Effect of Joining Compressive Stress on Solid State Bonding of Graphite to Nickel

Abstract

Graphite was bonded to nickel in a vacuum using an RF-induction furnace with temperature, keeping time and joining compressive stress as variables. The bending strength of graphite/nickel joints, the changes of microstructure and hardness of nickel near the interface were investigated. Thermal stress induced in the joints was estimated by means of finite element analysis. On the basis of these results, the influences of joining compressive stress on bonding were examined. Joining compressive stress and joining temperature are important factors for close contact and strong bonding between the joining surfaces of graphite and nickel. At a high joining temperature, nickel is plastically deformed even under low joining compressive stress so that full contact and strong bonding are accomplished. At a low joining temperature, strong bonding was not achieved when joining compressive stress is low, because plastic deformation of nickel is restrained. The plastic deformation of nickel plays an important role in enabling good contact and strong bonding. The strength of the joints increased with joining compressive stress. The plastic deformation of nickel near the joining interface increases with the joining compressive stress so that graphite is elastically deformed during annealing. Therefore, compressive stress is retained on the graphite surface near the joining interface. Thermal tensile stress induced on the graphite surface after cooling is relaxed by this compressive stress.