Tempering as a means to strengthen porcelain-fused-to-metal restorations.

Structural failures of porcelain-fused-to-metal (PFM) dental restorations still occur despite the known success of specific products and reliable techniques. The possible causes of these failures are varied but the actual fractures usually originate at a surface flaw. Although it is difficult to produce a porcelain that is free of surface flaws, it is possible to inhibit the growth of these flaws by inducing compressive stresses in the surface of the porcelain by a tempering process. The objectives of the present study were: 1) to develop an analytical model to calculate incompatibility stresses in metal-porcelain discs due to thermal contraction mismatch between metal and porcelain, and 2) to determine whether tempering stresses can retard the growth of induced cracks in the porcelain surface of metal-porcelain discs. Ni-Cr-Be alloy discs, 16mm in diameter and 0.3mm thick, were prepared with a 0.5mm thick layer of opaque porcelain and a 1.5mm-thick layer of body porcelain. The materials were selected to provide a range of thermal contraction mismatches. The discs were fired to the maturing temperature of body porcelain (982 degrees C) and then were subjected to three cooling procedures: slow cooling in a furnace (SC), fast cooling in air (FC) and tempering (T) by blasting the surface of the body porcelain with compressed air. The lengths of cracks induced in the surface of the body porcelain by a microhardness indenter were measured immediately after indentation at 30 points along diametral lines.(ABSTRACT TRUNCATED AT 250 WORDS)