Diffusion bonding of aluminium oxide to stainless steel using stress relief interlayers

Aluminium oxide was diffusion bonded to AISI 304 steel using Ti, Cu or Mo as interlayer materials. It was observed that Ti joins easily to both ceramic and steel parts, giving an average shear strength equal to 20 MPa. However, within the experimental conditions applied, the adhesion between Al2O3 and Cu or Mo was unsuccessful. A finite element modelling (FEM) was employed to evaluate the residual stresses in the joints and showed that the use of Mo and Cu reduced the stress level, when comparing to Ti. Results from the FEM suggests that similar behaviour is expected when an interlayer comprising a thin Ti foil in contact with the alumina and a thick Cu or Mo foil is used. Diffusion bonding experiments using such combinations substantiated the FEM results and the average strength of the joints reached 27 MPa with Ti/Mo and 65 MPa with Ti/Cu. Ti3Al formation was found to be responsible for the Al2O3/Ti adhesion whilst β-Ti phase stabilisation was detected at the Ti/steel interface and attributed to the intense diffusion of Fe into the Ti.