Objectives: The press-to-metal and press-to-Zirconia techniques offers the advantage to obtain a perfect marginal fit with only one thermal processing step according to the following procedure: wax-up with sprues upon the Zirconia or opaqued metal framework, investing, pressing the ceramic onto the framework, devesting. Finishing of the restoration requires further thermal treatments as veneering, staining and glazing. Thus the fit of a pressed ceramic margin may be affected by subsequent firings and a shoulder may become rounded if firing temperatures exceed a critical temperature (Tc). The aim of the study is to evaluate Tc of some experimental ceramics by high-temperature microscopy (HTM) and to develop a method to select ceramic compositions with high thermal stability.

Method: Ceramic powders with different properties concerning coefficient of thermal expansion (CTE) and pressing temperature were mixed and sintered to receive four pressable ceramics with CTE 13.8*10^-6/K and pressing temperature 930°C. Ceramic cylinders (3x3mm; diameter/length) were pressed and polished to obtain coplanar circular surfaces. Tc is measured with HTM. As temperatures exceed Tc the corners of the cylinders progressively become rounded which is recorded by HTM.

Results: Tc ranges between 755°C and 795°C for the experimental ceramics. Temperatures exceeding Tc causes progressive rounding of the corners due to viscose flow of the material. Using a heating rate of 0.5K/min between Tc and 850°C the corner angles increase almost linear with temperature. The increment of the experimentally found increase of the corner angles range between 1.4°/K to 2.5°/K, where the material having the lowest Tc exhibit the weakest increase with temperature and vice versa.

Conclusion: HTM offers the opportunity to measure the onset of thermal instability of pressed margins and to identify the most stable ceramic composition concerning subsequent firings. The results can further be used to calculate new compositions to optimize the thermal stability.

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