Specifying silicon carbide ceramic starts with grade choice: sintered SiC, reaction-bonded SiC, nitride-bonded SiC and recrystallized SiC each carry different density, porosity and maximum-use-temperature envelopes that govern thickness and diameter limits for a given service load [S2].
For industrial buyers, the practical scope is dense wear parts (nozzles, seals, liners), kiln furniture, mirror substrates, foundry deoxidant and recarburizer, plus foam filters for molten metal; power-device substrates form a parallel market and are typically sourced from wafer-grade 4H/6H crystal rather than the ceramic grades covered here [S1][S3].
Sintered SiC (SSiC) is the high-purity, high-density option with free silicon below roughly 1% by weight and bulk density commonly landing between 3.10 and 3.15 g/cm³, supporting working temperatures up to about 1600 °C in oxidizing atmospheres and giving the highest corrosion resistance of the family [S2][S5]. Reaction-bonded SiC (RBSiC, often called SiSiC) infiltrates a porous carbon preform with molten silicon, leaving 5–15% residual free silicon which caps the maximum service temperature near 1380 °C and limits use in strong alkali or hydrofluoric acid, while delivering lower cost and near-net-shape forming of complex parts like large kiln beams and burner nozzles [S2][S5]. Recrystallized SiC (RSiC) keeps porosity at roughly 10–20% with density around 2.6 g/cm³, trades mechanical strength for thermal-shock endurance, and is the standard pick for kiln furniture, radiant tubes and porous flame-detector elements [S2][S5].
Nitride-bonded SiC (NBSiC, or Si₃N₄-bonded SiC) bonds SiC grains with a silicon-nitride phase, sits in the 2.7–2.8 g/cm³ density band, and is widely used for aluminum-reduction cell sidewalls and blast-furnace trough liners where metal-wetting resistance matters more than ultimate strength [S2]. Oxide-bonded and mullite-bonded SiC variants, including SiC foam for molten-metal filtration, sit at the low end of the spec window (density often 0.4–0.8 g/cm³ for foams) and are chosen purely for filtration or thermal-insulation duty rather than load-bearing service [S4].
Geometry Limits: Wall Thickness, Diameter and Tolerance
For solid SiC parts, the practical wall-thickness floor for reaction-bonded forming is 3–5 mm and for sintered pressing is 5–8 mm; thinner sections must be specified as recrystallized or as foam, because sub-3 mm sintered walls crack during densification [S2]. Diameter capacity is grade-dependent: reaction-bonded tubes and rollers are routinely produced past 600 mm outer diameter and up to roughly 3 m in beam length, while sintered parts above 500 mm diameter push into bespoke tooling with longer lead times [S5].
Dimensional tolerance bands published by mainstream SiC suppliers cluster around ±0.1 mm per 100 mm for as-sintered parts and ±0.05 mm per 100 mm for ground reaction-bonded sealing faces; surface roughness of Ra 0.4–0.8 µm is achievable on lapped SiC mechanical seals, which is the usual reference for chemical-pump seal faces [S2][S5]. For mirror substrates, the relevant band is different: surface figure of λ/10 to λ/20 at 632.8 nm on reaction-bonded or chemical-vapor-deposited SiC optics, with areal density below 30 kg/m² for space-qualified mirrors — a separate selection exercise from wear-grade ceramic [S4].
Mechanical and Thermal Performance Bands
Hardness, flexural strength and thermal conductivity all shift with grade. Sintered SiC typically reports Vickers hardness of 2500–2800 HV, flexural strength 400–500 MPa at room temperature, and thermal conductivity of 120–180 W/m·K; reaction-bonded SiC lands lower at 2200–2500 HV, 250–350 MPa flexural, and 150–200 W/m·K; recrystallized SiC drops to 2000–2300 HV and 80–120 MPa flexural but compensates with the lowest Young's modulus-to-strength ratio, hence the best thermal-shock figure of merit in the family [S2][S5].
Coefficient of thermal expansion sits in a tight 4.0–5.0 × 10⁻⁶ /K band across all polycrystalline SiC grades, which is why SiC-to-metal joints in heat-exchanger tubes are typically designed around a 4.5 × 10⁻⁶ /K match and why SiC susceptors couple cleanly with induction coils at 10–100 kHz without thermal-stress ratcheting [S2]. Maximum service temperature ceilings line up with the phase map: SSiC at roughly 1600 °C in air, 2000 °C in inert, RSiC at 1600 °C in air, RBSiC at 1380 °C limited by the free-silicon phase, and NBSiC near 1450 °C before the bond phase softens [S2][S5].
Selection Matrix: Grade vs Application
For wear-and-abrasion duty — slurry nozzles, sandblasting nozzles, cyclone liners, mining hydrocyclones — sintered or reaction-bonded SiC is the default, with grade picked on part size: small nozzles under 100 mm in any dimension favor SSiC for purity, while large shells and complex shells favor RBSiC for shape and cost [S2]. Mechanical seals and chemical-pump faces go to SSiC lapped to Ra 0.4 µm or finer, paired with a matching carbon or SiC counter-face; reaction-bonded grades are used where the seal OD exceeds 200 mm and SSiC tool wear would blow the budget [S2][S5].
For high-temperature structural parts, RSiC and SiC-foam dominate kiln furniture, burner nozzles and flame-detector bodies, while NBSiC is the standard for aluminum smelting sidewalls and trough liners where molten aluminum wetting is the failure mode [S2]. For molten-metal filtration in iron and steel foundries, SiC foam filters with 10–30 ppi (pores per inch) cell counts and 0.3–0.8 g/cm³ bulk density are specified to remove slag inclusions above 1 mm; this is the same application chain where the foundry-grade SiC from [S1] is used upstream as a deoxidant and recarburizer for gray, ductile and malleable cast iron in electric furnaces.
Sourcing Levers and Supplier Signals
Lead time is the single biggest commercial lever: stock reaction-bonded plates, tubes and nozzles in standard sizes run 2–4 weeks, custom SSiC parts 8–14 weeks, and large custom RBSiC kiln furniture 6–10 weeks, with binder-phase purity and free-silicon control the most common cause of out-of-spec lots [S5]. A practical check at receiving is density by Archimedes and a quick acid test for free silicon: SSiC resists HF attack far better than RBSiC because the residual silicon phase in RBSiC dissolves, and this single 5-minute test routinely separates a mislabeled lot from a correctly graded one [S2][S5].
For wear parts in mining and heavy industry, SiC competes with alumina ceramic on cost and on thermal-shock behavior, with SiC favored above 400 °C service or where thermal conductivity matters for heat dissipation [S5]. Mirror-grade SiC substrates for aerospace optics overlap with the aerospace crossed roller guide sizing topic only in cleanroom assembly, not in material selection — a useful boundary to mark before procurement specs cross the desks of both teams.
Failure Modes and Specification Pitfalls
The three most common SiC ceramic failures in industrial service are thermal-shock fracture, free-silicon melt-out, and oxidation-driven strength loss above 1000 °C. Thermal-shock fracture is almost always a grade-mismatch problem: a part specced in SSiC for a 1200 °C-to-room-temperature cycle is more reliable when switched to RSiC, because RSiC's lower modulus and higher porosity absorb the strain-energy jump that cracks SSiC [S2].
Free-silicon melt-out is specific to RBSiC running above 1380 °C or in strong alkaline service; the failure signature is progressive loss of geometry rather than a single fracture event, and the spec fix is to step up to SSiC or to add a protective oxide coating. Oxidation-driven strength loss above 1000 °C is intrinsic: passive silica growth continues with time, the rate-law exponent shifting with alkali impurity, which is why high-purity SSiC is the only grade specified for long-life heat-exchanger tubes in the 1100–1400 °C band [S2]. Specification should always name the grade, the free-silicon limit, the density band, and the maximum service temperature with atmosphere; generic "SiC" line items invite the wrong grade and shorten service life by an order of magnitude.
For component-level specifications, see silicon carbide, and linear guide.