STUDIES ON DIFFUSION MAXIMUM IN X-RAY-DIFFRACTION PATTERNS OF PLASMA-SPRAYED HYDROXYAPATITE COATINGS

Study of an amorphous phase in plasma-sprayed hydroxyapatite (HA) coat ings is important owing to its unique characteristics and nonnegligibl e amount of the amorphous phase compared to crystalline HA. However, l ittle is known about the component parts of an amorphous phase. It is known that amorphous phase usually appears as the diffusion maximum (D -max) in X-ray diffraction (XRD) patterns. Analyzing D-max, including the position (P-max) and area of D-max, we can indicate the component parts of an amorphous phase and their transitions. In this study, the variation of D-max in XRD patterns of the coatings during plasma spray ing, in postheating, and in dissolving in vitro was studied with the a id of XRD. It was found that component parts of the amorphous phase in the coating varied with increasing thickness, consisting of two parts represented by D-max1, located between 29.4 and 29.8 degrees (2 theta ), and D-max2, located between 31.0 and 31.4 degrees (2 theta). It was concluded that D-max3, located between 32.0 and 32.4 degrees (2 theta ), should be referred to as nanocrystals of HA. In addition, the parti cle size of the starting powder may affect the component parts of the amorphous phase in the coating in addition to thickness. With vacuum h eating (650 degrees C) and water vapor treatment at a low temperature (125 degrees C) in a saturated vaporic atmosphere, transition of the a morphous components was not as efficient as that at 490 degrees C with water vapor. The reason might be that the amorphous-to-crystalline HA conversion is dependent on both temperature and water vapor pressure. It was found that amorphous components were transformed completely in to crystalline HA after heating at 490 degrees C with a partial water vapor pressure of 0.01 MPa for 2 h. It was concluded that the unstable amorphous components (D-max1, D-max2) converted into more stable nano crystals of HA (D-max3). Degradation in vitro showed that D-max3 was m ore stable than D-max1 and D-max2. It was concluded that nucleation of apatite in vitro should be attributed to nanocrystals of HA (D-max3) except for the amorphous components. It is recommended that the optima l phasic contents of the plasma-sprayed HA coating be mainly composed of crystalline HA and nanocrystals of HA (D-max3) in terms of the stab ility and biocompatibility of the coating. (C) 1998 John Wiley & Sons, Inc.