Alumina (Al2O3) is one of the most widely specified, general-purpose technical ceramics. All aluminas are very hard and wear resistant, with high compressive strength even against extreme temperatures and corrosive environments. Aluminas are also excellent electrical insulators and are gas tight.
Blends of zirconia with alumina such as ZTA have a performance combination that is tougher than alumina alone, but with improved hardness, strength and thermal properties compared to these other materials, especially at elevated temperature.
Alumina is produced by firing a tightly packed powder form of Al2O3 which includes some binder material. Commercially available grades range from 90% up to 99.95% with the higher purity generating somewhat higher hardness. It is possible to machine alumina using diamond grinding techniques. Polishing is also possible, with the degree attainable affected by alumina grain size and production technique, whether pressed or extruded. Alumina 90%-97% is best suited for metalizing (metal deposition which allows brazing) because of large grain structure. 98%-99.95% is the common range for isostatically pressed grades, with extruded shapes also available at low cost.
As-fired tolerances are generally only possible within a few percent of dimension. Extremely tight tolerances are attainable, but only by precision machining the fired part using diamond grinding techniques. This adds considerable cost, but tolerances to millionths of an inch are possible, and are often cost effective due to the extraordinary stability and durability of the finished piece.
See Also : Sialon (Si3N4-Al2O3)