Introduction
Chromium and titanium alloys are the main base alloys used under porcelain. They give a strong, stiff frame for metal ceramic restorations at a much lower price than noble alloys.
Requirements for metal ceramic alloys
If the alloy is too weak, the frame must be thicker, which means cutting more tooth or using a thinner, less esthetic layer of porcelain. If the expansion does not match, cooling after firing ruptures the bond, and the porcelain cracks and separates.
Chromium alloys
Chromium forms the passivating oxide layer. It must make up at least 17% for corrosion resistance and no more than 30% for good casting.
Cobalt-chromium
These alloys have the highest mechanical properties of all dental alloys and are used for fixed and removable work. Molybdenum raises strength and refines the grain. Up to 0.2% carbon increases hardness, but more makes the alloy brittle. Melt it quickly, and avoid acetylene flames and graphite.
Nickel-chromium
These are easier to process than cobalt-chromium, and they form a thin, sturdy oxide layer, so they replaced expensive noble alloys for metal ceramics. Their use is falling because of allergy and possible carcinogenic risk, so cobalt-chromium and titanium are taking over. Modern alloys contain no beryllium.