Pressureless sintering of ZrO2–ZrB2 composites: Microstructure and properties

In last couple of decades, consistent efforts in the materials community have been invested to develop tougher and stronger ceramic materials via cost effective processing techniques. In this perspective, the current work reports the development of ZrO2–ZrB2 composites via pressureless sintering. The sintering experiments were conducted on ZrO2–30 vol% ZrB2 composites in the temperature range of 1400–1600 °C for 1 h in argon atmosphere. The influence of yttria content in the zirconia matrix, as well as the sintering temperature, on the densification, microstructure and mechanical properties of the composites has been investigated. Careful analysis of the indentation data has been made and such analysis has provided an estimate of the toughness properties of the ZrO2-composites. Our experimental results reveal that the composites, processed at 1400 °C and 1500 °C exhibit excellent combinations of hardness (∼12 GPa) and fracture toughness (10–15 MPa m1/2). However, sintering at higher temperature of 1600 °C is observed to degrade mechanical properties. The mechanisms leading to the achievement of superior toughness and the possible reasons for toughness variation with yttria content, as well as with sintering temperature, are critically analyzed. The systematic measurements of the amount of m-ZrO2 phase present on polished and fracture surfaces have been used to determine the t-ZrO2 transformability and it has been observed that the measured toughness correlates well with the calculated t-ZrO2 transformability.