Effect of Fine Particle Peening Using Hydroxyapatite Shot Particles and Plasma Sprayed Hydroxyapatite Coating on Fatigue Properties of Beta Titanium Alloy

Abstract

Fine particle peening (FPP) and plasma spraying using hydroxyapatite particles were applied to a beta titanium alloy, Ti-22V-4Al, to form the hydroxyapatite (HAp) layer on the surface. As a result, HAp layer was formed on the specimen surface by the both treatments. The thicknesses were 5 m and 100m, respectively for FPP treated and plasma sprayed specimens. In the FPP treated specimen, Vickers hardness was increased by FPP compared with that of the untreated specimens, resulting in work-hardening. Rotary bending fatigue tests were carried out on both treated and untreated specimens. The FPP treated specimens exhibited higher fatigue strength than the untreated specimens. It is due to the increase in hardness and compressive residual stress by FPP. On the other hand, significant deteriorations of the fatigue strength for the plasma sprayed specimen was observed in comparison with the result for the untreated specimen. As a result of fracture surface observation in plasma sprayed specimen, the defect formed by blasting before the plasma spraying at interface between substrate and HAp layer was observed at crack initiation site. Thus, the defect plays a role as the crack starter in the case of plasma sprayed specimen. Keywords: fatigue, fine particle peening, plasma spray, titanium alloy, hydroxyapatite. INTRODUCTIONTitanium alloys have been used as implant components due to their high corrosion resistance, good biological compatibility and so on. In recent years, plasma and thermal spraying have been applied for titanium alloy to form the hydroxyapatite (HAp) layer on the surface and to improve osteo-conductivity [1]- [3]. However, Laonapakul et al. reported that delamination and spallation of the coating layer formed by plasma spraying occur under cyclic loading [3]. In this study, fine particle peening (FPP) treatment was applied for titanium alloy because the FPP treatment can improve the fatigue strength of metallic materials. In addition, the FPP treatment can create transferred layer consisted shot particle [4]- [6]. Thus, it is expected that the FPP treatment using hydroxyapatite shot particles can improve the fatigue strength and create HAp layer on the substrate.The aim of this study is to form HAp transferred layer on the substrate by FPP and to compare the fatigue strength of FPP using HAp particles treated specimen and HAp coated specimen by plasma spraying. Rotating bending fatigue tests were carried out for FPP treated specimens. Tests were also conducted on untreated specimens and hydroxyapatite coated specimens by plasma spraying for comparison. Fracture surface of all the failed specimens were examined in a scanning electron microscope (SEM). The effects of FPP and plasma spray on fracture mechanisms are also discussed in the light of fractography.