4 Types of Tests that Measure the Strength of a Coating
While many tests have been devised to measure the strength of a coating, they generally fall into one of four categories.
Ensuring the integrity of a protective coating is absolutely critical when designing, applying and monitoring the coating. There is a severe risk of coating failure if one does not know what a coating is capable of withstanding. Testing the mechanical properties of a coating is one of the best ways to ensure that the coating in question can handle the environmental and operational conditions into which it will be placed.
There are a multitude of tests that give many different measurements regarding the strength and other properties of coatings. Understanding the various tests and how they are used to determine the quality of a coating is a key factor in improving the odds of project success. (Learn about other proactive steps in Coating Failure: Why a Preventive Strategy is the Best Way to Avoid it.) While many tests have been devised, they generally fall into the following four categories.
Coating Hardness Testing
Hardness is a mechanical property that helps a coating resist indentation. (A wider discussion of hardness can be found in the article 5 Ways to Measure the Hardness of a Material.) A harder coating has a greater ability to withstand depressions. When indentation is a concern it is worthwhile to use a coating hardness tester to verify that the cured coating is at the desired hardness level.
A Buchholz indentation tester is an excellent method to perform this verification. During the Buchholz indentation test, a standard sized tool is depressed onto the cutting under a constant load for a set amount of time. After the time has expired, the tool is removed and the indentation is measured. The greater the size of the indentation, the lower the hardness of the coating.
Coating Abrasion Testing
Abrasion of a coating occurs when all or a portion of the coating is scraped off by another material, which is clearly undesirable because it leaves the base material unprotected after the coating has been removed. Therefore, testing for abrasion is necessary to understand how a coating will hold up when placed in contact with another object while the two objects are moving relative to each other.
A common test to determine the abrasion resistance of a coating is the Taber test. To perform the Taber test, the coating material being evaluated is applied to a solid, flat plate. After the coating has cured, the plate is then loaded into the test machine. Abrasive wheels are placed onto the coating and are moved around on the plate's surface for a predefined period of time. This motion gradually removes the coating material as the abrasive wheels travel along the coated surface. The weight of the specimen before and after the test is then used to calculate the abrasion resistance of the coating.
Another, less scientific approach to understanding the indentation and abrasion resistance of a coating is performing what is known as the fingernail test. To carry out this test, a fingernail (or other object with a well-defined edge such as a coin or knife) is dragged across the surface of a coating after it has cured. Once this has been done, the coating surface is evaluated to determine if any damage has occurred. If excessive harm has been done to the coating, then it would be considered unsatisfactory. If the movement of the fingernail or other edge type across the surface of the coating appeared to have little or no effect, then it would be deemed acceptable.
Because the force of the fingernail being applied to the coating is not measured, and because the sharpness of the fingernail is not captured either, the fingernail test method is typically only useful for rudimentary field tests. If numerical data is required for a coating abrasion test, a more appropriate evaluation method would be the Taber abrasion test.
Coating Adhesion Testing
The ability of a coating to adhere to a base material is extremely important because without an adequate adhesive bond, the coating could prematurely flake off entirely, leaving the base material exposed to the environment.
A frequently used test to determine the adhesive strength of a coating is the pull-off adhesion test. To perform this test, a special pull-off gauge is required. The gauge is attached to an adhesive pad, or dolly, which is then attached to the coating. A force is then applied to pull the dolly away from the coating. The amount of force used is recorded until coating failure occurs. This force is used in conjunction with the area measurement of the dolly to determine the adhesive strength of the coating.
Since coating adhesion strength is such an important consideration, there are other tests besides the pull-off adhesion test to measure this mechanical property. Another way to determine a coating's adhesive strength is a tape adhesion test, which is a popular evaluation method for coatings applied in the field when other, more digital options are not available. To perform a tape adhesion test, a portion of the coating that is adhered to the base material is cut with a sharp object. Then an adhesive tape is applied to the area of the coating that has been cut. The tape is removed in a rapid fashion and the amount of coating material removed is evaluated. Although it does not provide much in the way of quantifiable data, it is useful for quickly assessing the adhesive strength of a coating to some degree.
Coating Flexibility Testing
When coatings are applied to base materials that will undergo deformation then it is imperative that the coating be able to withstand the material's deformation without failure. Some coatings are hard and brittle, and therefore less likely to be able to remain intact when the base material begins to flex.
Testing the flexibility of the coating can be helpful to prevent this type of coating failure from occurring. A common way to test coating stiffness is to apply the coating to a base material, let it cure, and then bend the base material and the coating around a mandrel. The diameter of the mandrel is selected based on the amount of deformation the coating is expected to endure in the actual application. If cracking or chipping occurs, then that indicates that the coating is most likely not flexible enough for the intended purpose.