Phase evolution of hydroxapatite coatings suspension plasma sprayed using variable parameters in simulated body fluid

Fine, hydroxyapatite (HA) powder, synthesized using calcium nitrate and diammonium nitrate was formulated with water and alcohol to obtain a suspension used to plasma spray coatings onto titanium substrates. The deposition process was optimized using statistical design of 2ⁿ experiments with two variables: spray distance and electric power input to arc plasma. The sprayed coatings were soaked in simulated body fluid (SBF) for the periods of 3, 7, 14, 28, and 60 days at controlled temperature of 37 °C. The reference intensity ratio (RIR) method basing onto X-ray diffraction (XRD) data was used to determine quantitatively the phase composition of as-sprayed and soaked deposits. Electron probe microanalysis (EPMA) enabled to make the profiles of calcium to phosphorus atomic ratio along a line though the coatings' cross sections. Raman spectroscopy of selected samples enabled to localize the different crystal phases in sprayed coatings. The coatings included some porosity and have two characteristic zones: (i) dense zone corresponding to the lamellas, observed usually in thermally sprayed coatings; (ii) sintered zone containing fine hydroxyapatite grains which correspond to the fine solids from the suspension which were agglomerated in the cold regions of plasma jet and sintered on the substrate. The soaking in SBF homogenizes the morphology of coatings. The sintered zone disappears and the pores get filled by the reprecipitated calcium phosphates.