PGCEE MDS 2022: When porcelain is baked, is backed against metal, it should possess a:

 # When porcelain is baked, is backed against metal, it should possess a:

A. High fusion expansion

B. Linear coefficient of thermal expansion less than, but close to that of the metal

C. Linear coefficient of thermal expansion greater than, but close to that of the metal

D. Linear coefficient of thermal expansion greater than that of the metal

The correct answer is B.: Linear coefficient of thermal expansion less than, but close to that of the metal

The metal and porcelain should be selected with a slight mismatch in their coefficient of thermal contraction (the metal CTC being slightly higher) so that the metal contracts slightly more than the porcelain on cooling from the firing temperature to room temperature. This mismatch leaves the porcelain adjacent to the metal in a state of residual compression, which effectively protects the metal-ceramic restoration against fracture of the veneering ceramic.

Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, Anusavice, Shen, Rawls, 12th Edition, Page No: 441

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