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Silicon nitride

The specialist ceramic for special requirements

Silicon nitride: Hardly any abrasion despite permanent load

Silicon nitride (Si₃N₄) is a very light ceramic material that can respond to extreme requirements to provide resistance to wear, abrasion and temperature. These properties are in high demand in numerous industrial applications, such as in ball and cylindrical roller bearings in machine and vehicle construction. The microstructure of silicon nitride also makes it interesting for metal forming – such as cutting tools that are subject to thermal shock, the machining of carbide metal materials as forming and punching tools, as well as for aluminium smelting companies.

Silicon nitride as a ceramic material for roller bearings

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.

The capabilities of silicon nitride

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.

Typical applications and components of silicon nitride

  • In ball bearings for high loads at low friction
  • As a ball and rolling element for light and extremely precise hybrid bearings (rolling elements consist of Si₃N₄), as well as all-ceramic bearings (races and rolling elements made of Si₃N₄)
  • As a roller bearing material in turbocharger engines
  • For highly stressed wear parts (e.g. scraper strips in paper machines, conveyor belts, etc.)
  • For valves in engine construction
  • Ball valves
  • Medical applications (e.g. covering pacemakers)
  • Cam and crankshaft rollers
  • As dynamic gas sealing rings in compressors
  • As a ceramic platelet in high-pressure pumps in the injection system for common-rail diesel

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.

Other fields of application of silicon nitride  

  • 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:

Alumina (Al2O3)

1

Silicon carbide (SiC)

1-3

Zirconium dioxide (ZrO2)

5-20

Silicon nitride (Si₃N₄)

20

Manufacture and form

The basis of silicon nitride ceramics is silicon, which is extracted from pure quartz sand (SiO2). Si₃N₄ is then usually formed through a reaction of the pure silicon with nitrogen at around 1,200 °C / 2,192 °F. Ceramic components made of silicon nitride are then produced by means of liquid-phase sintering from fine Si₃N₄ powder, which is mixed with other sintering additives to enable complete compaction.

The compaction and needle-shaped recrystallisation of the silicon nitride particles is caused by sintering at high temperatures (1,800 to 1,900 °C / 3,272 - 1,652 °F) and under high pressure in a nitrogen atmosphere. This creates an interconnected structure, which gives the material its high degree of strength and fracture toughness, as well as very good resistance to temperature change. A wide range of component shapes can be achieved by using ceramic forming processes. Due to the toughness and hardness of the material, it’s usually only possible to process sintered components with the use of diamond tools.

Silicon nitride is available in the form of three modifications with different crystal structures:

  • Trigonal α-Si₃N₄
  • Hexagonal β-Si₃N₄
  • Cubic γ-Si₃N₄ – mostly used as a high-quality Si₃N₄ powder for technical applications

Properties of silicon nitride

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.

2-4

2-313-20

ST-37

Grey cast iron

Silicon nitride

Chemical resistance:

Silicon nitride has good resistance to almost all acids (except hydrofluoric acid) and alkalis. It’s also resistant to oxidation and corrosion at high temperatures, e.g. against liquid non-ferrous metal melts such as aluminium.

Conclusion: This versatile non-oxide ceramic offers numerous possible uses as a construction material. This makes it worth considering working with competent technology partners to expand the portfolio of applications of this material – because we have not yet exhausted the potential of this high-performance ceramic.

From 3M™ Ceramics Experts: Non-binding advice for your company

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