What a scratch test tells you
A scratch test answers a practical question: how much load can a coated surface take before it is damaged, and how does it fail? It draws a stylus across the surface under a controlled normal load, records the response, and then images the scratch so each failure can be located and assigned a load. Hard coatings on tools, paints and clearcoats, optical and electronic thin films, polymer and medical device coatings are all qualified this way.
Because the result is a load at which a defined failure occurs, scratch testing is fast to run and easy to compare between samples, processes or suppliers. That is why it is used both in coating development and in production quality control.
How an instrumented scratch tester works
- The sample is positioned under the stylus with the integrated video microscope.
- An optional pre-scan at a very low load records the surface profile along the future scratch.
- The stylus moves along the set length while the normal load is applied, increasing linearly in a progressive test or held constant.
- Penetration depth, friction and acoustic emission are recorded continuously.
- A post-scan at low load gives the residual depth. Compared with the pre-scan and the penetration depth, it separates elastic recovery from permanent deformation (True Depth, European Patent No. 0663068).
- The tracking zoomed view images the full scratch, and each failure is located and converted to a critical load.
Types of scratch test
- Progressive load: the load rises linearly along the scratch. It finds the critical loads for cohesive failure (cracking, chipping) and adhesive failure (delamination) in a single pass, per ASTM C1624, ISO 20502 or ASTM D7027.
- Constant load: the load is fixed. It is used for scratch hardness per ASTM G171, for mar resistance and to check uniformity across a surface.
- Multi-pass and reciprocating: the stylus passes repeatedly over the same track at a sub-critical load, showing how damage accumulates and how many passes a coating survives. See multi-pass wear.
Nano, micro or macro: choosing the load scale
The right scratch tester depends on the coating’s thickness and hardness. A load that is too high goes straight through a thin film; one that is too low never makes a hard coating fail.
- Nano scratch uses a piezoelectric actuator, load ranges from 80 mN to 4800 mN and fine tips, down to 2 µm radius in the published studies. It suits thin films, paints and polymer coatings, and small features such as stent struts.
- Micro scratch uses a servomotor, load ranges from 20 N to 400 N and Rockwell C or sphero-conical tips. It suits nitride, DLC and other hard coatings, paints on metal and electrode coatings.
- Macro scratch on a tribometer uses tips of 200, 500 or 800 µm radius for thick and very hard coatings such as anodized layers.
The PB1000 carries the nano and micro modules together, so a coating system can be tested at both scales without moving the sample to another instrument.
What to look for in a scratch tester
- Direct vertical loading with independent load and depth sensors and closed-loop load control, so the applied load is the measured load, with no cantilever or pivot point.
- Resolution and noise at the scale you need. NANOVEA modules use 24-bit load and depth acquisition, and acoustic emission sensing at 150 to 400 kHz with 0.005 aJ sensitivity.
- Imaging of the whole scratch, not just single frames, so every failure can be located and re-examined after the test.
- Positioning and automation: multi-sample testing that handles height differences of up to 50 mm, mapped scratch series and saved recipes, for repeatable QC.
- Environments: high temperature, low temperature, humidity and liquid modules, for coatings that work hot, cold or wet.
- Indentation on the same system, since hardness and modulus help explain why a coating fails where it does.
Reading scratch results
A critical load is not an absolute adhesion value. It depends on stylus radius, loading rate, coating thickness, substrate hardness and friction as well as on the bond itself. Scratch tests are therefore most useful as comparisons under identical, reported conditions: ranking coatings, comparing deposition processes and checking production batches. Depth and residual-depth data add what images alone cannot show, such as how much of the deformation was elastic and at what depth the coating gave way.
