K. Scott Weil

High‐purity aluminum foil was used to join alumina substrates directly in air at temperatures ranging from 800° to 1200°C and soak times of 1–100 h. It was found that the bend strengths of the resulting Al ₂ O ₃ /Al/Al ₂ O ₃ joints generally increase with increasing brazing temperature and time, with a maximum bend strength of 135 MPa on average achieved in samples joined at 1200°C for 100 h. Additionally it was determined that measurable ductility is retained in the joint even after exposure under extended high‐temperature conditions. During joining, an Al ₂ O ₃ scale forms along the interface between the aluminum and adjacent substrates. An increase in brazing temperature and/or time leads to intergrowth and sintering between this thermally grown oxide layer and the substrate surface, which appears to be the primary source of improved joint strength. Fracture analysis indicates that the Al ₂ O ₃ /Al/Al ₂ O ₃ joints generally fail via one of three mechanisms, (1) by de‐bonding along the foil/substrate interface in specimens that were joined at low temperature or held at temperature for an insufficient period of time; (2) by ductile rupture in specimens that were joined at conditions that promoted sintering between the oxidized foil and adjacent alumina faying surfaces, but left behind a continuous residual aluminum layer within the joint; or (3) by mixed‐mode fracture in specimens joined at high temperature and long exposure times, in which the thermally grown alumina that forms between the two substrates is interrupted by dispersed pockets of residual aluminum metal.

A silver‐based joining technique referred to as reactive air brazing has been recently developed for joining high‐temperature structural ceramic components of the type used in gas turbines, combustion engines, heat exchangers, and burners. It was found that additions of copper oxide to silver exhibit a tremendous effect on both the wettability and joint strength characteristics of the subsequent braze relative to polycrystalline alumina substrates. The effect is particularly significant at low copper oxide content, with substantial improvements in wetting observed in the 1–8 mol% range. The corresponding strength of the brazed polycrystalline alumina joints appears to be maximized at a copper oxide content of 8 mol%, with a maximum room temperature flexural strength approaching that of monolithic alumina. While further increases in oxide content lead to improved wetting on polycrystalline alumina, the effect on joint strength is deleterious. It appears that the formation of a continuous brittle copper‐based oxide layer along the interface between the braze and alumina faying surface is responsible for the poor mechanical behavior observed in joints fabricated with higher copper oxide content brazes.