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 Pd 3 (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 Pd 3 (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.

Effect of ice-quenching on the change in hardness of a Pd-Au-Zn alloy during porcelain firing simulation

“…2). Such rapid recovery in the hardness according to the cooling rate control during the porcelain firing process was also reported for Pd-Au-In-Ag and Pd-Ag-Au-Sn alloys, which had undergone a pre-softening heat treatment before porcelain firing 7,11) . From the above, if the readjustment is done for the ice-quenched metal substructure after degassing, it will increase the efficiency of processing without affecting the mechanical properties of the final prosthesis.…”

“…1), which is a relatively fast cooling rate. Such an effective cooling rate for alloy hardening was reported to vary according to the composition of the alloys 7,[10][11][12] . From the apparent hardening during cooling, the alloy was believed to have undergone precipitation or atomic ordering during cooling.…”

“…By cooling the ice-quenched specimen at stage 0 after the remaining firing process, the precipitates dissolved in the matrix re-precipitated from the grain boundary and interior (Fig. 3), resulting in apparent hardening 7,11,13,14) . Finally, in the ice-quenched specimen after degassing, the quantity of grain boundary precipitates increased significantly, and the particle-like precipitates in the grain interior became coarse by the complete firing simulation, showing a similar microstructure to that of the specimen cooled at stage 0 after all the firing process.…”