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Properties of Al2O3–40 wt.% ZrO2 composite coatings from ultra-fine feedstocks by atmospheric plasma spraying
Wear 265 (2008) 1642–1648
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Wear journal homepage: www.elsevier.com/locate/wear
Properties of Al2 O3 –40 wt.% ZrO2 composite coatings from ultra-fine feedstocks by atmospheric plasma spraying Xiaoqin Zhao a,b , Yulong An a,b , Jianmin Chen a,∗ , Huidi Zhou a , Bin Yin a,b a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No. 18, Tianshui Road, Lanzhou 730000, China b Graduate School, Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 25 October 2007 Received in revised form 9 March 2008 Accepted 26 March 2008 Available online 27 May 2008 Keywords: Al2 O3 –ZrO2 composite coating Plasma spraying Microstructure Friction and wear behavior Wear mechanism
a b s t r a c t In the present study, both ultra-fine and coarse Al2 O3 –40 wt.% ZrO2 grains were used as the starting materials to prepare ultra-fine structured and micro-structured Al2 O3 –40 wt.% ZrO2 composite coatings (coded as NZTA coating and MZTA coating, respectively) by atmospheric plasma spraying. The ultra-fine Al2 O3 –40 wt.% ZrO2 feedstocks for spraying were prepared by means of crushing sintered, starting from commercially availed powders of ultra-fine Al2 O3 and ZrO2 . The microstructures and phase compositions of the crushing sintered powders and the corresponding composites coatings were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The friction and wear behaviors of the composites coatings sliding against stainless-steel under dry friction conditions and at room temperature were investigated using an optimol SRV oscillating friction and wear tester. The wear mechanisms of the coatings were discussed based on the SEM observation of the worn surface morphologies and wear debris, and the elemental composition analysis of the wear debris by energy dispersive X-ray analysis as well. Results showed that aside from the typical splat lamellae, equiaxle grains were also observed in the Al2 O3 –40 wt.% ZrO2 composite coating made from the corresponding ultra-fine crushing sintered powders. The NZTA coatings had higher microhardness and better wear resistance than that of the MZTA coatings, which could be largely attributed to the better inter-splats bonding of the former. And the stainless-steel counterpart matched with the NZTA coatings had a smaller wear rate as well. Moreover, the two types of composites coatings were dominated by spalling and fracture as sliding against the stainless-steel counterpart, and the MZTA coatings experienced more severe worn surface damage at a larger load than the NZTA coatings tested under the same conditions, well corresponding to the difference in the wear resistance of the two types of composite coatings. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tetragonal zirconia in alumina matrix is known as zirconiatoughened alumina (ZTA). ZTA is a high purity combination of the low cost of alumina and high strength of zirconia. The enhanced strength and toughness have made the ZTAs more widely applicable and more productive than plain ceramics and cermets in machining steels and cast irons [1–4]. It was proved that the combination of high hardness alumina (19.3 GPa in the dense form) with the low thermal conductivity zirconia (2.2–2.6 W/mK in the dense form) contributed to the development of the microhardness and wear resistance of the as-sprayed coatings [5–8]. For example, ZTA exhibits a fracture toughness of 7 MPa, hardness of 15 GPa and flexural strength of 910 MPa. It is three to four times
∗ Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (J. Chen). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.03.019
more abrasion resistant than high purity alumina [9,10]. In addition, their mechanical properties are known to depend strongly on their microstructure [2]. With the development of nanoscience and nanotechnology, the interest in the preparation of ultra-structured coatings is growing, since they have improved mechanical properties and might find promising application in engineering [3,11–18]. There are usually two ways for transporting feedstock in the preparation of ultra-structured coatings. One is to use agglomerated ultra-structured powders, which is usually very expensive in terms of the feedstock cost. Another is to fuse and crush the agglomerated ultra-structured particles, which is the so-called crushing sintered and is less expensive in terms of the feedstock cost. Noticing that although the microstructures and properties of plasma sprayed ultra-structured alumina or zircon coatings have been extensively dealt with in many reports, only limited researches have been published on ultra-structured Al2 O3 –ZrO2 composite coatings, and in particular, few reports are currently available on the preparation of ultra-structured Al2 O3 –ZrO2 composite coatings by
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Fig. 1. Microscopic of the NZTA feedstock particles observed at low magnification (a), and at high magnification (b), and the MZTA feedstock particles observed at low magnification (c).
plasma spraying based on crushing sintered method and on the microstructure and properties of the composite coatings as well. Therefore, in the present study, both ultra-fine and coarse powders of Al2 O3 –40 wt.% ZrO2 were used as the starting materials for the feedstocks to prepare ultra-structured and micro-structured Al2 O3 –40 wt.% ZrO2 composite coatings by atmospheric plasma spraying. The microstructures, hardness and friction and wear behaviors of the composite coatings were comparatively investigated.
in the presence of ultrasonic stirring to allow the formation of slurry. Organic binder was then added into the mixed slurry in order to improve the bond force of ultra-fine grains. The content of the organic binder was about 4%. The slurry was then dehydrated, pressed, sintered and porphyrezed. The optimized parameters of
2. Experimental 2.1. Preparation of NZTA and MZTA composite coatings Commercial ultra-fine Al2 O3 powders with a mean diameter of 150 nm and ZrO2 powders with a mean diameter of 50 nm were used as the raw materials and mixed at a weight ratio of 60:40. The ultra-fine Al2 O3 and ZrO2 powders were wet-mixed and milled for 8 h in a ball mill and homogeneously dispersed in distilled water Table 1 Parameters for plasma spray Parameter
Value
Arc current (A) Voltage (V) Argon gas flow rate (SLPM) Hydrogen gas flow rate (SLPM) Carrier gas Ar (SLPM) Feedstock feed rate (g min−1 ) Spray distance (mm) Spray angle (◦ )
630 67 40 10 3.5 28 120 90
Fig. 2. XRD diffraction patterns of the NZTA and MZTA powders and coatings.
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Fig. 3. SEM images of the cross-section of the as-sprayed MZTA (a) and NZTA coatings (b).
crushing sintered process were pressing 250 MPa for 10 min and sintering temperature 1100 ◦ C for 2 h. At last, the powders were sifted by the sieves of 200 meshes and 800 meshes before being plasma sprayed; the obtained size distribution was about in the range of −75 + 10 m, which was expected to have a good flowability. The coarse powders of Al2 O3 and ZrO2 powders were also blended at weight ratio of 60:40, and then the mixed powders were ball milled for 12 h. The obtained size distribution was about −75 + 45 m. Prior to spraying, a GS-943 sand blasting machine (Beijing Changkong Sand Blasting Equipment Co., Ltd., China) was used to
obtain a rough surface so as to readily facilitate the plasma spraying process. The parameters about grit blasting were as follows: aluminum oxide grit (80–120 m), blasting distance 80–100 mm, angle 90◦ and pressure 0.4–0.7 MPa. The surface roughness of the results disk was about 8.10 ± 0.78 m Ra . The Metco A-2000 atmospheric plasma spraying system with a F4-MB plasma gun (Sulzer Metco AG, Switzerland) was used to deposit the ZTA coatings on 1Cr18Ni9Ti stainless-steel disk substrates (24 mm × 7.5 mm). The Twin-System 10-C (Plasma-Technick AG, Switzerland) was used for powder feeding. The powder-injected position locates out of the gun. Ar and H2 were used as primary and the sec-
Fig. 4. SEM micrographs of the fracture surface of the NZTA coatings at low magnification (a), and at high magnification (b), and the fracture surface of the MZTA coatings observed at low magnification (c) and at high magnification (d).
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3. Results and discussion 3.1. Characterization of the mixed powders and corresponding composite coatings
Fig. 5. Steady friction coefficients of the NZTA and MZTA coatings as a function of applied loads.
ondary plasma gases, respectively, and N2 was used as powder carrier gas. The thermal efficiencies the plasma spray is about 40–60%. The plasma spraying parameters are summarized in Table 1.
Fig. 1 shows the SEM morphologies of ultra-fine and coarse Al2 O3 –40 wt.% ZrO2 powders. It is seen that the agglomerated ultra-fine powders had granular shape and a size of 10–75 m (Fig. 1a). A high magnification SEM picture (Fig. 1b) demonstrated that the grain in the ultra-fine granules had a size of about 200 nm and the ultra-fine grains were closely connected among each other, which could be due to sintering effect and would contribute to improve the strength of the plasma sprayed coatings [20]. Besides, the SEM picture of the coarse Al2 O3 –40 wt.% ZrO2 powders shown in Fig. 1c indicated that they had an irregular blocky shape, and their size is ranged from 45 m to 75 m. Fig. 2 shows the XRD patterns of both the ultra-fine and the coarse starting powders and the corresponding as-sprayed coatings. It is seen that the reorganized ultra-fine powders were composed of ␣-Al2 O3 and tetragonal ZrO2 and the coarse powders were composed of ␣-Al2 O3 , monoclinic ZrO2 and tetragonal ZrO2 . The structure changes after depositing to coatings. It can be found that ␥-Al2 O3 was presented in the NZTA and MZTA coatings. The formation of meta-stable of ␥ (cubic) is due to the heat condition during the cooling process. Furthermore, the monoclinic
2.2. Friction and wear test The friction and wear tests were carried out on an optimol SRV oscillating friction and wear tester. Thus the steel disks deposited with the composite coatings were carefully polished to an average surface roughness of about 0.4 ± 0.1 m Ra and driven to slide against commercially obtained upper stationary stainlesssteel balls (1Cr18Ni9Ti 10 mm) in a ball-on-disk configuration. The reciprocal sliding was at an amplitude of 1 mm, normal loads of 20–60 N, a frequency of 10 Hz (corresponding to a linear velocity of 0.02 m/s), and for a duration of 20 min. All the test runs were carried without lubrication in laboratory. After the tests, the wear volume of each coating specimen was measured using an micro-XAM surface mapping microscope, and that of the counterpart steel ball was calculated from the wear scar diameter determined using an optical microscope. The wear rates of the two types of composite coatings and the counterpart steel balls were obtained from dividing the wear volumes by the applied load and sliding distance. All the experiments results are the average value of three tests, which were plotted with error bars to evaluate the scatter and repeatability of the results. The details about the friction and wear tester are given elsewhere [19].
2.3. Characterization of coatings The microhardness of the two types of ZTA coatings were measured using an MH-5-VH microhardness tester at a load of 200 g and for a loading duration of 15 s. Microhardness values were calculated by taking average of 10 measurements at each distance. The microstructures and worn surfaces morphologies of the coatings were analyzed using a JSM-5600LV scanning electron microscope (SEM) coupled with an energy dispersive X-ray analyzer (EDXA). The phase compositions of the coatings were determined using a Philips X’Pert MPD X-ray diffractometer (XRD) operating with Cu K␣ radiation at a potential of 40 kV and current of 20 mA.
Fig. 6. Wear rates of the NZTA and MZTA coatings (a) and their counterparts (b) as a function of applied loads.
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Fig. 7. SEM images of the NZTA and MZTA coatings at different loads: (a) NZTA coating at 20 N, (b) NZTA coating at 60 N, (c) MZTA coating at 20 N and (d) MZTA coating at 60 N.
ZrO2 was detected in the NZTA coating and the monoclinic ZrO2 was enhanced in the MZTA coating, while tetragonal ZrO2 was weakened. This could be rationally understood since plasma spraying could generate a high-rate cooling function, the tetragonal to monoclinic transformation occurs upon this process [16]. For ultra-fine powders have higher surface activity and surface energy than that of the coarse powders, phase deformation would be more readily occurred during high-rate cooling of plasma spraying [3]. The cross-sectional micrographs of the as-sprayed NZTA coating and MZTA coating are shown in Fig. 3, where the bimodal nature of the NZTA and MZTA coatings was observable. Namely, aside from the microcracks and pores, fully melted and partially melted areas were also observed in the coatings, which have been reported to be typical for all air plasma sprayed coatings [21]. Moreover, the NZTA coating had a larger fully melted area than that of the MZTA coating, which could be closely related to the higher surface activity and liability to be melted of the ultra-fine powders. The SEM morphologies of the fractured surfaces of the NZTA coating and MZTA coating shown in Fig. 4 indicates that both coatings contained typical splat lamellae. It is generally accepted that during plasma spraying, molten powders spread and solidify to form splat lamellae. Besides, ultra-fine grains were observed in the NZTA coating, and the high magnification micrograph of the equiaxle grain region showed that the equiaxle grains had a size of less than 100–300 nm. Combining the SEM observations of the powders and coatings, it could be inferred that ultra-fine grains in the NZTA coating were originated from the un-molten or partially melted ultra-fine raw materials powders.
3.2. Friction and wear behaviors of the composite coatings Fig. 5 shows the variations of the friction coefficients as functions of load for both NZTA coating and MZTA coating sliding against stainless-steel ball. The friction coefficients were obtained after 20 min wear testing, when the friction coefficients were stable. Generally speaking, the friction coefficient decreased with increasing load, which is attributed to the slow increase of actual contact area of the sliding couples. The NTA coating presents lower friction coefficient than that of the MZTA coating under 30 N, 40 N and 60 N. Furthermore, the friction coefficients of the NTA coating have smaller deviation than that of the MZTA coating. The wear rates of the NZTA and MZTA coatings and that of the corresponding steel ball counterpart as functions of load are given in Fig. 6. It is seen that the wear rates of the two types of ZTA composite coatings had a magnitude of 10−5 mm3 /(N m) and roughly increased with increasing load (Fig. 6a). And in particular, the wear rate of the NZTA coating was lower, by about 18–46% depending on the load, than that of the MZTA coating, indicating that the NZTA coating had better wear resistance than the MZTA coating. Besides, the stainless-steel ball sliding against the NZTA coating recorded a lower wear rate, by about 40% to 50%, as compared with sliding against the MZTA coating (Fig. 6b). Thus it could be concluded that the plasma sprayed NZTA composite coating composed of ultra-fine grains had improved wear resistance, which helped to effectively reduce the wear rate of the counterpart stainless-steel as well. Fig. 7 shows the SEM morphologies of the worn surfaces of NZTA and MZTA coatings sliding against the stainless-steel counterpart at a load of 20 N and 60 N, respectively. Spalling pits and signs of
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Fig. 8. Wear debris of the NZTA coating at 20 N (a), 60 N (b), and EDS analytical results for (a) and (b), corresponding to (c) and (d).
detachment were visible on the worn surfaces of the two types of ZTA composite coatings, and no substantial difference was observed in terms of the worn surfaces features, except that the spalling pits on the worn surface of the MZTA coating at a higher load were larger in size, corresponding to the higher wear rate as compared with the NZTA coating. Thus it was supposed that both the NZTA and MZTA coatings coupled with the stainless-steel were dominated by spalling which was induced by inter-layer cracks. Because the MZTA coating had poorer inter-splats bonding than the NZTA coating (Fig. 4a and c), the former would be more liable to delamination and fracture along the splat boundaries where cracks developed preferentially even under low loads [17], resulting an increased wear rate. In other words, aside from the grain size, microstructure, and
mechanical properties and so on [17,18,22], the better wear resistance of the plasma sprayed NZTA coating was largely attributed to its better inter-splats bonding. In addition, the microhardness could also play an important role in governing the friction and wear behaviors of the ceramic-matrix composite coatings [22]. The NZTA coating has a Vikers hardness of 887 ± 133 kg/mm2 , which is The microhardness approximately 1.1 times as large as that of MZTA coating (811 ± 169 kg/mm2 ). This might account for as well, to some extent, the better wear resistance of the NZTA coating with a higher microhardness than the MZTA coating with a lower microhardness. Fig. 8 shows the SEM micrographs of the wear debris of the NZTA coating matched with the stainless-steel counterpart at a load of 20 N and 60 N, respectively. It could be worth pointing out
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that the delamination-flake-like wear debris was more or less flattened in terms of the shape, indicating that the wear process was mainly dominated by delamination and brittle fracture (Fig. 8a and b). And interestingly, the wear debris generated at 20 N had a size of 10–100 m, smaller than 50–120 m, that of the wear debris generated at 60 N, and the Fe/Al (or Zr) atomic ratio of the wear debris at 60 N was lower than that of the wear debris at 20 N (Fig. 8c and d), which indicates that the delamination and fracture of the composite coating was enhanced at a higher load. The wear debris of the MZTA coating under the same test conditions had similar features as that of the NZTA coating. Therefore, it could be concluded, in combination with the SEM analyses of the worn surfaces, that the main wear mechanisms for the NZTA and MZTA coating were spalling and fracture caused by inter-layer cracks. 4. Conclusions From the above, the following conclusions can be drawn: (1) It was feasible to prepare ultra-fine Al2 O3 –40 wt.% ZrO2 powders for plasma spraying using crushing sintered. The resulting ultra-fine mixed powders composed of alpha alumina phase and tetragonal zirconia phase could be readily used to prepare plasma sprayed NZTA composite coating containing splat lamellae and ultra-fine grains. And the monoclinic zirconia phase presented in the as-sprayed ultra-fine composite coating after the plasma spraying. (2) The NZTA coating had better wear resistance than the MZTA coating, which could be mainly attributed to the better intersplats bonding and higher hardness of the former than the latter, which also contributed to reduce the wear rate of the counterpart stainless-steel. (3) Both NZTA and MZTA coatings were dominated by spalling and inter-layer fracture as they slide against the stainless-steel counterpart under dry sliding condition. And the MZTA coating experienced more severe spalling and fracture at a higher load than the NZTA coating, well corresponding to the difference in the wear resistance of the two types of composite coatings. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 50421502) and the 973 Project (Grant No.
2007CB607601) for financial support. The authors also appreciate the helpful comments of reviewers. References [1] S.T. Aruna, K.S. Rajam, Mixture of fuels approach for the solution combustion synthesis of Al2 O3 –ZrO2 nanocomposite, Mater. Res. Bull. 39 (2004) 157–167. [2] B. Yung-Koog Jang, Microstructure of nano-SiC dispersed Al2 O3 –ZrO2 composites, Mater. Chem. Phys. 93 (2005) 337–341. [3] B. Liang, H. Liao, C. Ding, C. Coddet, Nanostructured zirconia–30 vol.% alumina composite coatings deposited by atmospheric plasma spraying, Thin Solid Films 484 (2005) 225–231. [4] B. Kerkwijk, E.J. Mulder, Tribological properties of nanoscale alumina–zirconia composites, Wear 229 (1999) 1293–1302. [5] Michael V. Swain, Structure and Properties of Ceramics, Cambridge University Press Materials Science and Technology, 1994, p. 90. [6] A. Kyongjun, K.S. Ravichandran, S.L. Semiatin, Low thermal conductivity vapor deposited zirconia microstructures, J. Am. Ceram. Soc. 82 (1999) 299–319. [7] Y.J. He, A.J.A. Winnubst, A.J. Burggraaf, H. Verweij, P.G.T. van der Varst, G. de With, Sliding wear of ZrO2 –Al2 O3 composite ceramics, J. Am. Ceram. Soc. 17 (1997) 1371–1380. ` [8] L. Esposito, R. Moreno, A.J. Sanchez Herencia, A. Tucci, Sliding wear response of an alumina–zirconia system, J. Am. Ceram. Soc. 18 (1998) 15–22. [9] G.J. Liu, H.B. Qiu, R. Todd, R.J. Brook, J.K. Guo, Processing and mechanical behavior of Al2 O3 /ZrO2 nano-composites, Mater. Res. Bull. 33 (1998) 281–288. [10] L.E. McCandlish, B.H. Kear, B.K. Kim, Processing and properties of nanostructured WC–Co, Nanostruct. Mater. 1 (1992) 119–124. [11] Y.J. He, A.J.A. Winnubst, Effects of a second phase on the tribological properties of Al2 O3 and ZrO2 ceramics, Wear 210 (1997) 178–187. [12] B.H. Kear, P.K. Sakangi, M. Jain, R. Yao, Z. Kaiman, G. Skandan, W.E. Mayo, Thermal sprayed nanostructured WC/Co hard coatings, J. Therm. Spray Technol. 9 (2000) 399–406. [13] X. Lin, Y. Zeng, X. Zheng, C. Ding, Thermal diffusivity of plasma sprayed monolithic coating of alumina–13 wt.% titania produced with nanostructured powder, Surf. Coat. Technol. 195 (2005) 85–90. [14] P. Bansal, N.P. Padture, A. Vasiliev, Improved interfacial mechanical properties of Al2 O3 –13 wt.%TiO2 plasma-sprayed coatings derived from nanocrystalline powders, Acta Mater. 51 (2003) 2959–2970. [15] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Development ad implementation of plasma sprayed nanostructured ceramic coatings, Surf. Coat. Technol. 146 (2001) 48–54. [16] S. Tao, B. Liang, C. Ding, H. Liao, Wear characteristics of plasma-sprayed nanostructured yttria partially stabilized zirconia coatings, J. Therm. Spray Technol. 14 (2005) 518–523. [17] X. Lin, Y. Zeng, C. Ding, P. Zhang, Tribological behaviour of nanostructured Al2 O3 –3%TiO2 coating against steel in dry sliding, Tribol. Lett. 17 (2004) 19–26. [18] Y.-C. Zhu, K. Yukimura, C.-X. Ding, P.-Y. Zhang, Tribological properties of nanostructured and conventional WC–Co coatings deposited by plasma spraying, Thin Solid Films 388 (2001) 277–282. [19] P. La, Q. Xue, W. Liu, Effects of boron doping on tribological properties of Ni3 Al–Cr7 C3 coatings under dry sliding, Wear 249 (2001) 93–99. [20] G. Bolelli, V. Cannillo, L. Lusvarghi, T. Manfredini, Wear behaviour of thermally sprayed ceramic oxide coatings, Wear 261 (2006) 1298–1315. [21] D.I. Pantelis, P. Psyllaki, N. Alexopoulos, Tribological behaviour of plasmasprayed Al2 O3 coatings under severe wear conditions, Wear 237 (2000) 197–204. [22] K.H. Zum Gagr, W. Bundschuh, B. Zimmerlin, Effect of grain size on friction and sliding wear of oxide ceramics, Wear 162 (1993) 269–279.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Properties of Al2 O3 –40 wt.% ZrO2 composite coatings from ultra-fine feedstocks by atmospheric plasma spraying Xiaoqin Zhao a,b , Yulong An a,b , Jianmin Chen a,∗ , Huidi Zhou a , Bin Yin a,b a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No. 18, Tianshui Road, Lanzhou 730000, China b Graduate School, Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 25 October 2007 Received in revised form 9 March 2008 Accepted 26 March 2008 Available online 27 May 2008 Keywords: Al2 O3 –ZrO2 composite coating Plasma spraying Microstructure Friction and wear behavior Wear mechanism
a b s t r a c t In the present study, both ultra-fine and coarse Al2 O3 –40 wt.% ZrO2 grains were used as the starting materials to prepare ultra-fine structured and micro-structured Al2 O3 –40 wt.% ZrO2 composite coatings (coded as NZTA coating and MZTA coating, respectively) by atmospheric plasma spraying. The ultra-fine Al2 O3 –40 wt.% ZrO2 feedstocks for spraying were prepared by means of crushing sintered, starting from commercially availed powders of ultra-fine Al2 O3 and ZrO2 . The microstructures and phase compositions of the crushing sintered powders and the corresponding composites coatings were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The friction and wear behaviors of the composites coatings sliding against stainless-steel under dry friction conditions and at room temperature were investigated using an optimol SRV oscillating friction and wear tester. The wear mechanisms of the coatings were discussed based on the SEM observation of the worn surface morphologies and wear debris, and the elemental composition analysis of the wear debris by energy dispersive X-ray analysis as well. Results showed that aside from the typical splat lamellae, equiaxle grains were also observed in the Al2 O3 –40 wt.% ZrO2 composite coating made from the corresponding ultra-fine crushing sintered powders. The NZTA coatings had higher microhardness and better wear resistance than that of the MZTA coatings, which could be largely attributed to the better inter-splats bonding of the former. And the stainless-steel counterpart matched with the NZTA coatings had a smaller wear rate as well. Moreover, the two types of composites coatings were dominated by spalling and fracture as sliding against the stainless-steel counterpart, and the MZTA coatings experienced more severe worn surface damage at a larger load than the NZTA coatings tested under the same conditions, well corresponding to the difference in the wear resistance of the two types of composite coatings. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Tetragonal zirconia in alumina matrix is known as zirconiatoughened alumina (ZTA). ZTA is a high purity combination of the low cost of alumina and high strength of zirconia. The enhanced strength and toughness have made the ZTAs more widely applicable and more productive than plain ceramics and cermets in machining steels and cast irons [1–4]. It was proved that the combination of high hardness alumina (19.3 GPa in the dense form) with the low thermal conductivity zirconia (2.2–2.6 W/mK in the dense form) contributed to the development of the microhardness and wear resistance of the as-sprayed coatings [5–8]. For example, ZTA exhibits a fracture toughness of 7 MPa, hardness of 15 GPa and flexural strength of 910 MPa. It is three to four times
∗ Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (J. Chen). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.03.019
more abrasion resistant than high purity alumina [9,10]. In addition, their mechanical properties are known to depend strongly on their microstructure [2]. With the development of nanoscience and nanotechnology, the interest in the preparation of ultra-structured coatings is growing, since they have improved mechanical properties and might find promising application in engineering [3,11–18]. There are usually two ways for transporting feedstock in the preparation of ultra-structured coatings. One is to use agglomerated ultra-structured powders, which is usually very expensive in terms of the feedstock cost. Another is to fuse and crush the agglomerated ultra-structured particles, which is the so-called crushing sintered and is less expensive in terms of the feedstock cost. Noticing that although the microstructures and properties of plasma sprayed ultra-structured alumina or zircon coatings have been extensively dealt with in many reports, only limited researches have been published on ultra-structured Al2 O3 –ZrO2 composite coatings, and in particular, few reports are currently available on the preparation of ultra-structured Al2 O3 –ZrO2 composite coatings by
X. Zhao et al. / Wear 265 (2008) 1642–1648
1643
Fig. 1. Microscopic of the NZTA feedstock particles observed at low magnification (a), and at high magnification (b), and the MZTA feedstock particles observed at low magnification (c).
plasma spraying based on crushing sintered method and on the microstructure and properties of the composite coatings as well. Therefore, in the present study, both ultra-fine and coarse powders of Al2 O3 –40 wt.% ZrO2 were used as the starting materials for the feedstocks to prepare ultra-structured and micro-structured Al2 O3 –40 wt.% ZrO2 composite coatings by atmospheric plasma spraying. The microstructures, hardness and friction and wear behaviors of the composite coatings were comparatively investigated.
in the presence of ultrasonic stirring to allow the formation of slurry. Organic binder was then added into the mixed slurry in order to improve the bond force of ultra-fine grains. The content of the organic binder was about 4%. The slurry was then dehydrated, pressed, sintered and porphyrezed. The optimized parameters of
2. Experimental 2.1. Preparation of NZTA and MZTA composite coatings Commercial ultra-fine Al2 O3 powders with a mean diameter of 150 nm and ZrO2 powders with a mean diameter of 50 nm were used as the raw materials and mixed at a weight ratio of 60:40. The ultra-fine Al2 O3 and ZrO2 powders were wet-mixed and milled for 8 h in a ball mill and homogeneously dispersed in distilled water Table 1 Parameters for plasma spray Parameter
Value
Arc current (A) Voltage (V) Argon gas flow rate (SLPM) Hydrogen gas flow rate (SLPM) Carrier gas Ar (SLPM) Feedstock feed rate (g min−1 ) Spray distance (mm) Spray angle (◦ )
630 67 40 10 3.5 28 120 90
Fig. 2. XRD diffraction patterns of the NZTA and MZTA powders and coatings.
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Fig. 3. SEM images of the cross-section of the as-sprayed MZTA (a) and NZTA coatings (b).
crushing sintered process were pressing 250 MPa for 10 min and sintering temperature 1100 ◦ C for 2 h. At last, the powders were sifted by the sieves of 200 meshes and 800 meshes before being plasma sprayed; the obtained size distribution was about in the range of −75 + 10 m, which was expected to have a good flowability. The coarse powders of Al2 O3 and ZrO2 powders were also blended at weight ratio of 60:40, and then the mixed powders were ball milled for 12 h. The obtained size distribution was about −75 + 45 m. Prior to spraying, a GS-943 sand blasting machine (Beijing Changkong Sand Blasting Equipment Co., Ltd., China) was used to
obtain a rough surface so as to readily facilitate the plasma spraying process. The parameters about grit blasting were as follows: aluminum oxide grit (80–120 m), blasting distance 80–100 mm, angle 90◦ and pressure 0.4–0.7 MPa. The surface roughness of the results disk was about 8.10 ± 0.78 m Ra . The Metco A-2000 atmospheric plasma spraying system with a F4-MB plasma gun (Sulzer Metco AG, Switzerland) was used to deposit the ZTA coatings on 1Cr18Ni9Ti stainless-steel disk substrates (24 mm × 7.5 mm). The Twin-System 10-C (Plasma-Technick AG, Switzerland) was used for powder feeding. The powder-injected position locates out of the gun. Ar and H2 were used as primary and the sec-
Fig. 4. SEM micrographs of the fracture surface of the NZTA coatings at low magnification (a), and at high magnification (b), and the fracture surface of the MZTA coatings observed at low magnification (c) and at high magnification (d).
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3. Results and discussion 3.1. Characterization of the mixed powders and corresponding composite coatings
Fig. 5. Steady friction coefficients of the NZTA and MZTA coatings as a function of applied loads.
ondary plasma gases, respectively, and N2 was used as powder carrier gas. The thermal efficiencies the plasma spray is about 40–60%. The plasma spraying parameters are summarized in Table 1.
Fig. 1 shows the SEM morphologies of ultra-fine and coarse Al2 O3 –40 wt.% ZrO2 powders. It is seen that the agglomerated ultra-fine powders had granular shape and a size of 10–75 m (Fig. 1a). A high magnification SEM picture (Fig. 1b) demonstrated that the grain in the ultra-fine granules had a size of about 200 nm and the ultra-fine grains were closely connected among each other, which could be due to sintering effect and would contribute to improve the strength of the plasma sprayed coatings [20]. Besides, the SEM picture of the coarse Al2 O3 –40 wt.% ZrO2 powders shown in Fig. 1c indicated that they had an irregular blocky shape, and their size is ranged from 45 m to 75 m. Fig. 2 shows the XRD patterns of both the ultra-fine and the coarse starting powders and the corresponding as-sprayed coatings. It is seen that the reorganized ultra-fine powders were composed of ␣-Al2 O3 and tetragonal ZrO2 and the coarse powders were composed of ␣-Al2 O3 , monoclinic ZrO2 and tetragonal ZrO2 . The structure changes after depositing to coatings. It can be found that ␥-Al2 O3 was presented in the NZTA and MZTA coatings. The formation of meta-stable of ␥ (cubic) is due to the heat condition during the cooling process. Furthermore, the monoclinic
2.2. Friction and wear test The friction and wear tests were carried out on an optimol SRV oscillating friction and wear tester. Thus the steel disks deposited with the composite coatings were carefully polished to an average surface roughness of about 0.4 ± 0.1 m Ra and driven to slide against commercially obtained upper stationary stainlesssteel balls (1Cr18Ni9Ti 10 mm) in a ball-on-disk configuration. The reciprocal sliding was at an amplitude of 1 mm, normal loads of 20–60 N, a frequency of 10 Hz (corresponding to a linear velocity of 0.02 m/s), and for a duration of 20 min. All the test runs were carried without lubrication in laboratory. After the tests, the wear volume of each coating specimen was measured using an micro-XAM surface mapping microscope, and that of the counterpart steel ball was calculated from the wear scar diameter determined using an optical microscope. The wear rates of the two types of composite coatings and the counterpart steel balls were obtained from dividing the wear volumes by the applied load and sliding distance. All the experiments results are the average value of three tests, which were plotted with error bars to evaluate the scatter and repeatability of the results. The details about the friction and wear tester are given elsewhere [19].
2.3. Characterization of coatings The microhardness of the two types of ZTA coatings were measured using an MH-5-VH microhardness tester at a load of 200 g and for a loading duration of 15 s. Microhardness values were calculated by taking average of 10 measurements at each distance. The microstructures and worn surfaces morphologies of the coatings were analyzed using a JSM-5600LV scanning electron microscope (SEM) coupled with an energy dispersive X-ray analyzer (EDXA). The phase compositions of the coatings were determined using a Philips X’Pert MPD X-ray diffractometer (XRD) operating with Cu K␣ radiation at a potential of 40 kV and current of 20 mA.
Fig. 6. Wear rates of the NZTA and MZTA coatings (a) and their counterparts (b) as a function of applied loads.
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Fig. 7. SEM images of the NZTA and MZTA coatings at different loads: (a) NZTA coating at 20 N, (b) NZTA coating at 60 N, (c) MZTA coating at 20 N and (d) MZTA coating at 60 N.
ZrO2 was detected in the NZTA coating and the monoclinic ZrO2 was enhanced in the MZTA coating, while tetragonal ZrO2 was weakened. This could be rationally understood since plasma spraying could generate a high-rate cooling function, the tetragonal to monoclinic transformation occurs upon this process [16]. For ultra-fine powders have higher surface activity and surface energy than that of the coarse powders, phase deformation would be more readily occurred during high-rate cooling of plasma spraying [3]. The cross-sectional micrographs of the as-sprayed NZTA coating and MZTA coating are shown in Fig. 3, where the bimodal nature of the NZTA and MZTA coatings was observable. Namely, aside from the microcracks and pores, fully melted and partially melted areas were also observed in the coatings, which have been reported to be typical for all air plasma sprayed coatings [21]. Moreover, the NZTA coating had a larger fully melted area than that of the MZTA coating, which could be closely related to the higher surface activity and liability to be melted of the ultra-fine powders. The SEM morphologies of the fractured surfaces of the NZTA coating and MZTA coating shown in Fig. 4 indicates that both coatings contained typical splat lamellae. It is generally accepted that during plasma spraying, molten powders spread and solidify to form splat lamellae. Besides, ultra-fine grains were observed in the NZTA coating, and the high magnification micrograph of the equiaxle grain region showed that the equiaxle grains had a size of less than 100–300 nm. Combining the SEM observations of the powders and coatings, it could be inferred that ultra-fine grains in the NZTA coating were originated from the un-molten or partially melted ultra-fine raw materials powders.
3.2. Friction and wear behaviors of the composite coatings Fig. 5 shows the variations of the friction coefficients as functions of load for both NZTA coating and MZTA coating sliding against stainless-steel ball. The friction coefficients were obtained after 20 min wear testing, when the friction coefficients were stable. Generally speaking, the friction coefficient decreased with increasing load, which is attributed to the slow increase of actual contact area of the sliding couples. The NTA coating presents lower friction coefficient than that of the MZTA coating under 30 N, 40 N and 60 N. Furthermore, the friction coefficients of the NTA coating have smaller deviation than that of the MZTA coating. The wear rates of the NZTA and MZTA coatings and that of the corresponding steel ball counterpart as functions of load are given in Fig. 6. It is seen that the wear rates of the two types of ZTA composite coatings had a magnitude of 10−5 mm3 /(N m) and roughly increased with increasing load (Fig. 6a). And in particular, the wear rate of the NZTA coating was lower, by about 18–46% depending on the load, than that of the MZTA coating, indicating that the NZTA coating had better wear resistance than the MZTA coating. Besides, the stainless-steel ball sliding against the NZTA coating recorded a lower wear rate, by about 40% to 50%, as compared with sliding against the MZTA coating (Fig. 6b). Thus it could be concluded that the plasma sprayed NZTA composite coating composed of ultra-fine grains had improved wear resistance, which helped to effectively reduce the wear rate of the counterpart stainless-steel as well. Fig. 7 shows the SEM morphologies of the worn surfaces of NZTA and MZTA coatings sliding against the stainless-steel counterpart at a load of 20 N and 60 N, respectively. Spalling pits and signs of
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Fig. 8. Wear debris of the NZTA coating at 20 N (a), 60 N (b), and EDS analytical results for (a) and (b), corresponding to (c) and (d).
detachment were visible on the worn surfaces of the two types of ZTA composite coatings, and no substantial difference was observed in terms of the worn surfaces features, except that the spalling pits on the worn surface of the MZTA coating at a higher load were larger in size, corresponding to the higher wear rate as compared with the NZTA coating. Thus it was supposed that both the NZTA and MZTA coatings coupled with the stainless-steel were dominated by spalling which was induced by inter-layer cracks. Because the MZTA coating had poorer inter-splats bonding than the NZTA coating (Fig. 4a and c), the former would be more liable to delamination and fracture along the splat boundaries where cracks developed preferentially even under low loads [17], resulting an increased wear rate. In other words, aside from the grain size, microstructure, and
mechanical properties and so on [17,18,22], the better wear resistance of the plasma sprayed NZTA coating was largely attributed to its better inter-splats bonding. In addition, the microhardness could also play an important role in governing the friction and wear behaviors of the ceramic-matrix composite coatings [22]. The NZTA coating has a Vikers hardness of 887 ± 133 kg/mm2 , which is The microhardness approximately 1.1 times as large as that of MZTA coating (811 ± 169 kg/mm2 ). This might account for as well, to some extent, the better wear resistance of the NZTA coating with a higher microhardness than the MZTA coating with a lower microhardness. Fig. 8 shows the SEM micrographs of the wear debris of the NZTA coating matched with the stainless-steel counterpart at a load of 20 N and 60 N, respectively. It could be worth pointing out
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that the delamination-flake-like wear debris was more or less flattened in terms of the shape, indicating that the wear process was mainly dominated by delamination and brittle fracture (Fig. 8a and b). And interestingly, the wear debris generated at 20 N had a size of 10–100 m, smaller than 50–120 m, that of the wear debris generated at 60 N, and the Fe/Al (or Zr) atomic ratio of the wear debris at 60 N was lower than that of the wear debris at 20 N (Fig. 8c and d), which indicates that the delamination and fracture of the composite coating was enhanced at a higher load. The wear debris of the MZTA coating under the same test conditions had similar features as that of the NZTA coating. Therefore, it could be concluded, in combination with the SEM analyses of the worn surfaces, that the main wear mechanisms for the NZTA and MZTA coating were spalling and fracture caused by inter-layer cracks. 4. Conclusions From the above, the following conclusions can be drawn: (1) It was feasible to prepare ultra-fine Al2 O3 –40 wt.% ZrO2 powders for plasma spraying using crushing sintered. The resulting ultra-fine mixed powders composed of alpha alumina phase and tetragonal zirconia phase could be readily used to prepare plasma sprayed NZTA composite coating containing splat lamellae and ultra-fine grains. And the monoclinic zirconia phase presented in the as-sprayed ultra-fine composite coating after the plasma spraying. (2) The NZTA coating had better wear resistance than the MZTA coating, which could be mainly attributed to the better intersplats bonding and higher hardness of the former than the latter, which also contributed to reduce the wear rate of the counterpart stainless-steel. (3) Both NZTA and MZTA coatings were dominated by spalling and inter-layer fracture as they slide against the stainless-steel counterpart under dry sliding condition. And the MZTA coating experienced more severe spalling and fracture at a higher load than the NZTA coating, well corresponding to the difference in the wear resistance of the two types of composite coatings. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 50421502) and the 973 Project (Grant No.
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