15.6 Heat Resistant Refractory Ceramics

The first refractories were stones used for kilns and furnaces. Refractory ceramics are resistant to softening or deformation at high temperatures, but may also have to withstand abrasion and chemical attack. Useful refractories can withstand temperatures in excess of 1100 ºC. They are used in metal smelting, recovering petroleum from crude oil (lining the inside of catalytic crackers), in boilers for power generation, in kilns for ceramics and cement manufacture, and in incinerators.

Tunnel kiln and car for firing refractories, Shingwa;
refractory lining for incinerator, Saint-Gobain;
ceramic linings for process industry and Abresist
Kalenborn pipe linings

Fireclay is used for the manufacture of a huge range of refractory applications, including crucibles and kiln furniture. It is very hard and has to be crushed like rock before mixing with water. On its own it is too hard for pottery, having too low a flux content, and it is not easy to work as it has poor plasticity. It is predominantly made of aluminium silicate, basically kaolinite, usually containing around 40% alumina. In “high alumina” refractories the alumina can be increased to between 50 and 99% of the fired product, and at these higher alumina percentages, the ceramics are very inert and can be used up to 1930ºC.

Collection of Almath crucibles and
a boron nitride crucible

Apart from alumina, there is a range of other refractory ceramic ingredients that can be added to mixes to raise their “withstand” temperature and are selected to fit the particular application, such as high temperature furnace linings. These include mullite (melting temperature 1870 ºC), zirconia (2677), silicon carbide (2700) and hafnium carbide (4160). Even so, hostile applications such as contact with molten steel and slag can require replacement of the refractory every few weeks. Industrial consumption of refractories is huge. Large blast furnaces to produce iron required in excess of 300 tons of refractory bricks, steel open-hearth furnaces needed 6000 tons. Many more were also needed for coke ovens and electric furnaces.  

Ceramic fibre refractory insulation
- source Best-B2B.com

Thermally insulating, as opposed to refractory, firebricks have combustible material such as sawdust or powdered cork mixed with the clay that leaves voids after firing that lowers the thermal conductivity (utilising air/gas insulation, as with thermal clothing). Alternatively they can be made of ceramic fibre rather like high-temperature loft insulation.

Several other higher-technology applications take advantage of the robustness and temperature resilience of ceramics. Examples of applications that are extremely demanding include components for use as rocket exhausts, gas turbines or inside internal combustion engines for automobiles. Ceramics such as silicon nitride are used in turbocharger rotors, rocker arms, cam followers, valves, valve guides, pistons and rings, cylinder linings, brake pads and discs and exhaust port liners.

Ceramic rocket
thrusters nozzle
- source NASA

Silicon carbide brake disc on a
Porsche Carrera GT

High strength silicon nitride turbine rotors latter
1996 - courtesy Science Museum, SSPL

Space shuttle showing array of ceramic
tiles, a new tile and one from Atlantis
- source NASA

One of the most interesting high-temperature applications of ceramic materials is their use on the space shuttle. Almost the entire exterior of the shuttle is covered with ceramic tiles made of high purity amorphous silica fibres. Those exposed to the highest temperature have an added layer of black glazed, high emittance borosilicate glass, which sheds some 95% of the heat encountered. These tiles can tolerate temperatures up to 1480 ºC for a limited time. The hottest areas are the nose (1465) and tail (1175). The tiles keep the temperature of the aluminium shell of the shuttle below 176 ºC, compared with its melting temperature of 600 ºC. The tiles, which are between 0.5 and 3.5 inches thick, cool rapidly and can be handled within 10 seconds after exposure to the highest temperatures. The shuttle also uses ceramic fabrics for thermal barriers and high temperature glass for windows. The latest developments of glass foam provide extremely light but very thermally insulating materials.