Obtaining of uniform hydroxyapatite-based coatings by thermal substrate method

Abstract

The aim of our investigation is the processes of crystallization of calcium phosphate coatings for medical
implants obtained by the thermal substrate method on Ti6Al4V substrates, namely dependence of phase
analysis and coatings morphology on time and temperature of deposition. The thermal substrate method
allows obtaining coatings for medical implants, the main phase of which is hydroxyapatite, in a short
period of time (one hour). Coatings were deposited from the aqueous solution which included 10 mmol/dm³
CaCl₂ and 6 mmol/dm³ NaH₂PO₄ onto Ti6Al4V substrates, which were heated with the passage of
alternating current to the necessary temperature. This process based on decreasing hydroxyapatite solubility
with increasing temperature. The characteristics of deposited coatings such as crystal morphology depend
on the temperature of the substrate, solution pH, surface roughness, and heating time. During experiments, pH was held constantly in the range of 6,5 – 6,68. The heating time and temperature of the substrate were changed. X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDS), and scanning electron microscopy (SEM)were employed in the study of coatings morphology, phase, and elemental analysis after deposition. The influence of experimental conditions on coatings characteristics was examined. After 10 minutes of deposition, the calcium-phosphate coating has been precipitated locally on the substrate surface, but the coating was not dense and uniform. It has been shown, that for obtaining uniform hydroxyapatite coating with a well-developed structure quite enough time of deposition is 60 minutes at substrate temperature 105-110ºС. A substrate temperature 75ºC a little amount of nano amorphous hydroxyapatite has been obtained. Investigation of the process of crystallization allows controlling morphology, crystalline
structure, and molar ratio Ca/P changing experimental characteristics.