Wear Behavior of Plasma Spray Deposited and Post Heat-Treated Hydroxyapatite (HA)-Based Composite Coating on Titanium Alloy (Ti-6Al-4V) Substrate RENU KUMARI and JYOTSNA DUTTA MAJUMDAR The present study concerns a detailed evaluation of wear resistance property of plasma spray deposited composite hydroxyapatite (HA)-based (HA-50 wt pct TiO 2 and HA-10 wt pct ZrO 2 ) bioactive coatings developed on Ti-6Al-4V substrate and studying the effect of heat treatment on it. Heat treatment of plasma spray deposited samples has been carried out at 650 °C for 2 hours (for HA-50 wt pct TiO 2 coating) and at 750 °C for 2 hours (for HA-10 wt pct ZrO 2 coating). There is significant deterioration in wear resistance for HA-50 wt pctTiO 2 coating and a marginal deterioration in wear resistance for HA-10 wt pct ZrO 2 coating in as-sprayed state (as compared to as-received Ti-6Al-4V) which is, however, improved after heat treatment. The coefficient of friction is marginally increased for both HA-50 wt pct TiO 2 and HA-10 wt pct ZrO 2 coatings in as-sprayed condition as compared to Ti-6Al-4V substrate. However, coefficient of friction is decreased for both HA-50 wt pct TiO 2 and HA-10 wt pct ZrO 2 coatings after heat-treated condition as compared to Ti-6Al-4V substrate. The maximum improvement in wear resistance property is, however, observed for HA-10 wt pct ZrO 2 sample after heat treatment. The mechanism of wear has been investigated. https://doi.org/10.1007/s11661-018-4626-z Ó The Minerals, Metals & Materials Society and ASM International 2018 I. INTRODUCTION HYDROXYAPATITE HA (Ca 10 (PO 4 ) 6 (OH) 2 ) is osteoconductive and bioactive material and is widely used as coating on orthopedic implant material. [1–3] The limitation of this coating is its poor wear resistance and fracture toughness properties. [4,5] Fu et al. [6] studied the fretting wear behavior of thermal-sprayed hydroxyap- atite (HA) coating and found a coefficient of friction of 0.7 to 0.8 for HA against stainless steel (AISI 410) and concluded that the wear mechanism was delamination and abrasive in nature. In another study of Fu et al., [7] bovine albumin provides effective lubrication during fretting wear of HA coating against stainless steel surface and the mechanisms of wear were mainly delamination, pitting, and abrasive wear. Furthermore, hot isostatic pressing (HIP) post treatment densified the coating structure, decreased the generation of fretting wear debris, and improved the fretting wear resistance. Kalin et al. [8] studied the effect of counterface roughness on abrasive wear of hydroxyapatite. They found that the wear volume of hydroxyapatite increased as the surface roughness of the glass-infiltrated alumina and the load was increased. Xei et al. [9] studied the interface of HA/ ZrO 2 -graded composite in a dog lumbar vertebra bone defect model and observed high bone bonding rate (BBR) of HA/ZrO 2 -graded composite coating. Far- noush et al. [10] applied functionally graded HA/TiO 2 on Ti-6Al-4V by electrophoretic deposition method and reported improved corrosion resistance, adhesion strength, and fracture toughness. Single layer, double layer, and functionally graded coatings of HA/TiO 2 were deposited on Ti-6Al-4V titanium alloy substrate by electrophoretic deposition technique (EPD) which showed an improved adhesion strength (~ 31 MPa) in functionally graded coating as compared to HA single layer and HA/TiO 2 double layer coatings. [11] Daihua et al. [12] applied HA-TiO 2 coating on Ti-6Al-4V using hydrothermal-electrochemical method and reported that both the phase composition and the morphology of the coatings were significantly influenced by the variation in the anodizing voltage. Wang et al. [13] studied the nano-mechanical properties and bio-tribological behav- iors of nanosized HA/partially stabilized zirconia (PSZ) composites. They reported that HA-30 pct PSZ com- posite showed the highest hardness, lowest friction coefficient and wear rate. Balani et al. [14] found an improvement in fracture toughness and wear resistance RENU KUMARI is with the Department of Metallurgical and Materials Engineering, I. I. T. Kharagpur, Kharagpur, West Bengal, 721302, India and also with the Department of Metallurgical and Materials Engineering, NIFFT, Ranchi, Jharkhand, India. JYOTSNA DUTTA MAJUMDAR is with the Department of Metallurgical and Materials Engineering, I. I. T. Kharagpur. Contact e-mail: jyotsna@ metal.iitkgp.ernet.in Manuscript submitted July 7, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A