4 test types to test coating strength

When applying and monitoring coatings, the importance of ensuring the integrity of the protective coating cannot be overemphasized. If it is not known what the coating will withstand, there is a serious risk of coating failure. Testing the mechanical properties of a coating is one of the best ways to ensure that the coating in question can handle its environment and operating conditions.

There are many tests that give many different measurements regarding the strength and other properties of coatings. Understanding the various tests and how to use them to determine coating quality is a key factor in improving your project's chances of success. Although many tests have been devised, they generally fall into the following four categories.

Coating hardness test

Hardness is a mechanical property that helps a coating resist indentation. Harder coatings have a greater ability to withstand dents. When indentation needs to be considered, it is worth using a coating Hardness Tester to verify that the cured coating is at the desired hardness level.

A Buchholz indentation Tester is a better way to perform this verification. During a Buchholz indentation test, a standard-sized tool is pressed against a cutting edge under a constant load for a set period of time. After the time has expired, remove the tool and measure the indentation. The larger the size of the indentation, the lower the hardness of the coating.

Coating Abrasion Test

Coating abrasion occurs when all or part of the coating is scratched by another material, which is clearly undesirable because it leaves the substrate unprotected after the coating has been removed. Therefore, abrasion resistance testing is necessary to see how the coating holds up when it comes into contact with another object when the two objects move relative to each other.

A common test for determining the abrasion resistance of coatings is the Taber test. For the Taber test, the coating to be evaluated is applied to a solid panel. After the coating has cured, the board is loaded into the testing machine. Place the grinding wheel on the coating and move it over the surface of the board for a predetermined amount of time. As the wheel moves along the coated surface, the motion gradually removes the coating material. The sample weights before and after the test were then used to calculate the abrasion resistance of the coating.

Another, less scientific way to understand a coating's indentation and abrasion resistance is to conduct what is known as a nail test. To perform this test, a fingernail (or other sharp-edged object such as a coin or knife) is dragged across the coated surface after the coating has cured. Once this is done, the coated surface will be evaluated to determine if any damage has occurred. If substantial damage is done to the coating, it is not satisfactory. Movement of the nail or other edge type over the coated surface is considered acceptable if it appears to have little or no effect.

Because the force applied by the nail to the coating is not measured, and the sharpness of the nail is not captured, nail testing methods are generally only useful for basic field testing. If numerical data are required for coating abrasion testing, a more appropriate evaluation method is the Taber abrasion testing.

Coating Adhesion Test

The ability of a coating to adhere to a substrate is very important because without sufficient adhesion, the coating may peel off prematurely, exposing the substrate to the environment.

A common test to determine the adhesive strength of a coating is the peel adhesion test. To perform this test, a special pull-out gauge is required. The gauge is attached to a gel pad or dolly which is then attached to the coating. Force is then applied to pull the cart away from the coating. The force applied was recorded until coating failure occurred. This force is used in conjunction with the area measurement of the cart to determine the bond strength of the coating.

Since the adhesion strength of the coating is an important consideration, there are other tests besides the pull-off adhesion test to measure this mechanical property. Another method of determining coating adhesion strength is the tape adhesion test, a popular evaluation method for coatings applied in the field in the absence of other more numerical options. For tape adhesion testing, a sharp object is used to cut off a portion of the coating attached to the substrate. Then apply tape to the cut coating area. Remove the tape in a quick fashion and assess the amount of paint removed. Although it does not provide a lot of quantifiable data, it is useful for somewhat quick assessment of the bond strength of the coating.

Coating Flexibility Test

When applying a coating to a substrate that deforms, it is necessary for the coating to withstand the deformation of the material without failure. Some coatings are hard and brittle, so when the base material starts to bend, it's unlikely to stay intact.

Testing the flexibility of coatings can help prevent such coating failures from occurring. A common method of testing the stiffness of a coating is to apply the coating to a base material, allow it to cure, and then bend the base material and coating around a mandrel. The diameter of the mandrel is selected according to the amount of deformation the coating can withstand in the actual application. Cracks or chips indicate that the coating is most likely not performing as intended.