HVOF Sprayed Hydroxyapatite Coatings: Powders' Melting State and Mechanical Properties

In the present study, hydroxyapatite coatings were deposited on Ti-6Al-4V alloy substrate by high velocity oxy-fuel (HVOF) spray technique. The as sprayed HA powders and coatings were analyzed with the aim to reveal the melting state of HA powders and its influence on coating properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed for the characterization of the starting powders and as-sprayed coatings. Differential scanning calorimetry (DSC) was performed to determine the recrystallization temperature of the amorphous phase in HVOF HA coating. Results show that different melting state of HA powders can be achieved through altering HA powder size and/or spray parameters. XRD result reveals that the as sprayed HA coating made from large powders with size of ∼50μm is composed of crystalline HA and very small amount of α-tricalcium phosphate (TCP). While the coatings deposited using fine powders around 30 μm demonstrated a lot of amorphous phase besides crystalline HA and small amount of α-TCP. The recrystallization temperature of the amorphous phase in HA coating is ∼720°C. The adhesive strength of the HVOF sprayed HA coatings is ∼31MPa and is largely dependent on the melting state of HA powders. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase, and simultaneously decrease the adhesive strength. It also suggests that the melted fraction of the powders is the most critical factor influencing the adhesive strength and phase composition of HVOF HA coatings. The partial melting state of HA powders is beneficial in terms of adhesive strength and crystallinity.