15.7 High Strength Ceramics

Crystalline ceramics are brittle so break suddenly, usually along grain boundaries, although ceramics containing smaller crystals, such as glass ceramics, are stronger.

Ceramic fibres and whiskers have close to theoretical strength as they have no defects and have the highest theoretical strength of solid materials. These fibres, such as silicon carbide whiskers typically 1 micron diameter and 100 microns (0.1mm) long, are used to strengthen other products, increasing endurance at high temperatures, for example in satellite and missile structures and helicopter transmission parts. Conversely, carbon fibres can also be introduced into ceramics to improve their tensile strength.

Titanium carbide has four times the strength of steel, so a rod in an aerospace application could be made of this ceramic with half the diameter and 30% of the weight. Boron carbide is particularly hard and very wear resistant so is used in dies. Cubic boron nitride is as hard as diamond but more temperature resistant.

15.8 Abrasives and Cutting Tools

The first “abrasive paper” formed by gumming crushed shells to parchment was used by the Chinese in the 12^th century AD. Abrasives now have many forms including wheels, discs, sheets and belts used for precision machining, cutting, shaving and polishing. The use of ceramics as abrasives takes advantage of their hardness and temperature-withstand capability. The hardness of minerals is measured on the Mho scale from 1 (talc) to 10 (diamond). Corundum is 9 and quartz 7. Corundum and emery are natural alumina, although many abrasives now are made synthetically for consistency.

Carborundum (silicon carbide) cutting
discs - source Wikipedia via Albert

Ceramic surface finishing tools
- source Friatec

Over half the diamonds mined today are used industrially, for cutting and polishing materials such as the very hard silicon and boron carbide ceramic products. As the majority of abrasive ceramics are formed as a mixture of crystal grains bonded with a glassy phase, tailoring the characteristics of this mixture results in the various types of abrasives. Alumina is preferred for processing steel, and silicon carbide for most ceramics.

Ceramic cutting tools can also be readily tailored to meet particular requirements. Doped silicon nitride and alumina are fired with 4000-psi pressure at 1,600 ºC to maximise density and hardness. They can be coated with other ceramics such as titanium nitride or carbide, or be reinforced with silicon carbide whiskers. They can be formed as “chisels”, wheels and discs and are used for difficult-to-machine metals such as cast iron. The tools are often used at high temperatures.  In Germany some 70% of metal cutting tools in the casting process are ceramic. They are also used extensively in the automotive industry.

15.9 Wear Resistant

Conversely, ceramics have many applications to reduce friction in rubbing surfaces, as they can be sufficiently hard and have a high surface finish that wears very slowly. Doped zirconia is such a ceramic, as is silicon nitride, which can be used for ball bearings that can last ten times longer than steel ones.

Ceramic bearings, collection based on
silicon nitride and Ceradyne ball bearings

Ceramic spray nozzles - source Rojan

Also longer life can be obtained using ceramic nozzles reducing erosion. Another example is the use of silicon carbide pads as bearings in oil and gas drilling rig motors. They have a very good high temperature wear resistance, and are particularly used for directional drilling where the drill bit is steered in various directions (such as very accurate relief well drilling).