Background

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.
Alumina Production

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 Grades

90%-97% - Best suited for metalizing (metal deposition which allows brazing) because of large grain structure.

98%-99.95% - Common range for isostatically pressed grades, with extruded shapes also available at low cost.
Tolerances

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.
Alumina Blends

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.
Mechanical Properties

Table 1. Mechanical properties of alumina

Property


Value


Units

Density


3.88


gm/cc

Hardness


82


Rockwell 45N

Tensile Strength


54


Kpsi

Modulus of Elasticity


46


psi x 106

Flexural Strength


51


kpsi

Poisson’s Ratio


0.23


-

Compressive Strength


368


kpsi

Fracture Toughness


4.3


MPa m1/2
Electrical Properties

Table 2. Electrical properties of alumina

Property


Value


Units

Dielectric Strength


210


Ac volts/mil

Dielectric Constant


9.7


(at 1MHz)

Volume Resistivity


>1014


ohm-cm2/cm
Thermal Properties

Table 3. Thermal properties of alumina

Property


Value


Units

Coefficient of Thermal Expansion


7.2


X10-6/°C

Thermal Conductivity


30.3


W/m°K

Specific Heat


0.2


cal/g°C

Max Working Temp


1750


°C

Shock Resistance


200


°C diff