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

Dense and polished samples are often used to test the in vitro biological response of biomaterials. For one commonly used bone graft substitute, α-tricalcium phosphate (α-TCP), the production is challenging due to several factors including a density difference between α-to β-TCP, the phase transition temperature occurring within a temperature range ideal for sintering, and the sensitivity of the phase equilibria to impurity elements. The aim of this study was to assess the potential of nano-crystalline α-TCP to produce dense and pure bulk α-TCP. 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, even though the temperature was in the stability region of α-TCP. This unexpected behavior was attributed to the intrinsic pressure occurring within the material during sintering and which favoured the denser β-TCP phase.