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Phase compositions and microstructure of plasma sprayed wollastonite coating
Surface and Coatings Technology 141 Ž2001. 269᎐274
Phase compositions and microstructure of plasma sprayed wollastonite coating Xuanyong LiuU , Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 2 January 2001; accepted in revised form 13 March 2001
Abstract The phase compositions and microstructure of wollastonite coatings deposited onto Ti᎐6Al᎐4V substrates by plasma spraying were studied using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The plasma sprayed wollastonite coating was not homogeneous and had a rough surface. Some pores and microcracks existed in the coatings. The primary crystalline phase in the coating was triclinic wollastonite. Some crystalline wollastonite grains had dislocation. The glassy phase was also discovered with some small size crystals in the coating. In addition, tridymite ŽSiO 2 . and CaO were also found in the coating. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Wollastonite coatings; Plasma spraying; Microstructure; Phase compositions
1. Introduction Wollastonite ŽCaSiO 3 ., a common mineral, is an important compound in the ceramic and cement industries w1,2x. One of the possible applications of wollastonite ceramics is manufacturing of artificial bone and dental root because of its good bioactivity and biocompatibility w3᎐10x. The wollastonite was deposited onto Ti᎐6Al᎐4V substrate by plasma spraying and its bioactivity was studied in our previous work w11x. The obtained results demonstrated that the plasma sprayed wollastonite coatings possess excellent bioactivity thus may be a candidate for a biomaterial. In this paper, the phase composition and the microstructure of the plasma sprayed wollastonite coatings were investigated, using scanning electron microscopy ŽSEM., X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM..
U
Corresponding author. Tel.: q86-21625-12990; fax: q86-21-62513903. E-mail address: [email protected] ŽL. Xuanyong..
2. Experiments Commercially available wollastonite powder ŽMing Hua Minerals Co., Ltd of Liyang Jiangsu Province, China., with a typical size range of 10᎐100 m was used. Plasma spraying of the powder was made onto Ti᎐6Al᎐4V substrates with dimensions 20 mm= 10 mm= 4 mm. An atmosphere plasma spray ŽAPS. system ŽSulzer Metco AG, Switzerland. was applied to fabricate wollastonite coatings under the modified spray parameters which are shown in Table 1. The cross-section of the as-sprayed wollastonite coatings were polished using Al 2 O 3 abrasive powders and diamond grinding paste to prepare the metallographic samples. The surface and cross-section microstructure of the coatings was observed by scanning electron microscopy with electron probe X-ray microanalysis ŽEPMA-8705QH 2 .. The surface of samples was sputter-coated with gold for morphological observation and elemental analysis. X-Ray diffraction ŽXRD: RAX-10. using Cu-K ␣ radiation was used to measure the phase
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 6 9 - 0
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
270 Table 1 Spray parameters Plasma gas Ar Plasma gas H2 Spray distance Powder carrier gas Ar
40 slpm 12 slpm 90 mm 3.0 slpm
Powder feed rate Current Voltage
20 grmin 600 A 75 V
compositions of the starting powders and the as-sprayed coatings. Thin-foil samples were prepared to allow transmission electron microscopy ŽTEM. examination of the coating. A coating specimen with thickness of 1 mm was first prepared. The specimen was then attached to a tripod polisher with glue, and was polished from both sides until the thickness of the coating became approximately 30 m. Finally, the thickness of the specimen was further reduced with a low-angle ion-thinning precise ion polishing system ŽPIPS.. TEM examination was carried out using a JEM-2010 TEM at an accelerating voltage of 200 kV.
3. Results and discussion Fig. 1 shows the SEM photograph of the starting wollastonite powders. Most powders are granular particles. Some column-like particles are also found in the powders. The typical particle size range is approximately 10᎐100 m. The XRD pattern of the starting wollastonite powders is presented in Fig. 2. It can be seen that the starting powders are composed of high crystallinity wollastonite. Fig. 3 show SEM photographs of the surface and cross-section microstructure of the as-sprayed wollastonite coatings. The coating has a rough surface ŽFig. 3a.. Microcracks, interlinking of pores and close pores can be found in the cross-sectional view of the coating ŽFig. 3b.. In addition, a few black features indicated by an arrow in Fig. 3b were observed in the coating. Only silicon can be detected by EDS in these black patches, which indicates that they could be composed of SiO 2
Fig. 1. SEM photographs of the starting wollastonite powders.
Fig. 2. XRD spectra of the starting wollastonite powders.
ŽFig. 3c.. The Au peak in Fig. 3c results from the gold sputter-coated on the surface of the sample for morphological observation and elemental analysis. The XRD pattern of the as-sprayed wollastonite coating is shown in Fig. 4. It can be found that some sharp peaks coexist with an obvious glass bulge. This is an indication of the coexistence of the glassy phase with crystalline wollastonite in the coating. In addition,
Fig. 3. SEM photographs of the as-sprayed wollastonite coatings: Ža. the surface and Žb. the cross-section; Žc. EDS spectrum of the black area shown by an allow in Žb..
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
271
Fig. 4. XRD spectra of as-sprayed wollastonite coating.
CaO and tridymite ŽSiO 2 . are also found in the coating. TEM examination of the as-sprayed wollastonite coat-
Fig. 5. Calcium oxide grain in the wollastonite coating: Ža. TEM micrograph; Žb. SAD pattern and Žc. EDS spectrum of the grain indicated with an arrow in Ža..
ing reveals further the existence of calcium oxide ŽCaO. ŽFig. 5a᎐c.. This indicates that the tridymite and CaO are formed during the plasma spraying process. The plasma spraying process is far from being in equilibrium, during spraying the feedstock material can be overheated in some parts and not molten in other parts. Therefore, changes from the original phase and chemical composition are regularly observed. Furthermore, plasma spraying is a rapid heatingrrapid cooling and solidification process which often results in the formation of metastable deposits w12x. According to the CaO᎐SiO 2 binary system phase diagram w13x, the tridymite, metastable phase of quartz, is easy to precipitate from the wollastonite melt at 1436⬚C. The tridymite can be retained at room temperature resulting from the rapid cooling. At the same time, the phase separation could also occur in CaO᎐SiO 2-based melt during solidification w14x, which results in the formation of the SiO 2-rich areas. In addition, very small particles which overheated and vaporized in plasma could have been dissociated and decomposed from wollastonite to compounds like SiO 2 and CaO. The most frequently observed structures in TEM examination of the coatings are crystalline wollastonite grains ŽFig. 6a.. Analysis of selected-area diffraction ŽSAD. patterns and high-resolution electron microscopy ŽHREM. for these grains ŽFig. 6b,c. show that they are triclinic structure wollastonite, which corresponds with the result of XRD analysis in Fig. 4. Some crystalline wollastonite grains with dislocation are also found in the coating ŽFig. 7a.. The dislocation was obviously observed in the SAD pattern and the HREM images of the w012x zone of crystalline wollastonite, respectively ŽFig. 7b,c.. The dislocation is a stacking fault. One of the fault planes is shown with an arrow in Fig. 7c. The dislocation in wollastonite may come from
272
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
nanocrystals in amorphous materials could occur in situ under the action of the electron beam in the TEM w15x. Stobbs et al. w16x showed that the nucleation of such nanocrystals in FeNiB alloy was present before irradiation by electron beam because crystallization at the 1.0᎐2.0-nm level is normally only visible at 100 kV after approximately 10 min. Therefore, it is very possible that the formation of the nanocrystals in amorphous regions resulted from the rapid cooling of the molten wollastonite during the plasma spraying process. According to the CaO᎐SiO 2 binary system phase diagram, the nanocrystals could be CaSiO 3 and SiO 2 . Unfortunately, the nanocrystals are so small that it is difficult to determine their composition and structure. Kolman et al. w17x showed that plasma sprayed wollastonite coatings were less homogeneous and chemical variation might be observed even in the micrographs. During the plasma flame flight the wollastonite particles can be completely molten, semi-molten or not molten depending on their particle size. Unmelted particles and the core of semi-molten particles were still composed of crystalline wollastonite. Amorphous, glassy structures were also formed from the solidification of
Fig. 6. Crystalline wollastonite grains in the wollastonite coating: Ža. TEM micrograph; Žb. w111x SAD pattern and Žc. corresponding w111x HREM image.
the high velocity impact of melted wollastonite particles to the substrate and previously solidified materials. In conjunction with these crystalline wollastonite grains, amorphous regions are also observed ŽFig. 8a., giving rise to diffuse rings in the SAD pattern ŽFig. 8b.. The HREM image of the interface between crystalline wollastonite and amorphous regions is presented in Fig. 8c. The ordered and disordered structure regions represented crystalline wollastonite and amorphous regions, respectively. The phenomena may result from the unmelted core and molten surface of wollastonite particle during the plasma spraying process. Since there is rapid heatingrrapid cooling and solidification of semi-molten particles during plasma spraying process, the outer surface of these semi-molten particles will have a smooth glassy appearance, whereas their core has the structure and texture of the original powder. Fig. 9 shows the HREM image of amorphous regions with some local ordered structure, which indicates that some nanocrystals are formed in these amorphous regions. The size of the nanocrystals is approximately 5.0᎐10.0 nm. It is difficult to be certain of the extent to which these occur as a result of the spraying process because it is also possible that the nucleation of such
Fig. 7. Crystalline wollastonite grains with dislocation in the wollastonite coating: Ža. TEM micrograph; Žb. w012x SAD pattern and Žc. corresponding w012x HREM image.
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
273
4. Conclusions The highly crystalline wollastonite powders were deposited on Ti᎐6Al᎐4V substrate using plasma spraying methods. The plasma sprayed wollastonite coating was not homogeneous and had a rough surface. Some pores and microcracks existed in the coatings. The primary crystalline phase in the coating is triclinic structure wollastonite. Some crystalline wollastonite grains with dislocation are also observed in the coating, which may come from the high velocity impact of melted wollastonite particles to the substrate and previously solidified materials. The plasma spraying is a rapid heatingrrapid cooling and solidification process which often results in the formation of metastable in deposits. In addition, very small particles which overheated and vaporized in the plasma could have been dissociated and decomposed from wollastonite to compounds like SiO 2 and CaO. Therefore, glassy phase, tridymite ŽSiO 2 . and CaO were also found in the coating. During solidification if the thermal conditions do favor nucleation and growth of CaSiO 3 or SiO 2 , some nanocrystals could also be formed in glassy phase areas.
Fig. 8. Amorphous regions in conjunction with crystalline wollastonite grains in the wollastonite coating: Ža. TEM micrograph; Žb. SAD pattern of the amorphous regions and Žc. corresponding HREM image of the interface between crystalline wollastonite and amorphous regions.
completely molten particles and the outer surface of semi-molten particles. During solidification if the thermal conditions favor nucleation and growth of CaSiO 3 or SiO 2 , some microcrystals will be formed in these amorphous regions.
Fig. 9. HREM image of the amorphous regions with nanocrystals.
Acknowledgements This work is supported by National Basic Research Fund under grant G1999064706 and Shanghai Science and Technology R& D Fund under grant 995211020. The authors thank Ms Xiaming Zhou for kind cooperation in the preparation of the coating specimens and Ms Meiling Ruan for the TEM examination. References w1x W.A. Deer, R.A. Hoeie, J. Zussman, in: Chain Silicates, Longmans, Green and Co Ltd, London, 1963, pp. 167᎐175. w2x M. Vukovich, J. Am. Ceram. Soc. 39 Ž1956. 323᎐329. w3x P.N. De Aza, F. Guitian, S. De Aza, Scr. Metall. Mater. 31 Ž1994. 1001᎐1005. w4x P.N. De Aza, F. Guitian, S. De Aza, J. Microsc. 182 Ž1996. 24᎐31. w5x P.N. De Aza, F. Guitian, S. De Aza, Biomaterials 18 Ž1997. 1285᎐1291. w6x P.N. De Aza, F. Guitian, S. De Aza, Biomaterials 21 Ž2000. 1735᎐1741. w7x P. Siriphannon, S. Hayashi, A. Yasumori, K. Okada, J. Mater. Res. 14 Ž1999. 529᎐536. w8x T. Kokubo, S. Ito, M. Shigematsu, S. Sakka, T. Yamamuro, J. Mater. Sci. 22 Ž1987. 4067᎐4070. w9x T. Kokubo, J. Non-Cryst. Solids 120 Ž1990. 138᎐151. w10x Y. Abe, T. Kokubo, T. Yamamuro, J. Mater. Sci. Mater. Med. 1 Ž1990. 233᎐238. w11x X. Liu, C. Ding, Biomaterials Žaccepted..
274
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
w12x P. Chraska, J. Dubsky, ` ´ B. Kolman, J. ?avsky, ´ J. Forman, J. Thermal Spray Technol. 1 Ž1992. 301᎐306. w13x W. Eitel, The Physical Chemistry of The Silicates, Institute of Silicate Research, University of Toledo, 1954, p. 647. w14x O.V. Mazzurin, E.A. Porai-Koshits, Phase Separation In Glass, North-Holland, 1984, pp. 201᎐242.
w15x J.M. Guilemany, J. Nutting, M.J. Dougan, J. Thermal Spray Technol. 6 Ž1997. 425᎐429. w16x W.M. Stobbs, D.J. Smith, Nature 281 Ž1979. 54᎐55. w17x B.J. Kolman, K. Neufuss, J. Havsky, J. ´ J. Ddubsky, ´ P. Chraskka, ´ Anal. Atom. Spectrom 14 Ž1999. 471᎐473.
Phase compositions and microstructure of plasma sprayed wollastonite coating Xuanyong LiuU , Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 2 January 2001; accepted in revised form 13 March 2001
Abstract The phase compositions and microstructure of wollastonite coatings deposited onto Ti᎐6Al᎐4V substrates by plasma spraying were studied using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The plasma sprayed wollastonite coating was not homogeneous and had a rough surface. Some pores and microcracks existed in the coatings. The primary crystalline phase in the coating was triclinic wollastonite. Some crystalline wollastonite grains had dislocation. The glassy phase was also discovered with some small size crystals in the coating. In addition, tridymite ŽSiO 2 . and CaO were also found in the coating. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Wollastonite coatings; Plasma spraying; Microstructure; Phase compositions
1. Introduction Wollastonite ŽCaSiO 3 ., a common mineral, is an important compound in the ceramic and cement industries w1,2x. One of the possible applications of wollastonite ceramics is manufacturing of artificial bone and dental root because of its good bioactivity and biocompatibility w3᎐10x. The wollastonite was deposited onto Ti᎐6Al᎐4V substrate by plasma spraying and its bioactivity was studied in our previous work w11x. The obtained results demonstrated that the plasma sprayed wollastonite coatings possess excellent bioactivity thus may be a candidate for a biomaterial. In this paper, the phase composition and the microstructure of the plasma sprayed wollastonite coatings were investigated, using scanning electron microscopy ŽSEM., X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM..
U
Corresponding author. Tel.: q86-21625-12990; fax: q86-21-62513903. E-mail address: [email protected] ŽL. Xuanyong..
2. Experiments Commercially available wollastonite powder ŽMing Hua Minerals Co., Ltd of Liyang Jiangsu Province, China., with a typical size range of 10᎐100 m was used. Plasma spraying of the powder was made onto Ti᎐6Al᎐4V substrates with dimensions 20 mm= 10 mm= 4 mm. An atmosphere plasma spray ŽAPS. system ŽSulzer Metco AG, Switzerland. was applied to fabricate wollastonite coatings under the modified spray parameters which are shown in Table 1. The cross-section of the as-sprayed wollastonite coatings were polished using Al 2 O 3 abrasive powders and diamond grinding paste to prepare the metallographic samples. The surface and cross-section microstructure of the coatings was observed by scanning electron microscopy with electron probe X-ray microanalysis ŽEPMA-8705QH 2 .. The surface of samples was sputter-coated with gold for morphological observation and elemental analysis. X-Ray diffraction ŽXRD: RAX-10. using Cu-K ␣ radiation was used to measure the phase
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 1 6 9 - 0
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
270 Table 1 Spray parameters Plasma gas Ar Plasma gas H2 Spray distance Powder carrier gas Ar
40 slpm 12 slpm 90 mm 3.0 slpm
Powder feed rate Current Voltage
20 grmin 600 A 75 V
compositions of the starting powders and the as-sprayed coatings. Thin-foil samples were prepared to allow transmission electron microscopy ŽTEM. examination of the coating. A coating specimen with thickness of 1 mm was first prepared. The specimen was then attached to a tripod polisher with glue, and was polished from both sides until the thickness of the coating became approximately 30 m. Finally, the thickness of the specimen was further reduced with a low-angle ion-thinning precise ion polishing system ŽPIPS.. TEM examination was carried out using a JEM-2010 TEM at an accelerating voltage of 200 kV.
3. Results and discussion Fig. 1 shows the SEM photograph of the starting wollastonite powders. Most powders are granular particles. Some column-like particles are also found in the powders. The typical particle size range is approximately 10᎐100 m. The XRD pattern of the starting wollastonite powders is presented in Fig. 2. It can be seen that the starting powders are composed of high crystallinity wollastonite. Fig. 3 show SEM photographs of the surface and cross-section microstructure of the as-sprayed wollastonite coatings. The coating has a rough surface ŽFig. 3a.. Microcracks, interlinking of pores and close pores can be found in the cross-sectional view of the coating ŽFig. 3b.. In addition, a few black features indicated by an arrow in Fig. 3b were observed in the coating. Only silicon can be detected by EDS in these black patches, which indicates that they could be composed of SiO 2
Fig. 1. SEM photographs of the starting wollastonite powders.
Fig. 2. XRD spectra of the starting wollastonite powders.
ŽFig. 3c.. The Au peak in Fig. 3c results from the gold sputter-coated on the surface of the sample for morphological observation and elemental analysis. The XRD pattern of the as-sprayed wollastonite coating is shown in Fig. 4. It can be found that some sharp peaks coexist with an obvious glass bulge. This is an indication of the coexistence of the glassy phase with crystalline wollastonite in the coating. In addition,
Fig. 3. SEM photographs of the as-sprayed wollastonite coatings: Ža. the surface and Žb. the cross-section; Žc. EDS spectrum of the black area shown by an allow in Žb..
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
271
Fig. 4. XRD spectra of as-sprayed wollastonite coating.
CaO and tridymite ŽSiO 2 . are also found in the coating. TEM examination of the as-sprayed wollastonite coat-
Fig. 5. Calcium oxide grain in the wollastonite coating: Ža. TEM micrograph; Žb. SAD pattern and Žc. EDS spectrum of the grain indicated with an arrow in Ža..
ing reveals further the existence of calcium oxide ŽCaO. ŽFig. 5a᎐c.. This indicates that the tridymite and CaO are formed during the plasma spraying process. The plasma spraying process is far from being in equilibrium, during spraying the feedstock material can be overheated in some parts and not molten in other parts. Therefore, changes from the original phase and chemical composition are regularly observed. Furthermore, plasma spraying is a rapid heatingrrapid cooling and solidification process which often results in the formation of metastable deposits w12x. According to the CaO᎐SiO 2 binary system phase diagram w13x, the tridymite, metastable phase of quartz, is easy to precipitate from the wollastonite melt at 1436⬚C. The tridymite can be retained at room temperature resulting from the rapid cooling. At the same time, the phase separation could also occur in CaO᎐SiO 2-based melt during solidification w14x, which results in the formation of the SiO 2-rich areas. In addition, very small particles which overheated and vaporized in plasma could have been dissociated and decomposed from wollastonite to compounds like SiO 2 and CaO. The most frequently observed structures in TEM examination of the coatings are crystalline wollastonite grains ŽFig. 6a.. Analysis of selected-area diffraction ŽSAD. patterns and high-resolution electron microscopy ŽHREM. for these grains ŽFig. 6b,c. show that they are triclinic structure wollastonite, which corresponds with the result of XRD analysis in Fig. 4. Some crystalline wollastonite grains with dislocation are also found in the coating ŽFig. 7a.. The dislocation was obviously observed in the SAD pattern and the HREM images of the w012x zone of crystalline wollastonite, respectively ŽFig. 7b,c.. The dislocation is a stacking fault. One of the fault planes is shown with an arrow in Fig. 7c. The dislocation in wollastonite may come from
272
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
nanocrystals in amorphous materials could occur in situ under the action of the electron beam in the TEM w15x. Stobbs et al. w16x showed that the nucleation of such nanocrystals in FeNiB alloy was present before irradiation by electron beam because crystallization at the 1.0᎐2.0-nm level is normally only visible at 100 kV after approximately 10 min. Therefore, it is very possible that the formation of the nanocrystals in amorphous regions resulted from the rapid cooling of the molten wollastonite during the plasma spraying process. According to the CaO᎐SiO 2 binary system phase diagram, the nanocrystals could be CaSiO 3 and SiO 2 . Unfortunately, the nanocrystals are so small that it is difficult to determine their composition and structure. Kolman et al. w17x showed that plasma sprayed wollastonite coatings were less homogeneous and chemical variation might be observed even in the micrographs. During the plasma flame flight the wollastonite particles can be completely molten, semi-molten or not molten depending on their particle size. Unmelted particles and the core of semi-molten particles were still composed of crystalline wollastonite. Amorphous, glassy structures were also formed from the solidification of
Fig. 6. Crystalline wollastonite grains in the wollastonite coating: Ža. TEM micrograph; Žb. w111x SAD pattern and Žc. corresponding w111x HREM image.
the high velocity impact of melted wollastonite particles to the substrate and previously solidified materials. In conjunction with these crystalline wollastonite grains, amorphous regions are also observed ŽFig. 8a., giving rise to diffuse rings in the SAD pattern ŽFig. 8b.. The HREM image of the interface between crystalline wollastonite and amorphous regions is presented in Fig. 8c. The ordered and disordered structure regions represented crystalline wollastonite and amorphous regions, respectively. The phenomena may result from the unmelted core and molten surface of wollastonite particle during the plasma spraying process. Since there is rapid heatingrrapid cooling and solidification of semi-molten particles during plasma spraying process, the outer surface of these semi-molten particles will have a smooth glassy appearance, whereas their core has the structure and texture of the original powder. Fig. 9 shows the HREM image of amorphous regions with some local ordered structure, which indicates that some nanocrystals are formed in these amorphous regions. The size of the nanocrystals is approximately 5.0᎐10.0 nm. It is difficult to be certain of the extent to which these occur as a result of the spraying process because it is also possible that the nucleation of such
Fig. 7. Crystalline wollastonite grains with dislocation in the wollastonite coating: Ža. TEM micrograph; Žb. w012x SAD pattern and Žc. corresponding w012x HREM image.
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
273
4. Conclusions The highly crystalline wollastonite powders were deposited on Ti᎐6Al᎐4V substrate using plasma spraying methods. The plasma sprayed wollastonite coating was not homogeneous and had a rough surface. Some pores and microcracks existed in the coatings. The primary crystalline phase in the coating is triclinic structure wollastonite. Some crystalline wollastonite grains with dislocation are also observed in the coating, which may come from the high velocity impact of melted wollastonite particles to the substrate and previously solidified materials. The plasma spraying is a rapid heatingrrapid cooling and solidification process which often results in the formation of metastable in deposits. In addition, very small particles which overheated and vaporized in the plasma could have been dissociated and decomposed from wollastonite to compounds like SiO 2 and CaO. Therefore, glassy phase, tridymite ŽSiO 2 . and CaO were also found in the coating. During solidification if the thermal conditions do favor nucleation and growth of CaSiO 3 or SiO 2 , some nanocrystals could also be formed in glassy phase areas.
Fig. 8. Amorphous regions in conjunction with crystalline wollastonite grains in the wollastonite coating: Ža. TEM micrograph; Žb. SAD pattern of the amorphous regions and Žc. corresponding HREM image of the interface between crystalline wollastonite and amorphous regions.
completely molten particles and the outer surface of semi-molten particles. During solidification if the thermal conditions favor nucleation and growth of CaSiO 3 or SiO 2 , some microcrystals will be formed in these amorphous regions.
Fig. 9. HREM image of the amorphous regions with nanocrystals.
Acknowledgements This work is supported by National Basic Research Fund under grant G1999064706 and Shanghai Science and Technology R& D Fund under grant 995211020. The authors thank Ms Xiaming Zhou for kind cooperation in the preparation of the coating specimens and Ms Meiling Ruan for the TEM examination. References w1x W.A. Deer, R.A. Hoeie, J. Zussman, in: Chain Silicates, Longmans, Green and Co Ltd, London, 1963, pp. 167᎐175. w2x M. Vukovich, J. Am. Ceram. Soc. 39 Ž1956. 323᎐329. w3x P.N. De Aza, F. Guitian, S. De Aza, Scr. Metall. Mater. 31 Ž1994. 1001᎐1005. w4x P.N. De Aza, F. Guitian, S. De Aza, J. Microsc. 182 Ž1996. 24᎐31. w5x P.N. De Aza, F. Guitian, S. De Aza, Biomaterials 18 Ž1997. 1285᎐1291. w6x P.N. De Aza, F. Guitian, S. De Aza, Biomaterials 21 Ž2000. 1735᎐1741. w7x P. Siriphannon, S. Hayashi, A. Yasumori, K. Okada, J. Mater. Res. 14 Ž1999. 529᎐536. w8x T. Kokubo, S. Ito, M. Shigematsu, S. Sakka, T. Yamamuro, J. Mater. Sci. 22 Ž1987. 4067᎐4070. w9x T. Kokubo, J. Non-Cryst. Solids 120 Ž1990. 138᎐151. w10x Y. Abe, T. Kokubo, T. Yamamuro, J. Mater. Sci. Mater. Med. 1 Ž1990. 233᎐238. w11x X. Liu, C. Ding, Biomaterials Žaccepted..
274
X. Liu, C. Ding r Surface and Coatings Technology 141 (2001) 269᎐274
w12x P. Chraska, J. Dubsky, ` ´ B. Kolman, J. ?avsky, ´ J. Forman, J. Thermal Spray Technol. 1 Ž1992. 301᎐306. w13x W. Eitel, The Physical Chemistry of The Silicates, Institute of Silicate Research, University of Toledo, 1954, p. 647. w14x O.V. Mazzurin, E.A. Porai-Koshits, Phase Separation In Glass, North-Holland, 1984, pp. 201᎐242.
w15x J.M. Guilemany, J. Nutting, M.J. Dougan, J. Thermal Spray Technol. 6 Ž1997. 425᎐429. w16x W.M. Stobbs, D.J. Smith, Nature 281 Ž1979. 54᎐55. w17x B.J. Kolman, K. Neufuss, J. Havsky, J. ´ J. Ddubsky, ´ P. Chraskka, ´ Anal. Atom. Spectrom 14 Ž1999. 471᎐473.