Machining of the Ceramic Silicon Carbide - Process, Applications and Types

The Chemical Formula of Silicon Carbide, which is also known carborundum, is SiC. It is produced by the carbothermal reduction of silica to form an ultra-hard covalently bonded material. It is extremely rare in nature but can be found in the mineral moissanite, which was first discovered in Arizona in 1893.

Precision machined sintered silicon carbide component machined by Insaco.

Machining of Silicon Carbide

In all of the applications outlined above, where a high precision engineering components are required, it is important to recognize the difficulties of machining Silicon Carbide. Despite the high hardness values it displays, it is nevertheless a relatively brittle material and can only be machined using diamond grinding techniques.

Consequently, it is beneficial that a skilled and experienced operator conducts the machining operations as incorrect procedures can generate sub-surface damage and micro-cracks that may lead to premature failure once the component is subjected to operating stresses in service.

Synthesizing Silicon Carbide

Typically, Silicon Carbide is produced using the Acheson process which involves heating silica sand and carbon to high temperatures in an Acheson graphite resistance furnace. It can be formed as a fine powder or a bonded mass that must be crushed and milled before it can be used as a powder feedstock. Once the Silicon Carbide is in a powder form, the grains of the compound can be bonded together by sintering to form a very useful engineering ceramic, which has a wide range of uses in many manufacturing industries.

The Structure of Silicon Carbide

Many structures or polytypes have been identified for Silicon Carbide. These polytypes have different stacking arrangements for the atoms of silicon and carbon in the compound. One of the simplest structures is the diamond structure, which is known as b -SiC. There are more complex hexagonal or rhombic structures of the compound and these are designated as a -SiC.

The Discovery of Silicon Carbide

Dr. Edward Goodrich Acheson was a scientist who once worked for Thomas Edison. He first synthesized Silicon Carbide by chance in the process of trying to create artificial diamonds. Diamonds could be, at least in theory, baked in the laboratory and so he decided to attempt to synthesize them using carbon based materials. In his experiment he attached a lead from a dynamo to a plumber’s bowl, which was filled with clay and powdered coke.

When the mixture was subjected to the high heating temperature from the dynamo lead, he did not produce any diamonds, but he did notice a few bright specks on the end of the lead. He picked up the lead and drew it over a glass pane and it cut the pane like a diamond. What he had succeeded in developing was, the first man made substance that was hard enough to cut through glass.

He was also trying to dissolve carbon in molten corundum or alumina when he discovered the blue black colored crystals which he thought were a compound of corundum and carbon, hence why he called the material carborundum. This became the popular name for Silicon Carbide and was also the name of the company that Acheson founded. Although the first use of the compound was as an abrasive, it has since been subsequently developed to be used in electronic applications and many other engineering uses.

Types of Silicon Carbide

For use in commercial engineering applications Silicon Carbide products are produced in three forms. These are:

• Sintered silicon carbide (SSC)
• Nitride bonded silicon carbide (NBSC) and
• Reaction bonded silicon carbide (RBSC)

Other variations of the compound include clay bonded silicon carbide and SiAlON bonded silicon carbide. There is also chemical vapor deposited silicon carbide called CVD Silicon Carbide, which is an extremely pure form of the compound.

To sinter the Silicon Carbide its is necessary to add sintering aids which help to form a liquid phase at the sintering temperature which allows the grains of silicon carbide to bond together.

Key Properties of Silicon Carbide

Silicon Carbide has a refractive index that is greater than that of diamond. It has a high thermal conductivity and it has a low thermal expansion coefficient. This combination of these properties give it outstanding thermal shock resistance, which makes it useful to many industries. It is also a semiconductor and lends itself to a range of uses thanks to its electrical properties. It is also known for its extreme hardness and is very corrosion resistant.

The Table below provides further example data for Sintered Silicon Carbide.

Table 1. Properties of sintered silicon carbide.

Major Applications of Silicon Carbide

There are many uses of Silicon Carbide in different industries. Its physical hardness makes it ideal to be used in abrasive machining processes like grinding, honing, sand blasting and water jet cutting.

The ability of Silicon Carbide to withstand very high temperatures without breaking or distorting is used in the manufacture of ceramic brake discs for sports cars. It is also used in bulletproof vests as an armor material and as a seal ring material for pump shaft sealing where it frequently runs at high speed in contact with a similar silicon carbide seal. One of the major advantages in these applications being the high thermal conductivity of Silicon Carbide which is able to dissipate the frictional heat generated at a rubbing interface.

The high surface hardness of the material lead to it being used in many engineering applications where a high degree of sliding, erosive and corrosive wear resistance is required. Typically this can be in components used in pumps or for example as valves in oilfield applications where conventional metal components would display excessive wear rates that would lead to rapid failures.

The unique electrical properties of the compound as a semiconductor make it ideal for manufacturing ultra fast and high voltage light emitting diodes, MOSFETs and thyristors for high power switching.

The material’s low thermal expansion coefficient, hardness, rigidity and thermal conductivity make it an ideal mirror material for astronomical telescopes. Silicon Carbide fibers, known as filaments are used to measure gas temperatures in an optical technique called thin filament pyrometry.

It is also used in heating elements where extremely high temperatures need to be accommodated. It is even used in nuclear power to provide structural supports in high temperature gas cooled reactors.

This information has been sourced, reviewed and adapted from materials provided by INSACO Inc.

For more information on this source, please visit INSACO Inc.