Density and phase-purity of α-TCP obtained by sintering of nano-crystalline powder

Dense and polished samples are sometimes used to test the in vitro biological response of biomaterials. However, their production can be challenging, for example for α-tricalcium phosphate (α-TCP), a commonly-used bone graft substitute. In this particular case, the ideal sintering conditions are in a temperature range close to the β-α-TCP phase transition (1125 °C). This phase transition is characterized by a 7 % volume increase which typically leads to the formation of cracks. Additionally, the production of a powder suitable for ceramic processing is difficult because α-TCP can only be produced via a thermal process. The initial aim of this study was to produce dense and polished α-TCP samples using an innovative method to produce nano-crystalline α-TCP. However, after observing the formation of β-TCP phase above the β-α-TCP phase transition (1125 °C), the focus of this study was shifted towards this unexpected phenomenon. Specifically, uniaxially compressed bulk samples were sintered at between 1160 and 1240 °C for different durations up to one week. The resulting density reached up to 95 % and was positively associated with green body density and sintering time, while being initially highest when sintered at the lowest temperature. A transient appearance of up to 6 wt% β-TCP was observed during sintering. The higher the sintering temperature was, the shorter and the smaller this transient appearance was. This behavior was attributed to the intrinsic pressure occurring within the material during sintering and which favoured the denser β-TCP phase.