Improving the thermal shock resistance of zirconium diboride ceramics

Zirconium diboride (ZrB2) and ZrB2--SiC ceramics with densities greater than 99% were fabricated by hot pressing ZrB 2 and SiC powders and reactively hot pressing ZrH2, B 4C and Si to form ZrB2-27 vol% SiC. Thermophysical properties were investigated for hot pressed ZrB2 and ZrB2-30 vol% SiC ceramics. The thermal conductivity of ZrB2 increased from 56 W m-1 K-1 at room temperature to 67.0 W m -1 K-1 at 1675 K, whereas the thermal conductivity of ZrB2-SiC decreased from 62.0 W m-1 K-1 to 56 W m-1 K-1 over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends.; Thermal shock of high density ZrB2, ZrB2--30 vol% SiC and ZrB2--30 vol% SiC/graphite - 15 vol% SiC fibrous monoliths was studied. Experimental thermal shock values measured during a water quench test were the same for both materials (DeltaT crit∼400°C). A finite element model was used to estimate the temperature gradients and stresses in both ceramics during quench testing. The model predicted that maximum thermal stresses exceeded the strength of ZrB2 (568 MPa) but not ZrB2-30 vol% SiC (863 MPa). The lower than predicted thermal shock resistance of ZrB2-SiC was attributed to the non-uniform cooling between the ZrB2 matrix and the SiC particulate phase. Water quench thermal shock testing of ZrB2-based fibrous monolith ceramics had a critical thermal shock temperature (Delta Tcrit) of 1400°C, a 250% improvement over the previously reported DeltaTcrit values of ZrB2 and ZrB2-30 vol.% SiC of similar dimensions (4 x 3 x 45 mm). The improvement in thermal shock resistance was attributed to cell boundary crack propagation and crack deflection around the load bearing cells.