Silicon Nitride Sphere

This glossy sphere is made of silicon nitride, a ceramic compound of silicon and nitrogen. At first, it was thought to be a purely manmade material, its initial preparation having been achieved in the laboratory in the 1850s. It wasn’t until the 1990s that miniscule inclusions of silicon nitride were discovered as naturally-occurring deposits in meteorites. In this celestial form, silicon nitride was given the mineral name nierite, after Alfred O. C. Nier, a pioneering American physicist whose work contributed to the Manhattan Project and instruments on Martian space probes Viking 1 and Viking 2.

The pristine shininess of this sphere is thanks to silicon nitride’s supreme hardness – it is exceedingly difficult to scratch. This property, together with its wear resistance and good thermal stability, saw silicon nitride first used in abrasive cutting tools. But despite these tantalising materials properties, after its first synthesis in the 1850s, silicon nitride remained mostly a chemical curiosity over the ensuing century until it was picked up again in the 1950s by American and British researchers seeking high-temperature materials suitable for use in rockets, gas turbines and crucibles for melting metals.

Silicon nitride is a refractory material, meaning it is chemically inert and can resist extremely high temperatures. It is also very resistant to thermal shock. In NASA’s Space Shuttle project, silicon nitride spheres like this one were used as low-friction ball bearings in the equipment which supplied the main engines with cryogenic liquid hydrogen and liquid oxygen, the rocket’s propellants. These bearings outperformed traditional equivalents, boasting lower friction, lighter weight, better longevity and less lubrication needed. Today, thanks to lowered production costs, they can be found in more terrestrial settings, such as wind turbines, bicycles and high-end cars.

To make silicon nitride, powdered silicon is heated up to 1400°C in a nitrogen environment. But processing refractory materials can be a challenge, since they are quicker to decompose than melt when heated. Instead, one method is to sinter packed powders at high temperature and pressure in the presence of a binder which helps to fuse the powder particles together into a solid mass.

Silicon is a material integral to electronics, and two properties of silicon nitride make it a crucial component in integrated circuits. Firstly, it is an electrical insulator, so can be used to make insulating layers in capacitor components through processes such as chemical vapour deposition. Secondly, it is chemically inert, so can be used as a chemical barrier to mask certain areas of silicon from chemical etching, a common process used to remove material in patterns to create electronic components.

The strength, toughness, hardness and inertness of silicon nitride are properties exploited in the medical field to make it a popular material in orthopaedic implants, particularly in the spine. Silicon nitride is a naturally biocompatible material with a strong affinity for bone, and implants can be made dense, porous, or even in a structure mimicking that of natural bone.