Synthesis and characterization of plasma sprayed functional gradient bioceramic coating for medical implant applications

Abstract

The dissolution of plasma-sprayed hydroxyapatite (HA) coating in physiological media is complex and involves numerous factors. It is generally believed that the highly crystalline HA coatings are beneficial for long-term stability, whereas amorphous layers dissolve more quickly and promote early biointegration. In the present study, the functionally gradient coating (FGC) was formulated in three successive layers over Ti6Al4V alloy using an atmospheric plasma spray (APS) process. At the inner layer, the titania (TiO2) bond coat is applied to enhance adhesion strength. In the middle layer, crystalline hydroxyapatite (HA) coating was deposited to provide long-term stability. In contrast, the outer amorphous layer of as-sprayed HA coating was assumed to promote the early implant bonding with host bone. The crystallinity level of the middle HA layer was increased by post coating heat treatment at 700̊ C for 1 h. There is an improvement in microhardness value after heat treatment from 276.6 ± 91.2 HV to 316.3 ± 41.4 HV. The corresponding porosity and surface roughness values decreased from 15 to 11% and 5.81 ± 1.00 to 5.22 ± 0.84 μm, respectively. In addition, the post coating heat treatment of the middle layer minimized the plasma spray process defects, such as micro-cracks and voids. Furthermore, a corrosion study in the ringer solution shows that heat treatment effectively enhanced the corrosion resistance of the middle crystalline HA layer. Nevertheless, the TiO2 bond coat enhanced the adhesion strength of functional gradient coating from 21.70 MPa to 30.50 MPa. The findings of this study indicate that surface modification with functional gradient coating is a promising approach to improve the performance of the Ti6Al4V alloy for biomedical applications.