Characterization of chemical inhomogeneity in plasma-sprayed hydroxyapatite coatings

Successful applications of plasma-sprayed hydroxyapatite (HA) coating for implants rely on understanding characteristics of the coatingʹs microstructure, particularly its inhomogeneity. We explored three new techniques for characterizing the chemical inhomogeneity of sprayed HA coatings on titanium substrate: micro-Raman spectroscopy (MRS), positive and negative ion ratios of time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the energy loss peaks of X-ray photoelectron spectroscopy (XPS). The results showed that MRS effectively revealed a chemical gradient in the direction of the coating thickness and a decrease in crystallinity from the surface to interface within the as-sprayed coatings. The post-spray treatment effectively promoted homogeneity between surface and the coating/Ti interfaces. Elucidating the chemistry of the sprayed HA coatings using the ion ratios of ToF-SIMS and the energy loss peaks of XPS remains a challenge, even though such techniques can be used to identify certain calcium phosphate phases in pure powder form.