SILICON NITRIDE[Si3N]

 CAS Number 12033-89-5

·         Linear Formula Si₃N₄

·         Molecular Weight 140.28

·          EC Number 234-796-8

·          MDL number MFCD00011230

·          PubChem Substance ID 24860055

Silicon nitride is a chemical compound of the elements silicon and nitrogen, with the formula Si
3N
4. It is a white, high-melting-point solid that is relatively chemically inert, being attacked by dilute HF and hot H
2SO4.

Detailed description

 It is very hard (8.5 on the mohs scale). It is the most thermodynamically stable of the silicon nitrides. Hence, Si
3N
4 is the most commercially important of the silicon nitrides^[3] and is generally understood as what is being referred to where the term "silicon nitride" is used.

Applications

In general, the main issue with applications of silicon nitride has not been technical performance, but cost. As the cost has come down, the number of production applications is accelerating.^[18]

Automobile industry

One of the major applications of sintered silicon nitride is in automobile industry as a material for engine parts. Those include, in diesel engines, glowplugs for faster start-up; precombustion chambers (swirl chambers) for lower emissions, faster start-up and lower noise; turbocharger for reduced engine lag and emissions. In spark-ignition engines, silicon nitride is used for rocker arm pads for lower wear, turbocharger for lower inertia and less engine lag, and in exhaust gas control valves for increased acceleration. As examples of production levels, there is an estimated more than 300,000 sintered silicon nitride turbochargers made annually.

Bearings

Silicon nitride bearings are both full ceramic bearings and ceramic hybrid bearings with balls in ceramics and races in steel. Silicon nitride ceramics have good shock resistance compared to other ceramics. Therefore, ball bearings made of silicon nitride ceramic are used in performance bearings. A representative example is use of silicon nitride bearings in the main engines of the NASA's Space Shuttle.^[19][20]

Since silicon nitride ball bearings are harder than metal, this reduces contact with the bearing track. This results in 80% less friction, 3 to 10 times longer lifetime, 80% higher speed, 60% less weight, the ability to operate with lubrication starvation, higher corrosion resistance and higher operation temperature, as compared to traditional metal bearings.^[18] Silicon nitride balls weigh 79% less than tungsten carbide balls. Silicon nitride ball bearings can be found in high end automotive bearings, industrial bearings, wind turbines, motorsports, bicycles, rollerblades and skateboards. Silicon nitride bearings are especially useful in applications where corrosion, electric or magnetic fields prohibit the use of metals. For example, in tidal flow meters, where seawater attack is a problem, or in electric field seekers.^[11]

Si₃N₄ was first demonstrated as a superior bearing in 1972 but did not reach production until nearly 1990 because of challenges associated with reducing the cost. Since 1990, the cost has been reduced substantially as production volume has increased. Although Si
3N
4 bearings are still 2–5 times more expensive than the best steel bearings, their superior performance and life are justifying rapid adoption. Around 15–20 million Si
3N
4 bearing balls were produced in the U.S. in 1996 for machine tools and many other applications. Growth is estimated at 40% per year, but could be even higher if ceramic bearings are selected for consumer applications such as in-line skates and computer disk drives.^[18]

High-temperature material

Silicon nitride has long been used in high-temperature applications. In particular, it was identified as one of the few monolithic ceramic materials capable of surviving the severe thermal shock and thermal gradients generated in hydrogen/oxygen rocket engines. To demonstrate this capability in a complex configuration, NASA scientists used advanced rapid prototyping technology to fabricate a one-inch-diameter, single-piece combustion chamber/nozzle (thruster) component. The thruster was hot-fire tested with hydrogen/oxygen propellant and survived five cycles including a 5-minute cycle to a 1320 °C material temperature.^[21]

In 2010 silicon nitride was used as the main material in the thrusters of the JAXA space probe Akatsuki.^[22]

Medical

Silicon nitride has many orthopedic applications.^[23][24] The material is also an alternative to PEEK (polyether ether ketone) and titanium, which are used for spinal fusion devices.^[25][26] It is silicon nitride’s hydrophilic, microtextured surface that contributes to the materials strength, durability and reliability compared to PEEK and titanium.^[24][25][27]

Metal working and cutting tools

The first major application of Si
3N
4 was abrasive and cutting tools. Bulk, monolithic silicon nitride is used as a material for cutting tools, due to its hardness, thermal stability, and resistance to wear. It is especially recommended for high speed machining of cast iron. Hot hardness, fracture toughness and thermal shock resistance mean that sintered silicon nitride can cut cast iron, hard steel and nickel based alloys with surface speeds up to 25 times quicker than those obtained with conventional materials such as tungsten carbide.^[11] The use of Si
3N
4 cutting tools has had a dramatic effect on manufacturing output. For example, face milling of gray cast iron with silicon nitride inserts doubled the cutting speed, increased tool life from one part to six parts per edge, and reduced the average cost of inserts by 50%, as compared to traditional tungsten carbide tools.^[5][18]

Electronics

Silicon nitride is often used as an insulator and chemical barrier in manufacturing integrated circuits, to electrically isolate different structures or as an etch mask in bulk micromachining. As a passivation layer for microchips, it is superior to silicon dioxide, as it is a significantly better diffusion barrier against water molecules and sodium ions, two major sources of corrosion and instability in microelectronics. It is also used as a dielectric between polysilicon layers in capacitors in analog chips.^[28]

Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile stress, which may crack films thicker than 200 nm. However, it has higher resistivity and dielectric strength than most insulators commonly available in microfabrication (10¹⁶ Ω·cm and 10 MV/cm, respectively).^[6]

Not only silicon nitride, but also various ternary compounds of silicon, nitrogen and hydrogen (SiN_xH_y) are used insulating layers. They are plasma deposited using the following reactions:^[6]

2 SiH
4(g) + N
2(g) → 2 SiNH(s) + 3 H
2(g)

SiH
4(g) + NH
3(g) → SiNH(s) + 3 H
2(g)

These SiNH films have much less tensile stress, but worse electrical properties (resistivity 10⁶ to 10¹⁵ Ω·cm, and dielectric strength 1 to 5 MV/cm).

Silicon nitride is also used in xerographic process as one of the layer of the photo drum.^[30] Silicon nitride is also used as an ignition source for domestic gas appliances.^[31] Because of its good elastic properties, silicon nitride, along with silicon and silicon oxide, is the most popular material for cantilevers — the sensing elements of atomic force microscopes.

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