Frequently Asked Questions
Ceramics can be organised into types or categories in terms of composition (oxide, carbide and so on). The characteristics of some of the more common are given below.
Alumina (aluminium oxide, Al2O3) is by far the most commonly used engineering ceramic and is generally specified as the ceramic of first choice where operating conditions do not require a higher specification material. Alumina has a high hardness, is electrically insulating and can be produced in a wide variety of shapes and purities (typically from 80 to 99% where the remainder is composed of a mixture of intergranular glasses). Found both as a substrate in the electronics industry and as a high temperature engineering material, alumina can be used up to 1500°C in a wide range of applications.
Zirconia (zirconium oxide, ZrO2) undergoes two phase changes at around 1000°C and about 2370°C respectively. These are accompanied by significant changes in volume, which help to confer relatively high toughness (about half that of steel) to the material. Zirconias engineered to have attractive mechanical properties are generally partially stabilised, which means they consist of two phases. Due to these phase changes, the attractive mechanical properties of zirconia deteriorate above around 350°C. However, zirconia is also an ionic conductor and is particularly active at temperatures of approximately 600°C, useful for high temperature sensors.
Silicon nitride (Si3N4) and SiAlON (alumina substituted into silicon nitride) offer high hardness, low density, good strength and a low coefficient of thermal expansion. These materials find application in cutting tools and applications requiring excellent corrosion resistance. Strength is retained to about 1200°C.
Aluminium nitride (AlN) is highly stable in non-oxidising atmospheres to temperatures in excess of 2000°C, and its lack of wetting by molten metals means that it is commonly used in the refractories industry. Its high thermal conductivity also leads to its use in semi-conductors and electronics substrates.
Boron nitride (BN) comes in two forms: cubic and hexagonal. The hexagonal form resembles graphite (it is known as 'white graphite') and is a soft material good for heat and corrosion resistant products and electrical insulation. The cubic form has properties similar to diamond and its high hardness leads to its use in cutting tool applications.
Tungsten carbide (WC) is widely used in cutting tools and wear applications. Tungsten carbide is sintered to full density by adding 4-17% cobalt. In general the higher the cobalt, the easier it is to sinter the WC and the higher the toughness but there is a corresponding decrease in hardness.
Boron carbide (B4C) and diamond are two of the hardest materials known - and as such find application in cutting tools. Diamond is also becoming of interest to the electronics industry where its high thermal conductivity and low thermal expansion make it a contender as a high performance substrate.
Silicon carbide (SiC) is known for its strength, abrasion resistance, high thermal conductivity and resistance to corrosion which leads to its use in refractory bricks and tiles; pump parts; and heat exchangers for the power generation industry. With its low density and high hardness, silicon carbide is also found in the military sector as armour.
Table of properties
|Coefficient of thermal expansion|
x10 -6 /°C
1 4 point bend test data
* sublimes, decomposes or vaporises
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