Silicon nitride (Si3N4)

Silicon nitride (Si3N4) has made great strides in recent years beyond the biomedical industry. R&D has led to the development of a number of industrial applications for silicon nitride-based materials, paving the way to manufacture different types of silicon nitride. These methods of fabrication have governed the resulting properties and applications of silicon nitride, allowing for a great deal of flexibility and customization to create precisely the material needed for an application.

In this article, we review the history of silicon nitride, key properties, and the top applications for this material.

Silicon nitride (Si3N4) is a non-metallic compound composed of silicon and nitrogen, first discovered in 1857. The first synthetic silicon nitride was developed by Deville and Wohler in 1859. Since its discovery, silicon nitride had remained an academic curiosity for almost a century, until the 1950s when commercial interests started to increase and the material was developed in various refractory applications. Notable strides in manufacturing processes during the 1970s through the 1980s reduced the production cost of the compound, and silicon nitride quickly established a foothold in many industrial applications, particularly as a structural ceramic. It was also in the 1980s that a global effort started to expand the use of silicon nitride in internal combustion engines and high-temperature gas turbines. Throughout the years, further improvements to its synthesis, processing, and properties resulted in it being one of the most studied ceramics in history. Because its material properties are well studied and understood, silicon nitride commercial use has significantly expanded greatly in recent years.

Silicon nitride has been commercially produced and synthesized by the reaction of various silicon carriers with nitrogen or ammonia. Further R&D in the 1980s led to the development of a number of industrial applications for silicon nitride-based materials, paving the way to manufacture different types of silicon nitride – reaction-bonded silicon nitride (RBSN), hot-pressed silicon nitride (HPSN), sintered reaction-bonded silicon nitride (SRBSN), sintered silicon nitride (SSN), partial pressure sintered silicon nitride (PSSN) and hot isostatically pressed silicon nitride (HIPSN). These methods of fabrication have governed the resulting properties and applications of silicon nitride, allowing for a great deal of flexibility and customization to create precisely the material needed for an application.

Key Properties

Silicon nitride is considered a very important engineering ceramic due to its adaptability to many different forms, with each form having distinctive properties. At high temperature, silicon nitride shows excellent mechanical properties including low density, high bending strength, high elastic modulus and fracture toughness, and high abrasion wear, as well as solid particle erosion resistance. In other words, this material is extremely strong and extremely tough. The material also shows excellent thermal properties, with minimal expansion and contraction due to temperature and the ability to withstand thermal shock (fast, significant changes in temperature). Finally, silicon nitride possesses exceptional chemical properties including stability against most acids and bases, corrosive gases and liquid metals.

Rolling contact fatigue (RCF) tests conducted on several ceramic-based materials subjected to loads of high-performance bearings have shown that only fully dense silicon nitride could outperform bearing steels. Fully dense Si3N4 bearing materials have shown RCF has ten times the longevity of high-performance bearing steel. Rotating bodies at high speed could lead to significant centrifugal stress. Being a low-density material makes silicon nitride as light as aluminum. However, this property also gives the material another benefit. The low density of silicon nitride reduces the centrifugal stress on the outer races of these rotating bodies at high speeds. The high tensile strengths of silicon nitride ceramics can resist elongation and offer outstanding flexural strengths to withstand yielding or rupturing at elevated transverse stresses. Fully dense Si3N4 also exhibits high fracture toughness and high moduli properties, giving the material an excellent resistance to multiple wearing phenomena. This enables it to withstand severe operating conditions that may cause other ceramic materials to generate flaws, deform, or collapse.

Aside from its superior mechanical properties, silicon nitride also exhibits an array of exceptional thermal properties that makes it suitable for demanding industrial applications. Thermal conductivity is the intrinsic ability of a material to transfer or conduct heat. The heat transfer coefficient is a critical factor that characterizes the applicability of engineered materials to be used in any industrial application under extreme temperature requirements. Because of its unique chemical composition and microstructure, silicon nitride offers similarly low thermal conductivity as compared to metals.

Such properties have enabled silicon nitride to reduce thermal conductivity in severe temperature applications significantly. Thermal expansion is another concern and occurs when materials are heated and their size and volume increase in small increments. This expansion varies depending on the material being heated. The degree of expansion of a material per 1°C rise in temperature is indicated by its coefficient ratio of thermal expansion. The strong atomic bonds of silicon nitride are what give this material low coefficients of thermal expansion, making its distortion values very low with respect to changes in temperature.

Owing to its exceptional thermal qualities, silicon nitride radio frequency (RF) properties are not considerably affected during high-velocity applications as compared to other ceramics. Silicon nitride is the preferred material for various RF applications due to its moderate dielectric constant (the ability of a substance to store electrical energy in an electric field) and low RF loss in combination with its superior strength and thermal resistance.

It is the unique combination of properties possessed by silicon nitride that led to further research for its use as a structural ceramic in biomedical applications. The biocompatibility of silicon nitride was already established in the late 1980s, as confirmed by an initial in vitro study and followed by an in vivo investigation involving silicon nitride implantation in animals. A study in 1999 further supported the claim of the biocompatibility of silicon nitride for the propagation of functional human bone cells performed outside a living organism (in vitro). These findings further propelled silicon nitride as an emerging biomedical material. Aside from its biocompatibility, silicon nitride is known to have a surface chemistry favorable for bone formation (osteogenesis) and increased bone-to-implant contact, resulting in the improved structural and functional integration between the living bone and the surface of an artificial implant (osseointegration).

Silicon nitride’s excellent stability due to its strong atomic bonding makes the material highly resistant to corrosion by acidic and basic solutions at room temperature. This is of utmost importance when considering long term implants inside the aqueous, salty environment of the human body. Corrosion resistance is primarily attributed to the formation of the oxide layer at the surface of the material. The same resistance was observed when corrosion tests were carried out subjecting silicon nitride to hot gases, molten salts and metals and in complex environments. This formation of the oxide layer plays a central, complex role in the corrosion resistance of the material.

Due to its self-reinforced microstructure, exceptionally high strength and toughness and many excellent properties have made silicon nitride an attractive structural component for numerous applications in various industries including biomedical applications.

Applications

As one of the significant structural ceramics that have developed, silicon nitride has found valuable use in wear-resistant and high-temperature applications.

Automotive Industry

Currently, the predominant application of silicon nitride is in the automotive industry. The material is found in applications for engine parts and engine accessory units including turbochargers for lower inertia and reduced engine lag and emissions, glow plugs for faster startup, exhaust gas control valves for increased acceleration, and rocker arm pads for gas engines to lower wear. Regulatory restrictions on engine emissions and higher fuel costs have driven the use of silicon nitride in the automotive industry. Each application within the automotive industry requires subtly different yet tightly controlled attributes to be able to meet the requirements and attain the greatest improvement in performance versus other materials.

Bearing Industry

Owing to its excellent shock resistance compared to other ceramics, silicon nitride is considered an extremely promising material for the fabrication of high-performance all-ceramic or hybrid steel-ceramic rolling contact bearings. Due to its extreme strength, toughness, and resistance to chemical and thermal factors, silicon nitride offers significant benefits by extending contact fatigue life. As a low-density material, silicon nitride can greatly reduce the dynamic loading during ball contacts in very high-speed applications such as those in gas turbine engines. The material also has notable applications in severe lubrication and wear conditions including extreme temperature, large temperature differential, ultra-high vacuum as well as in safety-critical applications, enabling the material to respond to the specific requirements, i.e. aircraft maintenance operations. As evidenced by their outstanding performance in extreme environments, silicon nitride bearings are expected to continue to gain wide acceptance in many industrial applications.

Metal Working and Cutting Tools

Silicon nitride saw its first major application in abrasive and cutting tools. Due to its hardness, thermal stability and wear resistance, silicon nitride was found to work well with cutting tools especially during high-speed machining of cast iron. The superior material properties of Si3N4 can cut cast iron, hard steel as well as nickel-based alloys with surface speeds up to 25 times faster than conventional machining materials such as tungsten carbide. The benefits of using silicon nitride inserts range from doubling cutting speed, extending tool service life from one part to six parts per edge, and reducing the average cost of inserts by 50 percent as compared to conventional tungsten carbide tools.

Aviation and Aerospace Industry

Traditional aerospace applications used metals and other composites as the materials of choice for aircraft production. The advent of supersonic and hypersonic flights mean that these traditional materials can no longer meet the enormous demands of the extreme conditions and applications that hypersonic flights require. Hypersonic flight dictates the need for new materials possessing extremely exceptional material properties. The mechanical robustness of silicon nitride along with its ability to withstand and perform at high temperatures experienced during high-velocity flights have made it the material of choice for various aerospace applications. Silicon nitride can appropriately replace legacy materials used in ball bearings, radomes, and RF windows for extreme applications. Similar to the automotive industry but to a greater extreme, the ability to customize silicon nitride to a specific application is crucial to the overall optimization of these aerospace applications.

Electronics Industry

The unique electrical properties of silicon nitride have caused an increasing usage in microelectronics applications as an insulator and chemical barrier in the manufacture of integrated circuits for the protective packaging of devices. Silicon nitride is used as a passivation layer with a high diffusion barrier against water and sodium ions, both of which are major sources of corrosion and instability in microelectronics. The material is also used as an electrical insulator between polysilicon layers in capacitors of analog chips. Silicon nitride also found use in the xerographic process as one of the photo drum layers. Owing to its excellent elastic properties, silicon nitride has been the most accepted material for cantilevers, which are the sensing elements for atomic force microscopes.

The use of silicon nitride has also been recognized in the field of optics, having great transparency in the spectral range, giving rise to Si3N4 photonics. The broadband nature of Si3N4 has allowed it to be used in biochemical and biomedical optical applications, biophotonics, tele- and data communications, and optical signal processing, sensing from visible through near to mid-infrared wavelengths.

Biomedical Applications

There is a surprising array of material properties that makes silicon nitride the ideal biomaterial. Further scientific investigations have produced innovative methods in the potential use of silicon nitride in a variety of biomedical applications.

Biosensors

The last three decades have seen significant growth in the development of biosensing devices and their utilization in various fields of applications. While the biosensor technology has already reached maturity and has established a global market, problems surrounding stability, sensitivity, and size have limited the general use of optical biosensors for real field applications. Silicon-based integrated photonic biosensors have the potential to solve these shortcomings by offering early diagnostic tools with better sensitivity, specificity, and reliability and possibility of improving effectiveness of in-vitro and in-vivo diagnostics.

Recently, silicon nitride-based fabrication technology has been successfully integrated with biochemistry, facilitating the creation of innovative biosensing devices with high sensitivity and selectivity . Silicon nitride offers other advantages compared to other materials, with the absence of impurities and excellent control of film composition and thickness. These features proved important for ultra-thin layer applications in gate and tunnel dielectrics in the fabrication of biosensing devices. Silicon nitride’s high-grade electronic and mechanical properties have made the material exceptionally attractive for biosensor applications.

Spinal and Orthopedic Implants

With its superior mechanical, thermal and chemical properties, silicon nitride has other unique properties that may make it a valuable material for spinal implants and prosthetic replacements for hip and knee joints. As a bioinert material, silicon nitride is biocompatible per ISO 10993-01 standards (Internal data on file). The material has demonstrated antibacterial and osseointegrative properties to support bone formation. As a spinal implant, its partially radiolucent property produces no image distortions in plain radiography, allowing a more precise view of the implant during surgery and postsurgical evaluation.

SINTX, as the only contract manufacturer of biomedical silicon nitride implants, has had great success in the spinal implant arena with favorable results.

Dental Applications

Having demonstrated itself as an beneficial implant material for spinal implants , researchers have turned to the potential of silicon nitride for dental implants. In an article posted by The American Ceramic Society, silicon nitride was labeled as a “killer ceramic material,” due to its antibacterial property that inhibits Porphyromonas gingivalis, a bacterium found in oral cavities that causes gum disease. In this joint research invitro study, the bacteria underwent lysis (breaking down of the cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity) on polished surfaces of silicon nitride material. Silicon nitride demonstrated the ability to degrade the bacteria’s biological makeup in six days. This breakdown of cell function was primarily due to the formation of peroxynitrite, an oxidizing and nitrating agent capable of damaging molecules in cells, including DNA and proteins. The research study also found out that altering the surface chemistry of silicon nitride will influence the formation of peroxynitrite and thus affect bacteria metabolism in various ways. The unique properties of silicon nitride demonstrate potential in oral health applications.

There has been an increasing demand for metal-free dental implants and has contributed to the interest in silicon nitride material for the potential dental implant global market.

SINTX Technologies

As a leading manufacturer of biomaterials for medical applications, SINTX Technologies is the only FDA registered and ISO 13485:2016 certified silicon nitride medical device manufacturing facility in the world. SINTX has more than 20 years of industry experience combining scientific, technical, medical, and manufacturing expertise in silicon nitride. SINTX Technologies innovative technology has been published in more than 85 peer-reviewed papers. Recognized for its product versatility, the company has produced over 50,000 silicon nitride implants of different shapes and compositions since 2008.

The company’s drive for innovation has been proven by its intellectual property portfolio of 13 US patents issued, with three US patents pending and 10 international patents pending. With its 30,000 sq. ft. facility designed for rapid prototyping and development, equipped with dedicated research and development, product development laboratories, and vertically integrated manufacturing capabilities, SINTX is poised to lead pioneering work in industrial product design and biomaterial formulation.

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