Carbon-filled PTFE is usually black, hard and more conductive than glass-filled material. It is made with amorphous (petroleum coke) carbon, carbon fibre or both.
Typical content
Ten to thirty-five percent by weight, most commonly 15 to 25 percent.
What it improves
- Compressive strength and creep. Better than virgin and comparable to glass at the same loading.
- Wear. Good, especially against hard counterfaces and in water or steam.
- Chemical resistance. Carbon is inert in nearly everything that PTFE resists, including hydrofluoric acid and strong alkalis, where glass fails.
- Thermal conductivity. Higher than virgin PTFE, so frictional heat leaves the interface faster.
- Electrical behaviour. Adds some static dissipation. High loadings are conductive.
What it costs you
- Mechanical properties are slightly lower than glass at equal loading for pure strength, and the compound is a little more abrasive to soft mating metals than graphite-filled grades.
- Conductivity can be unwanted. In electrical insulation or in galvanic couples with stainless or aluminium in wet service, check corrosion risk.
- Colour hides wear debris and contamination.
- Cost. Carbon fibre grades cost more than glass.
Carbon fibre versus carbon powder
Fibre grades give the best compressive stiffness and lowest deformation under load. Powder grades give lower friction and better surface finish. Blends exist, sometimes with graphite to bring friction down further.
Typical applications
Compressor piston rings and rider rings, steam valve seals, pump bearings and wear parts for aggressive fluids, hydraulic wear rings in hard-chrome systems, and parts for cryogenic or hot-water service.
What to watch
Specify the carbon type: powder, fibre or carbon-graphite. State the percent by weight. For steam and hot water ask for supplier data at the actual temperature and pressure, because creep rises with temperature even in filled grades.
