The interface microstructures and bonding strengths of $ZrO_2$/Ag-Cu-Al-Ti brazed joints have been investigated at various brazing conditions. Microstructures developed near the C/M interface can be roughly classified into three different types A, B, and C. The interface microstructure "A", which was usually found at the lower brazing temperatures and shorter brazing times, consisted of the uniform $Cu_2(Ti,Al)_4O$ layer of which the thickness was more or less constant(0.3 ~ 0.4㎛). An interface microstructure intermediate to the type "A" or "C" microstructure was the type "B" microstructure. New reaction products of α-$Al_2O_3$ and γ-TiO which were formed at the $ZrO_2/Cu_2(Ti,Al)_4O$ interface and grown into the $Cu_2(Ti,Al)_4O$ phase, did not form continuous layers at the $ZrO_2/Cu_2(Ti,Al)_4O$ interface yet and the two layers almost always existed together and not separately. The type "B" microstructure was a transient one and ultimately changed to the type "C" microstructure in which the reaction products formed continuous layers between $ZrO_2$ and $Cu_2(Ti,Al)_4O$. The "C" type microstructure was found at higher temperatures and longer brazing times. By AES depth profiling of reaction layers, two layers of α-$Al_2O_3$ and γ-TiO were appeared parallel to the original $ZrO_2/Cu_2(Ti,Al)_4O$ interface.
The color of $ZrO_2$ near the C/M interface was initially white but turned black after brazing. Then the oxygen atoms necessary the formations of three oxides were supplied from $ZrO_2$, which were decomposed into nonstoichiometric $ZrO_2$ and [O].
The evolution of interface microstructures in the sequence of A→B→C is obvious and the phase transformation kinetics are basically governed by thermally activated diffusion process.
The bonding strength of $ZrO_2$/Ag-Cu-Al-Ti and the fracture path analyzed with in situ Auger electron spectroscopy, are related with the C/M interface microstructure. In the "A" type microstructure, fracture occured at $ZrO_2/Cu_2(Ti,Al)_4O$ interface and the interfacial strength decreased as the amount of S which was detected at the fracture surface exposed the $Cu_2$(Ti, Al)$_4$O phase, increased. It can be suggested that the origion of S impurity was in the brazing alloy and the S was segregated at the $ZrO_2/Cu_2(Ti,Al)_4O$ interface during the brazing process. In the "C" type microstructure, fracture occured at the γ-TiO/α-$Al_2O_3$ interface or the α-$Al_2O_3/Cu_2(Ti,Al)_4O$ interface and fracture path was mixed with two interfaces. The interfacial strength decreased as the total thickness of γ-TiO/α-$Al_2O_3$ increased with further brazing. In the "B" type microstructure, the $ZrO_2/Cu_2(Ti,Al)_4O$ interface was fractured in the region not formed new reaction products, but γ-TiO/α-$Al_2O_3$ interface or α-$Al_2O_3/Cu_2(Ti,Al)_4O$ interface was fractured in the region formed new reaction products between $ZrO_2$ and $Cu_2(Ti,Al)_4O$.
The bonding strength of the $ZrO_2$/Ag-Cu-Al-Ti systems are correlated with two interfacial properties, typically the amount of S segregation ; $X_s$ and the total thickness of γ-TiO/α-$Al_2O_3$ ; $X_s$. The flexural strength of $ZrO_2$/Ag-Cu-Al-Ti/$ZrO_2$ sandwich structure was evaluated such as σ($X_s$, $X_t$) = 443.8 - 44.3 $X_s$ - 223.3 $X_t$ + 42.4 $X_t^2$.
