Novel processing of hydroxyapatite-zirconia composites using nano-sized particles

Abstract

Hydroxyapatite, HA, is employed in the repair of skeletal defects, because it closely resembles the mineral phase in bone and is bioactive. However, its relatively poor strength has limited its use in major load-bearing applications. Attempts have been made to improve the strength of HA by the incorporation of a second ceramic phase, such as ZrO2 phases (e.g. yttria-doped tetragonal zirconia polycrystals, Y-TZP), but the addition of a second phase can have inherent disadvantages. Sintering HA {Ca10(PO4)6(OH)2} at high temperatures can result in the formation of decomposition products such as tri-calcium phosphate, TCP {Ca3(PO4)2} and tetracalcium phosphate, TeCP {Ca4(P2O9)}[1, 2]. TeCP can decompose further to TCP and calcium oxide [CaO] at higher temperatures. These secondary phases have, in certain instances, been reported to adversely affect the biological response [3, 4]. When a ZrO2 phase, such as Y-TZP, is added to HA, high temperatures of 1200–1400 ◦C are commonly required to sinter these composites to high density, but HA has been reported to be thermally unstable above 1300–1400 ◦C [5, 6]. It has commonly been observed that phase transformations/decompositions occur extensively during sintering of these composites at temperatures even below 1200 ◦C. For example, at 1150 ◦C, HA will react with the ZrO2 phase, with the formation of cubic calcia-stabilized ZrO2, and subsequently calcium zirconate (CaZrO3) [7]. Increasing the sinter temperature further increases the amounts of β-TCP and CaZrO3. At a sinter temperature of 1400 ◦C, virtually no HA remains and some α-TCP forms from β-TCP. This phase decomposition means that the advantage of combining a low-strength bioactive ceramic (HA) with a high-strength, bioinert ceramic (Y-TZP) is lost. This problem of decomposition reactions between the HA and the ZrO2 phases during sintering have limited the applications of this composite, as the mechanical and biological properties are compromised by the formation of undesirable secondary phases. In this study, a novel approach has been used to produce composites of HA with ZrO2 (Y-TZP) additions between 0 and 10 wt % at a low sinter temperature (750 ◦C), in order to limit the amount of reaction between the HA and ZrO2. The aim was to find a