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.
Silicon nitride plungers suit high-pressure service with vibration and thermal cycling, including diesel injection and hot fluid dosing. The low density reduces reciprocating mass and the toughness tolerates the pressure spikes that break a brittle plunger.
Silicon nitride pairs high strength of 700-900 MPa with 6-8 MPa·m½ toughness at only 3.2 g/cm³. It survives the thermal shock that cracks most ceramics, resists molten non-ferrous metals, and its fine grain allows bearing-grade finishes.
It is among the more expensive options, and unlike SiC its thermal conductivity is a modest 25-35 W/m·K, so it is specified for mechanical reliability rather than heat spreading.
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 |
Silicon Nitride Properties
| Composition | Gas-pressure sintered silicon nitride |
| Density | 3.20-3.25 g/cm³ |
| Vickers hardness | 1500-1600 HV |
| Flexural strength | 700-900 MPa |
| Fracture toughness | 6-8 MPa·m½ |
| Max. service temperature | 1000°C (1200°C short term) |
| Thermal conductivity | 25-35 W/m·K |
| Thermal expansion (20-1000°C) | 3.2 ×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.