The group of technical/engineering ceramics is large, with new materials continuously being added. Conventional basic materials improve all the time, while new refined sub-groups appear. New compositions are developed, as well as new ways of armouring and hardening, which lead to ceramics that meet the specifications as structural materials. The following is a general introduction to the most common and basic ceramic materials.

Aluminium oxide, Alumina, Al₂O₃
The most common technical ceramic is aluminium oxide, Al2O3, in varying degrees of purity and density. It is a common material, mainly because of its relatively low price in particular for simple designs where no processing is required after sintering.

The purest qualities are selected for temperatures up to about 1900°C and for the most corrosive environments. Dense materials (with low porosity) are used for high loads, while the porous qualities find their way into applications with high thermal shock exposure, as improved insulators, as filters and the like.

For applications with a high degree of wear, compositions with 92-98% Al2O3 are usually suitable. In this case they must always be dense sintered, i.e. with no open or interconnected porosity.

Al2O3 also exists in combination with MgO and SiO2, among others, in numerous technical ceramics. One example is cordierite, with a low thermal expansion coefficient and extraordinary thermal shock resistance. It is a material well suited as a carrier material in catalysts.

Silicon nitride, Si₃N₄
Ceramics based on silicon nitride are a relatively new group of materials. They are well suited to structural applications, as it has been possible to combine several positive characteristics. When hot-pressed, Si3N4 has among the highest hot strength of all ceramics. It shows small thermal expansion and exhibit a relatively high degree of thermal conductivity. These features makes it in particular suitable for applications involving extreme thermal shocks combined with high loads.

One sub-group is the Sialons, an Si3N4 and Al2O3 alloy that combines many of the best characteristics of each material.

Zirconium dioxide, Zirconia, ZrO₂
This ceramic is available hardened as "partially stabilised" ZrO2 (PSZ), giving it a level of strength and toughness rarely found in ceramics. This ceramic, however, should not be used at high temperatures - max. approx. 300°C, to retain its fulll strength. Fully stabilised zirconia is by far not as tough but can on the other hand be used in applications up to about 2400°C. Among structural ceramics, this is an unusually dense material at approx. 6 g/cm³.

High-strength varieties are used as cutting edges and other products under high stress, e.g. inserts in extrusion tools. It may out-perform much harder ceramics such as alumina in wear applications, thanks to its high toughness.

Because ZrO2 exhibit good heat insulation, and has almost identical thermal expansion as steel, it has become a suitable temperature-resistant heat barrier in combustion engines. Moreover, it is a good conductor not only of electrons, but also of oxygen ions at temperatures exceeding 500°C, which is why it is used as an oxygen sensor in monitoring various combustion processes.

Silicon carbide, SiC
Silicon carbide is characterised by its extreme hardness and low friction. It has a high fusing point (around 2700°C), and is also a good thermal conductor. Additionally, there are silicon-bearing varieties which also have good electrical conduction properties. The manufacturing method for so-called Si-SiC or Reaction Bonded Silicon Carbide (RBSC) makes it possible to obtain complicated shapes with high specification tolerances at relatively low cost.

Sintered or hot-pressed qualities lacking free silicon can be used at up to 2200°C and in rather severe corrosive environments. Due to their high SiC content, they do become somewhat brittle with poor edge retention and with low edge strength.

SiC is well suited as a bearing material, both toward itself and also with other ceramics, as well as together with steel, graphite, etc. This material is popular for use as gaskets in pumps subjected to severe environmental conditions, because it can endure dry runs before lubrication has started.

In simpler compositions or as relatively porous structures, this material is often used as crucibles, as furnace muffles, tubings etc. in the metallurgical field.

Glass ceramics
Generally these ceramics exhibit low thermal expansion. They are then used as cover tops on household stoves and for similar applications which require exceptional good thermal shock resistance in combination of translucency to radiant heat. Some of them can even be machined with conventional tools, making them appreciated choice for production of prototypes or parts with complex geometry, especially in short series.

Boron carbide, B₄C; Titanium diboride, TiB₂
These are extremely hard and light weight ceramics, and are used, among other things, as blasting spray nozzles and ceramic armour. TiB2 is electrically conductive and can thus effectively be machined by spark erosion.

Aluminium nitride, AlN
This ceramic is an excellent thermal conductor with high electrical resistivity. It is used, for example, as a substrate material in microelectronics when standard aluminium oxide cannot be employed.

Aluminium titanate, Al₂TiO₅
Being an excellent thermal insulator, aluminium titanate also has good resistance against fused aluminium and other light metals, copper, etc. As such it is used as pouring spouts, standpipes, spray nozzles, etc. The material has good thermal shock resistance, which renders it successful as an in-built heat barrier in the exhaust channels of combustion engines, on piston tops, in turbochargers etc.

Production of technical ceramics

Adopting various manufacturing methods, the aim is always to achieve as final a form as possible prior to sintering, including allowance for shrinkage. Since ceramics are usually extremely hard, all machining after sintering usually requires the use of diamond tools. Such processing unfortunately is very expensive and time-consuming, and this is one of the very reasons why engineering ceramics becomes considerably more expensive compared to conventional engineering materials.

Further, the post-firing machining requirement calls for selecting simple designs and geometries whenever possible, and for the number and quality of the worked surfaces to be kept to a bare minimum. It often makes overall economical sense to start from the manufacturing prerequisites of ceramics and alter the design of the metallic components involved.

Several ceramics exist in standard offerings as tubes, crucibles, plates, etc. As a rule of thumb, it is advisable to use these when making prototypes and other short runs in development work.

When merely sintered, the ceramics have a rather rough surface. In this condition, products with tolerances of approx. ±2% or ±2 mm can be obtained with a surface finish of Ra 5-10µm. Higher tolerances, up to around ±0.1 mm, can be achieved with standard machining, which also results in a surface finish of approx. Ra 0.1-1µm. It is thus neither simpler nor cheaper to manufacture the ceramic component to a tolerance specification of ±0.5 mm or an Ra of 3µm.

Thanks to the small particle size (0.1-10 µm) of ceramics and their extreme hardness, they can be lapped and polished to a high level of smoothness. Still, this is a matter of costly processing using diamond tools, and it is not recommended to strive for extreme surface finishes unless absolutely necessary. Old specifications of surface roughness of machined metal components, may often be eased-up when considering ceramics. The reason for this, is that the ceramic doesn't deform during machining. The ceramic surface will rather easy attain a high flatness, or low roughness, with residual surface imperfections extending down from an otherwise flat or smooth top.

When polished, the durability of ceramics and their mechanical strength improve. However, two facing surfaces may stick to each other when they are too smooth and flat. E.g., when used as seal rings or sliding bearings, it may become necessary to retain some surface roughness to avoid seizure.

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