Effect of Room and High Temperature Compaction on Optical and Mechanical Properties of HIPed Transparent Spinel Ceramics

Abstract

Spinel specimens are processed through Hot Pressing at 1620 °C and 20 MPa (designated as HPS) and uniaxial pressing at room temperature with 80 MPa followed by pressure‐less sintering at 1650 °C (designated as CS). HPS exhibit translucency with marginally higher density (99.8% of theoretical density (TD)) in comparison to CS with 99.5% of TD and negligible transmission value. HPS samples have shown a transmission of 40 and 76% in visible and midwave infrared (MWIR) region, respectively. Both HPS and CS samples are further subjected to Hot Isostatic Pressing (designated as HPS + HIP and CS + HIP, respectively), at identical conditions of 1800 °C and 195 MPa to achieve close to TD and transparency. HPS + HIP and CS + HIP samples enhance their transmission to 78 and 71% in visible region and 86 and 79% in MWIR region, respectively. Although most important parameter for transparent ceramics, i.e., transmission values are relatively high for HPS + HIP samples and exhibit substantially lower hardness and flexural strength values in comparison to CS + HIP samples at room and elevated temperature. Fractographic studies of samples fail under flexure at room and elevated temperature have exhibited cleavage fracture with longer facet length for HPS + HIP correlating well with microstructure. Difference in mechanical properties can be attributed to differences in grain size produced by two methodologies.