Powder processing effects on the rapid low-temperature densification of ZrB ₂-SiC ultra-high temperature ceramic composites using spark plasma sintering

Investigating the powder processing effects on a ZrB ₂-25 vol% SiC ceramic composite densified using spark plasma sintering (SPS) allows for identification of densification mechanisms and enables a reduction in sintering temperature to a minimum of 1650°C. Attrition milling (AM) and ball milling (BM) were investigated as processing methods to produce a fine and coarse powder densified using SPS with or without a tube furnace preheat treatment. Ceramics formed from AM and BM powders contain 1.66 wt% oxygen contamination, primarily ZrO ₂ and SiO ₂, and 0.35 wt% oxygen contamination as SiO ₂, respectively. Heat treatment slightly reduces oxygen contamination but has significant impacts on the densification mechanisms. Without heat treatment, powder coarsening dominates the initial sintering process in the SPS inhibiting densification until ∼1350°C. After heat treatment, sintering and densification is enabled at low temperature, 1000°C-1100°C. The densification of ZrB ₂-SiC composites can be broken into a two-step process with phase 1 as the sintering step based on powder surface area reduction and phase 2 as a forging step where high-temperature creep and pressure eliminate porosity after the primary grains have formed. A time-temperature-density plot illustrates the change in densification mechanism used to fully densify ZrB ₂-SiC composites in SPS.