Improving the processing of high-gold metal-ceramic frameworks by a pre-firing heat treatment

Objectives: High-gold metal-ceramic alloys are difficult to elaborate due to their soft surface. Overlappings after grinding cause porosities at the metal-ceramic interface. Poor mechanical strength at high temperatures leads to thermal distortion of the framework. During firing hardness and high temperature strength of the alloy improve, thus improving processing. Therefore the question arises if by a pre-firing heat treatment hardening can be anticipated without affecting the mechanical strength after firing. Methods: Four commercially available alloys were investigated with regard to hardness, thermal distortion, proof stress, and grain growth in the as-cast state, after a pre-firing heat treatment at 950°C for 10 min, after a complete firing cycle and after a combination of both heat treatments. Hardness was measured as HV5/30, thermal distortion by means of a three point bending test and proof stress according to ISO 9693: 1991. The results were compared by means of Studentʹs t-test (p<0.05). Results: For all alloys hardness, resistance to thermal distortion, and proof stress significantly increased after the pre-firing heat treatment compared to the as-cast state. When a pre-firing heat treatment was performed before the complete firing cycle, hardness may slightly drop, but proof stress and resistance to thermal distortion remained at least constant or even improved, compared to the results obtained after a complete firing cycle. Grain growth due to the pre-firing heat treatment was not observed. Significance: The results showed, that a heat treatment of high-gold frameworks performed directly after casting improves processing without affecting mechanical strength.