Role of Oxygen in Microstructure Development at Solid‐State Diffusion‐Bonded Cu/α‐Al 2 O 3 Interfaces

Microstructure development at solid‐state diffusion‐bonded Cu/α‐Al 2 O 3 interfaces has been studied using optical and electron microscopy. High‐purity Cu foil was bonded between basal‐oriented α‐Al 2 O 3 single‐crystal plates at 1040°C for 24 h in a vacuum of ∼1.3 × 10 −4 Pa (1 × 10 −6 torr). Optical microscopy of as‐bonded specimens revealed a large Cu grain size, fine pores, and long needles of Cu 2 O at the interface. Bulk specimens were annealed at 1000°C for various times under controlled oxygen partial pressures in CO/CO 2 mixtures. Consistent with a thermochemical analysis, CuAlO 2 could be formed at the interfaces. The CuAlO 2 was acicular and discontinuous, but occurred in a uniform distribution over the bulk specimen interfaces.