Phase transformation and densification of cBN-Y2O3 nanocomposites by high pressure and high temperature

Composite cBN- Y₂O₃ ceramics with a 30 vol% cBN content were successfully fabricated at sintering temperature ranging from 800 to 1600 °C under a high pressure of 7.2 GPa. The high sintering pressure was observed to prevent phase transformation of cubic BN phase, while simultaneously inducing the transformation from cubic Y₂O₃ to monoclinic Y₂O₃ phase. No significant change in the grain size of cBN within the composites was observed, attributed to the low sintering temperatures and short holding time. In contrast, the grain size of Y₂O₃ within the cBN- Y₂O₃ composite was reduced to less than 100 nm after sintering at 800 and 1000 °C. This observation indicates that the high pressure-induced phase transformation of Y₂O₃contributed to its grain refinement. As a result of the retained cubic BN phase and refined Y₂O₃ grains, the cBN- Y₂O₃ composite ceramics sintered for 60 s achieved a relative density of 94 % and a hardness of approximately 11 GPa. These results demonstrated that the high pressure and high temperature (HPHT) technique is a promising route in achieving fully dense cBN-Y₂O₃ composite ceramics with enhanced performance.