Scratch Testing for Cohesive Failure

What cohesive-failure testing tells you

Many coatings never delaminate in service; they crack, chip or get marked first. Paints and clear coats mar, hard coatings develop tensile cracks, glass and screen protectors fracture, polymer layers gouge. Measuring the loads at which these cohesive failures begin tells you how a coating will look and perform long before adhesion becomes the issue.

How the test works

1. A low-load pre-scan records the original surface profile.
2. The stylus scratches the surface under a progressively increasing load, recording penetration depth, friction and acoustic emission.
3. A low-load post-scan records the residual depth.
4. The full scratch is imaged; each failure is located and converted to a critical load, and depth data separate elastic from plastic deformation.

Failure modes to look for

• Tensile and conformal cracks behind or within the track, typical of hard coatings on softer substrates;
• Chipping along the track edges in brittle layers;
• Plastic deformation and pile-up in ductile polymers and paints;
• Marring: shallow, appearance-level damage in clear coats.

Each mode has its own critical load. The failure sequence and its loads characterise the coating far better than a single number.

Nano scratch for paints and thin films

Fine-tipped nano scratch separates the mechanisms of mar damage in paints. In NANOVEA’s paint-on-metal study, a 2 µm, 90° conical tip ramped from 0.015 to 20 mN across a 30–50 µm paint that had aged seven years. Pre- and post-scans at 0.2 mN gave the true scratch depth, and the weathered paint failed at lower loads than fresh paint would. Temperature matters too, as NANOVEA’s ASTM D7187 temperature study explores.

Micro scratch for hard and brittle surfaces

Hard coatings and glass need higher loads. In NANOVEA’s screen protector study, a 50 µm Rockwell tip ramped from 0.1 to 12 N. Tempered glass showed its first failure at 3.6 N and a second at 7.4 N, while TPU failed at 2.0 N in a ductile mode: different materials, different failure mechanisms, both quantified on the same instrument.