Dissolution of Plasma-Sprayed Wollastonite Coatings: The Effects of Microstructure Coupled with Stress
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JTTEE5 21:908–916 DOI: 10.1007/s11666-012-9793-7 1059-9630/$19.00 Ó ASM International
Dissolution of Plasma-Sprayed Wollastonite Coatings: The Effects of Microstructure Coupled with Stress Weize Wang, Fuzhen Xuan, Jiachun Liang, and Luobin Wang (Submitted October 28, 2011; in revised form April 11, 2012) Wollastonite coatings are deposited on the U-shape titanium alloy coupons by atmospheric plasma spraying at substrate temperatures of room temperature and 400 °C, respectively. The effects of applied stresses and microstructure on the dissolution behavior of wollastonite coatings have been investigated. The dissolution rate is characterized by the ion concentration changes of Ca, Si, and P in the SBF solution. The coatings deposited at room temperature show higher porosity and lower crystallinity, and further higher dissolution rate is observed, compared with the coatings deposited at a substrate temperature of 400 °C. Applied tensile stresses promote the coating dissolution. The effect of compressive stresses on the dissolution of coatings depends on the balance between the hindrance effect and the promotion action caused by the debonding and/or peeling off. The stress condition cannot change the phase transformation process when the substrate crystal structure is adverse for the apatite precipitation.
Keywords
atmospheric plasma spraying, dissolution, microstructure, stress, wollastonite coating
1. Introduction Thermal-sprayed bioactive coatings on the metallic substrate, especially hydroxyapatite (HA) coatings on titanium alloy substrate have been widely acclaimed and used for clinical application (Ref 1-3). The combination of the superior mechanical properties of titanium alloy and the unique biocompatibility of bio-coatings makes it an ideal choice for most biomedical applications. However, the long-term stability of bio-coatings needs to be paid more attention to for it decides the success of implants. The dissolution behavior is critical to understanding the bioactivity (Ref 4), and also has an important role in the long-term stability, so as that it has been the main focus of the investigation on bio-coatings. Up to now, the in vivo or in vitro dissolution behavior has been studied for HA coatings (Ref 5-7) and calcium phosphate coatings (Ref 8, 9), etc. It was pointed out that the crystallinity and the particle melting status in the coatings affect the dissolution behavior (Ref 10). In addition, bio-coatings bear internal residual stresses as well as applied stresses during in service. It is pointed out that the internal stress plays a significant role in the dissolution of coatings (Ref 11). A tensile residual stress Weize Wang, Fuzhen Xuan, Jiachun Liang, and Luobin Wang, Key Lab of Pressure System and Safety, Ministry of Education, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China. Contact e-mail: [email protected].
908—Volume 21(5) September 2012
enhances and a compressive residual stress hinders the dissolution of coatings when
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