Effect of ice quenching after oxidation with or without vacuum on the hardness of Pd–Ag–Au–In alloy during porcelain firing simulation

Abstract

This study examined the effect of ice-quenching after oxidation treatment with or without vacuum, on the change in the hardness of a Pd–Ag–Au–In metal–ceramic alloy during porcelain-firing simulation. The aim of the present study is to determine whether ice quenching after oxidation renders a Pd–Ag metal–ceramic alloy soft enough for easy processing without the need for an additional solution treatment and whether it affects the final hardness of the Pd–Ag metal–ceramic alloy after porcelain firing simulation. The oxidation treatment resulted in the as-cast specimen becoming homogenized, which was progressed further by vacuuming. The bench-cooling to room temperature after the oxidation resulted in precipitation. From the next firing step, the precipitation occurred during the firing process as well as subsequent bench-cooling; this was owing to the presence of a phase boundary wherein the single phase is separated into two phases between the temperatures of the oxidation treatment (1010 °C) and post-oxidation treatment (~960 °C). During the firing process until main bake, the specimen that was ice-quenched after oxidation maintained a higher hardness than the bench-cooled specimen, particularly for the oxidation treated specimen with vacuum (p < 0.05). The mechanism of ice-quenching after oxidation with vacuum to induce further hardening during the firing process involved more active precipitation; this was attributed to the further progress of homogenization during the oxidation. The precipitation reaction in the present study corresponded to the Pd–In binary phase diagram, illustrating that as the firing proceeded, the Pd3(In,Sn,Ga) phase was precipitated from the Pd–Ag–Au–rich matrix. The findings revealed that ice quenching after oxidation makes the alloy soft enough for easy processing without an additional heat treatment and does not affect the final hardness of the Pd–Ag metal–ceramic alloy after porcelain firing simulation.