Shear bond strength of nickel/alumina interfaces diffusion bonded by HIP

The diffusion bonding by hot isostatic pressing (HIPping) of nickel powder onto alumina tubing was investigated. In particular, the effects of HIP temperature, time and pressure on the shear strength of the ceramic/metal interface were examined. Without exception, the nickel aluminate spinel, NiAl₂O₄, was observed at the ceramic/metal interfaces of all the specimens, which establishes that the oxygen level was well above those of the oxygen threshold activity levels. Researchers differ in opinion as to the spinel effect on the integrity of the ceramic/metal bond interface. The results of this study show that spinel formation is necessary to 'reaction bond' the nickel/alumina interface. Excessive reaction, however, will cause a drop in the shear bond strength. It is believed that this is consequential of the thermal residual stresses induced by the differential CTEs at the interphase/metal junction during the HIP cooling process. Shear tests were performed on the HIPped specimens. A maximum shear bond strength of 38.2 MPa was achieved with a HIP temperature of 1200°C at 200 MPa pressure and 100 min soaking time. In all observable cases, fractures occurred at the spinel/nickel junction of the interface.