Silicon nitride

In industries such as power generation, vehicle production and chemical process engineering, fast-rotating machine components are subjected to the highest mechanical continuous loads – and furthermore, under changing operating temperatures. In this environment, materials that are highly resilient and corrosion-resistant over the long term are required for continuous use with long service lives.

Silicon nitride offers high wear resistance, strength and rigidity, extremely high abrasion resistance and good friction characteristics combined with a high resistance to temperature and corrosion, making it ideal as a ceramic roller bearing material for use in hybrid rollering bearings. Si₃N₄ components have good dry-running properties in hybrid storage applications and are therefore often used in low-maintenance and maintenance-free assemblies. However, it’s also ideally suited for use with media lubrication.

Extremely good high-temperature properties combined with a high degree of resistance to corrosion and strong acids (e.g. sulfuric acid, hydrochloric acid, hydrogen fluoride, alkalis) make Si₃N₄ ceramic the ideal material for, in particular, high-temperature applications in chemical process engineering and mechanical engineering – especially where extreme temperature changes occur.

Use of silicon nitride in the metalworking industry

Because of the combination of high strength values, low coefficient of thermal expansion and a relatively small modulus of elasticity, Si₃N₄ ceramic is particularly suited to thermal shock-stressed cutting tools such as indexable inserts. One application in which this combination of properties has proven particularly useful is machining of cast iron materials – such as cast iron or grey cast iron – with high-performance ceramic cutting tools. In contrast to carbide metals or other cutting materials, when using ceramic indexable inserts the machining processes can be carried out at the highest speed – even without coolants or lubricants.

Silicon nitride cutting tools are therefore suitable for machining materials that are particularly hard to cut, such as:

• Grey cast iron
• Nickel-based superalloys
• Austenitic steels (rustproof)
• For rough machining of chilled cast-iron rollers
• For machining mass-production components

Components for rollers and draw rings made of silicon nitride are used in metal forming because they can offer a noticeably longer service life than the steel or carbide metal rollers that are usually used. Furthermore, the rolling cycle time is shortened and work hardening on the rolled metal is also facilitated. For example, forming rollers made of Si₃N₄ for rolling of heat-resistant metal alloys impress with a service life that’s twice that of forming rollers made of conventional metal materials. Such tools can also be reground up to three times.

Forming and stamping tools made of silicon nitride that are subject to high mechanical loads have good chemical resistance and a long service life.

• Heat-resistant silicon nitride is used in aluminium casting technology as a material for melting channels, and riser and protective tubes (e.g. for immersion heating elements, pyrometers etc.). An increase in tool life of up to 20 times longer than conventional steel components can be expected. This also applies to risers made of Si₃N₄ for die casting of, for example, aluminium rims. In practice, this makes it possible to meet special requirements for quality and surface finish.
• Linings for electromagnetic PUMPS in aluminium melting plants
  Electromagnetic pumps play an increasingly important role in the search for developing faster and more energy-efficient processes for melting aluminium. These pumps enable the molten bath to be mixed quickly and evenly without mechanically moved parts.

• The key to optimal functioning of these pumps are wear-free, low-maintenance and chemically resistant linings (so-called pump liners). With good resistance to molten metal melts, silicon nitride fulfills these requirements to the greatest possible extent and because of the thin walls enables the best possible use of the electromagnetic field of the pump.

• Silicon nitride sputtering targets can be used to apply abrasion-resistant hard coatings to other materials.

• The low density of silicon nitride (~3.2 g/cm³) and the associated low heat capacity make it the preferred material for light heat exchanger plates in the chemical industry and process engineering.

• Silicon nitride's extremely good resistance to temperature exchange and high temperatures is also used in welding processes: for example, MIG/MAG gas nozzles, welding rollers, centering and fixing elements in the welding field are made of Si₃N₄. Thanks to silicon nitride, it’s possible to avoid metallic buildup such as weld spatter and welding build-up. Conventional centring pins made of metallic materials, on the other hand, have a very limited lifespan.
• Because of its temperature resistance and low oxidation behavior, SIC is also suitable as a material for glow starters.
• Measuring tips (cantilevers) for atomic force microscopes, with which samples up to atomic size can be resolved, often consist of tiny silicon wafers with a silicon nitride layer on the surface that offers particularly good protection against mechanical wear.
• In semiconductor technology, silicon nitride is used as an insulation and passivating material for the fabrication of integrated circuits. In so-called “charge trapping stores”, the non-conducting Si₃N₄ forms storage layers for electrically bound charges. Also, the ceramic is used in numerous manufacturing processes as a masking material, e.g. for chemical mechanical polishing or for local silicon oxidation (LOCOS process).
• Silicon nitride is also used in the form of carrier plates for mechanical processing and for etching of silicon wafers. In these environments, the highest degree of chemical purity, corrosion resistance, evenness and high surface quality are important.
• Silicon nitride is also used as a coating for crucibles to produce silicon wafers for multicyristalline solar cells and as a precursor for LED phosphors.
• Silicon nitride heating plates. Silicon nitride is an electrical insulator with a dielectric strength of approx. 20 kV/mm and a specific resistance of 1,014, Ωcm. Due to these properties, conductor tracks can easily be applied directly to the silicon nitride ceramic using screen printing. This has the advantage that there’s no need for an additional heat conductor carrier or for electrical insulation between the heat conductor and the heating plate.

• In addition, due to a very low coefficient of thermal expansion of 3*10^-6/K, a silicon nitride plate undergoes only minimal deformation during operation. This means that mechanical contact and heat flow between the plate and the bottom of the pot is guaranteed at all times. This makes it possible to produce novel, very thin ceramic hot plates made of silicon nitride for high-speed household cooking systems with virtually inertia-free heat transfer.

• In aerospace applications, good resistance and rigidity under high temperatures, as well as low weight and low thermal expansion are particularly important – all of which are properties found in structural components made of Si₃N₄. This makes the ceramic of interest for, among other things, housings of reconnaissance and observation cameras and for support structures of satellites. The advantage here is that the low thermal expansion, even in the case of extreme temperature fluctuations such as those that occur in near-earth space, hardly causes any shifts within the high-precision, optical lens system. This means the resolution and imaging accuracy of the camera can be significantly improved.

Comparative price levels of various technical ceramic materials:

After zirconium dioxide (ZrO₂), silicon nitride has the highest fracture and crack toughness values as well as flexural strengths (up to max. 1,000 MPa) of all industrial ceramic materials. With a hardness of 13–20 GPa, silicon nitride is several times harder than steel.