Working Together


Concepts Shown:

property matching, thermal expansion, bonding


Transparencies of pages needed Smile.



Metal is the most common material for making dental crowns. It is very durable and exact, yet it is unpleasant to look at. Porcelain has a better appearance but is brittle and tends to shrink when sintered. The idea of using both porcelain and metal together on a crown for dental purposes is fairly new. The metal provides strength, and the ceramic improves the aesthetics. There are three major challenges with using porcelain/metal crowns: Thermal expansion Chemical bonding Interface morphology 1. Thermal expansion. The thermal expansion coefficients must be similar to avoid cracking. The coefficients are not close enough. [eq]. High melting point � Low thermal expansion Metal has a lower melting point � higher thermal expansion Solution: Added potassium oxide to porcelain � lowered melting point � raised coefficient. Formed leucite K(AlSiO2)6 tetra-pseudo cubic structure. 2. Chemical Bonding. Gold is commonly used for crown because of biocompatibility and low corrosion. Gold would not bond to the porcelain. Solution: Dentists added small amounts of elements that are easily oxidized which provided better adhesion: Iron, Tin, Indium. Compounds form bonding sites because oxidation, or a loss of electrons, occur. Unlike the noble metal, gold, the material now has a positive charge and will accept electrons for bonding. 3. Interface morphology. The ideal interface between materials is a fractal interface. A fractal interface has infinite area in which to bond. The rougher the surface the better the bonding. Solution: To improve bonding between surfaces metal is etched (chemically scratched) to increase surface area. Comments: This presentation is an excellent way to show how various properties of materials are important in design. It is a great way to join the concepts together on one topic. [eq].


Marcia Muller

Related Equipment
Related Supplies