Lowering hot isostatic pressing temperature of ultrafine‐grained MgAl ₂ O ₄ transparent ceramics by two‐step pre‐sintering

Abstract Cubic‐phase magnesium‐aluminum spinel (MAS) has attractive optical, mechanical, and thermal properties and has been widely used as transparent ceramics in a number of applications. However, the processing of MAS transparent ceramics typically requires pre‐sintering and hot isostatic pressing (HIP) both at very high temperatures (e.g., above 1600°C) over an extended time, which coarsens the microstructure thus impairing mechanical properties. There is an urgent need to lower the sintering temperature while maintaining high optical transparency. Here we reported new progress in combining two‐step sintering in the pressureless pre‐sintering step and low‐temperature HIP in the post‐treatment step, using high‐quality green bodies produced by pressure filtration. With this strategy, high‐quality MAS transparent ceramics are delivered. The main benefits include 200°C lower HIP temperature, a 33% smaller average grain size of 350 nm, and a higher hardness of 14.8 GPa. Such a method could be extended to many other transparent ceramics.