Thermal Shock Failure Mechanisms and Improvement Strategies for Corundum-Mullite Kiln Setter Plates

Corundum-mullite setter plates are widely used in high-temperature kilns to support and isolate ceramic products during sintering. Their popularity stems from an ideal balance of mechanical strength, thermal stability, and chemical resistance. However, thermal shock failure remains one of the most common and costly issues faced by users in fast-firing cycles or aggressive thermal environments. This article presents a detailed analysis of the mechanisms behind thermal shock failure in corundum-mullite setter plates, and offers practical strategies for improving service life and minimizing downtime.

1. Understanding Thermal Shock in Ceramic Materials

Thermal shock occurs when a material is rapidly heated or cooled, causing internal stress due to uneven thermal expansion or contraction. Ceramics are particularly sensitive due to their:

Low thermal conductivity → Surface and core heat at different rates

Low fracture toughness → Brittle failure under stress

Anisotropic grain structures → Localized stress concentrations

When these stresses exceed the material’s tensile strength, cracks initiate and propagate, leading to failure.

2. Corundum-Mullite Plates: Strengths and Limitations

Corundum-mullite combines the advantages of:

Corundum (α-Al₂O₃): High refractoriness (>1,900 °C), mechanical rigidity

Mullite (3Al₂O₃·2SiO₂): Low thermal expansion (~4.5–5.5 × 10⁻⁶/°C), high thermal shock resistance

However, under fast-firing conditions (e.g., >10 °C/min ramps), setter plates can still experience thermal fatigue and eventual cracking.

3. Failure Modes Observed in Service

Based on real-world kiln operations and microscopic failure analysis, the most common thermal shock failure patterns in corundum-mullite setter plates include:

3.1 Surface Microcracking

Initial cracks often begin at the hot face of the setter plate, where rapid exposure to heat causes a steep temperature gradient between the surface and interior.

Fine radial or network-shaped cracks

Typically <1 mm deep initially, but propagate over cycles

Lead to spalling, dust generation, or fracture

3.2 Edge and Corner Cracking

Corners and unsupported edges experience stress concentration during heating/cooling. This results in:

Vertical or L-shaped cracks

Chipping, warping, or full corner loss

More common in square or rectangular plates without edge rounding

3.3 Delamination or Layer Separation

In thicker plates, the mismatch in expansion between the hot surface and the cooler interior can cause layer separation or internal voids.

Sub-surface cracks visible on cross-section

Weakens load-bearing capacity

Can lead to catastrophic failure under thermal and mechanical stress

4. Root Causes Behind Thermal Shock Failure

Understanding the root causes enables targeted improvements:

Cause	Description
High heating/cooling rates	Causes sharp thermal gradients across the thickness
Uneven heat distribution	Poor airflow or placement in kiln leads to local hot spots
Inadequate material composition	Excessive corundum increases stiffness but reduces thermal shock tolerance
Poor microstructure	Large grains or high porosity act as crack initiation sites
Improper handling or loading	Mechanical stress combined with thermal stress accelerates cracking

5. Strategies for Improving Thermal Shock Resistance

5.1 Material Optimization

Balance between corundum and mullite is key:

Corundum (≥60%) provides mechanical strength

Mullite (30–40%) introduces flexibility and crack-deflection capability

Fine-grained, interlocked microstructure reduces crack growth

Additives like zirconia (ZrO₂) or titania (TiO₂) can enhance fracture toughness by promoting energy absorption at crack tips.

5.2 Microstructural Control

Use fine, uniformly distributed particles

Limit open porosity to <18% to prevent thermal shock crack paths

Avoid exaggerated grain growth during sintering by optimizing firing profile

Advanced processing methods, such as isostatic pressing and spray drying, can help achieve consistent density and homogeneity.

5.3 Design Improvements

Rounded corners and chamfers reduce stress concentrations

Use reinforced ribs or curvature in large plates to distribute stress more evenly

Introduce ventilation holes or grooves to reduce thermal mass and speed heat transfer

Thin-walled designs (<15 mm) are lighter and more responsive to temperature changes, reducing ΔT across the plate thickness.

5.4 Operational Best Practices

Preheat setter plates gradually before loading to eliminate thermal shock from ambient conditions

Avoid placing cold plates directly into hot zones

Ensure even airflow and flame distribution in the kiln

Use thermal insulation or buffer plates for high-ramp programs

Monitor and control ramp rates, especially during the initial heating phase and final cooling period.

6. Monitoring and Maintenance

Regular inspection helps detect early signs of failure:

Visual checks after each cycle for surface cracks or edge damage

Use acoustic emission or infrared thermography for internal damage detection

Clean residues (e.g., metal oxides or sintering flux) that may chemically weaken the surface

Establish a rotation schedule for setter plates to equalize exposure and wear across positions in the kiln.

7. Real-World Improvement Outcomes

Manufacturers who adopt these strategies have reported:

Improvement Measure	Result
Material optimization	30–50% increase in average plate life
Microstructure refinement	60% reduction in early thermal cracking
Design upgrades (edges, thickness)	Lower failure rates and better weight distribution
Process adjustments (preheating, ramp rates)	Enhanced reliability across 80–100 sintering cycles

Conclusion

Thermal shock failure in corundum-mullite setter plates can severely impact production yield and increase operational costs. However, by understanding the mechanisms of crack initiation and propagation, and implementing material, design, and process-based improvements, manufacturers can significantly extend plate life and maintain kiln performance.

Corundum-mullite remains an excellent choice for kiln furniture—but like any engineered component, it performs best when tailored and maintained according to application needs. Investing in smart design and proactive care leads to longer-lasting plates, higher productivity, and lower cost-per-cycle.

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