The effect of annealing, precipitation-strengthening, and compressive coating processes on sapphire strength

In this study, annealing, precipitation-strengthening, and compressive coating processes were used to enhance the strength of c-axis sapphire crystal. In terms of the annealing process, the biaxial strength of the specimen was improved with increasing annealing temperatures. In particular the biaxial strength increased more significantly when the annealing temperature was beyond a certain value where the heat polishing occurred. After an Mg-sputtered sapphire crystal substrate went through a precipitation-strengthening process, Mg–Al spinel precipitation was found to occur. Sapphire that had undergone a precipitation-strengthening process had the best strength reliability in comparison with other strengthening processes. But the transmittance of sapphire decrease slightly when spinel precipitates developed. After implementing the compressive coating process, the crystallized silicon nitride layer made the sapphire approximately twice as strong as an uncoated sample. The results show that a crystallized silicon nitride layer produces the greatest strength, when compared with any other strengthening processes.