Residual stresses in plasma-sprayed hydroxyapatite coatings

Abstract

Plasma-sprayed hydroxyapatite (HAp) coatings on Ti6Al4V alloy implants are currently being investigated as an approach to achieving reliable fixation of an implant in bone. Many aspects of these coatings, such as morphology, crystalline, phase composition, bonding strength and behavior in vitro or in vivo, etc., have been extensively investigated [1–7]. Because plasma-spraying technique involves high temperature, and there is the difference of thermal expansion coefficient between HAp and Ti6Al4V alloy, residual stresses most probably exist in the coating, especially in the thick coatings. It is well known that residual stress status has an important role in adhesion, fatigue and stress corrosion behavior of coatings and further affects the service lifetime. Several researchers have reported a decrease of bonding strength between thicker plasmasprayed HAp coatings and substrates and/or bones as the result of residual stresses [2–4]. There has been a lack of information on the status and distribution of residual stresses in plasma-sprayed HAp coatings, however, necessitating further research. Here, the residual stresses in HAp coatings are measured for the first time by a hole-drilling method. Ti6Al4V alloy substrates were mechanically ground and subsequently blasted with Al2O3 grits to make surface roughness Ra equal to 9 μm prior to spraying. Highly pure HAp powders with sizes of 20–45 μm were used in the plasma-spraying process. The plasmaspraying conditions were as follows: primary gas (argon) flow rate, 41 l/min; carrier gas (argon) flow rate, 4 l/min; arc current, 800 A; powder transport, 10–15 g/min; and stand-off distance, 10 cm. Two types of coatings, 80 and 200μm in thickness, were prepared using a Metco system under the same spraying condition. The plasma-spraying procedure was carried out at UTT in France. X-ray diffraction (XRD) was used to analyze the phase composition, as shown in Fig. 1. To evaluate the degree of coating crystallinity, the relative crystallinity index (IOC) was introduced and defined as follows [1]: