The Role of MgAl2O4 Powder Packing on Phase Stability of Hydroxyapatite During Sintering

MgAl2O4 (spinel) was utilized as a packing powder in the sintering of hydroxyapatite (HAp) and the composite of HAp/3 mol% Y2O3-stabilized tetragonal zirconia (3Y-TZP). The influence of spinel on phase stability of HAp was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and electron probe microanalysis (EPMA) to reveal the reaction in the vicinity of the interface between HAp and spinel. When covered with spinel powder, decomposition temperatures for both HAp monolith and HAp in the composite were raised from 1360°C to 1470°C and from 930°C to 1280°C, respectively. SEM images supported the role of spinel on retardation of the decomposition, showing a dense cross section of the monolith after sintering for 2 h at 1400°C with the spinel as opposed to a porous feature without the covering. XRD results indicated that the increase in the decomposition temperatures was accompanied by a decrease in the a-axis dimension of the hexagonal structure of HAp, probably as a result of the substitution of F− for OH−. EPMA revealed that negligible reaction occurred between HAp and spinel even at 1500°C, but the Ca2+ in HAp diffused about 20 μm into 3Y-TZP to form a cubic zirconia solid solution at 1275°C, resulting in the decomposition. The involvement of F− ion in the contraction of a-axis parameter and the consequent phase stability were manifested by an increase in the Raman band of the symmetric stretching of the P–O bonds at 962.3 cm−1 and the appearance of a band for fluoroapatite at 3538 cm−1.