Effect of rare earth oxides on the microstructure and properties of mullite/hBN composites

Mullite/hBN composites were fabricated with different rare earth oxides additives (ReO: Er₂O₃, CeO₂, La₂O₃, Lu₂O₃) by pressureless sintering at 1600 °C for 4 h. The impacts of ReO on the phase composition, microstructure, mechanical, dielectric and tribological properties of the composites were investigated. XRD results showed that all the ReO additives were beneficial to the formation of mullite phase. With the decrease in the ionic radius of the ReO, the mullite grains of the composite were refined while their mechanical properties were increased. The sample sintered with Lu₂O₃ showed the smallest grain size and the most excellent mechanical properties, e.g., its relative density, flexure strength, fracture toughness and hardness reached 93.7%, 222 MPa, 3.2 MPa m^1/2 and 6.02 GPa, respectively. Due to the different porosity and phase composition of the composites, the sample sintered with La₂O₃ had the lowest dielectric constant while the sample sintered with Er₂O₃ exhibited the lowest dielectric loss. Attributing to the highest hardness and fracture toughness, the sample sintered with Lu₂O₃ showed the best tribological properties.