Sintering of mixed Ca-K-Na phosphates: Spark Plasma Sintering vs Flash-sintering

A comparison between different field-assisted sintering methods (FAST), namely Spark Plasma Sintering (SPS) and flash-sintering (FS), is reported for calcium and alkaline phosphates especially with regards to their microstructure. A reduced average grain size upon SPS is shown when pressure is decreased. Shrinkage curves during flash sintering are compared with similar conventional sintering curve. Flash sintered ceramics possess unusual microstructural features, including a density/grain size gradient where the surface of the specimen is denser and possesses a coarser grains with respect to the core. Such microstructural gradient is opposite to what generally observed, the surface being colder due to the heat radiation toward the environment. The change in the average grain size for each sintering method is related to the process parameters.