Solid State Bonding of Graphite to Nickel

Abstract

Graphite was bonded to nickel under joining compressive stress of 3 to 33 MPa in a vacuum at temperatures within the range of 973 to 1273 K using an RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/nickel joint, and changes in the microstructure and hardness of nickel near the joining interface, were investigated. Thermal stress induced in the joint was estimated using the finite element method. On the basis of these results, the influence of thermal stress on the bending strength of the joint was examined Completion of the graphite/nickel joint depends on both joining compressive stress and joining temperature. At high joining temperature, good solid-state bonding under relatively low joining compressive stress becomes feasible. Axisymmetric thermoelastic finite element analysis suggests that the maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of graphite anal is increased with increasing joining temperature. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. Part of the thermal stress in practical joints is relaxed and less than that calculated using the finite element method. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of the maximum tensile thermal stress depends on the amount of carbon which diffuse into nickel. This may be related to changes in plasticity of both nickel and graphite, that is, changes in the amount of carbon that dissolves in nickel to supersaturation and the point defects introduced in graphite.