An oxide is a chemical compound made up of oxygen combined with at least one other element. Most of the Earth’s crust consists of oxides.
Nitrides used in ceramics consist of nitrogen atoms bonded to elements such as silicon and boron.
Metal oxide ceramics
The most frequently used metal oxide ceramics are alumina (Al2O3) and zirconia (ZrO2
Alumina is readily extracted from the ore known as bauxite, and zirconia is produced from the mineral known as zircon. Vast deposits of both bauxite and zircon are found in Australia, the world’s number one producer. The presence of uranium and thorium makes zircon deposits radioactive, so care needs to be taken when mining the ore.
When milled to very fine powders and then subjected to a process called sintering, both alumina and zirconia are transformed into extremely hard corrosion-resistant durable ceramic materials.
Sintering is a high-temperature heating process that encourages the tiny grains of powdered material to fuse together. This decreases the porosity of the material as well as making it denser. Careful heating is needed to control the grain size so that the strength and fracture toughness of the final product is maximised.
The non–metal nitrides like boron nitride and silicon nitride rarely occur naturally and have to be manufactured.
Boron nitride (BN) is produced by reacting boron trioxide (B2O3) with ammonia (NH3) in an atmosphere of nitrogen.
There are two main structural forms of boron nitride – hexagonal-shaped and diamond-shaped.
The hexagonal-shaped form is used in cosmetics as a lubricant in foundations and lipsticks.
The diamond-shaped form is used in applications where mechanical strength, toughness and durability are desirable.
Silicon nitride (Si3N4) is produced by reacting elemental silicon with nitrogen at temperatures between 1,300–1,400°C.
The excellent wear resistance, toughness and thermal shock resistance of silicon nitride makes it the ideal material to use in a wide range of mechanical applications. One of the major uses is in the production of car engine parts such as turbocharger rotors.
Another use is in high-performance high-quality ball bearings. Top-end skateboard and rollerblade bearings are made of silicon nitride.
A silicon nitride bearing can withstand a compressive stress of nearly 4 million psi – that’s about equal to balancing 80 elephants on a sugar cube.