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.
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.
Best suited for metalizing (metal deposition which allows brazing)
because of large grain structure.
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.
Additional options include blends of zirconia with alumina and
silicon nitride with alumina. The result is 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.
SEE ALSO: Sialon (Si3N4-Al2O3)