How and why should I measure the strength of a protective coating?
Modern industrial processes continue to place heavy demands on tools, equipment and structures. As a result, the protective coatings used to shield these assets must be able to maintain their integrity, particularly in aggressive and challenging environments.
Coating strength is defined as the ability of the coating to withstand loading without a coating failure (e.g., peeling, adhesion loss, cracking, wrinkling, erosion, etc.). This property is a direct indicator of the effectiveness of the protection offered. If the coating does not possess enough strength to resist applied forces, then it can fail and put the asset at risk for severe corrosion. (Be sure to download our Coating Failures and Defects Guide.)
Coating strength is measured by several mechanical tests. These tests involve subjecting the coating to multiple load conditions while observing the coating's response. Various coating strength properties are measured using several tests methods, some of which include:
Coating hardness measures the ability of the coating to withstand indentation or deformation from external forces (e.g., a sudden impact). This property is measured using several tools, including the Buchholz indentation, pendulum hardness and pencil hardness tests.
The abrasion test measures the coating's resistance to removal via scraping. This test determines the coating's ability to remain in place when rubbed against another surface. The Taber abrasion test and scrub abrasion test are most commonly used to measure this property.
The adhesion test, as its name implies, measures the quality of the bond between the coating and the substrate. If the coating lacks sufficient adhesion to the surface, it can peel or flake off, leaving the substrate exposed and vulnerable to corrosion. Coating adhesion tests include the pull-off adhesion test, the tape adhesion test and the crosscut adhesion test. (Learn more about adhesion testing in 4 Types of Tests that Measure the Strength of a Coating.)
When a coated material undergoes deformation, the coating needs to be able to deform likewise while maintaining its integrity. The most common way of measuring the flexibility of a coating is to bend a coated panel over a cylindrical or conical-shaped bar. If the coating is too brittle, it will develop cracks as a result of the bending.