Spatial and depth-resolved studies of air plasma-sprayed hydroxyapatite coatings by means of diffraction techniques: Part I

• Hydroxyapatite was plasma-sprayed for spatial and depth-resolved investigations.
• Thermal decomposition products show distribution trends with coating depth.
• The amount of HAp phase and coating crystallinity decrease with depth of coating.
• The stress state changes from tensile to compressive around the coating midpoint.

Hydroxyapatite coatings (HAp, Ca10(PO4)6OH2) were deposited by air plasma spraying onto Ti6Al4V substrates and investigated to determine the depth-dependent behaviour of phase composition, crystallinity, and residual stress using diffractometric techniques. Through-thickness characterisation was carried out by conventional X-ray and synchrotron radiations in reflection and transmission geometries. Results showed HAp together with its thermal decomposition products, tetracalcium phosphate (TTCP), tricalcium phosphate (TCP) and calcium oxide to be present throughout the coating thickness. Quantitative phase identification employing Rietveld refinement showed HAp and TTCP to be the two major phases, with the former decreasing with depth whilst the latter increases. The largest changes were observed adjacent to the coating-substrate interface region. Crystallinity investigation showed a similar trend, revealing a more crystalline near-surface region and increasing amorphisation toward the coating-substrate interface. Residual stress investigation revealed the normal components σ11 and σ33 to be tensile and compressive, respectively. The stresses relax and increase to respective minimum and maximum within the first 145 μm. With further penetration depth, both normal stress components became tensile.