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Scratch resistance of coatings

The coating over the paint on a car is designed to resist some scratches. While it is not impenetrable, it helps with minor, everyday wear. This article explains how the scratch resistance of a coating is tested.

Historical ideas have provided a range of cheap methods for quick approximations of adhesion forces between coating and substrate. However, to develop a truly exquisite coating, an investment in instrumented scratch testing is essential. Find out about the advantages offered by instrumented scratch testing over pencil scratch testing, tape-peel testing, and cross-cut adhesion testing.

Classical adhesion testing methods

Classical scratch testing relies a lot on solutions such as scribing a surface with pencils of varying hardness, pressing a tape to the surface and peeling it off at a steady rate, or scoring orthogonal hatch marks through the coating and counting the squares that go missing. While efforts have been made to remove the human element from some of these methods, such as robotic tape peeling, subjectivity can remain an issue, not to mention the poor resolution provided (often at the level of pass/fail).

Results of a cross-cut adhesion test

Figure 1: Results of a cross-cut adhesion test Source

Instrumented scratch testing: a quantitative method

There is a need of improvement for product development and quality control engineers with monitoring adhesion strength of coatings rather than simply defining results with a yes or no, or on a scale of one to five. Modern instrumented scratch technology can supply a precisely actuated progressive normal force while a sample is dragged across a durable, diamond-tipped stylus. An integrated optical microscope can be used to collect panoramic image data, where the points of critical coating failure are readily identifiable. Scratch testers can additionally be equipped with sensors to monitor penetration depth, lateral friction force, and acoustic emission. Crucially, well-designed scratch test software will then synch this data and the normal force profile with the panoramic image, allowing the user to easily spot and report critical scratch events with excellent quantitative force resolution. Here critical coating failures are reported as “12.3 N” for instance rather than as a “Yes”. This ability allows the optimization of coating process parameters to a degree of far greater subtlety than can be detected with classical adhesion test methods. Moreover, the distribution of critical failure loads across individual samples or batches provides an excellent measure of the homogeneity or repeatability of the production process, respectively.

Results of a progressive instrumented scratch

Figure 2: Results of a progressive instrumented scratch

What constitutes “failure” of a coating depends on the application. In optics, any scratch, even one that does not result in full coating delamination, can cause undesirable diffractions. Scratches that deform the coating may affect finish sheen, even if invisible to the naked eye. These types of failures are known as cohesive coating failures. Adhesive failures involve loss of material contact and subsequent exposure of substrate. In the case of protective coatings and traces designed to conduct electricity, small amounts of cohesive damage over the lifetime of a product may fit within the tolerances of the performance, while adhesive coating failures will likely be quickly followed by loss of function. Typically it is assumed that the resistance to scratch damage is the desirable consumer feature when speaking of coatings (unless the purpose of it is to come off, e.g. scratch-off lottery tickets).

Whether aesthetic or functional, once the concerns of a product development or QC engineer are correlated with microscopic scratch phenomena, the challenge becomes to explore how changing conditions in the manufacturing process can push these phenomena to occur at ever higher normal loads. An instrumented scratch tester is the ideal tool for this job.

Cohesive failure (left) vs. adhesive failure of a coating

Figure 3: Cohesive failure (left) vs. adhesive failure of a coating

Conclusion

While classical adhesion testing methods still have a role to play in industry, the value of instrumented scratch testing has greatly added to the reproducibility, sensitivity, and quantification of what has traditionally been a field prone to subjectivity and qualitative benchmarks. Modern scratch testers are designed and manufactured to cover a wide range of coating conditions, from soft polymers to exceptionally hard industrial coatings, with thicknesses ranging from the nm to mm level. This equipment is a powerful diagnostic tool for ensuring that coatings are formulated and applied to a standard that will allow them to survive the expected rigors of their environments for the designed lifetime of a product.