Vacuum brazing of cubic boron nitride to medium carbon steel with Zr added passive and Ti activated eutectic Ag-Cu alloys

Active element like Ti is usually added to Ag-Cu passive alloy to braze cubic boron nitride (cBN) ceramic particles with steel substrate under high vacuum. Current work focusses on another group-IV element Zr, having more negative Gibbs free energy than Ti, concerning its reaction capability with cBN. In the present study, its effectiveness as an additive to the 72Ag28Cu alloy in brazing of cBN has been investigated. Zr is added to both passive and Ti-activated Ag-Cu alloys. Interestingly, the addition of 2 wt% Zr to the composition of eutectic alloy fails to braze cBN, unlike the addition of 2 wt% Ti. The underlying science is critically investigated. A new formulation of the alloy with the composition of (72Ag28Cu)-4Zr-4Ti alloy is found to be a beneficial alternative. Wetting of this new alloy on cBN is appreciable and simultaneously an additional benefit of 50% increase in wear resistance is achieved, compared to the (72Ag28Cu)-2Ti alloy. A significantly hard new intermetallic phase, AgCu4Zr is detected in the microstructure which contributes to the enhancement of wear resistance of the new alloy. The joint strength thus achieved in brazing cBN with medium carbon steel using (72Ag28Cu)-2Ti and (72Ag28Cu)-4Zr-4Ti alloys is also compared but found to be compromised in case of the later. The results are analyzed in the light of microstructure, alloy-cBN interfacial chemistry and interfacial fracture under load.