STRUCTURAL CHARACTERIZATION OF PLASMA-SPRAYED HYDROXYLAPATITE COATINGS

Plasma-sprayed hydroxylapatite coatings, widely used on metallic surgi cal implants to improve their adhesion to bone, are formed by rapid qu enching of molten, or partly molten, particles which impact the substr ate at high velocity. The performance of these coatings in the body de pends upon their structure, which is not well understood. Coatings pre pared under a range of spraying conditions have been studied by X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimet ric analysis (TGA) and solid-state nuclear magnetic resonance spectros copy (NMR). The results suggest that particles partly melt and lose co mbined water at lower plasma torch input powers forming a glass, by qu enching of the liquid phase, and an OH-depleted hydroxy]apatite residu al crystalline phase. At higher power inputs an increasing amount of P 2O5 is also lost and the coatings contain CaO and Ca4P2O9. Heat treatm ent of coatings in air at 600 degrees C results in crystallization of the glass phase and reaction with water vapour to form hydroxylapatite . The results show that XRD is relatively insensitive to some of the s tructural details of hydroxylapatite coatings which may be significant to their performance. NMR provides more structural information and is a significant tool for coating characterization.