Determination of Triaxial Residual Stress in Plasma-Sprayed Hydroxyapatite (HAp) Deposited on Titanium Substrate by X-ra
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REVIEW
Determination of Triaxial Residual Stress in Plasma-Sprayed Hydroxyapatite (HAp) Deposited on Titanium Substrate by X-ray Diffraction N. Bosh1,2 • H. Mozaffari-Jovein2 • C. Mu¨ller1
Submitted: 9 March 2018 / in revised form: 19 August 2018 Ó ASM International 2018
Abstract Measurement of residual stress in plasmasprayed coating is of key importance to optimize their microstructure and mechanical properties. In this study, the x-ray diffraction analysis was carried out using the sin2w method to evaluate the residual stress distribution of hydroxyapatite (HAp) coatings produced on titanium substrate by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS). The sin2w method measured strains at different tilt w and rotating u angles around the specimen surface normal. A non-uniform and inhomogeneous stress distribution was present in the both coatings. The measured strain ewu is plotted versus sin2w, showing a nonlinear (elliptical) behavior, which indicates the presence of inhomogeneous triaxial stress distributions within coating, due to the crystalline anisotropy, inhomogeneous cooling rate or solidification of the molten particles. The normal stress values within both HAp coatings produced were found to be tensile in nature, but the values of tensile stresses are significantly higher in APS coating than those values obtained for VPS coating. Keywords hydroxyapatite nanoindentation plasma spraying residual stress titanium x-ray diffraction
& N. Bosh [email protected] 1
Department of Microsystems Engineering, IMTEK, University of Freiburg, Freiburg, Germany
2
Institut fu¨r Werkstoffe and Anwendungstechnik Tuttlingen, IWAT, Hochschule Campus Tuttlingen, Tuttlingen, Germany
Introduction Hydroxyapatite (HAp) has been widely considered as the most common coating material on the metallic implants to enhance the long-term osseointegration and improve the bioactivity behavior of the metallic surfaces (Ref 1, 2). Because of the similar structural and chemical composition to the bone, HAp can be rapidly integrated into the body issue and improve the bone-to-implant contact (Ref 3). Among the commercial coating deposition techniques, plasma spraying is the most widely used technique for the deposition of HAp due to its high deposition rate, relative ease of operation and low cost (Ref 4). In addition, the major advantage of this process is the ability to deposit coatings onto substrates without significant heat input and varying the microstructure of substrates (Ref 5). Generally, the durability of thermal spray coating and its adherence to the substrate depend strongly on their microstructure and residual stress present within the coatings (Ref 6, 7). Residual stress often appears in the HAp coatings which were deposited by plasma spraying due to the mismatch of its thermal expansion coefficient (CTE) with the metallic substrate. Thermal stresses can be formed during the spraying process and the following cooling period which can affect the mechanical stabilities, spalling
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