Choose by working through four questions in order: the fluid, the counterface, the load and speed, and the approvals.
Step 1: the fluid
If the fluid attacks a filler, that filler is out. Hydrofluoric acid and hot strong alkalis attack glass. Many acids, ammonia, steam and seawater attack bronze. Oxidising acids attack MoS2. Carbon and graphite are inert in nearly all of these. Virgin PTFE is the safest chemical choice.
Step 2: the counterface
Soft metals (aluminium, brass, mild steel, bronze) and plastics want graphite, carbon-graphite or virgin PTFE. Hardened steel, hard chrome and nitrided surfaces tolerate glass, carbon fibre and bronze.
Step 3: load, speed and temperature
Heavy static load or heavy dynamic load points to bronze, carbon fibre or glass. Light load and low friction point to graphite or virgin. High speed adds heat, so thermal conductivity (bronze, carbon, graphite) helps.
Step 4: approvals and cleanliness
Food, pharma, drinking water and semiconductor contact normally need virgin PTFE or a specifically certified filled grade. Check the approval, not just the colour.
Comparison
| Grade | Wear | Creep | Friction | Chemical range | Thermal conductivity | Kind to soft counterface | Food use |
|---|---|---|---|---|---|---|---|
| Virgin | Poor | Poor | Very low | Widest | Low | Yes | Yes (certified) |
| Glass 15-25% | Very good | Good | Low | Narrowed by HF, hot alkali | Low | No | Rarely |
| Carbon 15-25% | Very good | Good | Low | Very wide | Medium | Moderate | Rarely |
| Graphite 5-15% | Good | Fair | Lowest filled | Very wide | Medium | Yes | Special grades |
| Bronze 40-60% | Excellent | Excellent | Low | Narrow | High | No | No |
| Glass + MoS2 | Excellent | Good | Low, stable dry | Narrowed | Low | No | No |
Shortcuts
- Dry running, vacuum: add MoS2.
- Steam or hot water: carbon or carbon-graphite.
- Heavy hydraulic load with oil: bronze or glass.
- Soft shaft: graphite.
- Clean, chemical or food: virgin or modified PTFE.
Then test
Selection tables narrow the field. A short pilot run on the real counterface, finish and fluid is cheap compared with a field failure.
