Influence of Mg-doping, calcium pyrophosphate impurities and cooling rate on the allotropic α ↔ β-tricalcium phosphate phase transformations

Alpha and beta-tricalcium phosphates are allotropic phases which play a very important role as bone graft substitutes, namely in calcium phosphate cements. Despite extensive research efforts, contradictory reports exist on the importance of quenching for maintaining α-TCP purity. The role of calcium pyrophosphate impurities derived from a certain calcium-deficiency, hydroxyapatite impurities derived from calcium excess, and various ionic substitutions on thermal stability of these phases was not yet fully disclosed. The present work reports on the kinetics of α ↔ β-TCP phase transformations of calcium-deficient TCP powders with different Mg-doping extents (0-5 mol%) prepared by precipitation. Mg clearly enhanced the thermal stability of β-TCP. The effect of cooling rate was more complex and interdependent on the Mg content and the heat treatment schedule. High α-TCP contents were retained upon cooling at 5 °C min−1 for Mg ≤ 1 mol% or upon quenching from 1550 °C for Mg contents ≤2 mol%.