Category:
Ceramic
Material Type:
ceramic
Alternative Names:
Tags:
Mullite Ceramic (3Al₂O₃·2SiO₂)
Ceramic
Basic Information
Composition & Structure
Composition:
3Al₂O₃·2SiO₂ (71.8% Al₂O₃, 28.2% SiO₂)
Microstructure:
Needle-like mullite crystals in aluminosilicate glassy matrix
Description
Mullite is the only stable intermediate phase in the alumina-silica system, making it one of the most important refractory ceramics. Its needle-like crystal morphology provides excellent thermal shock resistance and creep resistance at high temperatures. Widely used in kiln furniture, furnace linings, foundry molds, and refractory bricks. Metallographic examination reveals the characteristic acicular mullite grains and intergranular glassy phase, with porosity being a key quality indicator.
Mechanical Properties
Hardness:
1100-1300 HV
Hardness (HV):
1200 HV
Hardness Category:
very hard
Tensile Strength:
180-250 MPa (flexural)
Physical Properties
Density:
2.8-3.2 g/cm³
Melting Point:
1840°C
Material Characteristics
Work Hardening:
No
Magnetic:
No
Corrosion Resistance:
excellent
Sectioning
Diamond wafering blade - medium grit / low concentration. Low feed rate with continuous coolant to minimize grain pullout. Sectioning damage in ceramics can be falsely characterized as porosity.
Mounting
Cold castable mounting with epoxy or acrylic resin. Vacuum impregnation recommended for porous mullite to infiltrate open porosity and prevent edge damage during grinding.
Grinding
Diamond grinding discs required - SiC papers are too soft for efficient ceramic material removal. Use smallest practical diamond abrasive to minimize subsurface damage.
Grinding sequence:
Grinding sequence:
- 70 μm diamond grinding disc: Planar grinding to remove sectioning damage. Central force mode. 200/200 RPM. 5-10 lbs.
- 30 μm DIAMAT diamond on CERMESH metal mesh: Fine grinding (5 min). Individual or central force. 200/200 RPM. 5-10 lbs.
Recommended Sequence:
Polishing
Polishing sequence:
- 6 μm DIAMAT diamond on TEXPAN pad: 5 minutes with SIAMAT colloidal silica co-dispensed. 200/200 RPM. 10 lbs.
- 1 μm DIAMAT diamond on GOLDPAD or ATLANTIS pad: 5 minutes with SIAMAT colloidal silica. 200/200 RPM. 10 lbs.
- SIAMAT colloidal silica on TEXPAN pad: 5 minutes CMP finish. 200/200 RPM. 10 lbs.
Recommended Sequence:
Etching
Typically examined as-polished in bright field. Mullite grains, glassy phase, and porosity clearly visible without etching. Thermal etching at 1400-1500°C for 15-30 min in air reveals grain boundaries. HF (concentrated) selectively attacks the glassy phase.
Common Etchants:
Heat Treatment
Mullite is sintered at 1500-1700°C. Not heat treated in service — the material is inherently stable to its melting point (1840°C). Mullite formation occurs by reaction sintering of alumina and silica precursors or by sintering of mullite powder.
Applicable Standards
- ASTM C1161: Flexural Strength of Advanced Ceramics
- ASTM C1327: Vickers Hardness of Advanced Ceramics
- ASTM C20: Apparent Porosity, Water Absorption, Apparent Specific Gravity of Fired Whiteware
Applications
- Kiln furniture and refractory supports
- Furnace linings and insulation
- Foundry molds and cores
- Thermocouple protection tubes
Typical Uses
- Roller kiln supports for tile and sanitaryware firing
- Investment casting shell molds
- Glass industry refractory components
- High-temperature electrical insulators
- Catalytic converter substrate supports