Impact formation and microstructure characterization of thermal sprayed hydroxyapatite/titania composite coatings

Formation mechanism of hydroxyapatite (HA)/titania (TiO₂) composite coating deposited by high velocity oxy-fuel (HVOF) thermal spray process was studied, and its structural characterization was conducted and elaborated in this paper. The impact theory was employed to analyze the formation procedure of the HA/titania composite coatings. Results revealed that the crater caused by the impact of entirely unmelted TiO₂ particles on the HA matrix during coating formation was of smaller dimensions than the original size of the reinforcements. It was found that chemical reaction between the mechanically blended HA and TiO₂ powder took place exclusively during the impingement stage, and calcium titanate, CaTiO₃, was one notable by-product. The bonding between the HA matrix and TiO₂ reinforcement might have been achieved predominantly through a chemical bond that resulted from the mutual chemical reactions among the components. Differential scanning calorimetry analyses showed that the chemical reaction between HA and TiO₂ was at ∼1410°C. The TiO₂ addition was found to exert particular effects on the thermal behavior of HA at elevated temperatures, during both heating and cooling cycles. Transmission electron microscopy observation identified the chemical reaction zone between HA and TiO₂, which revealed an improved splats' interface. The reaction zone demonstrated some influence on the grain size of HA nearby during resolidification of the melted portion. A structural model was proposed to illustrate the location of the different phases in the HA/titania composite coating.