Why Thermal Shock Is Often Misdiagnosed in SiC Component Failure?

Problem

In high-temperature applications, when SiC components fail, the most common conclusion is:

“This is thermal shock failure."

This assumption is widely accepted because:

• Temperature changes are obvious
• SiC is known to be sensitive to rapid temperature variation

However, in many cases, this diagnosis is incorrect.

Initial Assumption

Typical reasoning:

• Rapid heating or cooling → thermal stress
• Thermal stress → cracking
• Therefore → thermal shock failure

This logic is simple, but incomplete.

Field Observation

Observed failure characteristics often include:

• Cracks initiating at edges or contact zones
• Localized damage instead of uniform cracking
• Failure occurring after long service time
• No clear evidence of sudden temperature change

These do not match classical thermal shock behavior.

What Real Thermal Shock Looks Like

True thermal shock failure typically shows:

• Sudden fracture
• Cracks distributed across the component
• Failure shortly after rapid temperature change

It is a short-term, rapid event.

Engineering Analysis

In real systems, failure is usually governed by:

• Thermal gradients (not shock)
• Structural constraints
• Contact conditions
• Long-term degradation

These factors interact over time.

Mechanism 1 — Thermal Gradient, Not Shock

In most cases:

• Temperature differences exist across the component
• Heating/cooling is not perfectly uniform

This creates:

• Internal stress over time
• Gradual damage accumulation

This is thermal stress, not thermal shock.

Mechanism 2 — Constraint-Induced Stress

Components are often:

• Supported
• Fixed
• Partially constrained

Thermal expansion is restricted, leading to:

• Stress buildup near supports
• Crack initiation at edges

Mechanism 3 — Contact Stress Amplification

In systems such as rollers and supports:

• Load is transferred through localized contact
• Contact areas experience high stress

Combined with temperature effects:

• Local stress becomes critical
• Damage starts at contact zones

Mechanism 4 — Material Degradation

At high temperature:

• Oxidation
• Chemical corrosion
• Surface weakening

Over time:

• Material strength decreases
• Cracks initiate more easily

Why Thermal Shock Is Overdiagnosed

Because:

• It is easy to understand
• It is widely known
• It appears to match the symptom (cracking)

But it ignores system-level factors.

Failure Characteristics Comparison

Engineering Insight

Failure is rarely caused by a single factor

Instead, it is the result of:

• Temperature
• Structure
• Contact
• Environment

Acting together over time.

Practical Example

In kiln roller systems:

• Cracks often start at roller ends
• Occur after extended operation

This is due to:

• Contact stress
• Thermal gradient
• Constraint

Not pure thermal shock.

Design Implications

To improve reliability:

• Reduce thermal gradients
• Optimize support conditions
• Improve contact design
• Consider environmental effects

Not just focus on “thermal shock resistance".

Conclusion

Thermal shock is not always the real cause because:

• Most failures are gradual, not sudden
• Stress is influenced by system conditions
• Multiple factors interact

Key Takeaway

If failure develops over time, it is not thermal shock

It is a system-level problem.