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TiO2 nanostructured ceramic composite coatings prepared by plasma spraying
Journal of Alloys and Compounds 544 (2012) 13–18
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure and properties of Al2O3/TiO2 nanostructured ceramic composite coatings prepared by plasma spraying R.G. Song ⇑, C. Wang, Y. Jiang, H. Li, G. Lu, Z.X. Wang School of Materials Science and Engineering, Changzhou University, Changzhou, 213164 Jiangsu Province, China Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou, 213164 Jiangsu Province, China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 5 July 2012 Accepted 5 July 2012 Available online 21 July 2012 Keywords: Nanostructured ceramic composite coating Plasma spraying Microstructure Property
a b s t r a c t Al2O3/TiO2 nanostructured ceramic composite coatings were prepared on 6063 aluminum alloy substrates by plasma spraying in this paper. The microstructure and properties such as micro-hardness, wear resistance and corrosion resistance were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness tester, frictional wear testing machine and salt spray test. The results showed that both a-Al2O3 and c-Al2O3 phases exist in the as-sprayed nanostructured ceramic composite coatings, and the content of c-Al2O3 phase increases with increasing the spraying power, while rutile TiO2 phase exists in the coatings. The as-sprayed coatings were composed of bimodal structure, i.e., completely melted lamellar and partially melted particulate structure. Effects of both plasma spraying power and TiO2 content on hardness and wear resistance as well as corrosion resistance of the as-sprayed coatings were obvious. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Plasma sprayed ceramic coatings are being developed with outstanding properties such as high hardness, excellent wear, corrosion, chemical, and thermal resistance [1–6]. However, its application is limited due to its brittleness and poor machining property. The unique properties of nanostructured materials (in general referring to a grain size smaller than 100 nm) have been reported for both bulk materials and coatings [7–11]. Plasma sprayed nanostructured ceramic coatings, by taking advantage of properties associated with nanostructures, can improve the performance and durability of conventional plasma sprayed coatings that already have a wide variety of applications in the aerospace, biomedical, automobile and chemical industries [12–16]. It has been previously demonstrated that many outstanding properties such as improved abrasive and sliding wear resistance, adhesion strength, spallation resistance during bend- and cup-tests, and indentation crack resistance can be obtained by the controlled deposition of nanostructured Al2O3–13 wt.%TiO2 agglomerates using plasma spraying technique [17,18]. Nevertheless, there have been less research works on the effect of TiO2 content on the properties of Al2O3/TiO2 nanostructured ceramic composite coatings prepared by plasma spraying technique until
⇑ Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou, 213164 Jiangsu Province, China. Tel.: +86 519 86330069. E-mail address: [email protected] (R.G. Song). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.032
now. The objective of the present work was to study the microstructure and properties of plasma-sprayed Al2O3/TiO2 nanostructured ceramic composite coatings on 6063 aluminum alloy systematically.
2. Experimental The substrate material used in this study was 6063 aluminum alloy with the chemical composition as shown in Table 1. The specimens of 60 40 3 mm used for plasma spraying were cut from 6063 aluminum alloy sheet. All the specimens were thoroughly cleaned in CCl4 and sand-blasted prior to plasma spraying. The nanostructured Al2O3 (a-Al2O3) and TiO2 (rutile) powders employed in this study were obtained from Shanghai Zhuerna High-tech Powder Materials Co. Ltd. The powders had a mean diameter of 60 and 10 nm, respectively. First of all, the nanostructured powders were dispersed by ultrasonic vibration to avoid nanoparticles agglomerating. These powders were then blended with polyvinyl alcohol aqueous solution to produce a powder mixture with composition of Al2O3– 3 wt.%TiO2 (abbreviated to AT3 hereafter), Al2O3–13 wt.%TiO2 (abbreviated to AT13 hereafter), Al2O3–20 wt.%TiO2 (abbreviated to AT20 hereafter), Al2O3– 40 wt.%TiO2 (abbreviated to AT40 hereafter), respectively. Finally, the mixed powders were reconstituted to form agglomerates (see Fig. 1) that were large enough (40–60 lm) for plasma spraying deposition. Fig. 1 showed that the reconstituted agglomerates have a spherical morphology. The process of reconstitution consists of spray drying the slurry that contains nano-Al2O3 and nano-TiO2 particles, and subsequent heat treatment at high temperatures (800 °C) for 1 h. Plasma spraying was performed on a DH-1080 plasma-spraying machine made in China. AT3, AT13, AT20 and AT40 agglomerates were sprayed onto the surface of 6063 aluminum alloy, respectively. The thickness of ceramic coatings was about 0.15 mm. The detailed spraying parameters are listed in Table 2. The coatings were examined by X-ray diffraction (XRD) to identify the phase present. A Thermo ARL X’TRA X-ray diffractometer operated at 40 kV, 40 mA with Cu Ka radiation was employed. The morphology of the coating surface was characterized by scanning electron microscopy (SEM) (Hitachi S-4700).
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R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
Table 1 Chemical composition of 6063 aluminum alloy (wt.%). Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.20.6
0.35
0.1
0.1
0.450.9
0.1
0.1
0.1
Bal.
Fig. 1. SEM micrographs of AT13 reconstituted agglomerates.
Table 2 Plasma spraying parameters. Spraying power (kW)
Ar gas flow rate (l/min)
N2 gas pressure (Mpa)
Spraying distance (mm)
20 25 30 35
120 120 120 120
0.6 0.6 0.6 0.6
70 70 70 70
The microhardness of the coatings and substrate was measured using a Vickers hardness tester under a load of 0.2 kg. The wear resistance of the coatings and substrate were examined on a HT-500 ball-on-disc tribometer in open air. The coating or substrate serves as the disc, and the paired part is Si3N4 ball (3 mm in diameter). The tests were carried out at a normal load of 4.9 N and a sliding velocity of 0.12 m s 1 until the test time reached 0.5 h. The wear weight loss was measured using a precision electronic balance with an accuracy of 1 10 4 g. The wear resistance is evaluated by the wear weight loss, i.e., the smaller the wear weight loss, the better the wear resistance. Three specimens were tested for the average value. The salt spray tests were performed to evaluate the corrosion resistance of the coatings on a LYW-015 salt spray tester using a standard salt fog cabinet with 5% NaCl solution, the pH level of the solution was adjusted to 6.5–7.2 by adding HCl or NaOH solution. The temperature of the solution and cabinet was maintained at 35 °C and the tests lasted for 120 h. The corrosion surfaces of the coatings after salt spray tests were observed by an optical microscopy.
flames. Simultaneously, high speed flames were too fast to melt a part of nanoparticles completely and reached the surface of the substrates, hence forming the partially melted region consisted of nanoparticles. Therefore, the as-sprayed nanostructured ceramic composite coatings were composed of lamellar structure and partially unmelted nanoparticles, i.e., dual microstructure [19]. In the case of 20 kW spraying power, a large amount of powders could not be melted or melted partially during flying due to the relative low temperature plasma flames, leading to much partially melted region in the as-sprayed coatings. In addition, a flat structure could not be formed by partially melted particles striking the surface of the substrates during the process of plasma spraying, while a prominent structure was formed. The porosity is easily to be formed in such a structure (see Fig. 2a). The temperature of plasma flames increases with increasing the spraying power, and then the melted particles increase. The micron-sized lamellar structure was formed when the complete melted droplets struck the substrates. Therefore, the lamellar structure increased while partially melted region decreased in the as-sprayed coatings with increasing the spraying power. In the case of 25 and 30 kW spraying power (see Fig. 2b and c), the lamellar structure increased but there existed still a part of unmelted or partially melted particles in the coating. In the case of 35 kW spraying power (see Fig. 2d), there were almost only the lamellar structure in the coating. This fact suggested that the micron-sized reconstituted agglomerates were completely melted during the spraying process. However, there were still some nano-sized particles embedded in the lamellar melted structure. This is because the cooling rate was too rapid for the melted nanoparticles to grow during the process of solidification. To summary, the density of the coatings increased first and then decreased with increasing the spraying power, and the porosities in the coating prepared under 25 kW were the least among all the coatings. Fig. 3 shows the surface morphologies of nanostructured ceramic composite coatings with various TiO2 content on 6063 aluminum alloy substrates under 30 kW spraying power. It can be seen that the roughness and porosity of the coatings decrease while the density of the coatings increases with increasing the TiO2 content. TiO2 was melted better than Al2O3 during spraying due to the melting point of TiO2 being lower than that of Al2O3, and the cohesion of TiO2 was also higher than that of Al2O3. As a result, the density of the coating increased while the porosity decreased. XRD analysis of AT13 nanostructured ceramic composite coatings under various spraying power, as shown in Fig. 4, revealed that most of a-Al2O3 transformed to c-Al2O3 while TiO2 was still rutile after plasma spraying. The diffraction peak intensity of a-Al2O3 decreased obviously whereas the diffraction peak intensity of c-Al2O3 increased dramatically with increasing the spraying power, which suggested that the content of c-Al2O3 increased with increasing the spraying power. This is because the transformation of a-Al2O3 to c-Al2O3 occurred easily at high temperature.
3. Results and discussion
3.2. Properties of as-sprayed coatings
3.1. Microstructure of as-sprayed coatings
The microhardness of plasma sprayed Al2O3/TiO2 nanostructured ceramic composite coatings is listed in Table 3. It is indicated that the microhardness of the coatings increased first and then decreased with increasing the spraying power except AT40 coatings. This is because the micron-sized lamellar structure increased and partially melted region decreased with increasing the spraying power, which induced the density of the coatings increased first and then decreased (see Fig. 2), hence resulting in the microhardness of the coating increasing first and then decreasing. It is worth noticing that the microhardness of Al2O3/TiO2 nanostructured ceramic composite coatings is much higher than that of plasma sprayed conventional Al2O3/TiO2 micron-sized ceramic composite coatings reported in the literature [20].
Fig. 2 shows the surface morphologies of AT13 nanostructured ceramic composite coatings on 6063 aluminum alloy substrates under various spraying powers. It is showed that there exists mainly micron-sized lamellar structure in the as-sprayed nanostructured ceramic composite coatings, and a large amount of partially melted micron-sized particles consisted of nanoparticles embedded in a lamellar melted structure. During the process of plasma spraying, a large amount of nanoparticles were melted because of the high temperature of flames (more than 104 K). These melting droplets struck the surface of the substrates to form micron-sized lamellar structure under the action of high speed
R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
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Fig. 2. Morphologies of AT13 nanostructured ceramic composite coatings under various spraying powers: (a) 20 kW; (b) 25 kW; (c) 30 kW; (d) 35 kW.
Fig. 3. Morphologies of nanostructured ceramic composite coatings with various TiO2 content: (a) AT3; (b) AT13; (c) AT20; (d) AT40.
In the process of microhardness measurement for AT13 coatings, it was found that there existed a small amount of hard particles, and its microhardness might be HV0.2 4000 in the case of 25 kW spraying power. However, there were no hard particles in the case of 35 kW spraying power. It should be pointed out that we could not identify what the hard particles are at the moment, therefore, further investigations are needed to clarify this. The microhardness did not change obviously when the spraying power was between 20 and 25 kW. These facts implied that the density of
the coatings changed with changing the spraying power, and the range influenced by spraying power was mainly between 25 and 35 kW. When the spraying power was 30 kW, the microhardness reached a maximum value of HV0.2 3567. The microhardness of AT20 coatings reached a maximum value when the spraying power was 25 kW, which was different from AT13 coatings. This is because the content of TiO2 in AT20 coatings was higher than that in AT13. Since the melting point of TiO2 was lower than that of Al2O3, it was easier to be melted. The partially
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R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
Fig. 5. Wear weight loss and friction coefficient of AT13 nanostructured ceramic composite coatings under various spraying power. Fig. 4. X-ray diffraction patterns of AT13 nanostructured ceramic composite coatings under various spraying power.
Table 3 The microhardness of Al2O3/TiO2 nanostructured ceramic composite coatings. Specimens
AT3 AT13 AT20 AT40
Hardness (HV0.2) 20 kW
25 kW
30 kW
35 kW
865 1134 1506 2920
1037 1265 3502 1483
1119 3567 3045 2163
1031 1483 2930 2957
melted region in AT20 coatings was less than those in AT13 coatings under the same spraying power, therefore, the spraying power for AT20 coatings to reach maximum microhardness value was lower than that for AT13 coatings. The change tendency of the microhardness of AT40 coatings with the spraying power was much different from AT3, AT13 and AT20 coatings. The content of TiO2 in AT40 coatings was far higher than those in AT3, AT13 and AT20 coatings, so the temperature for AT40 nanostructured powder melted completely during spraying was much lower than AT3, AT13 and AT20, i.e., the corresponding spraying power for AT40 was lower than the latter three. As a result, a suitable ratio of partially melted region to completely melted region for AT40 coatings was obtained when the spraying power was about 20 kW. In the case of 25 kW spraying power, the microhardness decreased obviously because the partially melted region was little and the hardness of TiO2 was lower than that of Al2O3. Nevertheless, there was almost no partially melted region in the coatings when the spraying power increased further, but a large amount nanoparticles embedded in the lamellar structure (see Fig. 3d), hence the microhardness increasing again when the spraying power were 30 and 35 kW. Fig. 5 shows the wear weight loss and friction coefficient of AT13 nanostructured ceramic composite coatings under various spraying power. In is indicated that the friction coefficient of the coatings decreased obviously with increasing the spraying power when it was lower than 30 kW, which implied that the anti-friction quality of the coatings increased dramatically with increasing the spraying power. However, the friction coefficient of the coatings did not change obviously when the spraying power was higher than 30 kW, i.e., the anti-friction quality of the coatings almost did not change. Simultaneously, it is also indicated that the wear weight loss of the coatings decreased first and then increased with increasing the spraying power, that is, the wear resistance of the
Fig. 6. Wear weight loss and friction coefficient of nanostructured ceramic composite coatings with various TiO2 content under 30 kW spraying power.
coatings increased first and then decreased with increasing the spraying power. This is essentially in agreement with the relationship between the microhardness and spraying power. In addition, it should be pointed out that the weight losses of the AT13 coatings prepared at 25 kW and 30 kW were almost the same although their microhardnesses were much different. This is because there existed a small amount of hard particles of which hardness might be HV0.2 4000 in the case of 25 kW power. Although the average microhardness was relative lower, such hard particles formed by partially melted nanoparticles may improve the wear resistance of the coatings greatly. Thus it can be seen that the wear resistance of the coatings was optimal when the spraying power was between 25 and 30 kW. Fig. 6 shows the wear weight loss and friction coefficient of nanostructured ceramic composite coatings with various TiO2 content under 30 kW spraying power. It is showed that the wear weight loss decreased obviously first and then almost did not change with increasing the TiO2 content. This is because the amount of TiO2 nanoparticles in the as-sprayed coatings increased with increasing the TiO2 content, the partially melted region cohered with the lamellar structure more compact, hence resulting in increasing the wear resistance. Moreover, although the microhardness of AT20 coating was higher than that of AT40 coating (see Table 3), the friction coefficient of AT40 coating was lower than
R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
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Fig. 7. Photographs of the corrosion surfaces of AT20 nanostructured ceramic composite coatings prepared under various spraying power: (a) 20 kW; (b) 25 kW; (c) 30 kW; (d) 35 kW.
Fig. 8. Photographs of the corrosion surfaces of nanostructured ceramic composite coatings with various TiO2 content: (a) AT3; (b) AT13; (c) AT20; (d) AT40.
that of AT20 coating, i.e., the anti-friction quality of AT40 coating was better than that of AT20 coating, thus resulting in the wear weight loss of the two coatings were almost the same. Fig. 7 shows the photographs of the corrosion surfaces of AT20 coatings prepared under various spraying power. It is indicated that pits initiated on the surfaces of all coatings, however, the number and depth of the pits on the surface of the coating prepared under 25 kW spraying power is the least among all the coatings. The reason maybe that the porosities in the coating
prepared under 25 kW were the least among all the coatings (see Fig. 2b). Fig. 8 shows the photographs of the corrosion surfaces of nanostructured ceramic composite coatings with various TiO2 content prepared under 30 kW spraying power. It is showed that pits appear on the surfaces of all coatings, and the number and depth of pits decrease with increasing the TiO2 content, i.e., the corrosion resistance of the coatings increases with increasing the TiO2 content.
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R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
4. Conclusions (1) Plasma sprayed A12O3/TiO2 nanostructured ceramic composite coatings are mainly consisted of c-Al2O3, a-A12O3, and rutile TiO2 phases, and the content of c-Al2O3 increased with increasing the spraying power. (2) A12O3/TiO2 nanostructured ceramic composite coatings are composed of bimodal structure, i.e., completely melted lamellar and partially melted particulate structure. The completely melted structure increases while the partially melted structure decreases with increasing the spraying power. (3) The microhardness and wear resistance of the as-sprayed coatings increase first and then decrease with increasing the spraying power. (4) The effects of TiO2 content on the microhardness and wear resistance of the as-sprayed coatings are obvious, but the effect tendency is complex. (5) The corrosion resistance of the as-sprayed coatings is influenced by both spraying power and TiO2 content dramatically.
Acknowledgments The financial aids of A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
(PAPD) and A Project Funded by Science and Technology Bureau of Changzhou City under Grant No. CE20110092 are gratefully acknowledged. References [1] R.G. Song, Surf. Coat. Technol. 168 (2003) 191. [2] H.K. Seok, E.Y. Choi, P.R. Cha, et al., Surf. Coat. Technol. 205 (2011) 3341. [3] L. Gonzalez-Fernandez, L. Del Campo, R.B. Perez-Saez, et al., J. Alloy. Compd. 513 (2012) 101. [4] L. Shaw, D. Goberman, R. Ren, et al., Surf. Coat. Technol. 130 (2000) 1. [5] D.A. Stewart, P.H. Chipway, D.G. McCartney, Acta Mater. 48 (2000) 1593. [6] Y. Wang, S. Jiang, M. Wang, et al., Wear 237 (2000) 176. [7] C.G. Li, Y. Wang, S. Wang, et al., J. Alloy. Compd. 503 (2010) 127. [8] M.E. Mendoza, I.G. Solorzano, E.A. Brocchi, Mater. Sci. Eng. A 544 (2012) 21. [9] H. Chai, D. Josell, Thin Sol. Film. 519 (2010) 331. [10] H. Gleiter, Nanostruct. Mater. 1 (1992) 1. [11] A.S. Hamdy, Mater. Lett. 60 (2006) 2633. [12] V.P. Singh, A. Sil, R. Jayaganthan, Trans. Indian Inst. Met. 65 (2012) 1. [13] X.G. Chen, Y. Yang, D.R. Yan, J. Mater. Sci. 46 (2011) 7369. [14] Y. Yang, D.R. Yan, Y.C. Dong, et al., J. Alloy. Compd. 509 (2011) L90. [15] F. Tarasi, M. Medraj, A. Dolatabadi, et al., Surf. Coat. Technol. 205 (2011) 5437. [16] C.G. Li, Y. Wang, L.X. Guo, et al., J. Alloy. Compd. 506 (2010) 356. [17] E.H. Jordan, M. Gell, Y.H. Sohn, et al., Mater. Sci. Eng. A 301 (2001) 80. [18] B.H. Kear, Z. Kalman, R.K. Sadangi, J. Thermal Spray Technol. 9 (2000) 483. [19] D. Goberman, Y.H. Sohn, L. Shaw, et al., Acta Mater. 50 (2002) 1141. [20] X. Zhou, G.G. Wang, D.B. Sun, Mater. Prot. 41 (2008) 7.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure and properties of Al2O3/TiO2 nanostructured ceramic composite coatings prepared by plasma spraying R.G. Song ⇑, C. Wang, Y. Jiang, H. Li, G. Lu, Z.X. Wang School of Materials Science and Engineering, Changzhou University, Changzhou, 213164 Jiangsu Province, China Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou, 213164 Jiangsu Province, China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 5 July 2012 Accepted 5 July 2012 Available online 21 July 2012 Keywords: Nanostructured ceramic composite coating Plasma spraying Microstructure Property
a b s t r a c t Al2O3/TiO2 nanostructured ceramic composite coatings were prepared on 6063 aluminum alloy substrates by plasma spraying in this paper. The microstructure and properties such as micro-hardness, wear resistance and corrosion resistance were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness tester, frictional wear testing machine and salt spray test. The results showed that both a-Al2O3 and c-Al2O3 phases exist in the as-sprayed nanostructured ceramic composite coatings, and the content of c-Al2O3 phase increases with increasing the spraying power, while rutile TiO2 phase exists in the coatings. The as-sprayed coatings were composed of bimodal structure, i.e., completely melted lamellar and partially melted particulate structure. Effects of both plasma spraying power and TiO2 content on hardness and wear resistance as well as corrosion resistance of the as-sprayed coatings were obvious. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Plasma sprayed ceramic coatings are being developed with outstanding properties such as high hardness, excellent wear, corrosion, chemical, and thermal resistance [1–6]. However, its application is limited due to its brittleness and poor machining property. The unique properties of nanostructured materials (in general referring to a grain size smaller than 100 nm) have been reported for both bulk materials and coatings [7–11]. Plasma sprayed nanostructured ceramic coatings, by taking advantage of properties associated with nanostructures, can improve the performance and durability of conventional plasma sprayed coatings that already have a wide variety of applications in the aerospace, biomedical, automobile and chemical industries [12–16]. It has been previously demonstrated that many outstanding properties such as improved abrasive and sliding wear resistance, adhesion strength, spallation resistance during bend- and cup-tests, and indentation crack resistance can be obtained by the controlled deposition of nanostructured Al2O3–13 wt.%TiO2 agglomerates using plasma spraying technique [17,18]. Nevertheless, there have been less research works on the effect of TiO2 content on the properties of Al2O3/TiO2 nanostructured ceramic composite coatings prepared by plasma spraying technique until
⇑ Corresponding author at: School of Materials Science and Engineering, Changzhou University, Changzhou, 213164 Jiangsu Province, China. Tel.: +86 519 86330069. E-mail address: [email protected] (R.G. Song). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.032
now. The objective of the present work was to study the microstructure and properties of plasma-sprayed Al2O3/TiO2 nanostructured ceramic composite coatings on 6063 aluminum alloy systematically.
2. Experimental The substrate material used in this study was 6063 aluminum alloy with the chemical composition as shown in Table 1. The specimens of 60 40 3 mm used for plasma spraying were cut from 6063 aluminum alloy sheet. All the specimens were thoroughly cleaned in CCl4 and sand-blasted prior to plasma spraying. The nanostructured Al2O3 (a-Al2O3) and TiO2 (rutile) powders employed in this study were obtained from Shanghai Zhuerna High-tech Powder Materials Co. Ltd. The powders had a mean diameter of 60 and 10 nm, respectively. First of all, the nanostructured powders were dispersed by ultrasonic vibration to avoid nanoparticles agglomerating. These powders were then blended with polyvinyl alcohol aqueous solution to produce a powder mixture with composition of Al2O3– 3 wt.%TiO2 (abbreviated to AT3 hereafter), Al2O3–13 wt.%TiO2 (abbreviated to AT13 hereafter), Al2O3–20 wt.%TiO2 (abbreviated to AT20 hereafter), Al2O3– 40 wt.%TiO2 (abbreviated to AT40 hereafter), respectively. Finally, the mixed powders were reconstituted to form agglomerates (see Fig. 1) that were large enough (40–60 lm) for plasma spraying deposition. Fig. 1 showed that the reconstituted agglomerates have a spherical morphology. The process of reconstitution consists of spray drying the slurry that contains nano-Al2O3 and nano-TiO2 particles, and subsequent heat treatment at high temperatures (800 °C) for 1 h. Plasma spraying was performed on a DH-1080 plasma-spraying machine made in China. AT3, AT13, AT20 and AT40 agglomerates were sprayed onto the surface of 6063 aluminum alloy, respectively. The thickness of ceramic coatings was about 0.15 mm. The detailed spraying parameters are listed in Table 2. The coatings were examined by X-ray diffraction (XRD) to identify the phase present. A Thermo ARL X’TRA X-ray diffractometer operated at 40 kV, 40 mA with Cu Ka radiation was employed. The morphology of the coating surface was characterized by scanning electron microscopy (SEM) (Hitachi S-4700).
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R.G. Song et al. / Journal of Alloys and Compounds 544 (2012) 13–18
Table 1 Chemical composition of 6063 aluminum alloy (wt.%). Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
0.20.6
0.35
0.1
0.1
0.450.9
0.1
0.1
0.1
Bal.
Fig. 1. SEM micrographs of AT13 reconstituted agglomerates.
Table 2 Plasma spraying parameters. Spraying power (kW)
Ar gas flow rate (l/min)
N2 gas pressure (Mpa)
Spraying distance (mm)
20 25 30 35
120 120 120 120
0.6 0.6 0.6 0.6
70 70 70 70
The microhardness of the coatings and substrate was measured using a Vickers hardness tester under a load of 0.2 kg. The wear resistance of the coatings and substrate were examined on a HT-500 ball-on-disc tribometer in open air. The coating or substrate serves as the disc, and the paired part is Si3N4 ball (3 mm in diameter). The tests were carried out at a normal load of 4.9 N and a sliding velocity of 0.12 m s 1 until the test time reached 0.5 h. The wear weight loss was measured using a precision electronic balance with an accuracy of 1 10 4 g. The wear resistance is evaluated by the wear weight loss, i.e., the smaller the wear weight loss, the better the wear resistance. Three specimens were tested for the average value. The salt spray tests were performed to evaluate the corrosion resistance of the coatings on a LYW-015 salt spray tester using a standard salt fog cabinet with 5% NaCl solution, the pH level of the solution was adjusted to 6.5–7.2 by adding HCl or NaOH solution. The temperature of the solution and cabinet was maintained at 35 °C and the tests lasted for 120 h. The corrosion surfaces of the coatings after salt spray tests were observed by an optical microscopy.
flames. Simultaneously, high speed flames were too fast to melt a part of nanoparticles completely and reached the surface of the substrates, hence forming the partially melted region consisted of nanoparticles. Therefore, the as-sprayed nanostructured ceramic composite coatings were composed of lamellar structure and partially unmelted nanoparticles, i.e., dual microstructure [19]. In the case of 20 kW spraying power, a large amount of powders could not be melted or melted partially during flying due to the relative low temperature plasma flames, leading to much partially melted region in the as-sprayed coatings. In addition, a flat structure could not be formed by partially melted particles striking the surface of the substrates during the process of plasma spraying, while a prominent structure was formed. The porosity is easily to be formed in such a structure (see Fig. 2a). The temperature of plasma flames increases with increasing the spraying power, and then the melted particles increase. The micron-sized lamellar structure was formed when the complete melted droplets struck the substrates. Therefore, the lamellar structure increased while partially melted region decreased in the as-sprayed coatings with increasing the spraying power. In the case of 25 and 30 kW spraying power (see Fig. 2b and c), the lamellar structure increased but there existed still a part of unmelted or partially melted particles in the coating. In the case of 35 kW spraying power (see Fig. 2d), there were almost only the lamellar structure in the coating. This fact suggested that the micron-sized reconstituted agglomerates were completely melted during the spraying process. However, there were still some nano-sized particles embedded in the lamellar melted structure. This is because the cooling rate was too rapid for the melted nanoparticles to grow during the process of solidification. To summary, the density of the coatings increased first and then decreased with increasing the spraying power, and the porosities in the coating prepared under 25 kW were the least among all the coatings. Fig. 3 shows the surface morphologies of nanostructured ceramic composite coatings with various TiO2 content on 6063 aluminum alloy substrates under 30 kW spraying power. It can be seen that the roughness and porosity of the coatings decrease while the density of the coatings increases with increasing the TiO2 content. TiO2 was melted better than Al2O3 during spraying due to the melting point of TiO2 being lower than that of Al2O3, and the cohesion of TiO2 was also higher than that of Al2O3. As a result, the density of the coating increased while the porosity decreased. XRD analysis of AT13 nanostructured ceramic composite coatings under various spraying power, as shown in Fig. 4, revealed that most of a-Al2O3 transformed to c-Al2O3 while TiO2 was still rutile after plasma spraying. The diffraction peak intensity of a-Al2O3 decreased obviously whereas the diffraction peak intensity of c-Al2O3 increased dramatically with increasing the spraying power, which suggested that the content of c-Al2O3 increased with increasing the spraying power. This is because the transformation of a-Al2O3 to c-Al2O3 occurred easily at high temperature.
3. Results and discussion
3.2. Properties of as-sprayed coatings
3.1. Microstructure of as-sprayed coatings
The microhardness of plasma sprayed Al2O3/TiO2 nanostructured ceramic composite coatings is listed in Table 3. It is indicated that the microhardness of the coatings increased first and then decreased with increasing the spraying power except AT40 coatings. This is because the micron-sized lamellar structure increased and partially melted region decreased with increasing the spraying power, which induced the density of the coatings increased first and then decreased (see Fig. 2), hence resulting in the microhardness of the coating increasing first and then decreasing. It is worth noticing that the microhardness of Al2O3/TiO2 nanostructured ceramic composite coatings is much higher than that of plasma sprayed conventional Al2O3/TiO2 micron-sized ceramic composite coatings reported in the literature [20].
Fig. 2 shows the surface morphologies of AT13 nanostructured ceramic composite coatings on 6063 aluminum alloy substrates under various spraying powers. It is showed that there exists mainly micron-sized lamellar structure in the as-sprayed nanostructured ceramic composite coatings, and a large amount of partially melted micron-sized particles consisted of nanoparticles embedded in a lamellar melted structure. During the process of plasma spraying, a large amount of nanoparticles were melted because of the high temperature of flames (more than 104 K). These melting droplets struck the surface of the substrates to form micron-sized lamellar structure under the action of high speed
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Fig. 2. Morphologies of AT13 nanostructured ceramic composite coatings under various spraying powers: (a) 20 kW; (b) 25 kW; (c) 30 kW; (d) 35 kW.
Fig. 3. Morphologies of nanostructured ceramic composite coatings with various TiO2 content: (a) AT3; (b) AT13; (c) AT20; (d) AT40.
In the process of microhardness measurement for AT13 coatings, it was found that there existed a small amount of hard particles, and its microhardness might be HV0.2 4000 in the case of 25 kW spraying power. However, there were no hard particles in the case of 35 kW spraying power. It should be pointed out that we could not identify what the hard particles are at the moment, therefore, further investigations are needed to clarify this. The microhardness did not change obviously when the spraying power was between 20 and 25 kW. These facts implied that the density of
the coatings changed with changing the spraying power, and the range influenced by spraying power was mainly between 25 and 35 kW. When the spraying power was 30 kW, the microhardness reached a maximum value of HV0.2 3567. The microhardness of AT20 coatings reached a maximum value when the spraying power was 25 kW, which was different from AT13 coatings. This is because the content of TiO2 in AT20 coatings was higher than that in AT13. Since the melting point of TiO2 was lower than that of Al2O3, it was easier to be melted. The partially
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Fig. 5. Wear weight loss and friction coefficient of AT13 nanostructured ceramic composite coatings under various spraying power. Fig. 4. X-ray diffraction patterns of AT13 nanostructured ceramic composite coatings under various spraying power.
Table 3 The microhardness of Al2O3/TiO2 nanostructured ceramic composite coatings. Specimens
AT3 AT13 AT20 AT40
Hardness (HV0.2) 20 kW
25 kW
30 kW
35 kW
865 1134 1506 2920
1037 1265 3502 1483
1119 3567 3045 2163
1031 1483 2930 2957
melted region in AT20 coatings was less than those in AT13 coatings under the same spraying power, therefore, the spraying power for AT20 coatings to reach maximum microhardness value was lower than that for AT13 coatings. The change tendency of the microhardness of AT40 coatings with the spraying power was much different from AT3, AT13 and AT20 coatings. The content of TiO2 in AT40 coatings was far higher than those in AT3, AT13 and AT20 coatings, so the temperature for AT40 nanostructured powder melted completely during spraying was much lower than AT3, AT13 and AT20, i.e., the corresponding spraying power for AT40 was lower than the latter three. As a result, a suitable ratio of partially melted region to completely melted region for AT40 coatings was obtained when the spraying power was about 20 kW. In the case of 25 kW spraying power, the microhardness decreased obviously because the partially melted region was little and the hardness of TiO2 was lower than that of Al2O3. Nevertheless, there was almost no partially melted region in the coatings when the spraying power increased further, but a large amount nanoparticles embedded in the lamellar structure (see Fig. 3d), hence the microhardness increasing again when the spraying power were 30 and 35 kW. Fig. 5 shows the wear weight loss and friction coefficient of AT13 nanostructured ceramic composite coatings under various spraying power. In is indicated that the friction coefficient of the coatings decreased obviously with increasing the spraying power when it was lower than 30 kW, which implied that the anti-friction quality of the coatings increased dramatically with increasing the spraying power. However, the friction coefficient of the coatings did not change obviously when the spraying power was higher than 30 kW, i.e., the anti-friction quality of the coatings almost did not change. Simultaneously, it is also indicated that the wear weight loss of the coatings decreased first and then increased with increasing the spraying power, that is, the wear resistance of the
Fig. 6. Wear weight loss and friction coefficient of nanostructured ceramic composite coatings with various TiO2 content under 30 kW spraying power.
coatings increased first and then decreased with increasing the spraying power. This is essentially in agreement with the relationship between the microhardness and spraying power. In addition, it should be pointed out that the weight losses of the AT13 coatings prepared at 25 kW and 30 kW were almost the same although their microhardnesses were much different. This is because there existed a small amount of hard particles of which hardness might be HV0.2 4000 in the case of 25 kW power. Although the average microhardness was relative lower, such hard particles formed by partially melted nanoparticles may improve the wear resistance of the coatings greatly. Thus it can be seen that the wear resistance of the coatings was optimal when the spraying power was between 25 and 30 kW. Fig. 6 shows the wear weight loss and friction coefficient of nanostructured ceramic composite coatings with various TiO2 content under 30 kW spraying power. It is showed that the wear weight loss decreased obviously first and then almost did not change with increasing the TiO2 content. This is because the amount of TiO2 nanoparticles in the as-sprayed coatings increased with increasing the TiO2 content, the partially melted region cohered with the lamellar structure more compact, hence resulting in increasing the wear resistance. Moreover, although the microhardness of AT20 coating was higher than that of AT40 coating (see Table 3), the friction coefficient of AT40 coating was lower than
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Fig. 7. Photographs of the corrosion surfaces of AT20 nanostructured ceramic composite coatings prepared under various spraying power: (a) 20 kW; (b) 25 kW; (c) 30 kW; (d) 35 kW.
Fig. 8. Photographs of the corrosion surfaces of nanostructured ceramic composite coatings with various TiO2 content: (a) AT3; (b) AT13; (c) AT20; (d) AT40.
that of AT20 coating, i.e., the anti-friction quality of AT40 coating was better than that of AT20 coating, thus resulting in the wear weight loss of the two coatings were almost the same. Fig. 7 shows the photographs of the corrosion surfaces of AT20 coatings prepared under various spraying power. It is indicated that pits initiated on the surfaces of all coatings, however, the number and depth of the pits on the surface of the coating prepared under 25 kW spraying power is the least among all the coatings. The reason maybe that the porosities in the coating
prepared under 25 kW were the least among all the coatings (see Fig. 2b). Fig. 8 shows the photographs of the corrosion surfaces of nanostructured ceramic composite coatings with various TiO2 content prepared under 30 kW spraying power. It is showed that pits appear on the surfaces of all coatings, and the number and depth of pits decrease with increasing the TiO2 content, i.e., the corrosion resistance of the coatings increases with increasing the TiO2 content.
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4. Conclusions (1) Plasma sprayed A12O3/TiO2 nanostructured ceramic composite coatings are mainly consisted of c-Al2O3, a-A12O3, and rutile TiO2 phases, and the content of c-Al2O3 increased with increasing the spraying power. (2) A12O3/TiO2 nanostructured ceramic composite coatings are composed of bimodal structure, i.e., completely melted lamellar and partially melted particulate structure. The completely melted structure increases while the partially melted structure decreases with increasing the spraying power. (3) The microhardness and wear resistance of the as-sprayed coatings increase first and then decrease with increasing the spraying power. (4) The effects of TiO2 content on the microhardness and wear resistance of the as-sprayed coatings are obvious, but the effect tendency is complex. (5) The corrosion resistance of the as-sprayed coatings is influenced by both spraying power and TiO2 content dramatically.
Acknowledgments The financial aids of A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
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