Change in hardness of an as-cast and softening heat-treated low-gold-content alloy for bonding porcelain by simulated porcelain firing and its mechanism

Abstract

This study examined the change in hardness of a low-gold-content alloy for bonding porcelain induced by simulated porcelain firing after casting and softening heat treatment along with its mechanism. In the as-cast specimen, the hardness was increased by oxidation, and as the specimen underwent a further porcelain-firing process, the hardness decreased gradually to be softer than the as-cast state. The hardening in the as-cast specimen by oxidation was induced by the grain interior precipitation of the ordered Pd3 (Sn,In) phase, which resulted in the formation of severe lattice strain in the matrix. Softening by the complete firing process in the as-cast specimen was due to the coarsening of the ordered Pd3 (Sn,In) precipitates, which resulted in release of lattice strain by the reduced interphase boundaries between the coarsened precipitates and surrounding matrix. In the softening heat-treated specimen before simulated porcelain firing, the decrease in hardness by the softening heat treatment, which was performed to allow easy alloy processing before porcelain firing, was almost recovered in the oxidation-treated state, which is the first stage of the porcelain-firing cycles. From the first opaque-treated state, there was little difference between the hardness of the as-cast specimen and softening heat-treated specimens. The mechanism of hardening by oxidation and subsequent softening by the complete firing process was unaffected by the softening heat treatment before porcelain firing in a low-gold-content alloy for bonding porcelain.