Bonding of zirconia to super alloy with the active brazing technique

Zirconia stabilized with yttria was bonded to a Ni-based superalloy by active brazing. An interlayer with the composition of the silver–copper eutectic plus 1.75 wt.% titanium was used as brazing material. Various brazing heating cycles were performed using a furnace with graphite heating elements and under flowing argon. The microstructure of the bonded couples is related to the chemical reactions occurring during the brazing cycles, to the properties of the specific compounds formed and to the interdiffusion phenomena occurring across the interfaces. Critical feature for the success of the joint is the nature of the interfaces that the brazing interlayer formed with the superalloy and the zirconia. At the interface with the superalloy adhesion is obtained under a relatively wide range of experimental conditions. On the contrary, at the interface with zirconia, a good wetting and adhesion occurred only when a titanium oxide sublayer with a specific thickness formed. The best results were obtained with a maximum temperature between 870 and 900°C, a soaking time of 10 min and a fast heating rate (10°C/min). The optimal thickness of the TiOx sublayer was less than 1 μm.