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.
Highlights
Porcelain firing for the fabrication of metal–ceramic prostheses is performed several times on dental alloys for bonding porcelain over the temperature range 890–980 °C
Effects of the cooling rates on change in hardness by simulated porcelain firing During porcelain firing, precious alloys for bonding porcelain are normally softened, and such softening can be minimized depending on the rate of cooling after porcelain firing
(1) In the as-cast specimen, the hardness 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
Summary
Porcelain firing for the fabrication of metal–ceramic prostheses is performed several times on dental alloys for bonding porcelain over the temperature range 890–980 °C. Dental alloys for bonding porcelain undergo processing, such as trimming for adaptation on a master model before porcelain firing. Several studies have been performed to improve the hardness of the casted high-gold-content alloy for bonding porcelain by proper heat treatment prior to processing and porcelain firing [7, 8]. Compared to the high-gold-content alloy, the low-gold-content alloy for bonding porcelain is much harder after casting, which makes processing difficult or time consuming. A softening heat treatment can be attempted prior to processing the casted low-gold-content alloy for bonding porcelain. In the softening heat-treated specimen for easy alloy processing, possible further softening by subsequent porcelain firing can occur, which will cause problems, such as sag and thermal distortion of the alloy during porcelain
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
