High strength bonding of sapphire

by Fiore, Daniel F.

New high performance optical systems require highly durable, broadband window materials in larger sizes than are currently available. To meet this need, a low cost edgebonding process was developed for producing large area sapphire windows from smaller, melt-grown blanks. The method uses a polycrystalline alumina interlayer to promote diffusion and single crystal conversion at the interface between two sapphire substrates and produce high strength bonds. The goal of the current research effort is to determine the optimal alumina composition for maximum bond strength. Polycrystalline alumina fillets containing various chemical additives were prepared by tape casting for use in bonding trials. Oriented sapphire blanks were edge bonded in a furnace with special fixtures to accurately align and apply a load to the components during heat treatment. This approach is consistent with and builds upon the methods used in the previous edge-bonding studies. Flexure strength of the bonded samples, as compared to monolithic sapphire, was used as the performance metric. Additional bonding runs were carried out using the highest performing fillet composition in order to provide a sufficient number of specimens to conduct a Weibull analysis of the failure probability of the bonded material as a function of applied stress. A high purity alumina composition containing 3 wt.% SiO2, 0.05 wt.% MgO, and 0.05 wt.% Ti, produced the highest strength bonds. This composition yielded an average fracture strength of 255 MPa (37 kpsi), a Weibull modulus of 8.2, and a characteristic i strength of 269 MPa (39 kpsi). These results compare favorably to monolithic sapphire specimens which yielded an average fracture strength of 284 MPa (41 kpsi). ii