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.
Highlights
Metal–ceramic alloy, used for the manufacture of dental prosthesis, functions as a substructure for porcelain to compensate for its fragile characteristics [1]
S00 and S30 were added to determine whether the hardness of the ice-quenched specimen after oxidation can be recovered during multiple firing at relatively high or low cooling rates
Even though there was a tendency for the hardness to decrease as the firing progressed, the Stage 0 (S0) specimens showed higher hardness than those of Stage 3 (S3) specimens through the glaze step (p < 0.05; Figure 2, Table 6)
Summary
Metal–ceramic alloy, used for the manufacture of dental prosthesis, functions as a substructure for porcelain to compensate for its fragile characteristics [1]. Pd-Ag-based alloys have a high modulus of elasticity, excellent bonding strength with porcelain, and stability against discoloration and corrosion [2,3,4,5]. Metal substructures for metal–ceramic prosthesis are subject to a trimming process after casting to enable fitting on a master model prior to layering porcelain on top. It has been reported that the hardness of Pd-based alloys decreases during the multiple porcelain firing process [6]. Pd-based alloys have a relatively high hardness after casting [7], which means more time is needed to trim them. It is advantageous to temporarily reduce the hardness of the alloy
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