AMORPHOUS PHASE-FORMATION IN PLASMA-SPRAYED HYDROXYAPATITE COATINGS

The amorphous phase content of air plasma-sprayed hydroxyapatite coati ngs is dependent upon spraying and deposition conditions. X-ray diffra ction and optical microscopy were used to investigate the influence of spray parameters on the formation of the amorphous phase. Results sho w three factors which most influence the formation of the amorphous ph ase: dehydroxylation of the molten particle during flight, the cooling rate as it impinges onto the metal substrate, and the substrate tempe rature. Crystalline regions were identified as unmelted particles and elongated recrystallized areas. Amorphous phase regions vary throughou t the coating but are more commonly found at the coating-substrate int erface, i.e., the regions decrease toward the surface of the coating. Such an inhomogeneous distribution of phase content is expected to aff ect the clinical process of bone deposition, and therefore successful implant fixation. (C) 1998 John Wiley & Sons, Inc.