Effect of Cooling Rate on Hardness and Phase Transformation of a Pd-Ag-Based Metal–Ceramic Alloy with or without Ice-Quenching

Abstract

The aim of this study was to investigate the effect of cooling rate on the hardness and phase transformation of a Pd-Ag-based metal–ceramic alloy with or without ice-quenching. A total of 28 test specimens, in an as-cast state, were fabricated. A multiple firing simulation was performed on the randomly selected specimens (n = 3/group) in a porcelain furnace; each firing was followed by cooling at the relatively low or high cooling rate. In addition, ice-quenching after oxidation was introduced before the normal firing process (n = 3/group). Microhardness, microstructure, phase transformation and elemental distribution were observed. Oxidation followed by ice-quenching allowed the alloy to be in a homogenized state. On the other hand, the oxidation-treated specimens followed by cooling at relatively high or low cooling speeds showed much higher hardness than the ice-quenched specimen after oxidation, which was resulted from the formation of the metastable precipitates based on the InPd3 phase with tetragonal structure. The hardness of ice-quenched alloy after oxidation was recovered in the very next firing step at both the relatively high and low cooling rates. In all specimens, the Pd-rich matrix and the InPd3-based precipitates were observed. The hardness of a Pd-Ag-based metal–ceramic alloy with and without ice-quenching depended on the cooling rate during the firing process.