EFFECT OF CEMENT FILM THICKNESS ON THE FRACTURE-RESISTANCE OF A MACHINABLE GLASS-CERAMIC

Objectives. The aim of this study was to determine the fracture resist ance of a machinable glass-ceramic plate cemented to a resin composite block as a function of the cement film thickness for two types of cem ent. Methods. Ceramic plates were cemented to resin composite blocks u sing either zinc phosphate cement or a resin composite cement. For the zinc phosphate cement, the film thickness was 33 +/- 8 mu m or 128 +/ - 8 mu m; for the resin composite cement, the thickness ranged from 26 +/- 11 mu m to 297 +/- 48 mu m. The elastic modulus was determined fo r each of the cements. Fracture loads were obtained by using a spheric al steel indenter in the center of the glass-ceramic plate. The Weibul l distribution was used for the statistical analysis. Results. For gla ss-ceramic plates cemented with zinc phosphate cement, the fracture re sistance was independent of the film thickness. When the resin composi te cement was used, a gradual decrease of the fracture strength was ob served that became statistically significant at a cement thickness of 300 mu m or more. The characteristic fracture strength of glass-cerami c plates cemented with the resin composite cement was about 75% higher than when using the zinc phosphate cement. This difference is attribu ted to the bonding of the resin cement to the ceramic plate and the su pporting structure. Significance. The findings of this study suggest t hat the resistance to fracture due to indentation of the glass-ceramic may not be affected by the cement film thickness as much as previousl y thought.