Analysis of brazing process and properties of transparent joints of sapphire based on glass filler

In view of the problems that the limited size of monolithic sapphire single crystals cannot meet the demands of transparent armor and optical windows, and existing sapphire single crystal joining methods cannot obtain visible-light transparent joints, the brazing process of visible-light transparent joints of sapphire was studied. Based on LaK6 glass filler, a “pre-melting + brazing” two-step method was adopted to braze the sapphire single crystals. The pre-melting process parameters were 975 ℃/15 min, and the brazing process parameters were (800 ~ 1 000 ℃)/15 min. The results indicate that the joints are visible-light transparent. The shear strength of the joints increases initially and then decreases with the increase in brazing temperature. The highest shear strength of 44 MPa is obtained at 900 ℃/15 min, and the transmittance of the joints is higher than 80%. Microstructure analysis results show that La2Al24.4O39.6 whiskers and ZrO2 bulk phases are diffusely distributed in the brazing seam under the process conditions of (850 ~ 950 ℃)/15 min. The reaction product in the brazing seam at 1 000 ℃/15 min is ZnAl2O4. In this paper, the brazing process for obtaining visible-light transparent joints of sapphire single crystals with high shear strength is preliminarily determined, which is of great significance for realizing large-size sapphire optical-grade joining.