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TiN composite coating on Ti-6Al-4V
Surface & Coatings Technology 205 (2010) 620–624
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Phase constituents and mechanical properties of laser in-situ synthesized TiCN/TiN composite coating on Ti-6Al-4V Yuling Yang ⁎, Wenming Yao, Hezhi Zhang College of Science, Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 22 October 2009 Accepted in revised form 16 July 2010 Available online 23 July 2010 Keywords: Laser deposition In-situ synthesis TiCN/TiN Coating
a b s t r a c t Laser in-situ synthesis technology at room temperature was applied to obtain TiCN/TiN composite coating. A pulsed Nd:YAG laser (wavelength 1064 nm) was used to melt the mixture of Ti and C powder. Pure nitrogen gas with a pressure of 0.4 MPa was introduced coaxially together with laser beam to the melting pool to react with Ti and C atoms and in-situ synthesize TiCN/TiN composite coating. The coating consists of TiC0.3N0.7, TiN and TiN0.3, but the proportions of these three constituents vary with the laser power density. SEM results revealed that dendrites were oriented in accordance with the heat flow and a metallurgical bonding between the coating and the substrate was achieved. The in-situ synthesized TiCN/TiN composite coating, with a thickness of about 200 μm, increased the hardness and wear resistance compared to the bare Ti-6Al-4V substrate. A remarkable improvement of the average microhardness (3–4 times) and an enhancement of the wear resistance (10–11 times) are observed by laser in-situ synthesizing TiCN/TiN composite coating. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium carbonitride (TiCN)-based cermets have been known for decades and been receiving significant attention in tribological applications [1–5], due to their chemical stability and superior mechanical properties such as low friction, high hardness (HV 2500–3000), high melting point (3050 °C), electrical conductivity, and enhanced wear resistance. The superior mechanical properties of TiCN are determined by its chemical composition and the structure. In the work by Ertuerk et al. [6], it was shown that TiCN is a solid solution of FCC TiN and FCC TiC incorporating the advantages and characteristics of both TiN and TiC. In contrast to TiN, TiCN coating predominates with their better anti-adhesive [7] and anti-abrasive [8] capabilities. In the past 15 to 20 years, several techniques, such as the moderate-temperature PVD and CVD processes [9–13], selfpropagating high temperature synthesis [14], and arc-evaporation method [15], have been developed for producing TiCN-based hard coatings with an improved wear resistance. The PVD or CVD methods have been able to generate a single-layer or graded TiCN coating [16] and multi-layered TiCN coating [17,18]. The development of these techniques makes a great advance, whereas they gives rise to many difficulties, such as poor adherence, lower bonding strength, serious cracks propagation at the interface and limitation of the thickness of the coating, due to the huge differences of thermo-mechanical
⁎ Corresponding author. Tel.: +86 24 83687658. E-mail address: [email protected] (Y. Yang). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.058
properties between the substrate and TiCN ceramic particles. In addition, almost all PVD and CVD coating techniques require the substrate being heated to a temperature higher than 150–200 °C for reaching sufficient adhesion strength to the substrate. It is well known that a higher temperature will destroy the properties of the substrate. Hence, there is a high demand in the development of large-area, highrate and no vacuum process to reach metallurgical bonding to the substrate, while the properties of the substrate will not be destroyed. A promising way to overcome these difficulties is to in-situ synthesize TiCN-based cermets on the substrate at room temperature (RT) and atmospheric ambient using laser technology. Recently, laser technology has been widely used in fabricating ceramic coatings taking advantage of its rapid cooling rate and high efficiency. It is well known that highly confined and controlled local heat generated by the laser beam can yield the in-situ fabrication of a ceramic composite coating with metallurgical bonding to the substrate [19] without destroying the properties of the substrate. This leads to the rapid development of the so-called laser in-situ synthesis technology in the recent decades. This technology uses a high energy laser beam to melt the precursors and such that they insitu react with each other to form a new phase on the surface of the substrate. The key feature of laser in-situ synthesis technology is the localized melting and solidification in shallow depth within a short time, meanwhile the substrate material remains cool and serves as an infinite heat sink. Thus, a wide variety of chemical and microstructural states cannot be destroyed owing to this rapid quench effect [20]. Moreover, the laser surface engineering owns the important advantages of short processing time, flexibility in operation, shallow heat affected zone and precision [21]. Laser in-situ synthesis technology
Y. Yang et al. / Surface & Coatings Technology 205 (2010) 620–624
has been used for fabricating many composite ceramic phases of TiC and TiB2 [22,23]. However, there are no published literatures of laser in-situ synthesis technology on the fabrication of TiCN ceramic. Although we have fabricated the TiCN/Ti composite coating on the surface of Ti alloy using laser cladding technology in our previous work [24], TiCN powder was added as an additional precursor. There is a disadvantage of uneven distribution of TiCN particles. This problem may be solved by in-situ fabrication of TiCN coating as presented in this work. The objectives of this paper are (i) to explore the in-situ synthesis of TiCN/TiN composite coating on the Ti-6Al-4V substrate using pulsed Nd:YAG laser and a precursor of Ti/C mixture powder with a certain mole ratio, and (ii) to investigate the phase constituents, microstructure, microhardness and wear resistance of the ceramic coating. 2. Experimental 2.1. Materials Ti-6Al-4V sheets with dimensions of 100 mm × 60 mm × 2 mm were used as the substrate. The titanium sheets were prepared for laser cladding by initially polished using grit silicon carbide emery paper to remove the oxide film, followed by rinsing with acetone to get a clean surface. Commercial titanium powder (99.5% purity, 75 μm) and graphite powder (99% purity, 20 μm), obtained from Shenyang reagent factory of China, were taken as the precursor materials. The mole ratio of Ti to C was set to 3:1. The precursors were mixed with an organic binder, then pre-pasted on the surface of cleaned coupons. The pre-pasted coupons were then dried in the air for about 1 day to remove the moisture. A pre-pasted precursor was obtained with a thickness about 240 μm for all the coupons. Finally, pure nitrogen (99.9% purity) with a pressure of 0.4 MPa was coaxially introduced into the melting pool to serve as the nitriding element and shielding gas to prevent the surface from oxidation during laser process.
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were then sectioned perpendicular to the laser track using a low speed saw, and then polished and etched with an etchant (10 ml HF, 30 ml HNO3, and 90 ml H2O) for SEM investigation. The structure of the as-in-situ synthesized coating was observed by means of SEM (SSX-550, Japan) instrument, which was operated at a volt of 15 kV. The element distribution was analyzed by Electron Probe Micro Analysis (EPMA). The Vickers test was performed on metallographic cross-sections at RT in a Microhardness 401 MVD (Olympus Corp.) to determine the hardness distribution, and estimate the thickness of the coatings. A normal load of 1.96 N was applied for 10 s for each sample. The wear resistance of the composite coatings was studied at RT using a pin-on-disk configuration without a lubricant. The applied load was set to 30 N. Annealed carbon steel disks with a diameter of 60 mm were used as counterparts. In all the friction tests, the sliding linear speed and the sliding distance were set to 0.3 m/s and 1000 m, respectively. The in-situ synthesized pins (6 mm diameter, 12 mm length) and the counterparts were cleaned with acetone prior to testing. To calculate the wear rate and evaluate the wear resistance, weight loss was measured at regular intervals. 3. Calculations To characterize the abrasion resistance of the TiCN/TiN coating, we define the relative wear resistance ε as follows: ε=
1 ; w
ð1Þ
where w is the wear rate (mg min−1), and calculated using the formula:
w=
total weight loss : total wear time
ð2Þ
However, in order to decrease the system error caused by variation of experimental parameters and the measurement error, we adopt a relative abrasion resistance, εR, which is defined as:
2.2. Laser processing A JHM-1GY-400 model pulsed Nd:YAG laser with an average power of 500 W and a wavelength of 1064 nm was used to carry out the laser in-situ synthesis TiCN/TiN ceramic composite and form a metallurgical bonding between the coating and the substrate. A 100 mm focal length convex lens was used to focus the laser beam, which gave a spot diameter of approximately 300 μm at focus position. During laser processing, the laser beam was defocused on the material to give a beam with a spot diameter of approximately 500 μm. Overlapped laser tracks were obtained with an overlapping ratio of 50% to cover a large area. The laser processing parameters used for the current work are listed in Table 1. 2.3. Coating characterization The phase identification for the surface of the coatings was performed using X-ray diffractometer (XRD) (Rigaku-D/MAX-A) with Cu-Kα radiation (wavelength 1.5406 Å), which was operated at 30 kV and 20 mA. The 2θ angle ranged from 20° to 70°. The cladded samples Table 1 Parameters for laser processing. 6
Laser power density(×10 W cm Repetition rate (Hz) Laser speed (mm/s) Defocus distance (mm) Laser beam diameter (mm) Pressure of N2 gas (MPa)
−2
)
0.51; 1.15; 2.04; 3.23 15 2 15 0.5 0.4
εR =
εC w = S εS wC
ð3Þ
where εC and wC (εS and wS) are the abrasion resistance and wear rate of the in-situ synthesized sample and the substrate, respectively. Thus, εR represents the increase of the wear resistance of the cladded sample compared to the substrate. A high εR value represents an enhanced wear resistance. It should be stated that the wear tests were performed under the same wear time for all of the samples. Therefore, εR was calculated using the total weight loss instead of the wear rate in this work. The effect of the laser power density on the phase constituents of the in-situ synthesized TiCN/TiN composite coating was investigated. The laser power density is described as following:
Power density =
D×P v×A×W
ð4Þ
where D is the laser spot diameter, P the average laser power, v the laser traverse speed, A the laser beam area (A = 14 πD2 ), and W the pulse width. In our experiment, we set D = 0.5 mm, v = 0.2 cm/s, and W = 3.0 ms. By setting the laser average power, P to be 76 W, 48 W, 27 W, and 12 W, different power densities of 3.23 × 106 W cm−2, 2.04 × 106 W cm−2, 1.15 × 106 W cm−2, and 0.51 × 106 W cm−2 were obtained accordingly.
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N2 molecules resulted in the formation of TiN0.3 as laser and nitrogen beam moved from one position to another one. The relative amount of each phase was semiquantitatively conducted using the following formula [25]:
4. Results and discussions 4.1. XRD results and discussions Phase identification was carried out using the XRD method. The effect of laser power density on the phase constituents was investigated. XRD patterns obtained from the surface of the laser processed samples were illustrated in Fig. 1, which demonstrates a significant phase evolution after the laser processing. The phase constituents vary with the different energy densities. After laser processing under a power density of 0.51 × 106 W cm−2, phases presented in the composite coating are TiN and TiN0.3, indicating no TiCN phase was in-situ fabricated during laser processing. This may be attributed to no formation of TiC at this lower power density due to the low temperature in the laser pool. At low temperature, the Gibbs free energy for TiN formation is less than that of TiC. Therefore, TiN has the priority to be formatted. With the increase of the laser power density (e.g. greater than 1.15 × 106 W cm−2), a TiC0.3N0.7 phase was observed. In general, the coatings produced using different relatively high power densities of 1.15 × 106 W cm−2, 2.04 × 106 W cm−2 and 3.32 × 106 W cm−2 demonstrate similar XRD patterns of the phase constituents, which contain TiC0.3N0.7, TiN and TiN0.3. However, the intensity of each phase varies. This indicates that a broad processing energy density can be employed without affecting the phase constituents of the composite coating in our work. The TiC phase, which can be presented in laser processed Ti and C powder, was not detected within the resolution of XRD. This is possibly attributed to the insufficient amount of C to sustain the following reactions as a result of in-situ fabrication of TiC0.3N0.7: Ti þ C→TiC
ð5Þ
Ti þ N→TiN
ð6Þ
Tiþ
3 N →TiN0:3 20 2
0:3½TiC þ 0:7½TiN→TiC0:3 N0:7 :
ð7Þ ð8Þ
%Ii =
Ii ∑31 Ii
ð9Þ
where, Ii is the integrated peak intensity of the ith phase in concern and ∑ 31Ii corresponds to the sum of integrated peak intensity of TiC0.3N0.7 (111), TiN (111) and TiN0.3 (101) in our work. These peaks were chosen to avoid overlapping too much from different phases. The results of semiquantitative analysis are presented in Fig. 2 and Table 2. Note that the data in Fig. 2 and Table 2 represents the relative amounts of each phase rather than the actual proportions. As shown in Table 2 and Fig. 2, the relative amounts of TiN and TiN0.3 have an initial decreasing and then increasing trend with the increase of the laser energy density, while the relative amount of TiC0.3N0.7 presents a reverse trend. We inferred that this distribution trend may be well understood by thermodynamics and the nonequilibrium mass transfer process in the laser melt pool, which is being performed and will be presented in the future. In turn, the increase of Gibbs free energy ΔG of Eqs. (5) to (9) depends on the laser pool temperature. As the laser energy density increases, the temperature in the laser melting pool increases due to the rate of solid solution between TiC and TiN. As indicated in Fig. 2, when the power density is up to 3.23 × 106 W cm−2, the relative ratio of TiC0.3N0.7 decreases. This can be understood by the larger depth of laser melting pool and the rapid convection caused by a high energy density. This leads to TiC0.3N0.7 and TiN to distribute in a large range. However, some conclusions can be made qualitatively by considering the formation mechanism and the physical characteristics of the phases. As the energy density increases, the depth of laser melting pool and the convection increase; it, in turn, results in the distribution of TiN and TiC0.3N0.7 to a deeper depth. Because the density of TiC0.3N0.7 (0.25 g/cm3) is larger than that of TiN (0.08 g/cm3), more TiC0.3N0.7 distributes to a deeper layer than TiN. Thus, with the increase of laser energy density, the relative amounts of TiN and TiN0.3 first decrease and then increase, while that of TiC0.3N0.7 shows a reverse trend.
It is worth mentioning that during the laser processing, nitrogen molecules reacted with Ti atom to form TiN or TiN0.3. It was conjectured that the TiN phase mainly comes from the adequate reaction between Ti atoms and N2 molecules at the beginning of the laser process. In this stage, there were sufficient N2 molecules to react with Ti atoms. While the inadequate reaction between Ti atoms and
Cross-sectional view, elemental distribution results, and the coating micrographs with a high magnification of the sample treated at a power density of 2.04 × 106 W cm−2 are presented in Fig. 3a, b, and c, respectively. As shown in Fig.3a, a sound metallurgical
Fig. 1. XRD patterns of the laser processed samples.
Fig. 2. Relative phase amount as a function of the laser power density.
4.2. SEM and EPMA results
Y. Yang et al. / Surface & Coatings Technology 205 (2010) 620–624
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Table 2 Phase constituents and properties of TiCN/TiN composite coating and substrate. Power density Relative amounts % HVmax Average HV Weight εR (×106 W cm−2) (kg mm−2) (kg mm−2) loss (mg) TiC0.3N0.7 TiN TiN0.3 0.51 1.15 2.04 3.23 Ti-6Al-4V
0 32.9 37.7 16.4 –
49.5 32.3 29.2 36.5 –
50.5 34.7 33.1 47.1 –
661 1168 1438 1211 310
517 1001.7 1276.5 907.5 306
0.150 0.097 0.090 0.103 1.063
7.53 10.95 11.81 10.32 1
bonding between the substrate and the coating was obtained. No cracks were found in the coating whose thickness is about 200 μm, indicating that the coating demonstrates enough toughness. Fig. 3c shows micrographs with a high magnification of the coating and the interface circled in Fig. 3a. It is observed that dendrites are oriented in accordance with the heat flow and grow into the coating area. However, in the coating area, it is mainly made up of dendrites with perfect distribution and size. The distribution of Ti, C and N atoms via the distance from the surface to the substrate is obtained from the result of EPMA line scanning (Fig. 3b). The scan position is indicated using a line in the middle of the picture. The scan direction is from the top of the coating to the substrate. It reveals that C and N atoms distribute within the distance of about 200 μm from the surface of the coating. Furthermore, the intensity of the C and N atoms decreases with the increase of the distance from the surface; however the ratio of N to C atom remains around 2.5, which is larger than the value of 2.3 in TiC0.3N0.7 due to the presence of TiN in the coating. A distribution of the elements in a depth of about 200 μm was observed. 4.3. Microhardness and wear properties Microhardness was measured on the cross-sectional plane transverse to the laser track. For a given processing power density, microhardness is a function of distance from the top of coating to the Ti-6Al-4V substrate. Fig. 4a shows the changes of microhardness to the distances for all 4 samples processed at different power densities. It reveals that the sample processed with a power density of 0.51 × 106 W cm−2 presents a much lower microhardness than the
Fig. 4. Microhardness variation (a), and wear rate (b) of the samples treated with different processing power densities.
samples processed with a relatively high power density. This can be well understood by the formation of the TiC0.3N0.7 phase in the coatings processed with a higher energy density. In the work by J. M. Lackner et al. [26], it was demonstrated that the replacement of
Fig. 3. SEM images of TiC0.3N0.7/TiN coating of the (a) cross-section, (b) EPMA line distribution of Ti, C and N elements, and (c) high magnification image of the coating and interface.
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nitrogen atoms by carbon atoms leading to distortion of the FCC lattice results in the increase of hardness. The microhardness of the samples processed with a relatively high power density presents a gradient decreasing from the coating to the substrate. However, compared to the sample processed with a power density of 2.04 × 106 W cm−2, the microhardness of the samples processed with a power density of 1.15 × 106 W cm−2 and 3.23 × 106 W cm−2 is a little lower. It is maybe because of the different relative amounts of TiC0.3N0.7 as shown in Fig. 2. The average microhardness of the samples processed with a po w e r d e n si ty o f 0 .5 1 × 10 6 W c m − 2 , 1 .1 5 × 10 6 W c m − 2 , 2.04 × 106 W cm−2 and 3.23 × 106 W cm−2 are 517, 1001.7, 1276.5 and 907.5 kg mm−2, respectively (as listed in Table 2). For the sample processed with a power density of 2.04 × 106 W cm−2, it possesses the largest amount of the TiC0.3N0.7 phase, resulting in a maximum value of the microhardness of 1438 kg mm−2. It exhibits 4–5 times higher microhardness than the Ti-6Al-4V substrates. While, that of the other two samples with lower amounts of TiC0.3N0.7 is around 2–4 times. This illustrates that TiC0.3N0.7/TiN composite coating possesses much better mechanical properties than TiN coating and the coating with lower amounts of TiC0.3N0.7. Hence, a general trend can be found that the microhardness first increases and then decreases with the increase of the energy density. It depends on the relative amounts of TiC0.3N0.7. This result agrees with that of J. M. Lackner [26]. Fig. 4b displays the pin-on-disk wear rate of the Ti-6Al-4V substrate and the samples processed with different laser power densities at an applied load of 30N. The mechanical and tribological properties such as mass loss and the relative abrasion resistance εR calculated from Eq. (3) are summarized and listed in Table 2. As expected, TiCN/TiN composite coating in-situ synthesized by laser processing improved the wear resistance of the Ti-6Al-4V control. As shown in Table 2, the relative abrasion resistance εR of the four samples treated with different power densities is 7.53, 10.95, 11.81 and 10.32. This demonstrates an improvement of the wear resistance by 7.5 times, 10.9 times, 11.8 times and 10.3 times compared to the bare Ti-6Al-4V substrate. The wear rate has the same trend with the phase constituents, the relative amount of TiC0.3N0.7 and the microhardness as a function of the laser power density. Compared to the substrate, the laser in-situ synthesized TiCN/TiN composite coating presents a substantial reduction of the wear rate. It indicates an improvement of the wear resistance for all composite coatings. This is also in accordance with the results of microhardness (Fig. 4a) since microhardness plays an important role in the wear behavior and is generally used to improve the wear resistance [27,28]. In general, wear rate and wear mechanism strongly depend on the microstructure and mechanical properties of the coatings, as well as the characteristics of the impacting particles. No significant difference in the relative abrasive resistance was observed for samples with the relative amount of TiC0.3N0.7 of 32.9% and 16.4%, corresponding to laser power densities of 1.15 × 106 W cm−2 and 3.32 × 106 W cm−2. It is well known that these phases that are distributed in a deeper depth cannot be detected by XRD, therefore, they will not contribute to the relative amount calculation. But they did contribute to the wear resistance. We conjectured that this is a possible reason that a significant difference in the relative amount, while no significant difference in the relative abrasive resistance was observed. It is concluded from our experiments and measurements that the presence of TiCN/TiN composite coating results in improvement of the microhardness, and is favorable for the wear resistance, in agreement with the previous work [29].
5. Conclusions TiCN/TiN composite coating was in-situ synthesized on the surface of Ti-6Al-4V using a laser process by employing Ti/C mixture precursor and pure nitrogen reaction gas. A metallurgical bonding between the coating and the substrate was achieved. The phase constituents, the microstructure, and the mechanical properties of the composite coating were investigated. The results showed that the coating consists of TiC0.3N0.7, TiN, and TiN0.3. Their relative amounts depend on the laser power density. As a result of in-situ synthesizing of TiCN/TiN composite coating, the microhardness of the substrate was improved significantly, which resulted in the significant improvement of the abrasion resistance. A significant different relative amount of TiC 0.3 N 0.7 , TiN, and TiN 0.3 was observed corresponding to different laser power densities. However, no significant change in the relative abrasion resistance was observed with the variety of laser power density from 1.15 × 106 W cm−2 to 3.23 × 106 W cm−2. After laser in-situ synthesizing TiCN/TiN composite coating, the average microhardness and the relative abrasion resistance εR were improved by 3–4 times and 10–11 times, respectively. SEM and EPMA scanning results showed that the microstructure of TiCN/TiN coating was characterized as dendrites with a distribution of the depth of 200 μm from the surface. Acknowledgement This work is supported by the National Science Foundation of China for Young Scholars under grant number 50801012. References [1] S.Y. Zhang, Mater. Sci. Eng. A 163 (1993) 141. [2] L. Karlsson, L. Hultman, M.P. Johansson, J.-E. Sundgren, H. Ljungcrantz, Surf. Coat. Technol. 126 (2000) 1. [3] A. Forn, J.A. Picas, G.G. Fuentes, E. Elizalde, Int. J. Refract. Met. Hard Mater. 19 (2001) 507. [4] D. Mari, S. Bologini, T. Viatte, W. Benoit, Int. J. Refract. Met. Hard Mater. 19 (2001) 257. [5] K.J. Ma, C.L. Chao, D.S. Liu, Y.T. Chen, M.B. Shieh, Mater. Process. Technol. 127 (2002) 182. [6] E. Ertuerk, O. Knotek, W. Bergmer, H.-G. Prengel, Surf. Coat. Technol. 46 (1991) 39. [7] F.J. Teeter, Ceramic Coating, Vol. 44, ASME, New York, 1993, p. 87. [8] B. Navinsek, D. Hanzel, W. Meisel, Vacuum 43 (1992) 325. [9] K. Narasimhan, S. Boppana, D.G. Bhat, Wear 188 (1995) 123. [10] S.J. Bull, D.G. Bhat, M.H. Staia, Surf. Coat. Technol. 163/164 (2003) 499. [11] W.D. Sproul, J. Vac. Sci. Technol. A12 (1994) 1595. [12] H.K. Tonshoff, C. Blawit, Surf. Coat. Technol. 93 (1997) 119. [13] J. Deng, M. Branun, Surf. Coat. Technol. 70 (1994) 49. [14] Z.A. Eslamloogrami, J. Munir, J. Mater. Res. 9 (1994) 431. [15] M. Rebelo de Figueiredo, J. Neidhardt, R. Kaindl, A. Reiter, R. Tessadri, C. Mitterer, Wear 265 (2008) 525. [16] C. Wei, J.F. Lin, T.H. Jiang, C.F. Ai, Thin Solid Films 381 (2001) 94. [17] E. Bemporad, C. Pecchio, S. De Rossi, F. Carassiti, Surf. Coat. Technol. 146/147 (2001) 363. [18] K. Narasimhan, S. Boppana, D.G. Bhat, Wear 188 (1995) 123. [19] K. Akira, Surf. Coat. Technol. 132 (2000) 152. [20] C.W. Draper, C.A. Ewing, J. Mater. Sci. 19 (1984) 3815. [21] J.D. Majumdar, B.R. Chandra, R. Galun, B.L. Modike, J. Manna, Surf. Coat. Technol. 63 (2004) 771. [22] X.L. Wu, Surf. Coat. Technol. 115 (1999) 111. [23] B.S. Du, Z.D. Zeng, X.H. Wang, S.Y. Qu, Mater. Lett. 62 (2008) 689. [24] Y.L. Yang, D. Zhang, W. Yan, Y.R. Zheng, Opt. Lasers Eng. 48 (2010) 119. [25] Y. Greish, P. Brown, J. Biomed. Mater. Res. 67 (2003) 633. [26] J.M. Lackner, W. Waldhauser, R. Ebner, Surf. Coat. Technol. 188–189 (2004) 519. [27] T.J. Chen, Y. Ma, B. Li, Y.D. Li, Y. Hao, Mater. Des. 30 (2009) 235. [28] K. Haugen, O. Kvernvold, A. Ronold, R. Sandberg, Wear 186–187 (1995) 179. [29] J.P. Immarigeon, D. Chow, V.R. Parameswaran, P.H. Saari, A.K. Koul, Adv. Perform. Mater. 4 (1997) 371.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Phase constituents and mechanical properties of laser in-situ synthesized TiCN/TiN composite coating on Ti-6Al-4V Yuling Yang ⁎, Wenming Yao, Hezhi Zhang College of Science, Northeastern University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 22 October 2009 Accepted in revised form 16 July 2010 Available online 23 July 2010 Keywords: Laser deposition In-situ synthesis TiCN/TiN Coating
a b s t r a c t Laser in-situ synthesis technology at room temperature was applied to obtain TiCN/TiN composite coating. A pulsed Nd:YAG laser (wavelength 1064 nm) was used to melt the mixture of Ti and C powder. Pure nitrogen gas with a pressure of 0.4 MPa was introduced coaxially together with laser beam to the melting pool to react with Ti and C atoms and in-situ synthesize TiCN/TiN composite coating. The coating consists of TiC0.3N0.7, TiN and TiN0.3, but the proportions of these three constituents vary with the laser power density. SEM results revealed that dendrites were oriented in accordance with the heat flow and a metallurgical bonding between the coating and the substrate was achieved. The in-situ synthesized TiCN/TiN composite coating, with a thickness of about 200 μm, increased the hardness and wear resistance compared to the bare Ti-6Al-4V substrate. A remarkable improvement of the average microhardness (3–4 times) and an enhancement of the wear resistance (10–11 times) are observed by laser in-situ synthesizing TiCN/TiN composite coating. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium carbonitride (TiCN)-based cermets have been known for decades and been receiving significant attention in tribological applications [1–5], due to their chemical stability and superior mechanical properties such as low friction, high hardness (HV 2500–3000), high melting point (3050 °C), electrical conductivity, and enhanced wear resistance. The superior mechanical properties of TiCN are determined by its chemical composition and the structure. In the work by Ertuerk et al. [6], it was shown that TiCN is a solid solution of FCC TiN and FCC TiC incorporating the advantages and characteristics of both TiN and TiC. In contrast to TiN, TiCN coating predominates with their better anti-adhesive [7] and anti-abrasive [8] capabilities. In the past 15 to 20 years, several techniques, such as the moderate-temperature PVD and CVD processes [9–13], selfpropagating high temperature synthesis [14], and arc-evaporation method [15], have been developed for producing TiCN-based hard coatings with an improved wear resistance. The PVD or CVD methods have been able to generate a single-layer or graded TiCN coating [16] and multi-layered TiCN coating [17,18]. The development of these techniques makes a great advance, whereas they gives rise to many difficulties, such as poor adherence, lower bonding strength, serious cracks propagation at the interface and limitation of the thickness of the coating, due to the huge differences of thermo-mechanical
⁎ Corresponding author. Tel.: +86 24 83687658. E-mail address: [email protected] (Y. Yang). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.07.058
properties between the substrate and TiCN ceramic particles. In addition, almost all PVD and CVD coating techniques require the substrate being heated to a temperature higher than 150–200 °C for reaching sufficient adhesion strength to the substrate. It is well known that a higher temperature will destroy the properties of the substrate. Hence, there is a high demand in the development of large-area, highrate and no vacuum process to reach metallurgical bonding to the substrate, while the properties of the substrate will not be destroyed. A promising way to overcome these difficulties is to in-situ synthesize TiCN-based cermets on the substrate at room temperature (RT) and atmospheric ambient using laser technology. Recently, laser technology has been widely used in fabricating ceramic coatings taking advantage of its rapid cooling rate and high efficiency. It is well known that highly confined and controlled local heat generated by the laser beam can yield the in-situ fabrication of a ceramic composite coating with metallurgical bonding to the substrate [19] without destroying the properties of the substrate. This leads to the rapid development of the so-called laser in-situ synthesis technology in the recent decades. This technology uses a high energy laser beam to melt the precursors and such that they insitu react with each other to form a new phase on the surface of the substrate. The key feature of laser in-situ synthesis technology is the localized melting and solidification in shallow depth within a short time, meanwhile the substrate material remains cool and serves as an infinite heat sink. Thus, a wide variety of chemical and microstructural states cannot be destroyed owing to this rapid quench effect [20]. Moreover, the laser surface engineering owns the important advantages of short processing time, flexibility in operation, shallow heat affected zone and precision [21]. Laser in-situ synthesis technology
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has been used for fabricating many composite ceramic phases of TiC and TiB2 [22,23]. However, there are no published literatures of laser in-situ synthesis technology on the fabrication of TiCN ceramic. Although we have fabricated the TiCN/Ti composite coating on the surface of Ti alloy using laser cladding technology in our previous work [24], TiCN powder was added as an additional precursor. There is a disadvantage of uneven distribution of TiCN particles. This problem may be solved by in-situ fabrication of TiCN coating as presented in this work. The objectives of this paper are (i) to explore the in-situ synthesis of TiCN/TiN composite coating on the Ti-6Al-4V substrate using pulsed Nd:YAG laser and a precursor of Ti/C mixture powder with a certain mole ratio, and (ii) to investigate the phase constituents, microstructure, microhardness and wear resistance of the ceramic coating. 2. Experimental 2.1. Materials Ti-6Al-4V sheets with dimensions of 100 mm × 60 mm × 2 mm were used as the substrate. The titanium sheets were prepared for laser cladding by initially polished using grit silicon carbide emery paper to remove the oxide film, followed by rinsing with acetone to get a clean surface. Commercial titanium powder (99.5% purity, 75 μm) and graphite powder (99% purity, 20 μm), obtained from Shenyang reagent factory of China, were taken as the precursor materials. The mole ratio of Ti to C was set to 3:1. The precursors were mixed with an organic binder, then pre-pasted on the surface of cleaned coupons. The pre-pasted coupons were then dried in the air for about 1 day to remove the moisture. A pre-pasted precursor was obtained with a thickness about 240 μm for all the coupons. Finally, pure nitrogen (99.9% purity) with a pressure of 0.4 MPa was coaxially introduced into the melting pool to serve as the nitriding element and shielding gas to prevent the surface from oxidation during laser process.
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were then sectioned perpendicular to the laser track using a low speed saw, and then polished and etched with an etchant (10 ml HF, 30 ml HNO3, and 90 ml H2O) for SEM investigation. The structure of the as-in-situ synthesized coating was observed by means of SEM (SSX-550, Japan) instrument, which was operated at a volt of 15 kV. The element distribution was analyzed by Electron Probe Micro Analysis (EPMA). The Vickers test was performed on metallographic cross-sections at RT in a Microhardness 401 MVD (Olympus Corp.) to determine the hardness distribution, and estimate the thickness of the coatings. A normal load of 1.96 N was applied for 10 s for each sample. The wear resistance of the composite coatings was studied at RT using a pin-on-disk configuration without a lubricant. The applied load was set to 30 N. Annealed carbon steel disks with a diameter of 60 mm were used as counterparts. In all the friction tests, the sliding linear speed and the sliding distance were set to 0.3 m/s and 1000 m, respectively. The in-situ synthesized pins (6 mm diameter, 12 mm length) and the counterparts were cleaned with acetone prior to testing. To calculate the wear rate and evaluate the wear resistance, weight loss was measured at regular intervals. 3. Calculations To characterize the abrasion resistance of the TiCN/TiN coating, we define the relative wear resistance ε as follows: ε=
1 ; w
ð1Þ
where w is the wear rate (mg min−1), and calculated using the formula:
w=
total weight loss : total wear time
ð2Þ
However, in order to decrease the system error caused by variation of experimental parameters and the measurement error, we adopt a relative abrasion resistance, εR, which is defined as:
2.2. Laser processing A JHM-1GY-400 model pulsed Nd:YAG laser with an average power of 500 W and a wavelength of 1064 nm was used to carry out the laser in-situ synthesis TiCN/TiN ceramic composite and form a metallurgical bonding between the coating and the substrate. A 100 mm focal length convex lens was used to focus the laser beam, which gave a spot diameter of approximately 300 μm at focus position. During laser processing, the laser beam was defocused on the material to give a beam with a spot diameter of approximately 500 μm. Overlapped laser tracks were obtained with an overlapping ratio of 50% to cover a large area. The laser processing parameters used for the current work are listed in Table 1. 2.3. Coating characterization The phase identification for the surface of the coatings was performed using X-ray diffractometer (XRD) (Rigaku-D/MAX-A) with Cu-Kα radiation (wavelength 1.5406 Å), which was operated at 30 kV and 20 mA. The 2θ angle ranged from 20° to 70°. The cladded samples Table 1 Parameters for laser processing. 6
Laser power density(×10 W cm Repetition rate (Hz) Laser speed (mm/s) Defocus distance (mm) Laser beam diameter (mm) Pressure of N2 gas (MPa)
−2
)
0.51; 1.15; 2.04; 3.23 15 2 15 0.5 0.4
εR =
εC w = S εS wC
ð3Þ
where εC and wC (εS and wS) are the abrasion resistance and wear rate of the in-situ synthesized sample and the substrate, respectively. Thus, εR represents the increase of the wear resistance of the cladded sample compared to the substrate. A high εR value represents an enhanced wear resistance. It should be stated that the wear tests were performed under the same wear time for all of the samples. Therefore, εR was calculated using the total weight loss instead of the wear rate in this work. The effect of the laser power density on the phase constituents of the in-situ synthesized TiCN/TiN composite coating was investigated. The laser power density is described as following:
Power density =
D×P v×A×W
ð4Þ
where D is the laser spot diameter, P the average laser power, v the laser traverse speed, A the laser beam area (A = 14 πD2 ), and W the pulse width. In our experiment, we set D = 0.5 mm, v = 0.2 cm/s, and W = 3.0 ms. By setting the laser average power, P to be 76 W, 48 W, 27 W, and 12 W, different power densities of 3.23 × 106 W cm−2, 2.04 × 106 W cm−2, 1.15 × 106 W cm−2, and 0.51 × 106 W cm−2 were obtained accordingly.
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N2 molecules resulted in the formation of TiN0.3 as laser and nitrogen beam moved from one position to another one. The relative amount of each phase was semiquantitatively conducted using the following formula [25]:
4. Results and discussions 4.1. XRD results and discussions Phase identification was carried out using the XRD method. The effect of laser power density on the phase constituents was investigated. XRD patterns obtained from the surface of the laser processed samples were illustrated in Fig. 1, which demonstrates a significant phase evolution after the laser processing. The phase constituents vary with the different energy densities. After laser processing under a power density of 0.51 × 106 W cm−2, phases presented in the composite coating are TiN and TiN0.3, indicating no TiCN phase was in-situ fabricated during laser processing. This may be attributed to no formation of TiC at this lower power density due to the low temperature in the laser pool. At low temperature, the Gibbs free energy for TiN formation is less than that of TiC. Therefore, TiN has the priority to be formatted. With the increase of the laser power density (e.g. greater than 1.15 × 106 W cm−2), a TiC0.3N0.7 phase was observed. In general, the coatings produced using different relatively high power densities of 1.15 × 106 W cm−2, 2.04 × 106 W cm−2 and 3.32 × 106 W cm−2 demonstrate similar XRD patterns of the phase constituents, which contain TiC0.3N0.7, TiN and TiN0.3. However, the intensity of each phase varies. This indicates that a broad processing energy density can be employed without affecting the phase constituents of the composite coating in our work. The TiC phase, which can be presented in laser processed Ti and C powder, was not detected within the resolution of XRD. This is possibly attributed to the insufficient amount of C to sustain the following reactions as a result of in-situ fabrication of TiC0.3N0.7: Ti þ C→TiC
ð5Þ
Ti þ N→TiN
ð6Þ
Tiþ
3 N →TiN0:3 20 2
0:3½TiC þ 0:7½TiN→TiC0:3 N0:7 :
ð7Þ ð8Þ
%Ii =
Ii ∑31 Ii
ð9Þ
where, Ii is the integrated peak intensity of the ith phase in concern and ∑ 31Ii corresponds to the sum of integrated peak intensity of TiC0.3N0.7 (111), TiN (111) and TiN0.3 (101) in our work. These peaks were chosen to avoid overlapping too much from different phases. The results of semiquantitative analysis are presented in Fig. 2 and Table 2. Note that the data in Fig. 2 and Table 2 represents the relative amounts of each phase rather than the actual proportions. As shown in Table 2 and Fig. 2, the relative amounts of TiN and TiN0.3 have an initial decreasing and then increasing trend with the increase of the laser energy density, while the relative amount of TiC0.3N0.7 presents a reverse trend. We inferred that this distribution trend may be well understood by thermodynamics and the nonequilibrium mass transfer process in the laser melt pool, which is being performed and will be presented in the future. In turn, the increase of Gibbs free energy ΔG of Eqs. (5) to (9) depends on the laser pool temperature. As the laser energy density increases, the temperature in the laser melting pool increases due to the rate of solid solution between TiC and TiN. As indicated in Fig. 2, when the power density is up to 3.23 × 106 W cm−2, the relative ratio of TiC0.3N0.7 decreases. This can be understood by the larger depth of laser melting pool and the rapid convection caused by a high energy density. This leads to TiC0.3N0.7 and TiN to distribute in a large range. However, some conclusions can be made qualitatively by considering the formation mechanism and the physical characteristics of the phases. As the energy density increases, the depth of laser melting pool and the convection increase; it, in turn, results in the distribution of TiN and TiC0.3N0.7 to a deeper depth. Because the density of TiC0.3N0.7 (0.25 g/cm3) is larger than that of TiN (0.08 g/cm3), more TiC0.3N0.7 distributes to a deeper layer than TiN. Thus, with the increase of laser energy density, the relative amounts of TiN and TiN0.3 first decrease and then increase, while that of TiC0.3N0.7 shows a reverse trend.
It is worth mentioning that during the laser processing, nitrogen molecules reacted with Ti atom to form TiN or TiN0.3. It was conjectured that the TiN phase mainly comes from the adequate reaction between Ti atoms and N2 molecules at the beginning of the laser process. In this stage, there were sufficient N2 molecules to react with Ti atoms. While the inadequate reaction between Ti atoms and
Cross-sectional view, elemental distribution results, and the coating micrographs with a high magnification of the sample treated at a power density of 2.04 × 106 W cm−2 are presented in Fig. 3a, b, and c, respectively. As shown in Fig.3a, a sound metallurgical
Fig. 1. XRD patterns of the laser processed samples.
Fig. 2. Relative phase amount as a function of the laser power density.
4.2. SEM and EPMA results
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Table 2 Phase constituents and properties of TiCN/TiN composite coating and substrate. Power density Relative amounts % HVmax Average HV Weight εR (×106 W cm−2) (kg mm−2) (kg mm−2) loss (mg) TiC0.3N0.7 TiN TiN0.3 0.51 1.15 2.04 3.23 Ti-6Al-4V
0 32.9 37.7 16.4 –
49.5 32.3 29.2 36.5 –
50.5 34.7 33.1 47.1 –
661 1168 1438 1211 310
517 1001.7 1276.5 907.5 306
0.150 0.097 0.090 0.103 1.063
7.53 10.95 11.81 10.32 1
bonding between the substrate and the coating was obtained. No cracks were found in the coating whose thickness is about 200 μm, indicating that the coating demonstrates enough toughness. Fig. 3c shows micrographs with a high magnification of the coating and the interface circled in Fig. 3a. It is observed that dendrites are oriented in accordance with the heat flow and grow into the coating area. However, in the coating area, it is mainly made up of dendrites with perfect distribution and size. The distribution of Ti, C and N atoms via the distance from the surface to the substrate is obtained from the result of EPMA line scanning (Fig. 3b). The scan position is indicated using a line in the middle of the picture. The scan direction is from the top of the coating to the substrate. It reveals that C and N atoms distribute within the distance of about 200 μm from the surface of the coating. Furthermore, the intensity of the C and N atoms decreases with the increase of the distance from the surface; however the ratio of N to C atom remains around 2.5, which is larger than the value of 2.3 in TiC0.3N0.7 due to the presence of TiN in the coating. A distribution of the elements in a depth of about 200 μm was observed. 4.3. Microhardness and wear properties Microhardness was measured on the cross-sectional plane transverse to the laser track. For a given processing power density, microhardness is a function of distance from the top of coating to the Ti-6Al-4V substrate. Fig. 4a shows the changes of microhardness to the distances for all 4 samples processed at different power densities. It reveals that the sample processed with a power density of 0.51 × 106 W cm−2 presents a much lower microhardness than the
Fig. 4. Microhardness variation (a), and wear rate (b) of the samples treated with different processing power densities.
samples processed with a relatively high power density. This can be well understood by the formation of the TiC0.3N0.7 phase in the coatings processed with a higher energy density. In the work by J. M. Lackner et al. [26], it was demonstrated that the replacement of
Fig. 3. SEM images of TiC0.3N0.7/TiN coating of the (a) cross-section, (b) EPMA line distribution of Ti, C and N elements, and (c) high magnification image of the coating and interface.
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nitrogen atoms by carbon atoms leading to distortion of the FCC lattice results in the increase of hardness. The microhardness of the samples processed with a relatively high power density presents a gradient decreasing from the coating to the substrate. However, compared to the sample processed with a power density of 2.04 × 106 W cm−2, the microhardness of the samples processed with a power density of 1.15 × 106 W cm−2 and 3.23 × 106 W cm−2 is a little lower. It is maybe because of the different relative amounts of TiC0.3N0.7 as shown in Fig. 2. The average microhardness of the samples processed with a po w e r d e n si ty o f 0 .5 1 × 10 6 W c m − 2 , 1 .1 5 × 10 6 W c m − 2 , 2.04 × 106 W cm−2 and 3.23 × 106 W cm−2 are 517, 1001.7, 1276.5 and 907.5 kg mm−2, respectively (as listed in Table 2). For the sample processed with a power density of 2.04 × 106 W cm−2, it possesses the largest amount of the TiC0.3N0.7 phase, resulting in a maximum value of the microhardness of 1438 kg mm−2. It exhibits 4–5 times higher microhardness than the Ti-6Al-4V substrates. While, that of the other two samples with lower amounts of TiC0.3N0.7 is around 2–4 times. This illustrates that TiC0.3N0.7/TiN composite coating possesses much better mechanical properties than TiN coating and the coating with lower amounts of TiC0.3N0.7. Hence, a general trend can be found that the microhardness first increases and then decreases with the increase of the energy density. It depends on the relative amounts of TiC0.3N0.7. This result agrees with that of J. M. Lackner [26]. Fig. 4b displays the pin-on-disk wear rate of the Ti-6Al-4V substrate and the samples processed with different laser power densities at an applied load of 30N. The mechanical and tribological properties such as mass loss and the relative abrasion resistance εR calculated from Eq. (3) are summarized and listed in Table 2. As expected, TiCN/TiN composite coating in-situ synthesized by laser processing improved the wear resistance of the Ti-6Al-4V control. As shown in Table 2, the relative abrasion resistance εR of the four samples treated with different power densities is 7.53, 10.95, 11.81 and 10.32. This demonstrates an improvement of the wear resistance by 7.5 times, 10.9 times, 11.8 times and 10.3 times compared to the bare Ti-6Al-4V substrate. The wear rate has the same trend with the phase constituents, the relative amount of TiC0.3N0.7 and the microhardness as a function of the laser power density. Compared to the substrate, the laser in-situ synthesized TiCN/TiN composite coating presents a substantial reduction of the wear rate. It indicates an improvement of the wear resistance for all composite coatings. This is also in accordance with the results of microhardness (Fig. 4a) since microhardness plays an important role in the wear behavior and is generally used to improve the wear resistance [27,28]. In general, wear rate and wear mechanism strongly depend on the microstructure and mechanical properties of the coatings, as well as the characteristics of the impacting particles. No significant difference in the relative abrasive resistance was observed for samples with the relative amount of TiC0.3N0.7 of 32.9% and 16.4%, corresponding to laser power densities of 1.15 × 106 W cm−2 and 3.32 × 106 W cm−2. It is well known that these phases that are distributed in a deeper depth cannot be detected by XRD, therefore, they will not contribute to the relative amount calculation. But they did contribute to the wear resistance. We conjectured that this is a possible reason that a significant difference in the relative amount, while no significant difference in the relative abrasive resistance was observed. It is concluded from our experiments and measurements that the presence of TiCN/TiN composite coating results in improvement of the microhardness, and is favorable for the wear resistance, in agreement with the previous work [29].
5. Conclusions TiCN/TiN composite coating was in-situ synthesized on the surface of Ti-6Al-4V using a laser process by employing Ti/C mixture precursor and pure nitrogen reaction gas. A metallurgical bonding between the coating and the substrate was achieved. The phase constituents, the microstructure, and the mechanical properties of the composite coating were investigated. The results showed that the coating consists of TiC0.3N0.7, TiN, and TiN0.3. Their relative amounts depend on the laser power density. As a result of in-situ synthesizing of TiCN/TiN composite coating, the microhardness of the substrate was improved significantly, which resulted in the significant improvement of the abrasion resistance. A significant different relative amount of TiC 0.3 N 0.7 , TiN, and TiN 0.3 was observed corresponding to different laser power densities. However, no significant change in the relative abrasion resistance was observed with the variety of laser power density from 1.15 × 106 W cm−2 to 3.23 × 106 W cm−2. After laser in-situ synthesizing TiCN/TiN composite coating, the average microhardness and the relative abrasion resistance εR were improved by 3–4 times and 10–11 times, respectively. SEM and EPMA scanning results showed that the microstructure of TiCN/TiN coating was characterized as dendrites with a distribution of the depth of 200 μm from the surface. Acknowledgement This work is supported by the National Science Foundation of China for Young Scholars under grant number 50801012. References [1] S.Y. Zhang, Mater. Sci. Eng. A 163 (1993) 141. [2] L. Karlsson, L. Hultman, M.P. Johansson, J.-E. Sundgren, H. Ljungcrantz, Surf. Coat. Technol. 126 (2000) 1. [3] A. Forn, J.A. Picas, G.G. Fuentes, E. Elizalde, Int. J. Refract. Met. Hard Mater. 19 (2001) 507. [4] D. Mari, S. Bologini, T. Viatte, W. Benoit, Int. J. Refract. Met. Hard Mater. 19 (2001) 257. [5] K.J. Ma, C.L. Chao, D.S. Liu, Y.T. Chen, M.B. Shieh, Mater. Process. Technol. 127 (2002) 182. [6] E. Ertuerk, O. Knotek, W. Bergmer, H.-G. Prengel, Surf. Coat. Technol. 46 (1991) 39. [7] F.J. Teeter, Ceramic Coating, Vol. 44, ASME, New York, 1993, p. 87. [8] B. Navinsek, D. Hanzel, W. Meisel, Vacuum 43 (1992) 325. [9] K. Narasimhan, S. Boppana, D.G. Bhat, Wear 188 (1995) 123. [10] S.J. Bull, D.G. Bhat, M.H. Staia, Surf. Coat. Technol. 163/164 (2003) 499. [11] W.D. Sproul, J. Vac. Sci. Technol. A12 (1994) 1595. [12] H.K. Tonshoff, C. Blawit, Surf. Coat. Technol. 93 (1997) 119. [13] J. Deng, M. Branun, Surf. Coat. Technol. 70 (1994) 49. [14] Z.A. Eslamloogrami, J. Munir, J. Mater. Res. 9 (1994) 431. [15] M. Rebelo de Figueiredo, J. Neidhardt, R. Kaindl, A. Reiter, R. Tessadri, C. Mitterer, Wear 265 (2008) 525. [16] C. Wei, J.F. Lin, T.H. Jiang, C.F. Ai, Thin Solid Films 381 (2001) 94. [17] E. Bemporad, C. Pecchio, S. De Rossi, F. Carassiti, Surf. Coat. Technol. 146/147 (2001) 363. [18] K. Narasimhan, S. Boppana, D.G. Bhat, Wear 188 (1995) 123. [19] K. Akira, Surf. Coat. Technol. 132 (2000) 152. [20] C.W. Draper, C.A. Ewing, J. Mater. Sci. 19 (1984) 3815. [21] J.D. Majumdar, B.R. Chandra, R. Galun, B.L. Modike, J. Manna, Surf. Coat. Technol. 63 (2004) 771. [22] X.L. Wu, Surf. Coat. Technol. 115 (1999) 111. [23] B.S. Du, Z.D. Zeng, X.H. Wang, S.Y. Qu, Mater. Lett. 62 (2008) 689. [24] Y.L. Yang, D. Zhang, W. Yan, Y.R. Zheng, Opt. Lasers Eng. 48 (2010) 119. [25] Y. Greish, P. Brown, J. Biomed. Mater. Res. 67 (2003) 633. [26] J.M. Lackner, W. Waldhauser, R. Ebner, Surf. Coat. Technol. 188–189 (2004) 519. [27] T.J. Chen, Y. Ma, B. Li, Y.D. Li, Y. Hao, Mater. Des. 30 (2009) 235. [28] K. Haugen, O. Kvernvold, A. Ronold, R. Sandberg, Wear 186–187 (1995) 179. [29] J.P. Immarigeon, D. Chow, V.R. Parameswaran, P.H. Saari, A.K. Koul, Adv. Perform. Mater. 4 (1997) 371.