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. However, this material also has another 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 exceptional flexural strength that can withstand rupture or yield under higher lateral stresses. The full density Si3N4 is also extremely resistant to fracture toughness and modulus. It is able to withstand severe operating conditions that could cause other ceramic materials such as cracks, deformation 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 enable silicon-nitride’s ability to significantly lower thermal conductivity in extreme temperatures. The thermal expansion problem occurs when materials heat up and their volume and size change in small increments. The temperature at which the material is heated affects how much expansion occurs. 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 many RF applications. It also has excellent strength and heat resistance.
This unique combination of properties has led to further research on 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 for functional human bone cell proliferation. 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.
Because of its strong and stable atomic bonds at room temperatures, silicon nitride is highly resistant to corrosion. This is important when considering long-term implant in a salty and watery environment. This is due to the formation on the material of an oxide layer. When silicon nitride was subjected to heat gases, molten metals, or other chemicals, the same resistance was observed. The corrosion resistance of materials is affected by the formation of the oxide layer.
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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