What a scratch test tells you
Coatings fail in characteristic ways when a hard stylus is dragged across them under increasing load: tensile cracks behind the stylus, chipping at the edges, buckling, then full delamination. The load at which each failure first appears is a critical load, a practical measure of coating adhesion and cohesion. Critical loads are used to compare coating processes, check production quality and screen substrate preparation.
A constant-load scratch answers a different question: how much the material resists plastic deformation. That is its scratch hardness, calculated from the scratch width.
How the test works
- The sample is fixed on the stage and the stylus (diamond or WC, sphero-conical or Rockwell geometry) is brought into contact.
- The stage moves the sample under the stylus while the load is held constant or increased linearly from a start load to an end load.
- Friction force, penetration depth and acoustic emission are recorded along the scratch.
- The scratch is inspected optically to locate each failure and convert its position to a critical load. On the T2000, video zoom imaging records the full scratch so it can be reviewed later alongside the data.
Reading the signals
Optical inspection is the reference for identifying failures, but the in-situ signals add confidence:
- Friction often steps up when the stylus breaks through a coating into a softer or rougher substrate.
- Penetration depth separates elastic from plastic deformation and shows sudden changes at failure.
- Acoustic emission picks up cracking events, sometimes before damage is visible on the surface.
Correlating all three with the scratch image makes the reported critical loads defensible, especially for brittle and transparent coatings.
Progressive versus constant load
Progressive load is the standard approach for adhesion (ASTM C1624, ISO 20502, EN 1071-3, ASTM D7027): a single scratch spans the full load range and reveals every failure stage in sequence. It needs active load control, so it runs on the pneumatic T200 and T2000.
Constant load is used for scratch hardness (ASTM G171) and for mapping: repeating scratches at one load across a surface checks uniformity. Constant-load scratches are also possible on the dead-weight T50.
Tribometer scratch versus micro and nano scratch
The tribometer scratch module covers macro-scale loads, suited to thick, hard coatings and paints, and to polymers and bulk materials. For thin films, NANOVEA’s mechanical testers run micro and nano scratch tests with lower loads and finer load resolution; see adhesive failure and cohesive failure testing. Choosing the platform by load range keeps the critical loads well within the resolved range of the instrument.
