"Knock bump!" Uh oh...you just accidentally knocked the car in front of you when you parallel parked. You walk uneasily to the bumper and see no sign of your infraction. Paint saves the day again! The paint on your car's bumper, as well as the rest of the car's coating, is designed to resist some scratches. Granted, it's not impenetrable, but it lends itself to light everyday wear. But what about the scratch resistance of the test coating?
Historical uniqueness provides us with a range of inexpensive methods for quickly approximating the adhesion between coatings and substrates. However, the importance of investing in instrumented scratch testing to develop truly delicate coatings cannot be overemphasized. Does instrumental scratch testing offer advantages over pencil scratch testing, tape peel testing, and cross-cut adhesion testing? Let's find out!
Classic Adhesion Test Method
Ask most people who work in the paint industry what comes to mind when they discuss scratch testing, and you'll hear solutions such as scoring the surface with pencils of varying hardness, pressing the tape against the surface and peeling it off at a steady rate, Or score orthogonal hatch marks by coating and count missing squares. While efforts have been made to remove the human element from some methods, such as robotic tape stripping, subjectivity is still an issue, not to mention the poor solutions offered (often at pass/fail levels).
Instrumented Scratch Testing: A Quantitative Approach
Product development and quality control engineers are often required to “do better” by monitoring the adhesion strength of coatings rather than simply reporting the data as a “yes” or “no” or on a scale of 1 to 5. Modern instrumented scraping technology provides a precisely driven progressive normal force while dragging the sample across a durable diamond-tipped stylus. An integrated optical microscope can be used to collect panoramic image data where points of critical coating failure are easily identified. Casual Testers can also be equipped with sensors to monitor penetration depth, lateral friction and acoustic emissions. The importance cannot be overemphasized, and well-designed scratch testing software will synchronize these data and common force profiles with panoramic images, allowing users to easily spot and report important scratch events with excellent quantitative force resolution. Welcome to a world where critical coating failures are reported as "12.3 N" instead of "yes"! This capability opens the door to optimizing coating process parameters to a degree far beyond what can be detected by traditional adhesion testing methods. Furthermore, the distribution of the critical failure load across a single sample or batch provides an excellent measure of the uniformity or repeatability of the production process, respectively.
What constitutes a coating "failure"? It depends on the application! If you work in optics, any scratches, even ones that don't cause complete coating delamination, can cause unwanted diffraction. Even if invisible to the naked eye, scratches that deform the coating can affect the gloss of the finish. These types of failures are known as cohesive coating failures. Adhesive failure involves loss of material contact and subsequent substrate exposure. For protective coatings and traces designed to be conductive, a small amount of adhesive damage over the lifetime of the product may meet performance tolerances, while adhesive coating failure is likely to be followed by loss of function. Unless you're making scratch-off lottery tickets, we generally think of scratch resistance as a desirable consumer feature. Whether aesthetic or functional, once the concerns of product development or QC engineers are linked to microscopic scratch phenomena, the challenge becomes exploring how changing conditions during manufacturing drive these phenomena to occur at higher normal loads. An instrumented scratch Tester is a desirable tool for the job.
in conclusion
While traditional adhesion testing methods still have a role to play in industry, the value of instrumented scratch testing has greatly increased repeatability, sensitivity and quantification in areas traditionally 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 particularly hard industrial coatings, with thicknesses ranging from nm to mm. In the hands of the astute scientist or engineer, this equipment is a powerful diagnostic tool that ensures that coatings are formulated and applied to standards that will hold up to the expected rigors of their environment during the product's designed life.
