Microstructure at the interface between AIN and a Ag–Cu–Ti braze alloy

Abstract

The joining of aluminum nitride by active metal brazing with a Ag–Cu–Ti foil has been investigated in cross section by transmission electron microscopy. The reaction of AIN with the braze alloy results in the formation of continuous TiN and (Ti, Cu, Al)₆N (η-phase) layers at the interface. AIN grains at the interface often display arrays of dislocations, presumably arising from thermal expansion mismatch between the AIN and the TiN (the coefficients of thermal expansion are 43 ⊠ 10⁻⁷/°C and 80 ⊠ 10⁻⁷/°C, respectively). The adjoining TiN contains small Cu precipitates and may also contain numerous defects. Titanium preferentially penetrates the AIN grain boundaries, resulting in finger-like TiN intrusions into the substrate, which sometimes cover entire AIN grains in a TiN shell. On the other side of the TiN, a continuous layer of equiaxed, defect-free η–nitride grains is found. Beyond this η–nitride layer is the remaining mixture of metallic Ag and Cu. High-resolution electron microscopy demonstrates that the AIN–TiN and TiN–η boundaries are abrupt and contain no additional crystalline or amorphous intervening phases. Particular orientation relationships are occasionally observed at the AIN–TiN interface; these are not always the ones that produce the minimum lattice mismatch. The implications of the observed morphology with respect to the reaction sequence, transitional phases, and structural integrity of the joint are discussed.