A review on processing polycrystalline magnesium aluminate spinel (MgAl2O4): Sintering techniques, material properties and machinability

Transparent polycrystalline magnesium aluminate spinel (PMAS), owing to its high optical transparency with acceptable mechanical properties and chemical stabilities, exhibits great potential for optical applications under extreme environments and operating conditions. The first stage of preparing highly transparent PMAS is attainable with the rapidly maturing sintering technology. However, the machining technologies aiming to achieve high shape accuracy and surface quality on PMAS are still restricted by its poor machinability and limited endeavours to survey. This paper provides a comprehensive collection of PMAS processing strategies. The state-of-the-art techniques for sintering PMAS are reviewed with a focus on the technical details and intimate relationship with the material properties. The fundamental material properties of PMAS including the crystal structure, mechanical properties and damage mechanisms are systematically discussed. These characteristics inherently govern the material removal mechanisms and the generation of surface damages in the subsequent machining processes. The surveyed fundamental studies signify that the combination of high brittleness, strong anisotropy within each single-crystal grain and polycrystalline structure is the major obstacle in achieving damage-free surface finish. In this regard, further explorations of improving machining performance with advanced machining technologies and enhancing the machinability through optimising sintering strategies are expected in the outlook.