Silicon Carbide Brick: Oxide-Bonded, Nitride-Bonded and Self-Bonded, and Where Each Belongs
The most common mistake when buying silicon carbide brick is to ask only "what is the SiC content" and then compare prices on that number. Two bricks both marked 75% can behave completely differently: one lasts five or six years in an aluminium cell sidewall, the other turns to powder in six months. The difference is not SiC content but the bonding phase — silicate, silicon nitride, or free-silicon self-bond. That phase sets porosity, thermal conductivity, oxidation resistance and alkali resistance, and therefore decides which kiln and which zone the brick belongs in.
The bonding phase is the real dividing line
Silicon carbide is very hard to sinter on its own, so a second phase has to hold the grains together. Oxide bonding uses a silicate or mullite phase: low firing temperature, low cost, but the bond is glassy at temperature, and once slag dissolves that glass the grains simply fall apart. Nitride bonding reacts metallic silicon in nitrogen to form Si3N4 bridges between grains; the bond itself is highly refractory and strongly resistant to alkali attack, which is why it dominates aluminium cell sidewalls. Self-bonding (reaction sintering) leaves free silicon in the body, which melts and infiltrates pores at temperature, giving the lowest porosity, the highest strength and the highest conductivity. Recrystallised SiC contains almost no second phase and relies on SiC-to-SiC recrystallisation: highest purity, highest service temperature, but moderate strength and porosity — kiln furniture territory. None of the four is better; each is matched to a duty.
Key properties side by side
Oxide-bonded: SiC 60%–75%, apparent porosity 16%–20%, cold crushing strength 60–100 MPa, modulus of rupture at 1400℃ 6–10 MPa, conductivity 8–14 W/(m·K), service limit around 1400℃, 10–15 water quench cycles from 1100℃. Lowest cost, suited to wear-dominated zones at moderate temperature.
Nitride-bonded: SiC 75%–80% with 15%–20% Si3N4, porosity 14%–17%, cold crushing 100–150 MPa, MOR 25–35 MPa, conductivity 12–20 W/(m·K), service limit around 1500℃, 20–30 quench cycles. Its strength is resistance to cryolite and to alkali penetration; price sits in the middle.
Self-bonded (SiSiC): SiC 85%–90% with 8%–12% free silicon, porosity 8%–12%, cold crushing 150–250 MPa, conductivity 20–30 W/(m·K), service limit around 1550℃. Highest strength and conductivity, but free silicon is attacked first in strong alkali and oxidising atmospheres — confirm the atmosphere before specifying it.
Recrystallised: SiC ≥98%, porosity 15%–18%, service above 1600℃, moderate strength, used for saggars, setters and rollers.
Rank the properties in this order: porosity first because it governs penetration rate, modulus of rupture second because it governs thermal shock, cold crushing strength last. Many enquiry sheets lead with crushing strength, but that is a room-temperature number and the weakest predictor of high-temperature behaviour.
Matching the five common duties
Aluminium cell sidewalls and ledge: nitride-bonded, SiC ≥75%, porosity ≤17%, Fe2O3 ≤0.5%. The enemy is cryolite and sodium vapour; sodium migrates through pores and forms expanding phases that crack the brick from inside, so low porosity and low iron matter more than high strength. Use 90–120 mm thickness with adequate backing insulation. Waste-to-energy grates, sidewalls and feed sections: oxide-bonded at 65%–75%, or nitride-bonded where shutdowns are frequent. At 1000–1200℃ the drivers are HCl, alkali and abrasion; oxide-bonded is sufficient and cheaper to replace. Blast furnace stack and belly: self-bonded or nitride-bonded, relying on high conductivity to keep the hot face cooler so a stable slag layer forms, while resisting alkali and thermal cycling. Zinc fuming kilns and copper smelter slag lines: self-bonded with good slag resistance, and watch the interaction between free silicon and iron oxide in the slag. CFB cyclones and return legs are pure wear duty — high-strength oxide-bonded brick is the economical answer, and nitride-bonding is money wasted. Kiln furniture goes to recrystallised material; no other route fits.
Joints, expansion gaps and isolation layers
SiC expands at 4.5–5.0×10⁻⁶/℃, roughly 0.5% at 1000℃ — about 5 mm per metre — so a 6 m wall without gaps will lift. Provide a 5 mm expansion gap every 1.5–2 m, filled with ceramic fibre paper or aluminosilicate blanket, never with mortar. Lay dry or with a thin joint, keeping joints at 1.0–1.5 mm; aluminium cell sidewalls are stricter, dry-jointed to ≤1 mm, because a wider joint lets bath and metal wick straight through. Where SiC brick meets high-alumina or fireclay brick, insert a 0.5–1 mm isolation layer: at temperature SiC reacts with iron-bearing silicate phases to form a low-melting liquid, and the interface fails first. One more point that gets missed: do not bond SiC brick directly against lightweight insulating brick. The conductivity mismatch is too large, thermal stress cracks the interface, and a medium-density transition layer is needed.
Three traps in service
First, oxidation. Above 1200℃ with oxygen and water vapour, SiC oxidises to SiO2; the dense film that forms is protective, but thermal cycling cracks and spalls it, exposing fresh surface, and the brick thins cycle after cycle. Control idle high-temperature holds in intermittently operated kilns. Second, alkali attack. Na2O and K2O react with SiC in reducing conditions to form glassy phases with volume expansion, which shows up as bulging and powdering — precisely why aluminium cells need low-porosity nitride-bonded brick. Third, shell overheating from conductivity. SiC conducts five to ten times better than fireclay brick, so the cold face needs 65–115 mm of lightweight insulating brick and shell thermocouples held at or below 350℃. These are not operating lessons; they are constraints to design in at selection.
Delivery inspection
Check seven items: SiC content by chemical analysis (±2% tolerance), bonding type with a phase analysis or micrograph from the supplier rather than a grade name alone, apparent porosity, cold crushing strength and modulus of rupture, Fe2O3 content (critical for cell brick), dimensional tolerance (±1 mm in length, width and thickness, warpage ≤0.5 mm) and chipped corners (depth ≤3 mm per brick). Run one 1100℃ water quench on a sample and look for cracking and grain loss. Packaging must be moisture-proof: nitride-bonded brick loses strength noticeably after damp storage, so stack at least 200 mm off the ground under a rain cover, and do not lay rain-soaked brick without a strength check. A final point: the life difference between two batches of the same brick usually comes from joint and expansion-gap workmanship, not from the brick — done well or badly, it can double or halve the campaign.
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