How to Test the Scratch Resistance Of a Coating

“Bump!” Uh oh… you just accidentally tapped the car in front of you when you were parallel parking. You uneasily walk up to the bumper and are relieved to find no sign of your infraction. Coatings save the day once again! The coating over the paint on a car bumper, as well as the rest of your car, is designed to resist some scratches. Of course, it isn’t impenetrable, but it helps with minor, everyday wear. But how is the scratch resistance of a coating tested?

Historical ingenuities have provided us with a range of cheap methods for quickly approximating adhesion forces between coating and substrate. However, to develop a truly exquisite coating, an investment in instrumented scratch testing is essential. What advantages over pencil scratch testing, tape-peel testing, and cross-cut adhesion testing are offered by instrumented scratch testing? Let’s find out!

Classical Adhesion Testing Methods

Ask most people working in the coatings industry what comes to mind when discussing scratch testing, and you’ll hear about 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
Results of a cross-cut adhesion test (Source: http://www.occa.org.za/paintopedia/testing/croscuttest.jpg)

Instrumented Scratch Testing: a Quantitative Method

There is often a need for product development and quality control engineers to “do better” with monitoring adhesion strength of coatings than simply reporting data as 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. Welcome to the world of reporting critical coating failures as “12.3 N” rather than “Yes”! This ability opens the door to optimizing 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
Results of a progressive instrumented scratch

What constitutes “failure” of a coating? This depends on the application! If you’re working 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. Unless you are manufacturing scratch-off lottery tickets, we typically think of resistance to scratch damage as being a desirable consumer feature. 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.

failure
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. In the hands of an astute scientist or engineer, 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.

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