Plasma-sprayed hydroxyapatite coating on titanium alloy with ZrO2 second phase and ZrO2 intermediate layer

The cohesive and adhesive strengths of plasma-sprayed hydroxyapatite (HA) coatings were strengthened by respectively, adding ZrO2 particles as a reinforcing second phase (HA+ZrO2 composite coating), and using an intermediate ZrO2 layer between the HA coating and the Ti alloy substrate as a bond coat (two-layer HA/ZrO2 coating). The phase contents and microstructure of the plasma-sprayed coatings were investigated by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. The surface roughness of the grit-blasted Ti alloy and the ZrO2 bond coat was measured by a surface recorder. Results indicate that the bond strength of the HA coatings increases from 28.6±3.2 MPa for HA coating to 32.5±4.2 MPa and 36.2±3.0 MPa for HA+ZrO2 and two-layer HA/ZrO2 coatings, respectively. It was found that the interface between the HA coating and the Ti alloy is a site of critical weakness when compared to the cohesive strength of the inter- and intra-lamellar structure of the HA coating. The strengthening mechanisms of the two systems are discussed.