Surface treatment of metals plays an important part in many industries. Different treatments have been used since ancient times in metalwork. The right treatment for the metal or metal alloy in question, depending on the intended use of the product, can help to fight rust and corrosion, improve luster, strength, and hardness, or simply add decoration. Treatments can not only preserve metals, but increase the quality and performance of metal products.
About Silicon Nitride (SiN)
This is a ceramic-like material that can both preserve metals and improve durability. Composed, as the name suggests, of silicon and nitrogen, it's typically gray-black in color, though some admixtures can be white. Several different methods are used in preparing this material for commercial use, all using powdered silicon. The exact chemical formula varies, with Si3N4 being common.
SiN has a high melting point of about 3,452 degrees Fahrenheit (1,900 degrees Celsius). Silicon nitride has a uniquely grainy, crystalline structure. The long crystals interlock within SiN's microstructure to provide exceptional strength and hardness. This also allows it to take a high polish for a very smooth surface.
Because it's a very hard material, silicon nitride is ideal for use in high-stress mechanical environments, such as engines and turbines. Silicon nitride also makes metals highly resistant to thermal shock, or extreme temperature.
As a ceramic compound, SiN is relatively light-weight. This combination makes it possible to create some very precise components that will stand up to hard use and extreme temperatures, such as in welding or cutting operations.
It also helps to extend the life of metals as it's chemically inert to oxidation and most industrial solvents and contaminants.
The SiN Coating Process
It's often applied through heating powdered silicon in a nitrogen-rich environment to temperatures as high as 1,400 degrees Celsius (2,552 degrees Fahrenheit). This process is known as RBSN (reaction bonded silicon nitride).
HPSN (Hot-Pressed Silicon Nitride) involves using a high-pressure graphite die to apply the SiN under high heat and pressure. This is effective only for simple shapes.
Sintering is the name for several different processes of adding powdered substances to a metal surface by applying heat. Silicon nitride treatments, however, are often done at lower temperatures, a concern since the nitrogen tends to separate out at temperatures around 1,850 degrees Celsius (3,362 Fahrenheit) or higher. These high temperatures can also cause a release of oxygen from the powdered silicon, which can affect the density of the coating.
SiN can be used as a powder that can be bonded directly to metallic surfaces by mixing it with "binders" such as magnesia or alumina, also called sintering aids. This creates a mixture which gives the silicon nitride some liquidity for coating more complex surfaces.
Alternately, SiN can be applied by subjecting it to controlled electrical pulses that heat the material at a certain point to 1700 degrees Celsius (3,092 Fahrenheit) and above. This technique is called spark plasma sintering and can help with more precise application in tight spaces.
In both approaches, the SiN is often compacted beforehand, and/or the metal is pre-treated with other forms of nitrides, such as boron or carbide. The result is a more effective layer that improves the SiN's physical properties, particularly for high-stress uses.
Even in dense layers, SiN normally doesn't suffer much volume change after application. This is typically a predictable 15 percent in reduction. That can allow metal fabricators to treat even complex shapes at a single pass with a liquid form of SiN.
Silicone Nitride: Applications and Materials
Because of the properties it brings to metal components, SiN is useful to a range of business needs. This includes the automotive, aircraft, construction, medical, tool, and metal fabrication industries. It's widely used for surface treatment of ball bearings, engines, and power tools where SiN-coated bits and spindles will perform better than traditional hard metal alloys like tungsten carbide. Because of its high stability and thermal resistance, silicon nitride is also used in vehicle emission systems to reduce pollutants.
SiN is commonly used with iron, steel, zinc, lead, nickel, aluminum, tin, or alloys derived from these base metals. Due to its toughness and hardness it's often used for sheathings as well as bearings. Because of it's thermal resistance, it can be used in kilns, crucibles, or welding nozzles for working other metals. Since ceramics are also excellent insulators, SiN is used in vehicles or motors where electrical resistance is essential to metal components.
The durability of everything from eyeglass frames to heavy equipment can be improved with silicon nitrides.
Advantages of the Silicon Nitride
The crystalline structure also adds a high resistance to fracture in metals where SiN is applied. This added strength holds true over a wide range of temperatures, making it a good choice in any application where high heat is involved.
Metal products using silicon nitride have a very hard surface. SiN rates about 8.5 on the Mohs hardness scale. By comparison, most stainless steels only rate about 6.5. Silicon nitride is consequently very resistant to wear from friction or even obstructions when high rates of spin for prolonged periods are involved, such as industrial tools and machinery.
SiN as a surface treatment can also be applied to accurate thicknesses and take on a very smooth finish, making it ideal where specific tolerances are also required.
Where nonferrous metals are not involved, as a ceramic silicon nitride makes an excellent insulator to prevent electrical shock. This not only improves safety but helps limit damage to electrical circuits from loose or damaged wiring.
Where electrical conductivity is required, some newer silicon nitrides can be prepared with the inclusion of carbon nanotubes (CNT) that are excellent conductors. This can also allow methods of application involving electrodes and electroplating.