Seven Ways to Measure Coating Thickness, and Which Ones Damage the Part

Four non-destructive methods: the field toolkit

Magnetic pull-off uses the force between a permanent magnet and a steel substrate, which varies with distance. No power, no calibration, cheap and rugged – good for a quick shop-floor verdict. The trade-offs are low accuracy, iron-only substrates and sensitivity to surface flatness.

Magnetic induction drives an alternating field through a non-magnetic coating into a ferromagnetic substrate; the thicker the coating, the higher the reluctance. A step up in accuracy from pull-off, and the workhorse for steel structures and automotive bodies.

Eddy current induces currents in a conductive substrate with a high-frequency coil; coating thickness changes their intensity. Used for non-conductive coatings on aluminium, copper and stainless. Slightly less accurate than magnetic induction.

Ultrasonic measures the reflection at the coating-substrate interface. It handles non-metallic substrates and multilayer stacks, and there is a related instrument family that ignores the coating entirely to read the steel wall beneath it – do not confuse the two. Expensive and only moderately accurate, but it covers cases the first three cannot.

Three destructive methods: sampling and arbitration

Wedge cut: section the coating and measure the cross-section under a microscope or with a light-section method. Highly accurate, but it destroys the part, so it is reserved for sampling and disputes.

Gravimetric: weigh before and after coating and divide by area and density. Suitable for small, regular parts and common in laboratories.

Coulometric: dissolve the coating in electrolyte and derive thickness from time. Used for gold and silver on fine wire; destructive by definition.

Two specialised techniques

X-ray fluorescence measures very thin, duplex and alloy coatings non-destructively and accurately. The equipment is costly and contains a radiation source, so it comes with a safety regime.

Beta backscatter suits coatings where the atomic number difference between coating and substrate exceeds about three. Also radioactive, also restricted.

Choosing

Ferrous and quick: magnetic induction. Aluminium or copper: eddy current. Mixed substrates: a dual-principle gauge. Non-metallic or multilayer: ultrasonic. Disputes: wedge cut. Very thin coatings: XRF.

One rule holds across all of them: non-destructive methods may be cross-checked against each other, but when they disagree with a destructive result, the destructive result wins – it is a direct measurement that depends on no physical model.