Coating Disbond: Thermography for Hidden Separation

Author: RAS Thermal Co., Ltd. · Updated 2026-10-07

TL;DR Strength: finds hidden subsurface separation a visual pass would miss. Limit: no depth, a 1.5 mm and a 4 mm scar look the same on the thermal feed. Handoff: ultrasonic thickness gauging returns remaining wall to plus or minus 0.1 mm. Pair: thermal to locate, ultrasonic to size.

The blade surface looks sound, but a thermal scan shows regions cooling differently from their surroundings. A coating or topcoat disbond is a separation between layers near the surface, so it hides under an intact skin. Active thermography heats the blade and watches the decay, and a subsurface air gap changes that decay enough to be seen. The method is good at finding the hidden separation a drone would miss. What it does not return is a depth, so where the decision needs remaining thickness it hands off to ultrasonic gauging rather than finishing the job.

Common root causes

Root cause	What you see	Where it bites
Topcoat disbond from impact	a region with altered thermal decay and no visible surface break	leading edge and tip zones
Water ingress under the coating	a decay anomaly that moves with wet weather	cut edges and impact sites
Coating applied over contamination	separation forming where the surface was not clean	shop-applied coatings
Thermal cyclic separation	a slow disbond spreading from an edge	long service in wide temperature swings

What the numbers allow

Thermography reads a contrast, and the contrast carries no scale. A 1.5 mm scar and a 4 mm scar look the same on the thermal feed, which is exactly the limit a structural decision runs into. So the thermal pass is used to find the hidden separation, and an ultrasonic thickness gauge maps the remaining wall to plus or minus 0.1 mm where the depth matters. On a 5 MHz dual-element transducer the number that separates a cosmetic touch-up from a grind and fill is available, and the thermal map is what sends the gauge to the right place. The spread from 1.5 mm to 4 mm on the same thermal anomaly is wider than any repair tolerance, so a thickness number, not a contrast, ends the argument.

Strength: finds hidden subsurface separation a visual pass would miss
Limit: no depth, a 1.5 mm and a 4 mm scar look the same on the thermal feed
Handoff: ultrasonic thickness gauging returns remaining wall to plus or minus 0.1 mm
Pair: thermal to locate, ultrasonic to size

How to confirm it

Work down this list in order. The first check that changes the result has found the cause.

Heat and watch the decay. Run the active thermal pass over the suspect zone and read how each region cools, because the separation shows as a change in decay rather than as a visible mark.
Correlate with weather and history. Check whether the anomaly tracks wet weather or an earlier impact, since a water-fed disbond moves differently from a dry separation.
Mark the anomalous zones. Outline each thermal anomaly so the follow-up gauge pass has a target rather than a whole blade to cover.
Hand off for a depth. Run the ultrasonic gauge over the marked zones, because the thermal feed cannot separate 1.5 mm from 4 mm and the repair decision needs that split.
Decide from thickness. Use the measured remaining wall to separate cosmetic from structural, and keep the thermal map as the record of where to look.

Prevention

Control coating application and surface preparation so the disbond does not start, and inspect the suspect zones after impact and before the wet season. Then let the thermal pass find the hidden regions and the gauge size them, because a contrast map locates a separation but only a thickness number sizes it.

The numbers below are drawn from the equipment records and field campaigns this site already publishes, not from a generic catalogue.

Frequently Asked Questions

What does thermography find that a visual pass misses?

The hidden separation. A coating disbond sits under an intact surface, and active thermography sees the change it makes to the thermal decay, which a drone camera cannot detect from the surface appearance alone.

Can thermography give an erosion depth?

No. It returns a contrast, and a 1.5 mm scar and a 4 mm scar look the same on the thermal feed. Where a depth is needed the follow-up ultrasonic gauge maps remaining thickness to plus or minus 0.1 mm.

Why pair thermography with ultrasonic gauging?

Because the two answer different questions in sequence. Thermography locates the hidden disbond across a whole blade, and the gauge sizes the marked zones, so the thermal map sends the thickness pass where it is needed.

Does a coating disbond always need repair?

It needs the same split as any erosion decision, cosmetic against structural, and that split comes from the remaining thickness rather than from the size of the thermal anomaly. That is why the gauge follows the thermal pass.

Bottom line: A coating disbond hides under an intact surface, and thermography is what shows the altered decay where a drone only shows a sound skin. Use the thermal map to locate the separation and the ultrasonic gauge to size it, because a contrast map points at the zone but only a thickness returns the number.