TBC spallation: what it looks like on a borescope
TBC spallation is the loss of the ceramic thermal barrier coating that protects hot-section parts such as HPT blades, nozzle guide vanes and combustor liners. On a borescope it shows up as patches where the coating has broken away, and it matters because the metal underneath then runs hotter.
By InspectView · Published October 1, 2026 · Updated October 1, 2026
What a thermal barrier coating does
Hot-section parts run in gas hotter than their nickel-based alloys could tolerate unprotected. A thermal barrier coating, typically a ceramic top coat of yttria-stabilized zirconia over a metallic bond coat, insulates the metal and lowers its temperature.
Why it spalls
- Thermal cycling. Every flight heats and cools the part, and stress builds where the coating and the metal expand at different rates.
- Bond coat oxidation. A thin oxide layer grows between the bond coat and the ceramic over time and weakens the bond.
- Erosion and impact. Particles and debris wear or chip the coating, often at leading edges.
- Deposits. Molten sand, dust or volcanic ash, known as CMAS, can infiltrate the ceramic and make it brittle.
How it looks on a borescope
- Patches where the coating has broken away, usually with a defined edge.
- A change in color or brightness where the bond coat or base metal shows through.
- Discoloration, oxidation or burning in and around the patch once the damage has progressed.
Lighting and probe angle change how spallation looks, so compare against reference images in the manual where they're provided, and against earlier inspections of the same engine.
Why it matters
Without the coating, the metal runs hotter. That can speed up oxidation, burning and cracking, and shorten part life. Whether a given patch is acceptable depends on its size and location and on the manual's limits for that part.
What to record
| Record | Why it matters |
|---|---|
| Engine, module, stage and blade or vane number | So the finding can be found and trended later |
| Location on the part: leading edge, pressure or suction side, tip or platform | Limits often depend on location |
| Size of the affected area, measured where the task requires it | Limits are usually set by size |
| What's exposed: bond coat or base metal | Exposed base metal is usually more serious |
| Adjacent damage: cracks, burning or missing material | It can change the disposition |
| Numbered images tied to the finding | Evidence for the disposition and a baseline for the next inspection |
| The manual task, the limit and the disposition | Shows the call was made against the manual |
Disposition
Common outcomes are serviceable within limits, serviceable with a shorter interval to the next inspection, or unserviceable, requiring repair or removal. Trending matters: measuring the same patch at successive inspections shows whether it's growing.
How AirCheck handles it
AirCheck flags possible TBC spall in borescope video, groups findings by stage, and cites the manual's limit when the manual is indexed. The inspector confirms the finding, sets the disposition and signs.
Frequently asked questions
Is TBC spallation the same as burning?
No. Spallation is loss of the coating; burning is damage to the metal itself from overheating. Spallation can lead to burning if the exposed metal runs hot.
Can thermal barrier coatings be repaired?
Coatings on many hot-section parts can be stripped and reapplied during a shop visit, where the manufacturer's repair procedures allow it.
Does spallation always take an engine out of service?
No. Small areas often fall within the manual's limits, sometimes with a shorter interval to the next inspection. The manual's limits decide.