The effect of gold coating agents on residual stress in alloy-ceramic specimens

The use of gold coating agents has been claimed to reduce residual stress in alloy-ceramic composites. The purpose of this study was to evaluate the efficacy of gold coating agents in preventing cracking of bonded porcelain. Residual stress in alloy-ceramic specimens could be increased by decreasing the coefficient of thermal expansion (alpha) of the porcelain. Vitadur-N porcelain (jacket crown type, low alpha) was mixed in increasing proportions with VMK porcelain (high alpha) to prepare five test blends with decreasing values for alpha. For each test blend, alpha was measured by means of a vitreous silica dilatometer, and varied from 13.6 x 10(-6)/K to 11.0 x 10(-6)/K. The test blends were fused around spherical specimens of a substrate alloy, Option (alpha = 14.0 x 10(-6)/K), both with and without the use of three gold coating agents, Wilkote, Spectra-Seal and Deck-Gold (normal). The composite spheres were examined under 20 x magnification after 24 hours and again after eight weeks for evidence of cracking. There was no significant difference in the failure rate of the alloy-ceramic specimens with or without any of the gold coating agents. In a bonding porcelain, alpha can be lowered in a controlled manner by the addition of varying amounts of jacket crown porcelain. Cracking of the ceramic coating occurred at and above an alpha mismatch of 1.8 x 10(-6)/K. Gold coating agents have an insignificant effect on residual stress in alloy-ceramic specimens.