A thermal barrier coating (TBC) is a ceramic top coat (yttria-stabilized zirconia) bonded to a metal substrate through a bond coat. Every high-temperature exposure oxidizes the bond coat surface, growing a thin layer of alumina — the thermally grown oxide (TGO) — right at the interface. Its thickness follows parabolic oxidation kinetics:
h(N) = h₀ + √(k_p · N)
where N is the cycle count and k_p is the parabolic rate constant (accelerated here for visualization). Because the TGO's thermal expansion coefficient differs from the metal beneath it, cooling from operating temperature back to room temperature locks in a large biaxial growth/misfit stress:
σ = E' · Δα · ΔT (E' = E / (1 − ν))
As the TGO thickens, the elastic strain energy stored in it grows too. The energy available to drive interfacial delamination — the strain energy release rate — scales as:
G = Z · σ² · h / E'
with Z a dimensionless geometry factor for buckling-driven spallation (Evans–Hutchinson model). Local roughness at the bond coat/TGO interface means h and σ vary from patch to patch. The moment a patch's G exceeds the interface fracture toughness Gc, the top coat there buckles and pops free, exposing bare bond coat underneath — exactly the failure mode that ends a real turbine blade coating's service life.
- Cycling rate — how fast simulated heat/cool cycles run when "Run Cycling" is active.
- TGO growth rate kp — oxidation kinetics; higher values reach critical thickness sooner.
- CTE mismatch Δα — thermal expansion mismatch between TGO and bond coat; controls stress magnitude.
- Interface toughness Gc — how much strain energy the interface can absorb before delaminating; higher values delay spallation.