Silicon nitride vs Zirconia
Silicon nitride and Zirconia are genuine alternatives to one another — a choice somebody actually makes — and differ on 4 measured properties, density most clearly.
Silicon nitride vs Zirconia
Silicon nitride
Si3N4
The ceramic that tolerates thermal shock — bearings that run without oil, and the one that nearly became an engine.
Zirconia
ZrO2
A ceramic that resists cracking by changing crystal structure at the crack tip — and a convincing diamond substitute in its cubic form.
| Property | Silicon nitride | Zirconia |
|---|---|---|
| Density | 3.17–3.25 g/cm³ | 5.68–6.1 g/cm³ higher |
| Melting point | 1900 °C | 2715 °C higher |
| Mohs hardness Mohs hardness is an ordinal scale: values can be ranked, but differences and ratios between them are not meaningful. | 8.5–9 higher | 8–8.5 |
| Molar mass | 140.28 g/mol higher | 123.22 g/mol |
| Refractive index Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | 2.15–2.18 |
| Thermal conductivity Only one of the two has a figure we can stand behind, so there is nothing to compare. | 20–30 W/(m·K) | Not known |
Choosing between them
Silicon nitride is recorded as a substitute for Zirconia — lighter, better at temperature.
What it gives up: Less tough, and difficult enough to densify that the cost follows from the process.
In this application, on the evidence cited above.