon as a function of ion energy, Phys. Rev. B 48 (1993) 4777–4782. [10] Y. Pauleau, Residual stresses in DLC films and adhesion to various substrates, Tribology of Diamond-like Carbon Films (2008) 102–136. [11] J.E.E. Baglin, Interface structure, adhesion and ion beam processing, in: Y. Pauleau (Ed.), Materials and Processes for Surface and Interface Engineering, Springer, Gers 1995, pp. 111–149. [12] C.Z. Zhang, Y.S. Li, Y. Tang, L. Yang, L. Zhang, Y. Sun, Q. Yang, A. Hirose, Nanocrystalline diamond thin films grown on Ti6Al4V alloy, Thin Solid Films 527 (2013) 59–64. [13] O.R. Monteiro, Synthesis, properties and applications of pure and covalently doped DLC films prepared by energetic condensation, Invited article for the 33rd IUVSTA Workshop and IV Brazilian Meeting on Diamond, Diamond-Like, Nanotubes, Nitrides and Silicon Carbide, November 28th–30th, 2001, Sao Paulo, Brazil. [14] C.A. Davis, A simple model for the formation of compressive stress in thin films by ion bombardment, Thin Solid Films 226 (1993) 30–34.
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Effect of ion energy on microstructure and adhesion of diamond-like carbon on Ti6Al4V by ion beam deposition C.Z. Zhang a, S. Bhattacharya a, Y.S. Li a,b, S. Khatir b, Y.F. Hu c, S. Shiri a, Q. Yang a,⁎ a b c
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, S7N 5A9, Canada Plasma Physics Laboratory, University of Saskatchewan, 116 Science Place, Saskatoon, SK S7N 5E2, Canada Canadian Light Source, University of Saskatchewan, 44 Innovation Boulevard, Saskatoon, SK, S7N 2V3, Canada
a r t i c l e
i n f o
Article history: Received 13 May 2016 Received in revised form 12 September 2016 Accepted 15 September 2016 Available online 19 September 2016 Keywords: Diamond-like carbon Ti6Al4V Adhesion Ion energy Diamond nanoparticles
a b s t r a c t The microstructure and adhesion of diamond-like carbon (DLC) thin films on Ti6Al4V substrates were investigated using direct ion beam deposition with ion energy varying from 65 eV to 90 eV. The samples prepared were characterized by Raman spectroscopy, synchrotron near-edge X-ray absorption fine structure spectroscopy, scanning electron microscopy, and X-ray diffraction. Indentation testing by Vickers hardness tester was used for adhesion evaluation. The results show that the adhesion of DLC thin films on Ti6Al4V substrates mainly depends on ion energy used in the deposition process. Higher ion energy results in higher sp3 concentration in DLC thin films, which firstly increases then decreases the adhesion of DLC on Ti6Al4V substrate depending on the sp3 fractions and ion energy of 70 eV provides the highest adhesion on both the bare and diamond nanoparticle coated Ti6Al4V. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Ti alloys are extensively used to make implantable medical devices [1]. A well-known example is the artificial hip joints that typically consist of a stem, a spherical femoral head articulating against an acetabular cup with a UHMWPE liner as the bearing surface. Wear of the articulating surfaces and corrosion at the Ti alloy/UHMWPE interfaces have been two major concerns in total hip replacement as they lead to early implant failure. Coating the implants with biocompatible inert hard materials is expected to significantly improve their biocompatibility and their tribological and corrosion performance, and thus their service lifetime. Diamond Like Carbon (DLC) possess many unique properties, including high hardness, low friction coefficient, high chemical stability, bio-inertness, high wear/corrosion resistance, and excellent biocompatibility, making them ideal for orthopedic implant applications [2]. Nevertheless, successful applications of DLC coatings on Ti alloys have been limited due to the high internal stress in DLC coatings, which together with insufficient adhesion induces early coating delamination or spallation [3,4]. Therefore, there is an increasing engineering demand for developing technologies that can achieve synthesis of well adherent DLC coatings on Ti alloys. A wide variety of methods, including optimization of deposition conditions, substrate pretreatment such as ion implantation and nitriding/ oxidizing, deposition of an interlayer, incorporation of a metal or metal compound layer to form alternative multilayered films, ⁎ Corresponding author. E-mail address: [email protected] (Q. Yang).
http://dx.doi.org/10.1016/j.diamond.2016.09.013 0925-9635/© 2016 Elsevier B.V. All rights reserved.
incorporation of foreign elements, have been developed to solve adhesion barriers of DLC thin films on Ti alloys. Although the adhesion has been improved, further enhancement of adhesion is still required for practical applications. Our previous work has demonstrated that diamond nanoparticle (DNP) incorporation is a promising method for adhesion enhancement of DLC on Ti6Al4V [5]. Nonetheless, DLC processing parameters plays important roles on the microstructure and adhesion properties of the coated Ti alloys. In order to optimize the performance of DNP incorporated DLC films on Ti alloys, we investigated the effect of ion energy on the adhesion of DLC thin films on Ti6Al4V substrate and the results show that ion energy has significant effect on the adhesion and the thin film exhibits the highest adhesion at an ion energy of 70 eV. 2. Experimental details Ti6Al4V sheets (Ra 926 ± 40 nm) with dimensions of 10 mm × 10 mm × 1 mm were used as substrate materials. The substrate surfaces were ground with silicon carbide paper (350 grit) and polished using 9 μm diamond slurry, and 3 μm diamond slurry sequentially. After polishing, the samples (Ra 24 ± 2 nm) were ultrasonically cleaned in ethanol for 15 min and dried in air. Nanodiamond deposition on Ti6Al4V was conducted in a Microwave Plasma Chemical Vapor Deposition (MPCVD) reactor at an excitation of 2.45 GHz, with a gas mixture of methane CH4 and hydrogen H2. Before deposition, the Ti6Al4V sheets were ultrasonically seeded for 1 h in a suspension of ethanol and nanodiamond powder to enhance diamond nucleation. The deposition parameters are listed in Table 1.
C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17 Table 1 Deposition condition of diamond nanoparticles (DNP) on Ti6Al4V by MPCVD. Parameters
First step (nucleation enhancement)
Second step (diamond growth)
H2 flow rate (sccm) CH4 flow rate (sccm) Total flow rate (sccm) Gas pressure (kPa) Microwave power (W) Temperature (°C) Deposition time (min)
40 10 50 4 600 350 10
199 1 200 4 600 350 60
Table 2 Deposition conditions and resulting thickness of DLC thin films on Ti6Al4V. Ion energy (eV)
CH4 flow rate (sccm)
Temperature (°C)
Deposition time (h)
Thickness (nm)
60 70 75 80
10 10 10 10
30 30 30 30
2 2 2 2
230 ± 22 217 ± 18 201 ± 25 192 ± 31
An ion beam deposition system with an End-Hall (EH) ion source (KRI EH-1000, manufactured by Kaufman & Robinson, Inc. USA) was used for DLC thin film deposition [6,7]. The sample stage was tilted 45° with respect to the ion beam, methane gas with a flow rate of 10 sccm was introduced into the ion source and the mean ion energy of the beam used was ranging from 60 eV to 90 eV, as listed in
13
Table 2. The deposition was performed at room temperature and at a pressure of 1.2 × 10−1 Pa for 2 h. The thickness of the as-deposited DLC thin films as listed in Table 2 was measured by an optical surface profiler manufactured by Zygo Corporation. The bonding characteristics of the as prepared DLC thin films were characterized by Raman spectroscopy. The Raman spectrometer was operated at a laser wavelength of 514 nm generated by an argon laser. The adhesion evaluation by indentation testing was conducted using a Vickers hardness tester at a load of 1000 N. The surface morphology, microstructure, and chemical bonding of the as deposited DNP and DNP/DLC thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and synchrotron near-edge X-ray absorption fine structure spectroscopy (NEXAFS). 3. Results and discussion Fig. 1 shows the Gaussian-fitted Raman spectra of DLC thin films directly deposited on Ti6Al4V by different ion energy ranging from 60 eV to 80 eV. All the spectra show two broad peaks centered at around 1350 cm−1 (D band) and 1580 cm−1 (G band), a typical DLC feature [5]. The intensity ratio of D and G band, Id/Ig, obtained from Gaussian line fitting for all the samples are listed in Table 3. It can be seen that the Id/Ig value depends on ion energy, as shown in Fig. 2. The Id/Ig value decreases with the increase of the ion energy used for DLC deposition. According to Ferrari et al., the sp3 bond content in DLC thin films increases with the decrease of Id/Ig value [8]. So it can be concluded that the content of sp3 bonded carbon increases with the increase of the ion energy used for deposition. This is probably due to the subplantation of
Fig. 1. Gaussian-fitted Raman spectra (514 nm) of DLC coatings grown on Ti6Al4V by different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV.
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C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Table 3 Id and Ig ratio of DLC thin films on Ti6Al4V deposited with varying ion energy. Ion energy (eV)
60
70
75
80
Id/Ig
1.0289
0.9109
0.8256
0.7184
Fig. 2. Variation of the Id/Ig with ion energy of the DLC coatings grown on Ti6Al4V.
incident ions. The ions with higher energy would produce more quenched-in strain, higher local density and compressive stress, which give rise to a higher sp3 concentration [9]. Indentation testing was conducted at a load of 1000 N to evaluate the adhesion of DLC thin films directly deposited on Ti6Al4V with different ion energy. Fig. 3 shows the typical SEM images of DLC thin films after indentation testing (Fig. 3a–c) and without indentation testing (Fig. 3d). The light area corresponds to the exposed substrate, and the dark area corresponds to the film remaining onto the substrate. For the samples #a to #c with ion energies ranging from 60 eV to 75 eV, partial spallation or cracking of the films is observed in the areas around the imprint. For sample #a with an ion energy of 60 eV, the spallation area
accounts for about half of the imprint area, and small pieces of delaminated film can be found at the edge of the imprint, as shown in Fig. 3a. With the ion energy increased to 70 eV, much less spallation of the film is observed on sample #b, and cracking instead of spallation can be found at most of the area in the imprint, as shown in Fig. 3b, suggesting relatively good adhesion of DLC on Ti6Al4V. With the ion energy further increased to 75 eV, more spallation area can be observed, as shown in Fig. 3c, indicating poor adhesion. With further increasing the ion energy to 80 eV, the DLC thin film cracks and delaminates from Ti6Al4V substrate before indentation. The spallation area accounts for almost half of the total film area. And with the ion energy increased up to 90 eV, no DLC deposition was observed, probably because of the delamination. These results show that the DLC thin films deposited by EH ion beam of 70 eV possess the highest adhesion. As Robertson reported, a carbon ion flux at around 100 eV per carbon atom provides DLC films with the highest sp3 bonded carbon [1]. However, in our experiments, no film deposition was observed on Ti6Al4V substrate at ion energy of 90 eV or above. It has been confirmed that high compressive stress remains in DLC thin film because of ion bombardment during film deposition and the magnitude of the compressive stress depends on the film-substrate structure and the impact energy per atom [10]. Higher energy usually results in higher attractive (adhesion) strength between the substrate and the film but higher stress. The synergetic effect of these two factors would provide an optimum energy for the film with the highest load against delamination. In the present experiments, at the optimum ion energy is 70 eV. When the energy is higher than 70 eV, the measured adhesion decreases with the increase of ion energy, indicating that the compressive stress in the films increases with the increase of ion energy and the excessive stress level induces the delamination of the film [10], causing film failure at ion energy of 90 eV or above. When adhesion failure for a given film-substrate system cannot be avoided, application of an interlayer would be a good approach to address the issue. Our previous paper has reported that incorporation of DNP into DLC can significantly improve the adhesion of DLC on the Ti alloy due to diamond's ability to form strong bonds with both DLC and Ti alloy substrate, which can reduce the interfacial energy γi [11]. Nevertheless, the performance of DNP incorporation depends on the ion energy used during DLC deposition. Therefore, the effect of deposition ion energy on adhesion of DLC thin films has been investigated to further improve the adhesion.
Fig. 3. Typical SEM images of DLC coatings on Ti6Al4V after indentation testing deposited by different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV (before indentation).
C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Typical SEM image and XRD pattern of the as-deposited DNP on Ti6Al4V substrate are shown in Fig. 4. It can be seen that DNPs (light dots) cover most of the Ti6Al4V surface (see Fig. 4a), indicating high nucleation density, which might result in good adhesion as reported in our previous work [5]. In Fig. 4b, diamond peaks are denoted as D. It can be seen that there are diamond peaks at 2θ = 51°, and 91°, Ti peaks at 44°, 62°, 75°, 84°, and 99° (from the substrate), and TiC peaks at 41° and 92°, indicating the formation of a TiC intermediate layer on the substrate surface. The formation of a TiC interfacial layer is beneficial to the enhancement of adhesion of NDPs to the substrate and the reduction of substrate damage caused by hydrogenation, as revealed by our recent investigations [5,12]. To evaluate the bonding states of as-deposited DNP/DLC composite thin film, synchrotron NEXAFS spectroscopy was used. Fig. 5 shows the C K-edge NEXAFS spectrum of NDP/DLC thin films grown on Ti6Al4V substrate recorded in total electron yield (TEY). The peak located around 286.3 eV corresponds to the transition C 1s→ π⁎ for the sp2 C_C bond, and the peak located around 289.8 eV is the most interesting one as it is associated with the excitations from C 1s to σ⁎ in sp3-rich material, characteristic feature of DLC [13]. Indentation testing by Vickers hardness tester was conducted at a load of 1000 N to evaluate the adhesion of the DLC on NDP coated Ti6Al4V. Fig. 6 shows the SEM images of DNP/DLC thin films deposited with different ion energy ranging from 60 eV to 80 eV. It has found that all the DLC composite thin films show better adhesion on Ti6Al4V substrate comparing with the DLC thin films directly deposited on Ti6Al4V without interlayer. For DLC grown on DNP coated Ti6Al4V as shown in Fig. 6, much less cracking in the imprint and much less spallation around the edge of the imprint is observed comparing with DLC samples without interlayer.
Fig. 4. Typical (a) SEM image, and (b) XRD patterns of DNP grown on Ti6Al4V.
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It should be noted that among all the DLC composite thin films, DLC sample deposited at 70 eV shows the best adhesion, no spallation was observed within or around the imprint area as shown in Fig. 6b; instead, only fine cracking lines are observed inside the imprint. This is probably caused by the compressive stress inside DLC films. This compressive stress is induced by the energetic ion bombardment of the film surface during the DLC deposition, thus closely related to the deposition ion energy. This process causes carbon atoms to be implanted into the growing films by knock-on, which lead to an expansion of the films outwards from the substrate resulting in macroscopic compressive stress. The magnitude of the stress is strongly dependent on impact energy per atom. It is often found to increase with the square root of the ion energy for low normalized fluxes [14]. A certain value of compressive stress favor the formation of dense films, however, excessive compressive stress cause adhesion failure. In this work, we found that an ion energy of 70 eV yields the best adherent DLC films on both bare and DNP coated Ti6Al4V substrates. Those results have also demonstrated that DNP incorporation is effective in enhancing the adhesion of DLC on Ti6Al4V, which can be attributed to the reduced internal stress, increased interfacial bonding, and enhanced toughness due to the formation of composite film by the incorporation of DNP into DLC [5]. Fig. 7a shows a typical SEM image of DLC thin film deposited at ion energy of 90 eV on DNP coated Ti6Al4V, in which the light area is the Ti6Al4V substrate and the dark area corresponds to the residual DNP/ DLC thin film fragments. This significant delamination is probably due to the ion bombardment at relatively high energy of 90 eV, which produces too high compressive stress in the thin film and leads to adhesion failure. This can be also explained by the strain energy release rate
Fig. 5. Typical (a) SEM image and (b) NEXAFS spectrum of NDP/DLC composite thin film grown on Ti6Al4V.
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C.Z. Zhang et al. / Diamond & Related Materials 70 (2016) 12–17
Fig. 6. Typical SEM images of NDP/DLC coated Ti6Al4V at different ion energy: (a) 60 eV, (b) 70 eV, (c) 75 eV, (d) 80 eV after indentation testing.
Fig. 7. Typical SEM images of DLC thin films deposited at deposition ion energy of 90 eV: (a) DNP/DLC on Ti6Al4V and (b) DLC on CrCoMo alloy after indentation testing. Þ 2 equation G ¼ ð1−ν 2E σ h where G is the strain energy release rate, υ is the Poisson's ratio of the film, E is the Young's modulus of the film, σ is the residual stress of the film, and h is the film thickness. With the ion energy of 90 eV, the high residual stress leads to a large strain energy release rate, easily exceeding the critical energy release rate or the fracture energy. On the contrary, at deposition ion energy of 90 eV, continuous DLC thin film can be deposited on CoCrMo with improved adhesion comparing to lower energy level as shown in Fig. 7b. This can be attributed to the much higher Young's Modulus of the CoCrMo substrate in comparison with the Ti alloy, which can result in lower compressive stress [14] and thus withstand ion bombardment with higher energy. These results indicate that the optimum ion energy for achieving adherent DLC films is dependent on substrate materials. 2
4. Conclusions The dependence of DLC properties upon deposition ion energy during direct ion beam deposition process has been investigated. The results show that the ion energy is a key factor affecting the microstructure and adhesion of DLC thin films on Ti6Al4V substrates. Higher ion energy usually results in a higher sp3 concentration in DLC thin films, but 70 eV is the optimum ion energy for DLC on Ti6Al4V substrate to achieve the highest adhesion. In addition to choosing appropriate ion energy, incorporation of DNP into DLC can further enhance the adhesion significantly confirming that DNP incorporation is an effective
approach to enhance adhesion of DLC on Ti6Al4V. Furthermore, this study reveals that the desirable ion energy used for adherent DLC deposition is dependent on the nature of the substrate materials, and case studies should be conducted for individual substrate material. Prime novelty statement This work first time studied the effect of ion energy on adhesion of DLC thin film on Ti6Al4V by ion beam deposition. An optimum ion energy of 70 eV is presented to achieve the best adhesion of DLC on Ti6Al4V by ion beam deposition. Acknowledgment This work is supported by Natural Science and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation and the China Scholarship Council. The authors are also very thankful to Prof. Akira Hirose, Plasma Physics Laboratory, University of Saskatchewan, for providing the MPCVD reactor. References [1] M. Allen, B. Myer, N. Rushton, In vitro and in vivo investigations into the biocompatibility of diamond-like carbon (DLC) coatings for orthopedic applications, J. Biome. Mater. Res. 58 (2001) 319–328. [2] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281.
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