The effect of palladium additions on the solidus/liquidus temperatures and wetting properties of Ag–CuO based air brazes

Abstract

As a means of increasing the use temperature of ceramic–ceramic and ceramic–metal air brazes, palladium was investigated as possible ternary addition to the currently employed silver–copper oxide system. The silver component was directly substituted with palladium to form the following series of alloys: (100 − y)[(100 − z)Pd − ( z)Ag] − ( y)CuO x where y = 0–34 mol% CuO x and z = 50–100 mol% silver and where x = 0, 0.5, or 1, respectively, denotes copper metal, Cu 2O, or CuO. From differential thermal analysis, it was determined that the addition of palladium causes an increase in the solidus and liquidus temperatures of the resulting Pd–Ag–CuO brazes. In general, the liquidus was found to increase by approximately 220 °C for the (100 − y)(25Pd − 75Ag) − ( z)CuO x filler metal compositions relative to comparable Ag–CuO x alloys. Likewise, the solidus was found to increase for these alloys, respectively, by 185 and 60 °C, respectively, for CuO x contents of y = 0–1 mol% and 4–10 mol%. For the (100 − y)(50Pd − 50Ag) − ( y)CuO x alloys, the solidus increased between 280 and 390 °C over a copper oxide compositional range of x = 0–8 mol%. It was determined from sessile drop experiments conducted on alumina substrates that in all cases the palladium causes an increase in the wetting angle relative to the corresponding binary braze. Alloy compositions of (100 − y)(25Pd − 75Ag) − ( y)CuO x displayed increased wetting angles of 5–20° relative to comparable binary compositions. (100 − y)(50Pd − 50Ag) − ( y)CuO x alloys exhibited an increase in contact angle of 10–60° and compositions containing less than 10 mol% CuO x were not able to wet the substrate. Metallographic analysis indicated that the microstructure of the braze consists of discrete CuO x precipitates in an alloyed silver–palladium matrix. In both the binary and ternary filler metal formulations, a reaction layer consisting of CuAlO 2 was observed along the interface with the alumina substrate. This reaction product appears to be beneficial in promoting wetting by the remaining braze filler metal. However the formation of this layer is hindered as the concentration of palladium in the filler metal is increased, which appears to be the primary cause of poor wettability in these compositions, as indicated by the substantial amount of porosity found along the braze/substrate interface.