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 TiO 2 and HA, that formed CaTiO 3 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 TiO 2 can effectively inhibit the decomposition of HA at elevated temperatures. The positive influence of TiO 2 addition on the shear strength was revealed. The incorporation of 10 vol% TiO 2 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 TiO 2 increased to 20 vol%. However, the addition of TiO 2 has a negative bias on the adhesive strength of HA coatings especially when the content of TiO 2 reached 20 vol%. 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) MPa m 0.5, 0.60 (±0.07) MPa m 0.5 and 0.67 (±0.06) MPa m 0.5 for the HA coating, 10 vol% TiO 2 blended HA coating and 20 vol% TiO 2 blended HA coating respectively. The addition of TiO 2 in HA coating with the amount of less than 20 vol% is suggested for satisfactory toughening effect in HVOF HA coating.