- All metallic (radiation shields)
- Combination radiation shields and other (ceramic) insulating material
- Multiple-layer (sandwich) insulation
- All graphite (board, fiber, carbon-carbon composite)
- Tungsten or tantalum having a maximum operating temperature of 4350°F (2400°C)
- Molybdenum having a maximum operating temperature of 3100°F (1700°C)
- Stainless steel or nickel alloys having a maximum operating temperature of 2100°F (1150°C)
Sandwich insulation is composed of one or more radiation shields typically with ceramic-wool insulation between them. Combinations of graphite-fiber sheets and ceramic-wool insulation are also used. These versions are cheaper to buy and maintain but absorb higher levels of water vapor and gases (due to the very large surface area of the insulation wool). Their heat losses are considerably lower than those of radiation shields.
Graphite-fiber insulation designs cost somewhat more than sandwich-insulation designs. Because their heat losses are lower, however, a smaller thickness is sufficient. In these designs, the absorption of gases and water vapor is considerably reduced. Furthermore, the heating costs are lower, and the lifetime of this type of insulation is much longer. The maximum operating temperature is around 3630°F (2000°C). The lifetime depends strongly on the purity of the graphite. In some applications, such as brazing, a sacrificial layer is used to protect the insulation beneath. For most heat treatments in vacuum furnaces, graphite insulation is used.
In general, the heating elements for heating systems in vacuum furnaces are made from one of the following materials:
- Nickel/chromium alloys that can be used up to 2100°F (1150°C). Above 1475°F (800°C) there is a risk of evaporation of chromium.
- Silicon carbide with a maximum operating temperature of 2200°F (1200°C). There is a risk of evaporation of silicon at high temperatures and low vacuum levels.
- Molybdenum with a maximum operating temperature of 3100°F (1700°C). Molybdenum becomes brittle at high temperatures and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.
- Graphite can be used up to 3630°F (2000°C). Graphite is sensitive to exposure to oxygen or water vapor, resulting in reduction in material thickness due to the formation of carbon monoxide (CO) that will be evacuated by the pumps. The strength of graphite increases with temperature.
- Tantalum has a maximum operating temperature of 4350°F (2400°C). Tantalum, like molybdenum, becomes brittle at high temperatures and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.