Spark and plasma in spark plasma sintering of rigid ceramic nanoparticles: A model system of YAG

Yttrium aluminum garnet (YAG) nano-particles were spark plasma sintered between 1100 °C and 1400 °C under 2–100 MPa pressure without isothermal treatments. The span of relative density between 48% and 99% enabled microstructural examination at different stages of the densification. Electron microscopy examination showed material jets with amorphous character connecting between the spherical nano-particles, which were related to surface melting. Aligned nano-particles within the nano-clusters, apparently affected by high local electric fields, were observed in the partially dense microstructure. Rapid densification from 1200 °C was related to densification by nano-particle sliding and rotation assisted by surface softening. The observed microstructural features were discussed with respect to sparking and plasma formation during the SPS. The occurrence of plasma was explained by means of the plasma formation- plastic deformation diagram.