Titanium dioxide reinforced hydroxyapatite coatings deposited by high velocity oxy-fuel (HVOF) spray

Hydroxyapatite (HA) coatings with titania addition were produced by the high velocity oxy-fuel (HVOF) spray process. Mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated to reveal the effect of the titania reinforcement on HA. Qualitative phase analysis with X-ray diffraction (XRD) showed that mutual chemical reaction between TiO₂ and HA, that formed CaTiO₃ occurred during coating formation. Differential scanning calorimetry (DSC) analysis of the starting powders showed that the mutual chemical reaction temperature was ∼1410°C and the existence of TiO₂ can effectively inhibit the decomposition of HA at elevated temperatures. The positive influence of TiO₂ addition on the shear strength was revealed. The incorporation of 10vol% TiO₂ significantly improved the Young's modulus of HA coatings from 24.82 (±2.44)GPa to 43.23 (±3.20)GPa. It decreased to 38.51 (±3.65)GPa as the amount of TiO₂ increased to 20vol%. However, the addition of TiO₂ has a negative bias on the adhesive strength of HA coatings especially when the content of TiO₂ reached 20vol%. This is attributed to the weak chemical bonding and brittle phases existing at the splats' interface that resulted from mutual chemical reactions. The fracture toughness exhibited values of 0.48 (±0.08)MPam^0.5, 0.60 (±0.07)MPam^0.5 and 0.67 (±0.06)MPam^0.5 for the HA coating, 10vol% TiO₂ blended HA coating and 20vol% TiO₂ blended HA coating respectively. The addition of TiO₂ in HA coating with the amount of less than 20vol% is suggested for satisfactory toughening effect in HVOF HA coating.