THE EFFECTS OF SINTERING ATMOSPHERE ON THE CHEMICAL COMPATIBILITY OF HYDROXYAPATITE AND PARTICULATE ADDITIVES AT 1200-DEGREES-C

According to Le Chatelier's principle, dehydration and the associated decomposition of hydroxyapatite (HAP) to biodegradable unhydrated calcium phosphates during sintering may be suppressed under a moist sintering atmosphere (thermodynamic effect), or possibly under a pressurized sintering atmosphere (physical effect), by opposing the release of water. The present study explored this possibility. High-purity powdered additives were used to minimize impurity and morphological effects. Al2O3, C, SiC, SiO2, ZrO2, and 316L stainless steel were all trialled at an addition level of 20 vol%. Heat treatment was at 1200 degrees C for 1 h under two experimental atmospheres and two corresponding control atmospheres: flowing H2O/O-2 mix-ambient air as a control; pressurized (1 MPa) argon-ambient argon (0.1 MPa) as a control. Specimens were analysed for decomposition by X-ray diffraction (XRD), for densification by porosity measurement, and for microstructural uniformity by energy dispersive spectroscopy (EDS) and image analysis. Significant decomposition occurred under all atmospheres with the exception of flowing H2O/O-2 which eliminated decomposition in the HAP-Al2O3, HAP-ZrO2, and HAP-316L systems, and reduced the decomposition levels from near completion to similar to 50% in the HAP-SiC and HAP-SiO2 systems. Moistureless pressurization had little effect. Microstructural uniformity was confirmed. No generalized atmosphere-densification interrelationships were observed.