In this work, using $(Ag-Cu)_{eut}+0.5Ti$(wt/0) and $ZrO_2/(50Ag-50Gu)+xTi$ systems, the effect of the process condition(time,temperature) and active element(Ti) concentrations in brazing alloys on the behavior of the interfacial reaction layers and the relationship between an interfacial reaction layer and the fracture toughness of joint were studied.
I. In the $Al_2O_3/(Ag-Cu)_{eut}+0.5Ti$(wt/o)system, to investigate behaviors of reaction layers and fracture toughness values of joints, a brazing temperature was varied from 850 C to 950 C with constant holding time at the each temperature. In the temperature ranges studied, reaction layers were not continuous at the overall interface, the regions(mainly $Al_2O_3/TiO$ interface) where reaction products were formed continuously and the region(mainly $Al_2O_3$/Brazing alloy interface) where reaction products were formed sparsely were always coexisted. Reaction products consisted of $\gamma$-TiO(NaCl structure) and $\delta$-TiO (hexagonal) without a $Cu_3Ti_3O$ phase. In the temperature ranges studied, Kc values of the joints varies from 3 $MPa(m)^{1/2}$ to 5 $MPa(m)^{1/2}$, and increased with the brazing temperature. The fracture surface was composed of two different paths, which correspond to the Ceramic/B.A. interface (I-path) and ceramic interiors(C-path). The area fraction of the C-path on the fracture surface (Ar = Ac/(Ac + Ar)) had linear correlations with the fracture toughness of the joints.
From the results mentioned previously, it is suggested that crack propagated along the Ceramic/B.A. interface for the area with the sparse reaction layer, but through the ceramic interior for the area with the uniform reaction layer. The relationship between the Kc values and the Ar indicated that the fracture toughness of the C-paht(Ar is 1.0) was about 6.0 $MPa(m)^{1/2}$ and that of the I-path(when Ar is zero) was 3.0 $MPa(m)^{1/2}$, and the total fracture toughness value increased with the area fraction(Ar) of the fracture surface which had the larger fracture toughness.
II. In the $ZrO_2/(50Ag-50Cu)-xTi$ (wt/o) systems, the effects of Ti concentrations in brazing alloys and the holding time on the behavior of the reaction layer at the interface were studied. The behaviors of reaction layers can be distinguished into three groups with respect to the Ti concentration in the brazing alloy.
In case the Ti concentration was less than 0.75\%, reaction products were composed of only the first layer(TiO) without the second layer($Cu_3Ti_3O$), crystal structures of first layers were changed from $\gamma$-TiO(10 min) to $\gamma$-TiO/$\delta$-TiO (30 min), but total thickness was nearly constant independent of the brazing time. In the second case of $1.0 \leq Ti \leq 1.5$, the first layer(TiO) and the second one($Cu_3Ti_3O$) always coexisted, and the thickness of the only first layer increased with the holding time, also the crystal structures of the first one changed from the $\gamma$-TiO(10 min) to $\gamma$-TiO/$\delta$-TiO (30 min) with increasing the holding time. In case the Ti concentration was larger than 2.0\%, the reaction layer was composed of the only second layer at the beggining of the reaction (1 min), but the first layer nucleated and grew with increasing the holding time, and the change of the crystal structure in the first layers was same as previous two cases. From the results of the behaviors of the reaction layers stated above, it is found that the second layer could only be formed when the Ti concentration was larger than the critical amount, on the other hand the formation of $\delta$-TiO layer depended entirely on the holding time.
In order to investigate the relationship between reaction layers and Kc values of joints, variations of Kc values of specimens joined for constant holding time(10 min) by barzing alloys containing various Ti concentrations were tested. The fracture toughness value was about 6.0 - 6.5 $MPa(m)^{1/2}$ for $Ti \leq 0.75%$, 2.5 - 4.0 $MPa(m)^{1/2}$ and decreased with increasing Ti concentrations for $Ti \geq1.0%$. Fracture surfaces were ceramic interiors and TiO/$Cu_3Ti_3O$ interface which corresponds to $Ti\leq0.75%$ and $Ti\geq1.0%$, reapectively. Because the fracture toughness of the $ZrO_2$/TiO interface is higher than that of TiO/$Cu_3Ti_3O$ interface, the Ti concentration in the brazing alloy must be less than 0.75\% to obtain optimum conditions of reaction layers.
