Aj. Ruys et al., THE EFFECTS OF SINTERING ATMOSPHERE ON THE CHEMICAL COMPATIBILITY OF HYDROXYAPATITE AND PARTICULATE ADDITIVES AT 1200-DEGREES-C, Journal of materials science. Materials in medicine, 6(5), 1995, pp. 297-301
According to Le Chatelier's principle, dehydration and the associated
decomposition of hydroxyapatite (HAP) to biodegradable unhydrated calc
ium phosphates during sintering may be suppressed under a moist sinter
ing atmosphere (thermodynamic effect), or possibly under a pressurized
sintering atmosphere (physical effect), by opposing the release of wa
ter. The present study explored this possibility. High-purity powdered
additives were used to minimize impurity and morphological effects. A
l2O3, C, SiC, SiO2, ZrO2, and 316L stainless steel were all trialled a
t an addition level of 20 vol%. Heat treatment was at 1200 degrees C f
or 1 h under two experimental atmospheres and two corresponding contro
l atmospheres: flowing H2O/O-2 mix-ambient air as a control; pressuriz
ed (1 MPa) argon-ambient argon (0.1 MPa) as a control. Specimens were
analysed for decomposition by X-ray diffraction (XRD), for densificati
on by porosity measurement, and for microstructural uniformity by ener
gy dispersive spectroscopy (EDS) and image analysis. Significant decom
position 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 comp
letion to similar to 50% in the HAP-SiC and HAP-SiO2 systems. Moisture
less pressurization had little effect. Microstructural uniformity was
confirmed. No generalized atmosphere-densification interrelationships
were observed.