Sintering Furnaces

Sintering Furnaces

Sintering is a process whereby solid objects are created by heating a mass of small particles to a high temperature that is less than the melting point of the small particle mass. Unlike melting processes, sintering allows the creation of complex shapes that are different from the shape of the containment vessel. In contrast to melting, the lower temperatures allow the use of lower temperature furnaces. Low temperatures prevent chemical interactions that can occur between the containment vessel and any molten material.

For millenia, ceramic materials have been made with sintering processes. Sintering is particularly useful for ceramics for two reasons. First, it is difficult and expensive to reach the high melting point of many oxide ceramics. Secondly, it is impractical to reach the extremely high melting point of covalently bonded non-oxide ceramics, many of which sublime into the gas phase rather than melt into the liquid phase at elevated temperatures and readily achievable pressures.

With the increased importance of non-oxide ceramics in the later half of the twentieth century, vacuum sintering furnaces have been widely used. Cobalt-bonded tungsten carbide, silicon carbide, silicon nitride, boron carbide, and many other carbides, nitrides, and borides are now routinely fabricated in vacuum furnaces. Near net shape sintering of metallic components is also commonly used to reduce machining and overall component costs.

Vacuum sintering furnaces readily provide the non-oxidizing environment and extreme temperatures required by these materials. At the same time, they provide considerable process flexibility by allowing reducing and inert atmospheres, as well as vacuum. Vacuum can be used at lower temperatures during removal of organic binders and lubricants. This is because it effectively volatilizes and removes these materials before they are “cracked” to lower molecular weight constituents. As a result vacuum avoids leaving excess carbon in the material. At higher temperatures, inert gas atmospheres can be introduced to minimize sublimation.

Sublimation can affect both the material being sintered and the material the sintering furnace hot zone is made of. Sublimation of furnace heating elements generally requires backfilling of the furnace with inert gas between 2000°C to 2500°C , depending on choice of heating element material, element construction, element watt density, and longevity desired.

The following figure shows the RD-G, a widely used benchtop vacuum sintering furnace manufactured by Webb Vacuum Furnaces. Operating up to 2000°C in vacuum and 2200°C in argon gas, it consumes less than two kW of power.

RD-G Vacuum Furnace

RD-G Vacuum Sintering Furnace