Ceramic plungers and pistons reciprocate inside a bore, displacing fluid or transmitting motion. They are used where steel would score, corrode or wear the bore out.
Alumina plungers are the standard for high-pressure water jetting and general dosing pumps. The fine finish they take is what gives long seal life, and the material is unaffected by most process fluids.
Alumina is the default engineering ceramic. It combines high hardness (1500-1700 HV) with good electrical insulation and chemical inertness at a lower cost than most technical ceramics, and in 99.8% purity it holds strength to 1700°C while resisting most acids, alkalis and solvents.
It is not the toughest option. At 3-4 MPa·m½ fracture toughness, sharp impact or point loading will chip an edge, so chamfers and radii belong in the drawing from the start.
Specifications
| Diameter | 5-80 mm |
| Length | up to 400 mm |
| Cylindricity | 0.005 mm |
| Diameter tolerance | ±0.005 mm ground |
| Surface finish | Ra 0.4 µm ground, Ra 0.2 µm polished |
| Straightness | 0.02 mm per 100 mm |
| End form | flat, domed, grooved or threaded |
Alumina Properties
| Composition | Aluminium oxide, 96% / 99.5% / 99.8% |
| Density | 3.70-3.95 g/cm³ |
| Vickers hardness | 1500-1700 HV |
| Flexural strength | 300-400 MPa |
| Fracture toughness | 3-4 MPa·m½ |
| Max. service temperature | 1500°C (99.8%: 1700°C) |
| Thermal conductivity | 25-35 W/m·K |
| Thermal expansion (20-1000°C) | 8.0 ×10⁻⁶/K |
Machining and Tolerances
Plungers are centreless ground then lapped to size and finish. Surface finish matters more than the number suggests: a plunger polished to Ra 0.2 µm will outlast one at Ra 0.4 µm by a wide margin because it stops abrading the seal.
Where It Is Used
- High-pressure pump plungers in water jetting and dosing
- Metering pump pistons in chemical and pharmaceutical dosing
- Valve and dispensing pistons in filling equipment
- Pump components in HPLC and analytical instruments
- Wear pistons for slurry and abrasive fluid service
Frequently Asked Questions
What finish do you recommend for a seal running face?
Ra 0.2 µm polished, and smoother where the seal material allows it. Seals fail from abrasion long before the ceramic itself wears, so the best value in a plunger is usually the finish rather than the grade.
Can the plunger be joined to a metal drive?
Yes, and it is worth designing carefully. We recommend a compliant interface or a bonded joint that does not load the ceramic in tension, since ceramic carries compression far better than tension or bending.