Drawing No. EH–MT–022 // Engineering Materials
Silicon Nitride Ceramic Properties
Reviewed September 2026
Silicon nitride combines low density, high stiffness/strength, low thermal expansion and exceptional thermal-shock resistance.
Representative properties
The stated basis, temperature and direction are part of the value. Switch units without changing the physical reference.
What changes these properties?
Engineering variability
- Sintering additives and processing route affect thermal conductivity and high-temperature creep.
- Flexural strength differs from uniaxial tensile strength and should not be substituted directly.
- Mechanical properties remain good at elevated temperature but are product-specific.
- Oxidation and glassy grain-boundary phases can govern long-duration high-temperature use.
Selection note
Used for bearings, turbocharger rotors, molten-metal handling, cutting tools and high-temperature structural components.
Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.
Silicon nitride in practice
The structural ceramic with the best combination of toughness and thermal shock resistance — and the one that made ceramic bearings viable.
Where it is specified
Hybrid rolling-element bearings (ceramic balls in steel races) for machine tool spindles, turbochargers, electric motors and aerospace; cutting tool inserts for cast iron and nickel alloys; turbocharger rotors; welding and forming tooling; molten metal handling components; and glow plugs and diesel injector parts.
It is specified where a ceramic must tolerate mechanical and thermal cycling rather than merely resist wear. Its relatively low density and high stiffness also matter in bearings, where centrifugal loading on the rolling elements limits speed.
Chosen over what
Against bearing steel: 40% the density, so centrifugal loads at high speed fall sharply and DN values can roughly double. Silicon nitride balls are also electrically insulating, which prevents the electrical discharge bearing damage that afflicts inverter-driven motors — now a major reason for their adoption in EV traction motors.
Against alumina: roughly twice the fracture toughness, much better thermal shock resistance and higher strength, at significantly greater cost.
Against silicon carbide: tougher and far better in thermal shock, but lower thermal conductivity, lower hardness and a lower oxidation ceiling.
How it fails
Rolling contact fatigue in bearing use, initiating at surface or near-surface defects. Ball quality grade is therefore the specification that matters, and inspection standards are tighter than for steel.
Brittle fracture from flaws, with the same Weibull statistics as other ceramics; silicon nitride typically shows a Weibull modulus of 15–25, better than alumina but still meaning strength depends on stressed volume.
Oxidation and slow crack growth above about 1200 °C, accelerated by the grain-boundary glass phase from sintering aids. Higher-purity grades with refractory grain-boundary chemistry extend this.
Sensitivity to the sintering aid chemistry: yttria and alumina additions that allow densification also form the intergranular phase that limits high-temperature strength — an unavoidable trade.
Fabrication notes
It does not sinter without aids because silicon nitride decomposes before it densifies. Routes are reaction bonding (RBSN, porous and lower strength), sintering, gas-pressure sintering and hot isostatic pressing (HIP) — HIPed material is the premium grade used for bearings.
Post-fire machining is diamond grinding only, and ball manufacture involves extended lapping to achieve sphericity measured in tenths of a micrometre.
Surface finish dominates performance. For bearings, a fine lapped finish is what suppresses rolling contact fatigue, and handling damage before assembly is a real risk given the material's edge sensitivity.
Design in compression as with all ceramics, with generous radii and no point loading against hard surfaces.
Specifications and standards
ISO 26602 for silicon nitride bearing balls; ASTM F2094 for ceramic bearing balls; ASTM C1161, C1421 and C1239 for strength, toughness and Weibull analysis; ISO 3290-2 for ball dimensional grades; ASTM C1322 for fractographic flaw analysis.
References
Richerson, Modern Ceramic Engineering; Bhushan, Modern Tribology Handbook on hybrid bearings; CeramTec and Toshiba Materials technical data.
How to interpret this material
Stiffness
Young’s modulus describes elastic stiffness. For crystals and composites, orientation can make one scalar modulus only a directional reference.
Strength / failure
Ductile metals can use yield and tensile strength. Brittle ceramics/semiconductors and anisotropic composites require fracture, flexural or directional strength descriptions instead.
Thermal response
Conductivity, heat capacity and expansion are temperature dependent. The last column is a melting or transition descriptor only where that physical concept is appropriate.
Technical sources
The database favors manufacturer, industry, government or academic technical references and labels secondary condition-specific datasets when a primary source does not expose a complete property set.