Effect of post-CVD thermal treatments on crystallographic orientation, microstructure, mechanical and optical properties of ZnS ceramics

Chemical vapor deposited (CVD) zinc sulphide samples were subjected to hot isostatic pressing (HIP) and heat treatment under pressureless (HT) sintering conditions in argon. The effects of temperature and pressure on the crystallographic orientation, microstructure, optical and mechanical properties were studied. The removal of Zn–H absorption band present in CVD ZnS can be effected by temperature increase alone as observed for CVD + HT samples. However, a significant decrease in transmission was observed in mid IR region and is attributed to the creation of residual porosity during this process. During HIPing, on simultaneous application of temperature and pressure, the Zn–H band together with the residual porosity generated during the removal of Zn–H was also eliminated. This resulted in an increase of transmission in the mid wave IR as well as in the visible spectral region due to the considerable reduction in scatter points. A substantial increase in grain size along with a highly preferred orientation along the (1 1 1) crystallographic plane was observed as evidenced by the increase in the transverse I ratio after HIP as well as heat treatment under pressureless (HT) conditions. A decrease in the mechanical properties of the samples was also observed.