Silicon nitride ceramic
Specify silicon nitride by grade, forming and densification route; interpret specimen properties and qualify the finished component for its loads, environment and interfaces.
Tags: materials, ceramics, silicon-nitride, engineering-ceramics
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CeramTec’s Cyrol SL900 bearing ceramic has a mean linear expansion coefficient of 2.5 × 10⁻⁶ K⁻¹ from 20 to 200°C. A free 100 mm length heated uniformly over that range expands by 0.045 mm (100 × 2.5 × 10⁻⁶ × 180). The 1200°C maximum is specified without load. Assess fits, preload and mating materials separately. CeramTec Cyrol SL900 material table, p. 4
Specify a grade, not only Si₃N₄
Silicon nitride ceramic is a material family. CeramTec describes an interlocking, elongated-crystal microstructure and applications including bearing rolling elements, forming tools and welding components. This combination of properties makes it useful where low mass, mechanical performance and thermal shock resistance matter together. It does not identify one interchangeable composition or manufacturing route. CeramTec material overview
Order a named manufacturer grade, stock or finished form, drawing revision and acceptance specification. Record density/porosity requirements, critical surfaces, loads, operating atmosphere, temperature history and interfaces. Distinguish a dense structural part from a porous reaction-bonded component and from a deposited silicon-nitride thin film: a shared chemical formula does not establish equivalent engineering behavior.
Manufacturing route changes the material
San Jose Delta describes reaction-bonded silicon nitride (RBSN) as the product of reacting silicon compacts with nitrogen. Green-state machining enables complex shapes. Its listed typical RBSN density is 2.4–2.6 g/cm³, with properties dependent on forming technique. The same supplier separately lists fully dense pressureless-sintered grades, with differences in their intergranular phases. These are distinct material/process choices. San Jose Delta process and grade guidance
Do not convert an RBSN quotation into a dense-grade design by changing only a density entry. Establish which forming, densification and finishing operations belong to the offered grade, and whether the supplier controls relevant additives and microstructure. Ask which operations occur before and after densification, which features can be formed near net shape, and where finishing allowance is required.
For production transfer, define what constitutes a material or process substitution. A new powder/additive system, densification route or finishing operation may require renewed qualification even if nominal dimensions remain the same. Keep material identity linked to the finished lot rather than retaining only a generic ceramic description.
Named example: SL900 bearing material
CeramTec's Cyrol® brochure identifies SL900 for ceramic bearing rollers and publishes:
| Quantity | SL900 brochure value |
|---|---|
| Density | 3.20 g/cm³ |
| Bending strength at 20°C | >800 MPa |
| Young's modulus | 320 GPa |
| Thermal conductivity at 20°C | 30 W/(m·K) |
| Mean linear expansion, 20–200°C | 2.5 × 10⁻⁶ K⁻¹ |
| Mean linear expansion, 20–400°C | 3.2 × 10⁻⁶ K⁻¹ |
| Maximum usable temperature, without load | 1200°C |
The brochure states that values come from test samples and are not guarantees for specific applications. It does not assign a temperature to every mechanical entry; do not silently label the entire table “at 20°C.” CeramTec Cyrol material table and disclaimer, p. 4
A roller material's flexural strength is not a bearing's load rating, speed limit or fatigue-life prediction. Select the bearing assembly using its actual geometry, contact stresses, mating materials, lubrication, fits and operating conditions. Likewise, the no-load temperature is not an authorization for a loaded roller or arbitrary structural component to operate there.
For a ceramic insert in a metal housing, calculate differential expansion for both materials and evaluate restraint, gradients and property variation.
Interpret strength and test conditions
ASTM C1161 concerns ambient-temperature flexural testing of advanced ceramics with prescribed specimens and loading fixtures. Its public guidance explains the influence of specimen preparation, size, test environment and loading rate, and the role of flaw size and severity in strength scatter. A report should preserve those conditions rather than retain only the highest MPa value. ASTM C1161 scope and significance
San Jose Delta's tables explicitly distinguish room-temperature four-point average flexural strength from characteristic strength for certain grades. For example, its PSG entries are 689 MPa average and 730 MPa characteristic. Do not relabel either as an individual guaranteed minimum or combine data from separate grades into a synthetic material specification. San Jose Delta typical-property tables
For a critical design, identify relevant failure modes and obtain the appropriate statistical and application evidence. Keep bending, contact fatigue, impact, wear and thermal cycling separate. Define whether test surfaces represent the production finish and whether the tested population represents the delivered process. A smooth-looking surface alone is not a complete acceptance basis.
Finish, environment and assembly
Agree critical dimensions, surface finish, edge radii, chip/crack criteria and any defect inspection before production. Define how the supplier verifies inaccessible or highly stressed features. Preserve traceability through grinding, lapping and assembly. Do not assume that machining guidance for a readily machinable glass ceramic applies to a dense silicon-nitride grade.
CeramTec identifies SL200B/BG for welding rollers, centering pins and gas nozzles. That named application material should not inherit SL900's bearing table merely because both belong to the silicon-nitride family. CeramTec welding-process materials
Describe chemical exposure by substance, concentration, temperature, duration and contact conditions. For molten-metal service, identify the actual alloy and process rather than specifying generic “corrosion resistance.” For hot service, retain atmosphere, thermal cycles and applied load in the qualification record. For electrical service, establish voltage, temperature, geometry and environment for the finished assembly instead of treating room-temperature bulk resistivity as system insulation approval.
Assess mounting loads, contact geometry, differential thermal expansion and joining processes using the actual parts. A press fit, bonded joint or braze needs its own qualification; parent-ceramic strength cannot establish joint capacity. Inspect parts after operations that can alter critical surfaces, and define handling and packaging that protect them until installation.
Source and release record
Retain manufacturer/grade, lot, process and drawing revisions, inspection results and the applicable design/acceptance basis. Require explicit review of substitutions. Separate catalog material data from demonstrated component performance.
Sources reviewed November 1, 2026. The Cyrol PDF prints ST130038 • EN • 200 • 1804 • Lorenz; no publication date is inferred from that production code or its file metadata. The cited manufacturer/fabricator web pages provide no explicit technical revision date. The ASTM link identifies the 2018 edition; select the contract-required edition for actual testing.