Solid state bonding of graphite to nickel

Graphite was bonded to Ni under a compressive stress range from 3 to 33 MPa in a vacuum over a temperature range from 973 to 1273 K using a RF-inducti on furnace. The influence of joining conditions on the bending strength of the graphite/Ni joint, and changes of microstructure and hardness near the joining interface of Ni, were investigated. Thermal stress induced in the j oint was estimated with a finite element method. On the basis of these resu lts the influence of thermal stress on the bending strength of the joint as examined. Completion of the graphite/Ni joint depends on both the compressive stress and the joining temperature. Good solid state bonding becomes feasible unde r low compressive stress in case of higher joining temperatures. Axisymmetr ic thermoelastic finite element analysis suggests that maximum tensile ther mal stress is induced at a distance of 0.64 mm from the joining interface o n the surface of the graphite and is increased with increasing joining temp eratures. The position of fracture in a bending test corresponds approximat ely to that of the maximum tensile thermal stress. A part of the thermal st ress in practical joints is relaxed and less than that calculated by finite element method. The bending strength of the joint increases with decreasin g residual tensile stress on the surface of graphite. Relaxation of maximum tensile thermal stress depends on the amount of carbon atoms which diffuse into Ni. This may be related to the changes in plasticity of Ni and graphi te, that is, the changes in the amounts of C super-saturately dissolved in Ni and the point defects introduces in graphite.