1985 年 3 巻 3 号 p. 483-490
The effect of an insert metal on the diffusion bonding of alumina ceramics to carbon steel has been invsetigated with particular reference to the relaxation of thermal stress which is set up in the cooling process from bonding temperature owing to the difference in thermal expansion between the alumina and steel. The insert metal used was a combination of a titanium foil 20μm thick and a copper foil 0.05-4mm thick. All joints obtained were fractured in the alumina on tensile test. However, the fracture strength increased with the increase in the thickness of the copper foil, approaching a saturated value (30MPa) for foil thickness of 2-4mm. In the alumina ceramics cracks were observed in the as-bonded state. The crack decreased in length with the increase of the thickness of the copper foil and could not be observed when the foil thickness was more than 2mm. According to a stress analysis using finite element method (FEM), the crack was initiated at the position where the thermal stress took a maximam value, and the direction of the crack was nearly normal to that of the maximam stress. The analysis using FEM indicated also that the thermal stress decreased with increasing the thickness of the copper foil because the plastic flow of the copper foil relaxed the stress. Consequently, the formation of the crack in the alumina ceramics is attributable to the thermal stress caused by the difference in the thermal expansion between the alumina and steel. The crack and thermal stress are considered to cause the change in the fracture strength of the joint with the thickness of copper foil. A combination of the titanium foil and a nickel foil 2mm thick was also applied as an insert metal. However, the fracture strength of the joint was considerably low.