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Effect of heat treatment on high-hardness zirconia coatings formed by gas tunnel type plasma spraying
VAC=1803=Susan=Venkatachala=BG
Vacuum 59 (2000) 194}202
E!ect of heat treatment on high-hardness zirconia coatings formed by gas tunnel type plasma spraying A. Kobayashi *, T. Kitamura Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan College of Industrial Technology, 1-27-1 Nishi-koya, Amagasaki, Hyogo 661-0047, Japan
Abstract By means of gas tunnel-type plasma spraying, a high-hardness zirconia coating could be obtained at a short spraying distance. For example, Vickers hardness of zirconia coating was H "1050 at the spraying distance of ¸"30 mm, when the power input was P"20 kW. This high-hardness coating had a hardness layer near the surface. In this paper, the e!ect of heat treatment on the characteristic of such a high-hardness zirconia coating is investigated, and the in#uence on the Vickers hardness and structure of the coating is discussed. The hardness of this high-hardness layer became lower after heat treatment. The crystal form of the cubic ZrO of powder was transformed into a tetragonal phase through this plasma spraying. After heat treatment, the angular separation between the two tetragonal peaks was broader and clearer. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Zirconia coating; High hardness; Heat treatment; Gas tunnel-type plasma spraying; Short distance spraying; Vickers hardness; X-ray di!raction
1. Introduction Recently, high functional ceramic coatings are applied to many "elds such as electronics and so on. In this context, plasma spray method has recently been noticed for making use of the excellent characteristics of ceramics such as corrosion resistance, thermal resistance, wear resistance and also its density [1,2]. Then, the study on the ceramic sprayed coatings using the newly developed gas tunnel-type plasma spraying had been started by the authors [3]. The mechanism and the fundamental properties of the gas tunnel-type plasma spraying were investigated in the previous studies [4,5]. * Corresponding author. Tel.: #816-6879-8694; fax: #816-6879-8689. E-mail address: [email protected] (A. Kobayashi). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 0 - 0
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And the characteristics of Vickers hardness of ceramic sprayed coatings, which were formed by gas tunnel-type plasma spraying, was investigated in detail. For example, the relations between the spraying distance L and the Vickers hardness H on the cross sections of ceramic (alumina, etc.) sprayed coatings were clari"ed [6]. In the case of alumina coatings, high hardness of 1500 was obtained at spraying distance L"30 mm when P"30 kW [7]. It had a very high-hardness layer near the surface of the coating. In zirconia coating by means of the gas tunnel-type plasma spraying, the Vickers hardness of the cross section was also increased with decreasing spraying distance, and a higher Vickers hardness could be obtained in the case of a shorter spraying distance (¸(¸ ). The critical spraying distance ¸ was about ¸ "40 mm, when P"20 kW. At a short spraying distance of ¸"30 mm, when P"33 kW, the Vickers hardness of zirconia coating was about H "1200 in the case of gas divertor nozzle diameter of d"20 mm [8]. In this case, the Vickers hardness of this sprayed coating became 20}30% higher than that of the conventional plasma spraying. The color of sprayed coating changed under the spraying conditions [9]. The original color of the zirconia powder is yellow. After plasma spraying at P"20 kW, ¸"30 mm, the coating color was gray and/or black, in the case of coating thickness of 100}150 lm. It is thought that the zirconia coating as-sprayed was deoxidized by the heat of plasma and that ZrO or Zr was generated in the coating after the loss of oxygen. This can be con"rmed by the heat treatment of the coating in the air. In this paper, the e!ects of heat treatment on the high-hardness zirconia coating produced at ¸"30 mm, were investigated in detail. The in#uence of thermal process on the Vickers hardness of the zirconia coating was clari"ed. The crystal form of the zirconia coating was discussed from the results by X-ray di!raction method.
2. Experimental procedure The gas tunnel-type plasma spraying apparatus used in this study is shown in Fig. 1. The experimental method to form high-hardness ceramic coatings by means of the gas tunnel-type plasma spraying has been described in the previous papers [4}6]. In this case, the gas divertor nozzle diameter was d"15 mm. The power input to the pilot plasma torch, which was supplied by the power supply, PS-1 was zero after starting of the gas tunnel-type plasma jet. The experimental conditions for this plasma spraying of zirconia powder are shown in Table 1. The power input to the plasma torch was about P"20 kW, and the spraying distance was a short distance of ¸"20}50 mm. Ar gas #ow rate for gas tunnel-type plasma spraying torch was Q"220 l/min, and the powder feed rate was w"18 g/min. The traverse speed of the substrate had a rather low value of about v"100 cm/min. Table 2 shows the chemical composition and the size of zirconia powder used in this study. This zirconia powder was the commercially prepared type of K-90. The measurement of distribution of the Vickers hardness in the cross section of the zirconia coating was carried out at each distance from the coating surface in the thickness direction. The Vickers hardness of the sprayed coatings was measured at the non-pore region in those cross sections under the condition that the load weight was 100 g and its load time was 25 s. The Vickers hardness was calculated as a mean value of 10 point measurements.
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Fig. 1. Gas tunnel-type plasma spraying apparatus used in this study; L: spraying distance. Table 1 Experimental conditions Power input Working gas (Ar) #ow rate Powder feed gas (Ar) Spraying distance Traverse speed Powder feed rate
Table 2 Chemical composition and size of zirconia powder used P"20 kW Q"220 l/min Qen"10 l/min L"30 mm v"96 cm/min w"18 g/min
Composition (wt%)
Size (lm)
ZrO Y O Al O SiO Fe O
90.78 8.15 0.38 0.20 0.11
The X-ray di!raction method was carried out on the surface of the coating. The X-ray source was Co, and the tube voltage was 30 kV and the tube current was 14 mA. For zirconia coatings produced at ¸"30 mm when P"20 kW, the heat treatment was carried out in air in an electric furnace maintaining a temperature of 8003C for 1 h. After heat treatment the specimens were naturally cooled in the furnace. The change of the weight of the coating before and after heat treatment was measured to know the rate of the shortage of oxygen in the zirconia coating. And also, the change of hardness distribution and X-ray di!raction patterns of the coatings before and after heat treatment were measured. 3. Results and discussion 3.1. Vickers hardness of zirconia coating formed by the gas tunnel type plasma spraying Fig. 2 shows the relation between the spraying distance and the Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying.
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Fig. 2. Dependences of Vickers hardness on cross section of zirconia coatings on spraying distance at P"17, 22 kW.
In this case, the spraying condition was as follows: the working gas (Ar) #ow rate was Q"220 l/min, the power input: P"17, 22 kW and the spraying distance: ¸"20}70 mm. This coating was formed by a two times traverse (2 pass sprayed) with the traverse speed of v"96 cm/min. And the powder feed rate had a small value of w"18 g/min in the case of the gas divertor nozzle diameter of d"15 mm. As described in the previous paper, the Vickers hardness of the cross section was increased with decreasing spraying distance, and a higher Vickers hardness was obtained at a short spraying distance of ¸(¸ for each power input. In this case, the critical spraying distance ¸ was about ¸ "35 mm, when P"17 kW, and ¸ "44 mm, when P"22 kW. At a short spraying distance of ¸"30 mm, when P"22kW, the Vickers hardness of zirconia coating was H "1050. While, in the case that the power input was lower value of P"17 kW, the Vickers hardness of the coating became H "1000 at the shortest spraying distance of ¸"20 mm. 3.2. Distribution of Vickers hardness on the cross section of zirconia coating At a short spraying distance of ¸"30 mm, high-hardness zirconia coating of about H "1000 can be obtained by the gas tunnel-type plasma spraying. Fig. 3 shows distribution of Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying at the short spraying distance of ¸"30 mm. The measurement was carried out at each distance from the coating surface in the thickness direction. In this case, the working gas #ow rate was Q"220 l/min, the power input P"20 kW and the spraying distance ¸"30 mm. This coating was formed by a two times traverse (2 pass sprayed).
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Fig. 3. Distribution of Vickers hardness on cross section of zirconia coating sprayed at L"30 mm when P"20 kW.
And the powder feed rate had a small value of w"18 g/min. The coating thickness was 120 lm and the surface was gray. The distribution of Vickers hardness on the cross section of zirconia coating consists of two parabolic curves as shown in Fig. 3. From this distribution, it is found that the Vickers hardness of the coating surface layer (correspondings to the second pass) is the highest value of about H "950 at a distance from the coating surface of l"40 lm. The hardness near the substrate had a lower value of H "600 than that near the coating surface. From the observation of the cross section of this zirconia coating by an optical microscope, the coating structure was the same as described in the former paper. 3.3. Ewect of heat treatment on high hardness zirconia coating The heat treatment was carried out for the as-sprayed coating produced under the condition: the spraying distance of ¸"30 mm, and the traverse speed of about v"100 cm/min. Fig. 4 shows the distribution of Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying, after the heat treatment of the same coating in Fig. 3. In this "gure, the distribution of Vickers hardness of the sprayed coating after the heat treatment consists of two parabolic curves. The hardness near the substrate did not change as compared with the as-sprayed coating in Fig. 3. But the Vickers hardness near the coating surface became much lower than that of the as-sprayed coating. The Vickers hardness of the coating changed from H "950 to 650 through heat treatment. It was assumed that the internal stress in the zirconia coating was relaxed by the heat treatment. In this way, the zirconia coating after heat treatment has a #at distribution of Vickers hardness on the cross section in the thickness direction, whose value is about H "650, and a high-hardness layer does not exists in this zirconia coating.
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Fig. 4. Distribution of Vickers hardness on cross section of zirconia coating after heat treatment, for sprayed coating in Fig. 3.
Fig. 5. Microphotograph of cross section of zirconia coating after heat treatment.
Fig. 5 shows the microphotograph of the cross section of zirconia coating after heat treatment. This had almost the same structure as-sprayed coating produced at the power input: P"20 kW and the spraying distance: ¸"30 mm. In the coating, the pore was a little smaller and the microstructure was rather dense than in the conventional spraying coating. But, there exist some large pores in this coating: the reason is due to the low power input. 3.4. Deoxidization of high-hardness zirconia coating The color of zirconia coating was gray under the spraying condition: P"20 kW, ¸"30 mm, and the thickness was 120 lm. But after heat treatment of this coating, this color changed to light yellow, which is the color of zirconia powder. According to the change in the weight of the coating before and after heat treatment, the shortage of oxygen in the zirconia coating as-sprayed was calculated. The result showed that the
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Fig. 6. X-ray di!raction patterns of zirconia coating. (a): X-ray di!raction pattern for as sprayed coating, and (b): for after heat treatment.
rate of the shortage of oxygen was 1.25 at%. This value was small, but the deoxidization in zirconia coating could be con"rmed and, the rate of the shortage of oxygen was increased to 3 at% in the case of zirconia coating with darker gray. This means that the zirconia coating as-sprayed was deoxidized by the heat of plasma and that ZrO or Zr was generated in the coating after the oxygen loss and the color changed from yellow to gray. This also means that by the heat treatment in the air the coating was oxidized again and the yellow color was restored. 3.5. Structure of high hardness zirconia coating The results obtained by X-ray di!raction patterns on the surface of high-hardness zirconia coating were as follows: Fig. 6 shows the X-ray di!raction patterns of the surface of the coating, which is the same coating as shown in Fig. 3. Here, (a) is an X-ray di!raction pattern for as-sprayed coating; and (b) is that for after heat treatment.
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Fig. 7. Zirconia peaks near 2h"403 in X-ray di!raction patterns; (a) for as sprayed coating, and (b) for after heat treatment.
The peaks near the di!raction angles of 40 and 703 appear to be splitting, which shows the existence of tetragonal ZrO . In the tetragonal phase, two peaks obtained from (0 0 2) and (2 0 0) planes are 40.7 and the main peak of 41.1, respectively. The crystal form of ZrO of powder was thought to be the cubic phase containing a small amount of tetragonal ZrO [10]. The crystal form of this zirconia coating (tetragonal ZrO ) was clearer than the original powder by plasma spraying. And, through heat treatment two peaks separated in the di!raction angle and became much clearer than that of the as-sprayed coating. This was also con"rmed using the laser raman spectroscopy. Fig. 7 shows the details of X-ray di!raction patterns in the di!raction angle near the two ZrO peaks from (0 0 2) and (2 0 0) planes of zirconia coating before and after heat treatment. The amount of angular separation between the two peaks was measured. As a result, the angular separation of the coating after heat treatment is larger than that of the plasma sprayed coating. The angular separation between the two tetragonal peaks was changed from *2h"0.35 degrees to *2h"0.39 by heat treatment. Thus, after heat treatment, the angular separation between the two tetragonal peaks from (0 0 2) and (2 0 0) planes was broader and became clearer than that of the as-sprayed coating. This means that crystallization of zirconia, which is a tetragonal phase for as-sprayed, was advanced by heat treatment. However the crystal form of this zirconia coating (cubic#tetragonal ZrO ) did not change largely before and after heat treatment. 4. Conclusion The e!ects of heat treatment on high-hardness zirconia coating had been clari"ed, and the following results were obtained.
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(1) At the short spraying distance of L"30 mm, high-hardness zirconia coating of about H "1000 can be obtained by the gas tunnel-type plasma spraying. (2) The distribution of Vickers hardness on the cross section of zirconia coating consists of two parabolic curves in the thickness direction for two traverse spraying. (3) The high-hardness layer of H "950 appeared near the surface under the spraying condition: P"20 kW, L"30 mm. On the other hand, the hardness near the substrate had the lower value of H "600. The coating thickness was 120 lm and the surface was gray. (4) After heat treatment of this zirconia coating, the Vickers hardness of the high-hardness layer became a lower value, which was changed from H "950 to 650. The hardness distribution became #atter as compared with as-sprayed coating. But, clear change did not appear before and after heat treatment in the microstructure of this zirconia coating. (5) The deoxidation of ZrO by plasma spraying was con"rmed throughout the heat treatment. The rate of shortage of oxygen was 1.25 at% in this zirconia coating. Oxidization by heat treatment of the as-sprayed zirconia coating changed the color of the surface from gray to yellow of zirconia powder. (6) In the crystal form of the cubic ZrO and tetragonal phase, the angular separation between the two tetragonal peaks from (0 0 2) and (2 0 0) planes was broader and became clearer than that of the as-sprayed coating after the heat treatment. It is thought that the tetragonal phase of zirconia coating was more crystallized by heat treatment.
Acknowledgements The authors would like to thank Mr. K. Yamafuji for the assistance during experiment, and also Prof. N. Miyata for his valuable discussion. This study was "nancially supported in part by a Grant-in-Aid for Scienti"c Research from the Japanese Ministry of Education, Science and Culture.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Araya T. J Weld Soc Jpn 1988;57:216}22. Hasui J, et al. J Weld Soc Jpn 1987;35:216}22. Arata Y, Kobayashi A, Habara Y, Jing S. Trans JWRI 1986;15:227}31. Arata Y, Kobayashi A, Habara Y. J High Temp Soc 1987;13:116}24. Kobayashi A, Kurihara S, Habara Y, Arata Y. J Weld Soc Jpn 1990;8:457}63. Arata Y, Kobayashi A, Kurihara S. J High Temp Soc 1989;15:210}6. Kobayashi A. Proceedings of the ITSC, Orlando, FL, 1992. p. 57}62. Kobayashi A. Surface Coating Technol 1990;90:197}202. Kobayashi A, Kitamura T. J IAPS 1997;5:62}8. Miller RA, Smialek JL, Garlick RG. Am Ceram Soc 1981;3:241.
Vacuum 59 (2000) 194}202
E!ect of heat treatment on high-hardness zirconia coatings formed by gas tunnel type plasma spraying A. Kobayashi *, T. Kitamura Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan College of Industrial Technology, 1-27-1 Nishi-koya, Amagasaki, Hyogo 661-0047, Japan
Abstract By means of gas tunnel-type plasma spraying, a high-hardness zirconia coating could be obtained at a short spraying distance. For example, Vickers hardness of zirconia coating was H "1050 at the spraying distance of ¸"30 mm, when the power input was P"20 kW. This high-hardness coating had a hardness layer near the surface. In this paper, the e!ect of heat treatment on the characteristic of such a high-hardness zirconia coating is investigated, and the in#uence on the Vickers hardness and structure of the coating is discussed. The hardness of this high-hardness layer became lower after heat treatment. The crystal form of the cubic ZrO of powder was transformed into a tetragonal phase through this plasma spraying. After heat treatment, the angular separation between the two tetragonal peaks was broader and clearer. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Zirconia coating; High hardness; Heat treatment; Gas tunnel-type plasma spraying; Short distance spraying; Vickers hardness; X-ray di!raction
1. Introduction Recently, high functional ceramic coatings are applied to many "elds such as electronics and so on. In this context, plasma spray method has recently been noticed for making use of the excellent characteristics of ceramics such as corrosion resistance, thermal resistance, wear resistance and also its density [1,2]. Then, the study on the ceramic sprayed coatings using the newly developed gas tunnel-type plasma spraying had been started by the authors [3]. The mechanism and the fundamental properties of the gas tunnel-type plasma spraying were investigated in the previous studies [4,5]. * Corresponding author. Tel.: #816-6879-8694; fax: #816-6879-8689. E-mail address: [email protected] (A. Kobayashi). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 0 - 0
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And the characteristics of Vickers hardness of ceramic sprayed coatings, which were formed by gas tunnel-type plasma spraying, was investigated in detail. For example, the relations between the spraying distance L and the Vickers hardness H on the cross sections of ceramic (alumina, etc.) sprayed coatings were clari"ed [6]. In the case of alumina coatings, high hardness of 1500 was obtained at spraying distance L"30 mm when P"30 kW [7]. It had a very high-hardness layer near the surface of the coating. In zirconia coating by means of the gas tunnel-type plasma spraying, the Vickers hardness of the cross section was also increased with decreasing spraying distance, and a higher Vickers hardness could be obtained in the case of a shorter spraying distance (¸(¸ ). The critical spraying distance ¸ was about ¸ "40 mm, when P"20 kW. At a short spraying distance of ¸"30 mm, when P"33 kW, the Vickers hardness of zirconia coating was about H "1200 in the case of gas divertor nozzle diameter of d"20 mm [8]. In this case, the Vickers hardness of this sprayed coating became 20}30% higher than that of the conventional plasma spraying. The color of sprayed coating changed under the spraying conditions [9]. The original color of the zirconia powder is yellow. After plasma spraying at P"20 kW, ¸"30 mm, the coating color was gray and/or black, in the case of coating thickness of 100}150 lm. It is thought that the zirconia coating as-sprayed was deoxidized by the heat of plasma and that ZrO or Zr was generated in the coating after the loss of oxygen. This can be con"rmed by the heat treatment of the coating in the air. In this paper, the e!ects of heat treatment on the high-hardness zirconia coating produced at ¸"30 mm, were investigated in detail. The in#uence of thermal process on the Vickers hardness of the zirconia coating was clari"ed. The crystal form of the zirconia coating was discussed from the results by X-ray di!raction method.
2. Experimental procedure The gas tunnel-type plasma spraying apparatus used in this study is shown in Fig. 1. The experimental method to form high-hardness ceramic coatings by means of the gas tunnel-type plasma spraying has been described in the previous papers [4}6]. In this case, the gas divertor nozzle diameter was d"15 mm. The power input to the pilot plasma torch, which was supplied by the power supply, PS-1 was zero after starting of the gas tunnel-type plasma jet. The experimental conditions for this plasma spraying of zirconia powder are shown in Table 1. The power input to the plasma torch was about P"20 kW, and the spraying distance was a short distance of ¸"20}50 mm. Ar gas #ow rate for gas tunnel-type plasma spraying torch was Q"220 l/min, and the powder feed rate was w"18 g/min. The traverse speed of the substrate had a rather low value of about v"100 cm/min. Table 2 shows the chemical composition and the size of zirconia powder used in this study. This zirconia powder was the commercially prepared type of K-90. The measurement of distribution of the Vickers hardness in the cross section of the zirconia coating was carried out at each distance from the coating surface in the thickness direction. The Vickers hardness of the sprayed coatings was measured at the non-pore region in those cross sections under the condition that the load weight was 100 g and its load time was 25 s. The Vickers hardness was calculated as a mean value of 10 point measurements.
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Fig. 1. Gas tunnel-type plasma spraying apparatus used in this study; L: spraying distance. Table 1 Experimental conditions Power input Working gas (Ar) #ow rate Powder feed gas (Ar) Spraying distance Traverse speed Powder feed rate
Table 2 Chemical composition and size of zirconia powder used P"20 kW Q"220 l/min Qen"10 l/min L"30 mm v"96 cm/min w"18 g/min
Composition (wt%)
Size (lm)
ZrO Y O Al O SiO Fe O
90.78 8.15 0.38 0.20 0.11
The X-ray di!raction method was carried out on the surface of the coating. The X-ray source was Co, and the tube voltage was 30 kV and the tube current was 14 mA. For zirconia coatings produced at ¸"30 mm when P"20 kW, the heat treatment was carried out in air in an electric furnace maintaining a temperature of 8003C for 1 h. After heat treatment the specimens were naturally cooled in the furnace. The change of the weight of the coating before and after heat treatment was measured to know the rate of the shortage of oxygen in the zirconia coating. And also, the change of hardness distribution and X-ray di!raction patterns of the coatings before and after heat treatment were measured. 3. Results and discussion 3.1. Vickers hardness of zirconia coating formed by the gas tunnel type plasma spraying Fig. 2 shows the relation between the spraying distance and the Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying.
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Fig. 2. Dependences of Vickers hardness on cross section of zirconia coatings on spraying distance at P"17, 22 kW.
In this case, the spraying condition was as follows: the working gas (Ar) #ow rate was Q"220 l/min, the power input: P"17, 22 kW and the spraying distance: ¸"20}70 mm. This coating was formed by a two times traverse (2 pass sprayed) with the traverse speed of v"96 cm/min. And the powder feed rate had a small value of w"18 g/min in the case of the gas divertor nozzle diameter of d"15 mm. As described in the previous paper, the Vickers hardness of the cross section was increased with decreasing spraying distance, and a higher Vickers hardness was obtained at a short spraying distance of ¸(¸ for each power input. In this case, the critical spraying distance ¸ was about ¸ "35 mm, when P"17 kW, and ¸ "44 mm, when P"22 kW. At a short spraying distance of ¸"30 mm, when P"22kW, the Vickers hardness of zirconia coating was H "1050. While, in the case that the power input was lower value of P"17 kW, the Vickers hardness of the coating became H "1000 at the shortest spraying distance of ¸"20 mm. 3.2. Distribution of Vickers hardness on the cross section of zirconia coating At a short spraying distance of ¸"30 mm, high-hardness zirconia coating of about H "1000 can be obtained by the gas tunnel-type plasma spraying. Fig. 3 shows distribution of Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying at the short spraying distance of ¸"30 mm. The measurement was carried out at each distance from the coating surface in the thickness direction. In this case, the working gas #ow rate was Q"220 l/min, the power input P"20 kW and the spraying distance ¸"30 mm. This coating was formed by a two times traverse (2 pass sprayed).
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Fig. 3. Distribution of Vickers hardness on cross section of zirconia coating sprayed at L"30 mm when P"20 kW.
And the powder feed rate had a small value of w"18 g/min. The coating thickness was 120 lm and the surface was gray. The distribution of Vickers hardness on the cross section of zirconia coating consists of two parabolic curves as shown in Fig. 3. From this distribution, it is found that the Vickers hardness of the coating surface layer (correspondings to the second pass) is the highest value of about H "950 at a distance from the coating surface of l"40 lm. The hardness near the substrate had a lower value of H "600 than that near the coating surface. From the observation of the cross section of this zirconia coating by an optical microscope, the coating structure was the same as described in the former paper. 3.3. Ewect of heat treatment on high hardness zirconia coating The heat treatment was carried out for the as-sprayed coating produced under the condition: the spraying distance of ¸"30 mm, and the traverse speed of about v"100 cm/min. Fig. 4 shows the distribution of Vickers hardness on the cross section of zirconia coating produced by the gas tunnel-type plasma spraying, after the heat treatment of the same coating in Fig. 3. In this "gure, the distribution of Vickers hardness of the sprayed coating after the heat treatment consists of two parabolic curves. The hardness near the substrate did not change as compared with the as-sprayed coating in Fig. 3. But the Vickers hardness near the coating surface became much lower than that of the as-sprayed coating. The Vickers hardness of the coating changed from H "950 to 650 through heat treatment. It was assumed that the internal stress in the zirconia coating was relaxed by the heat treatment. In this way, the zirconia coating after heat treatment has a #at distribution of Vickers hardness on the cross section in the thickness direction, whose value is about H "650, and a high-hardness layer does not exists in this zirconia coating.
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Fig. 4. Distribution of Vickers hardness on cross section of zirconia coating after heat treatment, for sprayed coating in Fig. 3.
Fig. 5. Microphotograph of cross section of zirconia coating after heat treatment.
Fig. 5 shows the microphotograph of the cross section of zirconia coating after heat treatment. This had almost the same structure as-sprayed coating produced at the power input: P"20 kW and the spraying distance: ¸"30 mm. In the coating, the pore was a little smaller and the microstructure was rather dense than in the conventional spraying coating. But, there exist some large pores in this coating: the reason is due to the low power input. 3.4. Deoxidization of high-hardness zirconia coating The color of zirconia coating was gray under the spraying condition: P"20 kW, ¸"30 mm, and the thickness was 120 lm. But after heat treatment of this coating, this color changed to light yellow, which is the color of zirconia powder. According to the change in the weight of the coating before and after heat treatment, the shortage of oxygen in the zirconia coating as-sprayed was calculated. The result showed that the
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Fig. 6. X-ray di!raction patterns of zirconia coating. (a): X-ray di!raction pattern for as sprayed coating, and (b): for after heat treatment.
rate of the shortage of oxygen was 1.25 at%. This value was small, but the deoxidization in zirconia coating could be con"rmed and, the rate of the shortage of oxygen was increased to 3 at% in the case of zirconia coating with darker gray. This means that the zirconia coating as-sprayed was deoxidized by the heat of plasma and that ZrO or Zr was generated in the coating after the oxygen loss and the color changed from yellow to gray. This also means that by the heat treatment in the air the coating was oxidized again and the yellow color was restored. 3.5. Structure of high hardness zirconia coating The results obtained by X-ray di!raction patterns on the surface of high-hardness zirconia coating were as follows: Fig. 6 shows the X-ray di!raction patterns of the surface of the coating, which is the same coating as shown in Fig. 3. Here, (a) is an X-ray di!raction pattern for as-sprayed coating; and (b) is that for after heat treatment.
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Fig. 7. Zirconia peaks near 2h"403 in X-ray di!raction patterns; (a) for as sprayed coating, and (b) for after heat treatment.
The peaks near the di!raction angles of 40 and 703 appear to be splitting, which shows the existence of tetragonal ZrO . In the tetragonal phase, two peaks obtained from (0 0 2) and (2 0 0) planes are 40.7 and the main peak of 41.1, respectively. The crystal form of ZrO of powder was thought to be the cubic phase containing a small amount of tetragonal ZrO [10]. The crystal form of this zirconia coating (tetragonal ZrO ) was clearer than the original powder by plasma spraying. And, through heat treatment two peaks separated in the di!raction angle and became much clearer than that of the as-sprayed coating. This was also con"rmed using the laser raman spectroscopy. Fig. 7 shows the details of X-ray di!raction patterns in the di!raction angle near the two ZrO peaks from (0 0 2) and (2 0 0) planes of zirconia coating before and after heat treatment. The amount of angular separation between the two peaks was measured. As a result, the angular separation of the coating after heat treatment is larger than that of the plasma sprayed coating. The angular separation between the two tetragonal peaks was changed from *2h"0.35 degrees to *2h"0.39 by heat treatment. Thus, after heat treatment, the angular separation between the two tetragonal peaks from (0 0 2) and (2 0 0) planes was broader and became clearer than that of the as-sprayed coating. This means that crystallization of zirconia, which is a tetragonal phase for as-sprayed, was advanced by heat treatment. However the crystal form of this zirconia coating (cubic#tetragonal ZrO ) did not change largely before and after heat treatment. 4. Conclusion The e!ects of heat treatment on high-hardness zirconia coating had been clari"ed, and the following results were obtained.
Vac=1803=SK=VVC
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A. Kobayashi, T. Kitamura / Vacuum 59 (2000) 194}202
(1) At the short spraying distance of L"30 mm, high-hardness zirconia coating of about H "1000 can be obtained by the gas tunnel-type plasma spraying. (2) The distribution of Vickers hardness on the cross section of zirconia coating consists of two parabolic curves in the thickness direction for two traverse spraying. (3) The high-hardness layer of H "950 appeared near the surface under the spraying condition: P"20 kW, L"30 mm. On the other hand, the hardness near the substrate had the lower value of H "600. The coating thickness was 120 lm and the surface was gray. (4) After heat treatment of this zirconia coating, the Vickers hardness of the high-hardness layer became a lower value, which was changed from H "950 to 650. The hardness distribution became #atter as compared with as-sprayed coating. But, clear change did not appear before and after heat treatment in the microstructure of this zirconia coating. (5) The deoxidation of ZrO by plasma spraying was con"rmed throughout the heat treatment. The rate of shortage of oxygen was 1.25 at% in this zirconia coating. Oxidization by heat treatment of the as-sprayed zirconia coating changed the color of the surface from gray to yellow of zirconia powder. (6) In the crystal form of the cubic ZrO and tetragonal phase, the angular separation between the two tetragonal peaks from (0 0 2) and (2 0 0) planes was broader and became clearer than that of the as-sprayed coating after the heat treatment. It is thought that the tetragonal phase of zirconia coating was more crystallized by heat treatment.
Acknowledgements The authors would like to thank Mr. K. Yamafuji for the assistance during experiment, and also Prof. N. Miyata for his valuable discussion. This study was "nancially supported in part by a Grant-in-Aid for Scienti"c Research from the Japanese Ministry of Education, Science and Culture.
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