Aluminium and magnesium melts are corrosive, thermally punishing, and unforgiving of anything that flakes, wets or dissolves. Here is why silicon nitride keeps showing up in the parts that live inside the melt.
| Property in molten Al / Mg service | Steel / Cast iron | Graphite | Si₃N₄ (NitriCeram) |
|---|---|---|---|
| Continuous service temperature | Moderate | Good | Excellent |
| Thermal-shock resistance | Poor | Good | Excellent |
| Corrosion by molten Al/Mg | High | Moderate | Very low |
| Wetting by molten metal | Yes (erosion) | Yes | No (non-wetting) |
| Typical degassing-rotor life | Days–weeks | Weeks | Months |
In aluminium casting, dissolved hydrogen is the enemy — it comes out of solution as the metal solidifies and leaves porosity. A spinning rotor breaks an inert gas (argon or nitrogen) into fine bubbles that carry hydrogen out of the melt. The rotor runs fully submerged at 700–760 °C and is spun at thousands of rpm, so it must resist erosion, thermal shock from every dip, and attack by the flux-laden melt. Steel rotors dissolve and erode within days; graphite survives longer but is brittle and wets easily. Silicon nitride is non-wetting, chemically stable against molten aluminium, and shrugs off the thermal cycles — which is why our Si₃N₄ rotors typically run for months, not weeks, between change-outs.
In low-pressure die casting for wheels and structural parts, a riser or stalk tube lifts molten aluminium from the furnace up into the die. It sits hot on the bottom and cold on the top, with the melt creeping up its bore — a brutal thermal gradient. A Si₃N₄ riser tube resists the gradient without cracking, does not react with the aluminium, and keeps the bore clean so flow stays consistent. That means fewer inclusions, fewer scrap parts, and less downtime for tube swaps.
Melt temperature is the single most-controlled variable in a foundry, and the sheath decides whether the reading is trustworthy. A steel sheath scales and reacts; a Si₃N₄ sheath stays inert and dimensionally stable, so the thermocouple reads true and the sheath does not contaminate the bath. For magnesium and high-temperature aluminium alloys the difference is even larger.
For these parts we typically recommend a high-purity, fully densified GPS/HIP Si₃N₄ with a glazed or fine-ground surface so the melt cannot wet or key into the ceramic. Tell us your alloy, temperature and cycle time and we will pick the grade and finish — and, for rotors, the vane geometry that gives you the finest bubble distribution.
No. Si₃N₄ is essentially non-wetting and chemically stable against molten aluminium and most aluminium alloys, which is the main reason it outlasts steel and cast iron in the melt.
Yes — magnesium is more aggressive than aluminium, but silicon nitride remains stable in magnesium service where many metals rapidly corrode. We confirm the exact alloy and temperature before recommending a grade.
Yes. Rotor performance is set by vane design and surface finish; send us your current rotor or duty cycle and we will propose a geometry optimised for bubble size and gas usage.
