Mullite–cordierite: microstructure, thermal shock and refractory design

Two phases with complementary roles

Mullite–cordierite refractories belong to the MgO–Al₂O₃–SiO₂ system. Cordierite (nominally 2MgO·2Al₂O₃·5SiO₂) provides very low thermal expansion. Mullite (close to 3Al₂O₃·2SiO₂) expands more, but retains strength and creep resistance at high temperature. Fired bodies also contain glass, porosity and sometimes corundum, spinel or cristobalite.

The name does not define a composition. The cordierite-to-mullite ratio, coarse pre-fired grain, glass chemistry and porosity govern service behaviour, so grade data for the specific component are required.

Crack initiation versus damage tolerance

For a fully restrained layer, thermal stress is approximately σ ≈ EαΔT/(1−ν). Low expansion and low modulus reduce stress, but real components are not fully restrained, so the relation is a qualitative guide only.

Dense, strong bodies may resist crack initiation, yet cracks propagate quickly once started. Porous bodies with coarse grain and pre-existing microcracks, partly created by the expansion mismatch between cordierite and mullite, have lower cold strength but tolerate repeated cycling better. Thickness, heating and cooling rates, gas flow, edges and chipped corners control where damage begins.

Refractoriness, creep and allowable service temperature

Refractoriness is measured without load; refractoriness under load and creep tests apply a defined stress; the allowable service temperature also depends on span, load, cycle and atmosphere. In practice, service temperature is below refractoriness. Cordierite melts incongruently to mullite and liquid, and the glassy bond softens earlier, especially when alkalis or iron oxide lower its viscosity.

For a uniformly loaded plate on two supports, elastic deflection follows δ = 5wL⁴/(384EI) with I = bh³/12: doubling the span raises deflection about sixteen-fold and halving thickness about eight-fold. Creep deflection is permanent and accumulates with each firing, which is why aligned supports and periodic turning of batts extend life.

Furnace atmosphere and processing

The family is stable in oxidising atmospheres. Strongly reducing conditions, particularly hydrogen at high temperature, can reduce silica to volatile SiO. Alkali vapours, volatile glaze components and salt or soda firing form new glass at the surface. Separating coatings, often alumina- or zirconia-based, must be qualified with the fired product.

Cordierite may form in situ from talc, kaolin and alumina or be added as pre-synthesised grain. Pressing, extrusion and slip casting are common. The narrow gap between densification and cordierite decomposition demands tight firing control. Porous parts absorb moisture and must be dried before first heating above 100°C.

Specify and verify the component

Define maximum temperature, dwell, heating and cooling rates, atmosphere, span, load and the product in contact. Typical tests include bulk density and open porosity (ISO 5017), modulus of rupture at ambient and elevated temperature (ISO 5014, ISO 5013), refractoriness under load (ISO 1893), creep in compression (ISO 3187), dilatometry and a defined thermal-shock method. Laboratory values do not guarantee component life: qualify the actual part in its real setting and firing schedule.