The results of rolling contact fatigue (RCF), tests on several ceramic-based materials that were subjected to high performance bearing loads have shown that only fully dense silicon nitride is capable of outperforming bearing steel. RCF has a ten-fold longer service life than high-performance steel bearings, thanks to its compact Si3N4 bearing. A high-speed rotating body may cause significant centrifugal stresses. Because it is a low-density metal, silicon nitride can be as light as aluminium. This material has an additional benefit. The low density Si3N4 helps to reduce the centrifugal stresses on the outer rings of the high speed rotating body. The high tensile strength and flexibility of silicon-nitride ceramics allows for resistance to elongation as well as high lateral stress resistance. This allows for the material to withstand rupture or yield under increased lateral stresses. The full density Si3N4 is also extremely resistant to fracture toughness and modulus. This material can withstand severe operating conditions that could cause other ceramic materials such as cracking, deforming, or collapse.
It is capable of displaying superior mechanical properties and thermal properties. This makes silicon nitride suitable for demanding industrial applications. Thermal conductivity refers to the material’s ability to conduct or transfer heat. The heat transfer coefficient plays a critical role in determining the suitability for industrial applications. Silicon nitride, due to its unique chemical structure and microstructure, has the same low thermal conductivity of metals.
These properties allow silicon-nitride greatly reduces thermal conductivity in extreme temperatures applications. The thermal expansion problem occurs when materials heat up and their volume and size change in small increments. This expansion is dependent on the temperature of the material. The ratio of thermal expansion coefficients is a measure of the material’s expansion per 1degC temperature increase. Because of the strong atomic bonds of Si3N4, this material has a low coefficient thermal expansion. It also experiences very little deformation when heated.
The superior thermal properties of silicon nitride make it less sensitive to high-speed applications. Because of its low dielectric constant (the ability to store electric energy in an electrical field), silicon nitride has been chosen for a wide range of RF applications. It also offers excellent strength and heat resistance.
This unique combination of properties has driven further research into silicon nitride’s use in structural ceramics for biomedical applications. In vitro and in vivo studies, as well as later studies involving the injection of silicon dioxide into animals, have established the biocompatibility. An in vitro study from 1999 further confirmed the biocompatibility claim of Si3N4 in the proliferation of functional human bone cells. These findings support silicon nitride being a promising biomedical material. Additionally to its biocompatibility silicon nitride has surface chemical characteristics that encourage bone formation (osteogenesis), as well as increased bone contact with implants.
Silicon nitride exhibits excellent stability because of its strong Atomic Bonds. This material is highly resistant to corrosion in acidic and alkaline environments at room temperatures. This is important when long-term implantation of silicon nitride in a watery or salty environment is considered. This is due to the formation on the surface of the material of an oxide layer. When silicon nitride was subjected to hot gases, molten metals, or other chemicals, the same resistance was observed. This is how materials resist corrosion.
Its unique microstructure that self-reinforces, its high strength, toughness and other excellent properties make silicon nitride a desirable structural component for many applications across a range of industries, including biomedical.
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