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Effect of heat treatment on the structure and properties of ion-plated TiN films
Surface and Coatings Technology 168 (2003) 43–50
Effect of heat treatment on the structure and properties of ion-plated TiN films Wen-Jun Chou, Ge-Ping Yu, Jia-Hong Huang* Department of Engineering and System Science, National Tsing Hua University, 101 Kuang Fu Road, Sector 2, Hsinchu 300, Taiwan, ROC Received 28 June 2002; accepted in revised form 20 December 2002
Abstract Heat treatment processes were widely applied to improve the properties of hard coatings. However, the mechanisms that enhance thin film properties are still unclear. In this study, titanium nitride (TiN) films were deposited on 304 stainless steel using a hollow cathode discharge ion-plating technique. The specimens were heat-treated at 400 and 700 8C for 1 h under controlling atmosphere to reduce the oxidation of the thin films. After heat treatment, the microstructure and packing factor inside the TiN thin films were not significantly changed; however, the surface grain size was enlarged and surface roughness decreased. The variation of texture coefficient was more distinct at 700 than 400 8C. The hardness of heat-treated specimens increased 10– 30% more than the as-deposited specimens with corresponding thickness. The 400 8C-treated specimens were all slightly harder than those treated at 700 8C, except one specimen. This can be attributed to the fact that the specimens treated at 400 8C have higher residual stress than those treated at 700 8C. The higher residual stress may be from the insufficient supply of thermal energy at 400 8C such that the rearrangement of atoms is incomplete in the processing time, which may lead to the poor accommodation of the atoms, and thereby increasing the residual stress. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Heat treatment; Stainless steel; Thin films
1. Introduction During the past decade, transition metal nitrides have been extensively studied and used in industry. Nitrides of various elements play a significant role in industry, science and technology for their attractive properties. Many transition metal nitrides based on titanium, chromium, and nitrogen have stimulated commercial interest due to their high hardness, wear and corrosion resistance, thermal stability and electrical properties. Among the transition metal nitrides, owing to its superior properties and well-established coating technology, TiN coating is usually chosen as the protective film for many metals to prolong their service life. One of the major applications for the protection purpose is the coating on stainless steels. Stainless steels are widely used in industry and daily life because of their high corrosion *Corresponding author. Tel.: q886-35715131x4274; fax: q88635720724. E-mail address: [email protected] (J.-H. Huang).
and oxidation resistance. By coating TiN film, both corrosion resistance and surface hardness of stainless steel are increased, and the golden decorative color adds the product value as well. Physical vapor deposition (PVD) methods have been popularly applied on the deposition of transition metal nitride films. PVD is normally divided into three categories: evaporation, sputtering, and hybrid PVD processes (e.g. ion plating, reactive evaporation and ion beam assisted deposition, etc.) Ensinger et al. w1x reported that TiN films produced by ion plating have a higher hardness, better corrosion resistance and adhesion, and higher structure density than those produced by other PVD techniques. In this research, the hollow cathode discharge ion-plating (HCD-IP) method was chosen to deposit TiN film on 304 stainless steel. Hardness is one of the superior properties of nitrides. The variation of hardness has been related to microstructure, such as defects, grain size, texture, packing factor, and residual stress of the thin film w2–7x. It has been reported that N1 1 1M is the mechanically strongest direc-
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tion for the NaCl-type nitride crystals w8,9x and the higher (1 1 1) texture coefficient will enhance the hardness of TiN thin film w6,7,10x. In addition, our previous studies w5x showed that grain size and Ti2N precipitation may also play important roles in hardness. Heat treatment processes have been widely applied to improve the properties of hard coatings by reducing the residual stress and defects in the deposited film w11– 14x. However, the mechanisms that enhance thin film properties are still unclear. Studies have reported that the oxidation on the film surface during heat treatment may reduce the hardness of TiN film w15,16x. To improve the heat treatment process, we utilized a process with controlling atmosphere for reducing the contamination and oxidation of TiN films. The purpose of this study was to investigate the relationship between the change of structure and the enhancement of mechanical properties of TiN films by heat treatment. The as-deposited TiN specimens were selected from our previous research w7x, and heat treatment was conducted at 400 and 700 8C for 1 h, respectively, in a controlled atmosphere of Ar. 2. Experimental details The substrate materials used in this study was mirrorpolished 304 stainless steel. Prior to the coating process, the specimens underwent ultrasonic cleaning progressively in acetone and ethanol and then dried in a vacuum dryer. The coating process was carried out in a HCD-IP system with deposition conditions: gun power 6 kW, Ar pressure 0.16 Pa, N2 pressure 0.13 Pa, negative bias 40 V, and the substrate temperature 300 8C. The coating system and process has been detailed elsewhere w7x. Five specimens with different film thickness were deposited for further heat treatment. A LINDBERG high temperature tube furnace equipped with a mechanical pumping system was used to conduct the heat treatment process. After inserting the specimen, the quartz tube was sealed and pumped down to 1.33 Pa, and purged with Ar to 101 kPa. The process was repeated three times and the final pressure was controlled at 0.133 Pa by flowing high purity Ar and H2 mixing gas. Titanium-coated Si wafers were used as oxygen getters to further reduce the oxygen partial pressure. The oxygen partial pressure was monitored using a zirconia oxygen gauge (15% CaO-doped ZrO2) in the gas mixture. During heat treatment, by adjusting the ratio of AryH2, the oxygen partial pressure was maintained at 1.33=10y20 Pa. Heat treatment was carried out at two different temperatures, 400 and 700 8C, for 1 h. The crystal structure of the TiN films was identified by X-ray diffraction (XRD). The Cu Ka line at 0.15405 nm was used as the source for diffraction. The extent of (1 1 1) preferred orientation is quantified by a texture
coefficient defined as I(1 1 1)y wI(1 1 1)qI(2 0 0)x, where I is the integrated intensity of the corresponding Bragg peak. The residual stress of TiN films was determined by modified XRD sin2 c method w17,18x using a 4-circle diffractometer with psi-goniometer geometry. X-ray was incident at an angle of 0.58 to increase the diffraction volume of the thin film specimen. (2 2 0) peak was found to be the one with the weakest oscillation for the sin2 c method and also provided sufficient intensity for precisely determining the peak position when the thickness of the films was below 600 nm, and, therefore, the (2 2 0) diffraction was selected to derive the variation of lattice parameter among different c angles. The columnar structure of the deposited TiN film was observed from the cross-sectional specimens using a transmission electron microscope (TEM) (JOEL2000 FX-2) operated at 200 keV. The TiN film thickness was measured from the observation of scanning electron microscopy. The NyTi ratio was obtained from the results of Rutherford backscattering spectroscopy (RBS). The calculation method of the RBS results was described elsewhere w7x. Since large substrate effect may occur for the TiN film deposited on stainless steel substrate when using normal microhardness tester, the film hardness was measured using a Hysitron nanoindenter attached on a Digital atomic force microscope (AFM). The applying loads were ranging from 1000 to 4000 mN, depending on the depth displacement that should be less than onetenth of the film thickness. The procedures of applying load and calibration of the nanoindenter were detailed in our previous paper w7x. The hardness was calculated using Oliver–Pharr technique w19x, which could be executed from the software provided by Hysitron Inc. Ten indentations were made and the average value was reported. The precision of each value was within 10%. The surface roughness and surface grain size of the TiN films were determined by AFM. 3. Results Figs. 1 and 2 reveal the microstructure of the specimens after heat treatment. Fig. 1 shows the plan-view TEM image and diffraction pattern for specimen S5 heat-treated at 700 8C for 1 h, and the corresponding XTEM bright field image accompanied with diffraction pattern and dark field image are shown in Fig. 2. The columnar width of the thin film is approximately 60– 100 nm, which can be estimated from Fig. 2c. Compared with our previous study w7x, there is no significant difference in the columnar structure and columnar width between the as-deposited specimens and the specimens after heat treatment. From the diffraction patterns shown in Fig. 1b and Fig. 2b, no oxidation product, such as TiO2, in the TiN film can be found.
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Fig. 1. (a) The plan-view TEM image; and (b) diffraction pattern for specimen S5 heat-treated at 700 8C for 1 h.
Fig. 3 depicts the XRD patterns of the as-deposited, heat-treated at 400 and at 700 8C for 1 h for specimen S5, respectively. From the XRD patterns, the (1 1 1) texture coefficient and the full width at half maximum peak (FWHM) of (1 1 1) diffraction are determined. The results of the texture coefficients with respect to the film thickness for the as-deposited and the two different heat-treating conditions are shown in Fig. 4. It can be seen that the texture coefficient generally increases with increasing film thickness. The texture coefficients obviously increase when the specimens are heat-treated at 700 8C, compared with those of the asdeposited specimens and the specimen treated at 400 8C with corresponding thickness. However, compared with the as-deposited specimens, the specimens treated at 400 8C do not always have higher texture coefficients; the texture coefficient decreases as the film thickness is below 0.8 mm, while the coefficient increases as the film thickness is above 0.8 mm. This indicates that the effect of heat treatment on the variation of texture coefficient is more distinct at higher temperature. Since the migration of atoms in the film is a thermally activated process, in the same period of time the migration distance of the atoms is larger at higher temperature. It is expected that at 700 8C the rearrangement of the atoms in the film is more complete than at 400 8C. Fig. 5 depicts the variation of (1 1 1) FWHM with respect to film thickness. It can be seen that there is almost no change in (1 1 1) FWHM between the asdeposited and the 400 8C-treated specimens; on the other hand, the (1 1 1) FWHM decreases for the 700 8C-treated specimens. From the XRD results, no oxidation product of TiN is found in the diffraction patterns.
Fig. 2. The XTEM image for specimen S5 heat-treated at 700 8C for 1 h. (a) The bright field image; (b) diffraction pattern; and (c) dark field image of (1 1 1) diffraction.
Fig. 3. XRD patterns of (a) as-deposited; (b) heat-treated at 400 8C; and (c) heat-treated at 700 8C for 1 h for specimens S5.
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Fig. 4. The texture coefficients with respect to the film thickness for the as-deposited and the two different heat-treating conditions.
Fig. 5. The variation of (1 1 1) FWHM with respect to film thickness for the as-deposited and the two different heat-treating conditions.
The values of the residual stress for all specimens are listed in Table 1. Fig. 6 shows the effect of heat treatment on the residual stress of the TiN-coated stainless steel specimens. Interestingly, heat treatment does not always release residual stress. The 400 8C-treated specimens have much higher residual stress than the asdeposited and 700 8C heat-treated specimens. For the specimens treated at 700 8C, the residual stress remains at a nearly constant level ranging from y3.11 to y3.56 GPa. The residual stress can be classified into two categories: the intrinsic stress (growth stress) and the extrinsic stress (thermal stress). The thermal stress is from the difference of thermal expansion coefficient between film and substrate materials, and can be evaluated by the following equation w20x:
sths
E Žafyas.DT 1yn
where E is the Young’s modulus, n is the Poisson’s ratio, af is the thermal expansion coefficient of the film, as is the thermal expansion coefficient of the substrate, and DT is the temperature difference between the deposition or heat treatment temperature and the room temperature. Thermal stresses calculated from this equation are y1.41 GPa for the as-deposited specimens, y1.92 GPa for the 400 8C-treated specimens and y3.45 GPa for the 700 8C-treated specimens. Apparently, the intrinsic stress is almost relieved for the 700 8C-treated specimens and, therefore, the residual stress is about equal to the thermal stress.
Table 1 Summary of experimental results Specimen No.
Thickness (mm)
Texture coefficient
FWHM (1 1 1)
Residual stress (GPa)
Hardness (GPa)
NyTi ratio
Roughness (nm)
Surface grain size (nm)
as-S1 as-S2 as-S3 as-S4 as-S5 400-S1 400-S2 400-S3 400-S4 400-S5 700-S1 700-S2 700-S3 700-S4 700-S5
0.22 0.32 0.75 1.24 1.68 0.22 0.32 0.75 1.24 1.68 0.22 0.32 0.75 1.24 1.68
0.51 0.69 0.70 0.81 0.85 0.44 0.53 0.62 0.88 0.92 0.55 0.75 0.88 0.95 0.94
1.62 1.06 0.80 0.60 0.53 1.76 0.90 0.77 0.64 0.61 0.67 0.47 0.40 0.38 0.38
y5.93 y3.12 y3.12 y2.78 y2.70 y6.39 y7.19 y5.31 y5.72 y5.87 y3.11 y3.14 y3.43 y3.56 y3.45
14.9 24.5 33.6 32.4 32.1 18.2 30.7 37.9 39.2 38.9 16.2 29.8 35.5 36.8 43.3
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
3.4 2.4 3.1 3.5 2.9 1.7 1.0 1.9 1.7 1.8 1.7 1.0 1.5 1.3 –
42.9 51.8 77.1 116.7 125.0 70.5 79.6 93.1 162.2 156.9 102.1 134.2 135.6 210.4 –
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Fig. 6. The effect of heat treatment on the residual stress of the TiNcoated stainless steel specimens.
Roughness values and surface grain size measured by AFM are listed in Table 1. Fig. 7 depicts the effect of heat treatment on the roughness of TiN thin films. Obviously, the surface roughness decreases with increasing heat treatment temperature. This effect can be clearly seen in Fig. 8, which shows the AFM images of sample S3 at three conditions. Compared with the as-deposited specimen, the surface roughness rapidly decreases and the surface grain size is enlarged for the specimens subjected to heat treatment. The reduction of surface energy may play an important role in smoothing the film surface. For the specimens heat-treated at 400 and 700 8C, there is no significant difference in the surface roughness; however, the surface grain size becomes larger when the specimens are heat-treated at 700 8C.
Fig. 8. AFM images of sample S3 at (a) as deposited; (b) heat-treated at 400 8C; and (c) heat-treated at 700 8C for 1 h.
Fig. 7. The effect of heat treatment on the roughness of TiN thin film.
It should be noted that grain sizes displayed by AFM and by XTEM are different from each other. The former is identified by surface morphology changes given the condition that each column exhibits a cellular top. The latter is identified by electron diffraction imaging given the condition that each column presents an identical lattice orientation, which is not true in case of PVD
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Fig. 9. The variation of hardness of TiN thin film with respect to film thickness at three heat-treating conditions.
coatings because some columnar grains contain several sub-grains. Fig. 9 depicts the variation of hardness of TiN thin films with respect to film thickness at the three heattreating conditions. It can be found that heat treatment increases the film hardness; however, the hardness difference in the two heat-treated conditions is not distinct, and the error bar of most of the hardness values are overlapped. 4. Discussion The purpose of the present study is to improve mechanical properties of TiN film using heat treatment. Fig. 9 indicates that the increase of hardness ranging from 10 to 30%, depending on the TiN film thickness and heat treatment temperatures. The increase of film hardness can be related to many factors as mentioned earlier, and worth to be further discussed. Previous studies have reported w13,15,21–23x that the lowest limit of detrimental heat treatment temperature for TiN film would range from 400 to 700 8C, depending on different deposition techniques and heat treatment processes. Some phenomena have been observed if the detrimental heat treatment occurs, such as forming porosities w22x and oxidation products w13,15,21–23x, or roughening film surface w24x, which may lead to the deterioration of film hardness and some other properties. The decrease of film hardness may be due to substrate effect, since the softening of the substrate by heat treatment can be incorporated into the decrease of film hardness if the measurement method does not avoid substrate effect. As mentioned in the experimental detail, nanoindentation was used in this study for avoiding the substrate effect, and hence the effect of substrate softening is not supposed to occur. The results of TEM and
XRD (Figs. 1–3) indicate that porosities and oxidation do not apparently occur after heat treatment. From AFM observation (Figs. 7 and 8), the film surface becomes smoother, not rougher, after heat treatment. Furthermore, the surface grain size is increased (Table 1) and the columnar width inside the film is not significantly changed by heat treatment. Summarizing these results, one may reason that the increase of film hardness should be from the change of structure or residual stress of the TiN thin film by the heat treatment process. Two apparent trends can be seen in Fig. 9: the heattreated specimens have higher hardness than the asdeposited specimen with corresponding thickness, and 400 8C-treated specimens have slightly higher hardness than 700 8C-process, except for specimen S5. It has been reported w6,7,10x that TiN film hardness increases with increasing (1 1 1) texture coefficient. This is attributed to the fact that the Schmid factor of all slip systems is 0 if the applied load is at N1 1 1M direction on the (1 1 1) plane, and thereby increasing the hardness of TiN thin film. This argument can be partially applied in the present case. Combining the results in Fig. 4 and Fig. 9, it is found that with higher (1 1 1) texture coefficients, the 700 8C-process does produce harder specimen than the as-deposited; however, with lower (1 1 1) texture coefficients, the 400 8C-treated specimens are slightly harder than 700 8C-treated specimens. The crystallinity of the film also cannot fully explain the variation of hardness by heat treatment. The FWHM of (1 1 1) of the as-deposited specimens is not significantly different from that of 400 8C-treated specimens, while the (1 1 1) FWHM obviously decreases for the 700 8C-treated specimens. In general, the film hardness increases with increasing crystallinity; however, the smallest FWHM is not associated with the highest film hardness at 400 8C treatment. Microstructure and packing factor may affect film hardness. Compared with our previous study w7x, the microstructure of TiN thin film shows no apparent difference, the columnar width all ranging from 60 to 100 nm, between the two extreme conditions, as-deposited and 7008-treated specimens. This agrees with previous studies w14,25x that the microstructure and grain size of TiN film would not change for the heat treatment temperature below 1300 8C. The RBS results indicate that the packing factors of the TiN film also does not show significant variation due to heat treatment. Therefore, the variation of film hardness is not from the change of microstructure or packing factor. Film hardness may be related to defect density. It has been reported w2,11,14x that high defect density, vacancies or interstitials, created during deposition may increase the film hardness or compressive residual stress. In the HCD ion-plating process, the ionization degree of the metal vapor is quite high (;40–70%), which implies that not only argon ions but also metal ions
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with high energy strike the growing layer. Since the titanium ions, often multiply charged, have a similar radius to the nitrogen atoms (r2q Ti s0.76 nm, rNs0.70 nm), the accelerated metal ions could punch nitrogen atoms near the surface into interstitial sites and occupy the previous nitrogen sites. As a result, even the understoichiometric TiN film (NyTi-1) may have excess interstitial nitrogen atoms due to ion bombardment. The incorporation of metal atoms on nitrogen sites and the nitrogen interstitials thereby produce high compressive residual stress. Numerous studies have reported w11,13,15,26x that the residual stress could be released in a wide range of heat treatment temperature. For the bulk material, the recrystallization temperature is higher than 0.4 of melting point (in Kelvin scale), which is approximately 1150 8C for TiN. The recovery of the stresses in the present study has to be explained by the rearrangement of defects with atomic dimensions, because the microstructure of the heat-treated specimens does not change and the heat treatment temperatures are much less than the recrystallization temperature of TiN. However, surface diffusion is obviously active at 400 8C and above, which is evidenced by the fact that the surface smoothing and the increasing of surface grain size, as shown in Fig. 8. The surface diffusion may contribute to the stress relief at near surface region. In Fig. 6, the relief of residual stress is obvious by comparing the curves of as-deposited and 700 8C-treated specimens. In contrast, the 400 8C-treated specimens have much higher residual stress than the specimens at the other two conditions. In an early research, Perry reported w27x that residual stress increases in TiN-coated cemented carbide specimens after heat treatment, which was attributed to a concomitant contraction in the cemented carbide substrate causing a contractile stress in the film. However, the amount of residual stress is an order of magnitude less than that in the present study. For the 400 8C-treated specimens, the residual stress is increased to above y6 GPa, which is well above the yield strength of the stainless steel substrate. Therefore, the increase of the residual stress after 400 8C treatment cannot be due to the contraction in the stainless steel substrate. It is also much larger than the thermal stress of 400 8C-process. From Figs. 7 and 8, the surface roughness decreases with increasing heat treatment temperature. In addition, the surface grain size is enlarged by the heat treatment. On the other hand, TEM images, Figs. 1 and 2, show that the grain size is not significantly changed inside the film. This indicates that the stress recovery process can proceed by surface diffusion, and the rearrangement of defects in the TiN thin film. Due to the insufficient supply of thermal energy at 400 8Cprocess, the rearrangement of defects is incomplete in the processing time. The partial rearrangement of the atoms at 400 8C may lead to the poor accommodation
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of the atoms, and thereby increasing the residual stress. The thermal stress of the 700 8C-process is calculated to be y3.45 GPa, which is about the same value as the residual stress. This fact implies that the growth stress is totally released, also means that the 700 8C process provides sufficient thermal energy for the rearrangement of the atoms. Therefore, the reason that specimens treated at 4008 have slightly higher hardness than those treated at 700 8C could be due to the difference in residual stress. However, the reason that specimen S5 treated at 700 8C has the highest hardness among all specimens is still not understood. 5. Conclusion The heat treatment with controlling atmosphere is an effective process to enhance the properties of TiN-coated 304 stainless steel. Summarized the experiment results, it is concluded as follows. After heat treatment, the microstructure and packing factor inside the TiN thin films are not significantly changed; however, the surface grain size is enlarged and surface roughness decreases. The effect of heat treatment on the variation of texture coefficient is more distinct at 700 than 400 8C. The heat-treated specimens have 10– 30% higher hardness than the as-deposited specimen with corresponding thickness, and 400 8C-process produces specimens have slightly higher hardness than 700 8C-process, except for specimen S5. The specimens treated at 400 8C have higher residual stress and, therefore, are harder than those treated at 700 8C. This is probably due to insufficient supply of thermal energy at 400 8C such that the rearrangement of atoms is incomplete in the processing time, which may lead to the poor accommodation of the atoms and, thereby increasing the residual stress. Acknowledgments This research was funded by the National Science Council of the Republic of China under the contracts 89-2216-E-007-036 and NSC 88-3011-B-007-001-NU. Residual stress measurement and nanoindentation were carried out in the Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, University of Illinois. The authors would like to thank Mr Cheng-Hsin Ma for his help in residual stress and nanoindentation measurements. Dr M.H. Shiau was appreciated for the assistance of taking TEM pictures. References w1x W. Ensinger, A. Schroer, G.K. Wolf, Nucl. Instrum. Methods Phys. Res. B 80–81 (1993) 445. w2x J.-E. Sundgren, Thin Solid Films 218 (1985) 21. w3x F.S. Shieu, L.H. Cheng, Y.C. Sung, J.H. Huang, G.P. Yu, Mater. Chem. Phys. 50 (1997) 248.
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w4x T. Matsue, T. Hanabusa, Y. Miki, K. Kusaka, E. Maitani, Thin Solid Films 343–344 (1999) 257. w5x J.-H. Huang, Y.-P. Tsai, G.-P. Yu, Thin Solid Films 355–356 (1999) 440. w6x W.-J. Chou, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol. 140 (2001) 206. w7x W.-J. Chou, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol. 149 (2002) 7. w8x H. Ljungcrantz, M. Oden, L. Hultman, J.E. Greene, J.-E. Sundgren, J. Appl. Phys. 80 (1996) 6725. w9x B.O. Johansson, J.-E. Sundgren, J.E. Greene, A. Rockett, S.A. Barnett, J. Vac. Sci. Technol. A 3 (2) (1985) 303. w10x C.T. Chen, Y.C. Song, G.-P. Yu, J.-H. Huang, J. Mater. Eng. Perform. 7 (1998) 324. w11x H. Oettel, R. Wiedemann, S. Preißler, Surf. Coat. Technol. 74– 75 (1995) 273. w12x Cheng Zhao, Peng Hong-rui, Xie Guang-wen, Shi Yu-long, Surf. Coat. Technol. 138 (2001) 237. w13x X. Kewei, C. Jin, G. Rensheng, H. Jiawen, Surf. Coat. Technol. 58 (1993) 37. w14x W. Mader, H.F. Fischmeister, E. Bergmann, Thin Solid Films 182 (1989) 141.
w15x M. Tamura, H. Kubo, Surf. Coat. Technol. 49 (1991) 194. w16x I. Milosev, H.-H. Strehblow, B. Navinsek, Thin Solid Films 303 (1997) 246. w17x C.I. Noyan, B.J. Cohen, Residual Stress Measurement by Diffraction and Interpolation, Spring, New York, 1987. w18x C.-H. Ma, J.-H. Huang, H. Chen, Thin Solid Films 418 (2002) 73. w19x W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. w20x D. Burgreen, Elements of Thermal Stress Analysis, C.P. Press, New York, 1971, p. 462. w21x A. Tarniowy, R. Mania, M. Rekas, Thin Solid Films 311 (1997) 93. w22x T.F. Page, J.C. Knight, Surf. Coat. Technol. 39–40 (1989) 339. w23x M. Wittmer, J. Noser, J. Appl. Phys. 52 (1992) 980. w24x S. Santucci, P. Giuliani, P. Picozzi, et al., Thin Solid Films 360 (2000) 89. w25x J.C. Knight, T.F. Page, I.M. Hutchings, Surf. Eng. 5 (1989) 213. w26x K. Pantleon, O. Kessler, F. Hoffann, P. Mayr, Surf. Coat. Technol. 120–121 (1999) 495. w27x A.J. Perry, Thin Solid Films 146 (1987) 165.
Effect of heat treatment on the structure and properties of ion-plated TiN films Wen-Jun Chou, Ge-Ping Yu, Jia-Hong Huang* Department of Engineering and System Science, National Tsing Hua University, 101 Kuang Fu Road, Sector 2, Hsinchu 300, Taiwan, ROC Received 28 June 2002; accepted in revised form 20 December 2002
Abstract Heat treatment processes were widely applied to improve the properties of hard coatings. However, the mechanisms that enhance thin film properties are still unclear. In this study, titanium nitride (TiN) films were deposited on 304 stainless steel using a hollow cathode discharge ion-plating technique. The specimens were heat-treated at 400 and 700 8C for 1 h under controlling atmosphere to reduce the oxidation of the thin films. After heat treatment, the microstructure and packing factor inside the TiN thin films were not significantly changed; however, the surface grain size was enlarged and surface roughness decreased. The variation of texture coefficient was more distinct at 700 than 400 8C. The hardness of heat-treated specimens increased 10– 30% more than the as-deposited specimens with corresponding thickness. The 400 8C-treated specimens were all slightly harder than those treated at 700 8C, except one specimen. This can be attributed to the fact that the specimens treated at 400 8C have higher residual stress than those treated at 700 8C. The higher residual stress may be from the insufficient supply of thermal energy at 400 8C such that the rearrangement of atoms is incomplete in the processing time, which may lead to the poor accommodation of the atoms, and thereby increasing the residual stress. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Heat treatment; Stainless steel; Thin films
1. Introduction During the past decade, transition metal nitrides have been extensively studied and used in industry. Nitrides of various elements play a significant role in industry, science and technology for their attractive properties. Many transition metal nitrides based on titanium, chromium, and nitrogen have stimulated commercial interest due to their high hardness, wear and corrosion resistance, thermal stability and electrical properties. Among the transition metal nitrides, owing to its superior properties and well-established coating technology, TiN coating is usually chosen as the protective film for many metals to prolong their service life. One of the major applications for the protection purpose is the coating on stainless steels. Stainless steels are widely used in industry and daily life because of their high corrosion *Corresponding author. Tel.: q886-35715131x4274; fax: q88635720724. E-mail address: [email protected] (J.-H. Huang).
and oxidation resistance. By coating TiN film, both corrosion resistance and surface hardness of stainless steel are increased, and the golden decorative color adds the product value as well. Physical vapor deposition (PVD) methods have been popularly applied on the deposition of transition metal nitride films. PVD is normally divided into three categories: evaporation, sputtering, and hybrid PVD processes (e.g. ion plating, reactive evaporation and ion beam assisted deposition, etc.) Ensinger et al. w1x reported that TiN films produced by ion plating have a higher hardness, better corrosion resistance and adhesion, and higher structure density than those produced by other PVD techniques. In this research, the hollow cathode discharge ion-plating (HCD-IP) method was chosen to deposit TiN film on 304 stainless steel. Hardness is one of the superior properties of nitrides. The variation of hardness has been related to microstructure, such as defects, grain size, texture, packing factor, and residual stress of the thin film w2–7x. It has been reported that N1 1 1M is the mechanically strongest direc-
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(03)00007-0
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tion for the NaCl-type nitride crystals w8,9x and the higher (1 1 1) texture coefficient will enhance the hardness of TiN thin film w6,7,10x. In addition, our previous studies w5x showed that grain size and Ti2N precipitation may also play important roles in hardness. Heat treatment processes have been widely applied to improve the properties of hard coatings by reducing the residual stress and defects in the deposited film w11– 14x. However, the mechanisms that enhance thin film properties are still unclear. Studies have reported that the oxidation on the film surface during heat treatment may reduce the hardness of TiN film w15,16x. To improve the heat treatment process, we utilized a process with controlling atmosphere for reducing the contamination and oxidation of TiN films. The purpose of this study was to investigate the relationship between the change of structure and the enhancement of mechanical properties of TiN films by heat treatment. The as-deposited TiN specimens were selected from our previous research w7x, and heat treatment was conducted at 400 and 700 8C for 1 h, respectively, in a controlled atmosphere of Ar. 2. Experimental details The substrate materials used in this study was mirrorpolished 304 stainless steel. Prior to the coating process, the specimens underwent ultrasonic cleaning progressively in acetone and ethanol and then dried in a vacuum dryer. The coating process was carried out in a HCD-IP system with deposition conditions: gun power 6 kW, Ar pressure 0.16 Pa, N2 pressure 0.13 Pa, negative bias 40 V, and the substrate temperature 300 8C. The coating system and process has been detailed elsewhere w7x. Five specimens with different film thickness were deposited for further heat treatment. A LINDBERG high temperature tube furnace equipped with a mechanical pumping system was used to conduct the heat treatment process. After inserting the specimen, the quartz tube was sealed and pumped down to 1.33 Pa, and purged with Ar to 101 kPa. The process was repeated three times and the final pressure was controlled at 0.133 Pa by flowing high purity Ar and H2 mixing gas. Titanium-coated Si wafers were used as oxygen getters to further reduce the oxygen partial pressure. The oxygen partial pressure was monitored using a zirconia oxygen gauge (15% CaO-doped ZrO2) in the gas mixture. During heat treatment, by adjusting the ratio of AryH2, the oxygen partial pressure was maintained at 1.33=10y20 Pa. Heat treatment was carried out at two different temperatures, 400 and 700 8C, for 1 h. The crystal structure of the TiN films was identified by X-ray diffraction (XRD). The Cu Ka line at 0.15405 nm was used as the source for diffraction. The extent of (1 1 1) preferred orientation is quantified by a texture
coefficient defined as I(1 1 1)y wI(1 1 1)qI(2 0 0)x, where I is the integrated intensity of the corresponding Bragg peak. The residual stress of TiN films was determined by modified XRD sin2 c method w17,18x using a 4-circle diffractometer with psi-goniometer geometry. X-ray was incident at an angle of 0.58 to increase the diffraction volume of the thin film specimen. (2 2 0) peak was found to be the one with the weakest oscillation for the sin2 c method and also provided sufficient intensity for precisely determining the peak position when the thickness of the films was below 600 nm, and, therefore, the (2 2 0) diffraction was selected to derive the variation of lattice parameter among different c angles. The columnar structure of the deposited TiN film was observed from the cross-sectional specimens using a transmission electron microscope (TEM) (JOEL2000 FX-2) operated at 200 keV. The TiN film thickness was measured from the observation of scanning electron microscopy. The NyTi ratio was obtained from the results of Rutherford backscattering spectroscopy (RBS). The calculation method of the RBS results was described elsewhere w7x. Since large substrate effect may occur for the TiN film deposited on stainless steel substrate when using normal microhardness tester, the film hardness was measured using a Hysitron nanoindenter attached on a Digital atomic force microscope (AFM). The applying loads were ranging from 1000 to 4000 mN, depending on the depth displacement that should be less than onetenth of the film thickness. The procedures of applying load and calibration of the nanoindenter were detailed in our previous paper w7x. The hardness was calculated using Oliver–Pharr technique w19x, which could be executed from the software provided by Hysitron Inc. Ten indentations were made and the average value was reported. The precision of each value was within 10%. The surface roughness and surface grain size of the TiN films were determined by AFM. 3. Results Figs. 1 and 2 reveal the microstructure of the specimens after heat treatment. Fig. 1 shows the plan-view TEM image and diffraction pattern for specimen S5 heat-treated at 700 8C for 1 h, and the corresponding XTEM bright field image accompanied with diffraction pattern and dark field image are shown in Fig. 2. The columnar width of the thin film is approximately 60– 100 nm, which can be estimated from Fig. 2c. Compared with our previous study w7x, there is no significant difference in the columnar structure and columnar width between the as-deposited specimens and the specimens after heat treatment. From the diffraction patterns shown in Fig. 1b and Fig. 2b, no oxidation product, such as TiO2, in the TiN film can be found.
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Fig. 1. (a) The plan-view TEM image; and (b) diffraction pattern for specimen S5 heat-treated at 700 8C for 1 h.
Fig. 3 depicts the XRD patterns of the as-deposited, heat-treated at 400 and at 700 8C for 1 h for specimen S5, respectively. From the XRD patterns, the (1 1 1) texture coefficient and the full width at half maximum peak (FWHM) of (1 1 1) diffraction are determined. The results of the texture coefficients with respect to the film thickness for the as-deposited and the two different heat-treating conditions are shown in Fig. 4. It can be seen that the texture coefficient generally increases with increasing film thickness. The texture coefficients obviously increase when the specimens are heat-treated at 700 8C, compared with those of the asdeposited specimens and the specimen treated at 400 8C with corresponding thickness. However, compared with the as-deposited specimens, the specimens treated at 400 8C do not always have higher texture coefficients; the texture coefficient decreases as the film thickness is below 0.8 mm, while the coefficient increases as the film thickness is above 0.8 mm. This indicates that the effect of heat treatment on the variation of texture coefficient is more distinct at higher temperature. Since the migration of atoms in the film is a thermally activated process, in the same period of time the migration distance of the atoms is larger at higher temperature. It is expected that at 700 8C the rearrangement of the atoms in the film is more complete than at 400 8C. Fig. 5 depicts the variation of (1 1 1) FWHM with respect to film thickness. It can be seen that there is almost no change in (1 1 1) FWHM between the asdeposited and the 400 8C-treated specimens; on the other hand, the (1 1 1) FWHM decreases for the 700 8C-treated specimens. From the XRD results, no oxidation product of TiN is found in the diffraction patterns.
Fig. 2. The XTEM image for specimen S5 heat-treated at 700 8C for 1 h. (a) The bright field image; (b) diffraction pattern; and (c) dark field image of (1 1 1) diffraction.
Fig. 3. XRD patterns of (a) as-deposited; (b) heat-treated at 400 8C; and (c) heat-treated at 700 8C for 1 h for specimens S5.
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Fig. 4. The texture coefficients with respect to the film thickness for the as-deposited and the two different heat-treating conditions.
Fig. 5. The variation of (1 1 1) FWHM with respect to film thickness for the as-deposited and the two different heat-treating conditions.
The values of the residual stress for all specimens are listed in Table 1. Fig. 6 shows the effect of heat treatment on the residual stress of the TiN-coated stainless steel specimens. Interestingly, heat treatment does not always release residual stress. The 400 8C-treated specimens have much higher residual stress than the asdeposited and 700 8C heat-treated specimens. For the specimens treated at 700 8C, the residual stress remains at a nearly constant level ranging from y3.11 to y3.56 GPa. The residual stress can be classified into two categories: the intrinsic stress (growth stress) and the extrinsic stress (thermal stress). The thermal stress is from the difference of thermal expansion coefficient between film and substrate materials, and can be evaluated by the following equation w20x:
sths
E Žafyas.DT 1yn
where E is the Young’s modulus, n is the Poisson’s ratio, af is the thermal expansion coefficient of the film, as is the thermal expansion coefficient of the substrate, and DT is the temperature difference between the deposition or heat treatment temperature and the room temperature. Thermal stresses calculated from this equation are y1.41 GPa for the as-deposited specimens, y1.92 GPa for the 400 8C-treated specimens and y3.45 GPa for the 700 8C-treated specimens. Apparently, the intrinsic stress is almost relieved for the 700 8C-treated specimens and, therefore, the residual stress is about equal to the thermal stress.
Table 1 Summary of experimental results Specimen No.
Thickness (mm)
Texture coefficient
FWHM (1 1 1)
Residual stress (GPa)
Hardness (GPa)
NyTi ratio
Roughness (nm)
Surface grain size (nm)
as-S1 as-S2 as-S3 as-S4 as-S5 400-S1 400-S2 400-S3 400-S4 400-S5 700-S1 700-S2 700-S3 700-S4 700-S5
0.22 0.32 0.75 1.24 1.68 0.22 0.32 0.75 1.24 1.68 0.22 0.32 0.75 1.24 1.68
0.51 0.69 0.70 0.81 0.85 0.44 0.53 0.62 0.88 0.92 0.55 0.75 0.88 0.95 0.94
1.62 1.06 0.80 0.60 0.53 1.76 0.90 0.77 0.64 0.61 0.67 0.47 0.40 0.38 0.38
y5.93 y3.12 y3.12 y2.78 y2.70 y6.39 y7.19 y5.31 y5.72 y5.87 y3.11 y3.14 y3.43 y3.56 y3.45
14.9 24.5 33.6 32.4 32.1 18.2 30.7 37.9 39.2 38.9 16.2 29.8 35.5 36.8 43.3
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
3.4 2.4 3.1 3.5 2.9 1.7 1.0 1.9 1.7 1.8 1.7 1.0 1.5 1.3 –
42.9 51.8 77.1 116.7 125.0 70.5 79.6 93.1 162.2 156.9 102.1 134.2 135.6 210.4 –
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Fig. 6. The effect of heat treatment on the residual stress of the TiNcoated stainless steel specimens.
Roughness values and surface grain size measured by AFM are listed in Table 1. Fig. 7 depicts the effect of heat treatment on the roughness of TiN thin films. Obviously, the surface roughness decreases with increasing heat treatment temperature. This effect can be clearly seen in Fig. 8, which shows the AFM images of sample S3 at three conditions. Compared with the as-deposited specimen, the surface roughness rapidly decreases and the surface grain size is enlarged for the specimens subjected to heat treatment. The reduction of surface energy may play an important role in smoothing the film surface. For the specimens heat-treated at 400 and 700 8C, there is no significant difference in the surface roughness; however, the surface grain size becomes larger when the specimens are heat-treated at 700 8C.
Fig. 8. AFM images of sample S3 at (a) as deposited; (b) heat-treated at 400 8C; and (c) heat-treated at 700 8C for 1 h.
Fig. 7. The effect of heat treatment on the roughness of TiN thin film.
It should be noted that grain sizes displayed by AFM and by XTEM are different from each other. The former is identified by surface morphology changes given the condition that each column exhibits a cellular top. The latter is identified by electron diffraction imaging given the condition that each column presents an identical lattice orientation, which is not true in case of PVD
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Fig. 9. The variation of hardness of TiN thin film with respect to film thickness at three heat-treating conditions.
coatings because some columnar grains contain several sub-grains. Fig. 9 depicts the variation of hardness of TiN thin films with respect to film thickness at the three heattreating conditions. It can be found that heat treatment increases the film hardness; however, the hardness difference in the two heat-treated conditions is not distinct, and the error bar of most of the hardness values are overlapped. 4. Discussion The purpose of the present study is to improve mechanical properties of TiN film using heat treatment. Fig. 9 indicates that the increase of hardness ranging from 10 to 30%, depending on the TiN film thickness and heat treatment temperatures. The increase of film hardness can be related to many factors as mentioned earlier, and worth to be further discussed. Previous studies have reported w13,15,21–23x that the lowest limit of detrimental heat treatment temperature for TiN film would range from 400 to 700 8C, depending on different deposition techniques and heat treatment processes. Some phenomena have been observed if the detrimental heat treatment occurs, such as forming porosities w22x and oxidation products w13,15,21–23x, or roughening film surface w24x, which may lead to the deterioration of film hardness and some other properties. The decrease of film hardness may be due to substrate effect, since the softening of the substrate by heat treatment can be incorporated into the decrease of film hardness if the measurement method does not avoid substrate effect. As mentioned in the experimental detail, nanoindentation was used in this study for avoiding the substrate effect, and hence the effect of substrate softening is not supposed to occur. The results of TEM and
XRD (Figs. 1–3) indicate that porosities and oxidation do not apparently occur after heat treatment. From AFM observation (Figs. 7 and 8), the film surface becomes smoother, not rougher, after heat treatment. Furthermore, the surface grain size is increased (Table 1) and the columnar width inside the film is not significantly changed by heat treatment. Summarizing these results, one may reason that the increase of film hardness should be from the change of structure or residual stress of the TiN thin film by the heat treatment process. Two apparent trends can be seen in Fig. 9: the heattreated specimens have higher hardness than the asdeposited specimen with corresponding thickness, and 400 8C-treated specimens have slightly higher hardness than 700 8C-process, except for specimen S5. It has been reported w6,7,10x that TiN film hardness increases with increasing (1 1 1) texture coefficient. This is attributed to the fact that the Schmid factor of all slip systems is 0 if the applied load is at N1 1 1M direction on the (1 1 1) plane, and thereby increasing the hardness of TiN thin film. This argument can be partially applied in the present case. Combining the results in Fig. 4 and Fig. 9, it is found that with higher (1 1 1) texture coefficients, the 700 8C-process does produce harder specimen than the as-deposited; however, with lower (1 1 1) texture coefficients, the 400 8C-treated specimens are slightly harder than 700 8C-treated specimens. The crystallinity of the film also cannot fully explain the variation of hardness by heat treatment. The FWHM of (1 1 1) of the as-deposited specimens is not significantly different from that of 400 8C-treated specimens, while the (1 1 1) FWHM obviously decreases for the 700 8C-treated specimens. In general, the film hardness increases with increasing crystallinity; however, the smallest FWHM is not associated with the highest film hardness at 400 8C treatment. Microstructure and packing factor may affect film hardness. Compared with our previous study w7x, the microstructure of TiN thin film shows no apparent difference, the columnar width all ranging from 60 to 100 nm, between the two extreme conditions, as-deposited and 7008-treated specimens. This agrees with previous studies w14,25x that the microstructure and grain size of TiN film would not change for the heat treatment temperature below 1300 8C. The RBS results indicate that the packing factors of the TiN film also does not show significant variation due to heat treatment. Therefore, the variation of film hardness is not from the change of microstructure or packing factor. Film hardness may be related to defect density. It has been reported w2,11,14x that high defect density, vacancies or interstitials, created during deposition may increase the film hardness or compressive residual stress. In the HCD ion-plating process, the ionization degree of the metal vapor is quite high (;40–70%), which implies that not only argon ions but also metal ions
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with high energy strike the growing layer. Since the titanium ions, often multiply charged, have a similar radius to the nitrogen atoms (r2q Ti s0.76 nm, rNs0.70 nm), the accelerated metal ions could punch nitrogen atoms near the surface into interstitial sites and occupy the previous nitrogen sites. As a result, even the understoichiometric TiN film (NyTi-1) may have excess interstitial nitrogen atoms due to ion bombardment. The incorporation of metal atoms on nitrogen sites and the nitrogen interstitials thereby produce high compressive residual stress. Numerous studies have reported w11,13,15,26x that the residual stress could be released in a wide range of heat treatment temperature. For the bulk material, the recrystallization temperature is higher than 0.4 of melting point (in Kelvin scale), which is approximately 1150 8C for TiN. The recovery of the stresses in the present study has to be explained by the rearrangement of defects with atomic dimensions, because the microstructure of the heat-treated specimens does not change and the heat treatment temperatures are much less than the recrystallization temperature of TiN. However, surface diffusion is obviously active at 400 8C and above, which is evidenced by the fact that the surface smoothing and the increasing of surface grain size, as shown in Fig. 8. The surface diffusion may contribute to the stress relief at near surface region. In Fig. 6, the relief of residual stress is obvious by comparing the curves of as-deposited and 700 8C-treated specimens. In contrast, the 400 8C-treated specimens have much higher residual stress than the specimens at the other two conditions. In an early research, Perry reported w27x that residual stress increases in TiN-coated cemented carbide specimens after heat treatment, which was attributed to a concomitant contraction in the cemented carbide substrate causing a contractile stress in the film. However, the amount of residual stress is an order of magnitude less than that in the present study. For the 400 8C-treated specimens, the residual stress is increased to above y6 GPa, which is well above the yield strength of the stainless steel substrate. Therefore, the increase of the residual stress after 400 8C treatment cannot be due to the contraction in the stainless steel substrate. It is also much larger than the thermal stress of 400 8C-process. From Figs. 7 and 8, the surface roughness decreases with increasing heat treatment temperature. In addition, the surface grain size is enlarged by the heat treatment. On the other hand, TEM images, Figs. 1 and 2, show that the grain size is not significantly changed inside the film. This indicates that the stress recovery process can proceed by surface diffusion, and the rearrangement of defects in the TiN thin film. Due to the insufficient supply of thermal energy at 400 8Cprocess, the rearrangement of defects is incomplete in the processing time. The partial rearrangement of the atoms at 400 8C may lead to the poor accommodation
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of the atoms, and thereby increasing the residual stress. The thermal stress of the 700 8C-process is calculated to be y3.45 GPa, which is about the same value as the residual stress. This fact implies that the growth stress is totally released, also means that the 700 8C process provides sufficient thermal energy for the rearrangement of the atoms. Therefore, the reason that specimens treated at 4008 have slightly higher hardness than those treated at 700 8C could be due to the difference in residual stress. However, the reason that specimen S5 treated at 700 8C has the highest hardness among all specimens is still not understood. 5. Conclusion The heat treatment with controlling atmosphere is an effective process to enhance the properties of TiN-coated 304 stainless steel. Summarized the experiment results, it is concluded as follows. After heat treatment, the microstructure and packing factor inside the TiN thin films are not significantly changed; however, the surface grain size is enlarged and surface roughness decreases. The effect of heat treatment on the variation of texture coefficient is more distinct at 700 than 400 8C. The heat-treated specimens have 10– 30% higher hardness than the as-deposited specimen with corresponding thickness, and 400 8C-process produces specimens have slightly higher hardness than 700 8C-process, except for specimen S5. The specimens treated at 400 8C have higher residual stress and, therefore, are harder than those treated at 700 8C. This is probably due to insufficient supply of thermal energy at 400 8C such that the rearrangement of atoms is incomplete in the processing time, which may lead to the poor accommodation of the atoms and, thereby increasing the residual stress. Acknowledgments This research was funded by the National Science Council of the Republic of China under the contracts 89-2216-E-007-036 and NSC 88-3011-B-007-001-NU. Residual stress measurement and nanoindentation were carried out in the Center for Microanalysis of Materials, Frederick Seitz Materials Research Laboratory, University of Illinois. The authors would like to thank Mr Cheng-Hsin Ma for his help in residual stress and nanoindentation measurements. Dr M.H. Shiau was appreciated for the assistance of taking TEM pictures. References w1x W. Ensinger, A. Schroer, G.K. Wolf, Nucl. Instrum. Methods Phys. Res. B 80–81 (1993) 445. w2x J.-E. Sundgren, Thin Solid Films 218 (1985) 21. w3x F.S. Shieu, L.H. Cheng, Y.C. Sung, J.H. Huang, G.P. Yu, Mater. Chem. Phys. 50 (1997) 248.
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