Joining with ceramic materials is one of the important subjects in the application of vanadium for the fusion environment. In order to develop and understand the solid-state bonding of vanadium with oxide ceramics, conditions of diffusion bonding between vanadium and alumina were examined, and fundamental properties of bonded couples were studied. As alumina has a linear thermal expansion coefficient relatively close to the value of vanadium, enough bond strength is expected between them.
Couples of vanadium and alumina disks with 8-mm diameter and 3-mm thickness were heat-treated in a vacuum of 1 × 10-3 Pa in the temperature range of 1673 to 1873 K under the compression of 2.8 to 14.6 MPa. Both single crystals and polycrystals of alumina and a polycrystalline vanadium were used. The degree of bonding depended on the temperature and stress. In the case of alumina single crystals, relaxation of residual stress could be accompanied with crack nucleation and also twin-like deformation.
Transmission electron microscopy and electron-probe X-ray microanalysis were performed in the region including the metal/ceramic interface. Both straight and curved interfaces were observed along with several orientation relationships across the interface. No intermediate phase was observed by transmission electron microscope (TEM) and electron probe micro-analysis (EPMA), nor was a hardened layer on the vanadium side near the interface observed by the micro Knoop indentation technique.
The maximum strength of bonded couples was determined to be 83 MPa for the polycrystalline alumina by means of a miniature three-point bend test. Scanning electron microscopy showed a relatively wavy fracture mode along the interface.
The bonding mechanism and the method to improve bond properties are discussed.