SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB2 powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB2 powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB2 and a compositionally similar ZrB2–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB2 reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB2 powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB2 powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth.