Study on brazing process and properties of sapphire transparent joint based on filter glass

Due to the limited dimension of monolithic sapphire is unsatisfied with the demands of transparent armor and optical window, moreover, none of existed brazing methods could obtain transparent joints. Brazing process of transparent joint were studied in present study. Based on LaK6 filter glass, the “pre-melting and brazing” method was adopted to braze the sapphire with the pre-melting process parameter of 975 °C/15 min and brazing process parameter of (800 °C-1000 °C)/15 min. The results of the study show that the joints were transparent. The shear strength of the joints increased initially and then decreased with the increase of brazing temperature. The highest shear strength of 44 MPa was obtained at 900 °C/15 min and 950 °C/15 min, meanwhile, the transmittance of the joints was higher than 80%. The microstructure analysis show that the La₂Al_24.4O_39.6 whiskers and ZrO₂ bulk phases were diffusely distributed in the brazing region under the process conditions of (850 ℃-950 ℃)/15 min. The generators in the brazing region at 1000 ℃/15 min were ZnAl₂O. In this paper, the brazing process for obtaining transparent sapphire single-crystal joints with high shear strength has been preliminarily determined, which is of great significance for realizing large-size sapphire optical-grade joints.