Densification behavior involving creep during spark plasma sintering of ZrB2-SiC based ultra-high temperature ceramic composites

A comparative study has been carried out on the densification behavior of the ZrB2-SiC composites with B4C and C as additives along with varying amounts of LaB6 (7, 10 and 14 vol%) during spark plasma sintering (SPS) at 1600 degrees C and 1800 degrees C under applied ram pressures of 50 MPa and 70 MPa, respectively. The densification kinetics for both the sintering conditions has been analyzed with emphasis on the role of additives and LaB6 for oxide reduction as well as deformation by creep. For SPS at both temperatures, the relative density is increased with increasing volume fraction of LaB6, with higher values being observed on sintering at 1600 degrees C. The creep during SPS at 1600 degrees C being analyzed using the Bernard-Granger model has exhibited stress exponent (n) similar to 5-6 suggesting dislocation-climb as the operating mechanism. However, for sintering at 1800 degrees C, observation of n similar to 2 for the composites with 7 and 10 vol% LaB6 suggests grain boundary sliding aided by diffusion through grain boundary glassy phase, whereas n similar to 3 for the 14 vol% LaB6 composite indicates dislocation glide-controlled creep. Lower densification on SPS at 1800 degrees C has been ascribed to formation and escape of gaseous products like B2O3, SiO and CO during sintering. For the composites sintered at 1600 degrees C with density 97.5%, hardness increases with decreasing grain size, whereas it is related to relative density in the composites with lower density being processed by SPS at 1800 degrees C.