Graphite is a layered form of carbon whose sheets slide easily over each other. Added to PTFE it lowers friction, forms a smooth transfer film on the counterface and protects softer metals.
Typical content
Five to fifteen percent by weight, often combined with glass or carbon when more strength is needed.
What it improves
- Friction. Low, and stable across a wide speed range.
- Mating-surface protection. It is kinder to aluminium, brass, soft steel and plastics than glass or bronze.
- Chemical resistance. Inert, so it keeps the broad PTFE range, including hydrofluoric acid and caustics.
- Thermal conductivity. Better than virgin, which helps carry away frictional heat.
- Wear. Better than virgin, though not as good as glass at equal percentage.
What it costs you
- Moderate strength. It reduces creep less than glass, carbon or bronze. For heavy loads, add glass or carbon.
- Dry-vacuum limits. Graphite depends on adsorbed moisture or gas to lubricate. In very dry or vacuum service it can wear faster than MoS2-filled grades. See the MoS2 article.
- Conductivity. Mildly conductive at higher loading.
Typical applications
Oil-free compressor rings, sliding bearings with soft shafts, rotary seals at low to medium speed, valve stem packing and some food and pharma-equipment wear parts where black colour is acceptable and the grade is certified.
Combining fillers
A common balance is about 20 percent glass with 5 percent graphite. The glass supplies load capacity, the graphite smooths the sliding film. Other blends include carbon with graphite, and bronze with graphite or MoS2.
Practical tip
If a soft shaft is wearing in service with glass-filled PTFE, graphite-filled or carbon-graphite filled PTFE is usually the first change to try, together with a better finish and a hardness check on the shaft.
