Influence of dopants on thermal stability and densification of β-tricalcium phosphate powders

In this work, β-tricalcium phosphate (β-TCP) is doped with Mg2+ and Sr2+ in order to postpone the problematic β-TCP → α-TCP transition occurring from 1125 °C. Indeed, this phase transition occurs with a large lattice expansion during sintering causing microcracks and a reduced shrinkage leading to poor mechanical properties of ceramic parts. The substitution of calcium by cations like Mg2+ and Sr2+ allows to increase the temperature corresponding to β→α-TCP transition and therefore to increase the sintering temperature and achieve higher densification level. Three doping rates for each dopant individually (2.25, 4.50 and 9.00 mol%) and two co-doped compositions (2.00 mol% and 4.00 mol% of Mg2+ and Sr2+ simultaneously) were tested. Thermal and dilatometric analyses were used to evaluate the effects of Mg2+ and Sr2+ doping on the thermal stability of β-TCP. It has been shown that all doping, except the 2.25 mol% Sr-TCP, postpone the β→α transition. These results were confirmed after conventional and microwave sintering. Indeed, X-ray diffraction analyses of sintered pellets showed that the only phase present is β-TCP up to 1300 °C in all compositions except for the 2.25 mol% Sr-TCP with both sintering ways. Moreover, a higher densification rate is observed with the presence of dopants compared to undoped β-TCP according to the microstructures and relative densities close to 100%. Finally, the duration of microwave sintering is almost sixteen times shorter compared to conventional sintering allowing rapid densification with similar final relative densities and microstructures with finer grains.