Working Together

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Concepts Shown:

property matching, thermal expansion, bonding

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Transparencies of pages needed Smile.

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Science:

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].

References:

Author:
Marcia Muller
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