Cracking and checking.

Cracks are breaks or fissures in the surface of a coating that reach down to the next layer below or beyond, as shown in Figure 1. They occur in different sizes and geometries (crow's feet, three pointed and five pointed stars, angles, long single cracks, curves, etc.). Examples are shown in Figures 2-4. Cracks are seen in auto topcoats, appliance and other industrial metal coatings, wood furniture coatings and architectural finishes. Cracking occurs when stresses exceed the strength of the coating. The coating pulls apart to release the stress. However, even a coating with high tensile strength may crack if there are defects in the surface (pinholes, bubbles, pops, pits, acid etch spots) that allow the concentration of stress. Also, fatigue-inducing cyclic stresses or embrittlement caused by natural or accelerated weathering may lead to failure at lower than expected stress.

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Checking (Figure 5) involves shallow breaks in the surface of a coating, often in a fairly regular grid or checkerboard pattern. The gaps between the segments may be narrow or broad. The latter situation often is called alligatoring. I have never seen a good explanation of the mechanism for checking, but there must be a thin top layer (a few [micro]m) of the coating that is different from the rest of the film. That thin layer must undergo more stress or be less resistant to stress (less elastic) than the rest of the film. One scenario would be differential curing within the film such that the top layer cures later or more slowly. It tries to shrink but is prevented from doing so by the rigid, already cured lower portion and, therefore, is pulled apart. Checking also could occur where there is stressing of a rigid top layer over a more flexible lower portion.

Regarding stress, some comes from the curing process, but most comes from external effects such as temperature, humidity, and physical impact or flexing. Coatings undergo thermal stress on cooling and heating due to thermal expansion coefficient mismatches between coatings and between the coatings and the substrate. Natural heating (as in summer sunlight) can raise the coating temperature above the softening point and release stresses, but cooling restresses the film and freezes in the stress. Coatings also expand and contract as they absorb and lose water and stress is caused by humidity expansion coefficient mismatches. A rapid decrease in temperature and humidity may cause an abrupt increase in internal stress in a coating that can lead to cracking and delamination. A coating may be struck by stones or otherwise receive high stress impacts. The substrate under the coating may move and flex, thereby stressing the coating. For example, wood expands and contracts considerably as it picks up water, then dries out. Plastic parts and objects may deform under use putting stress on any coatings on them. Coil coatings undergo great stresses as the steel or aluminum sheet is stamped into shapes.

Cracking is also dependent on film thickness. The thicker the coating, the more likely it is to crack. I have seen severe cracking and lifting in thick (>10 mils) architectural coatings whether they were applied in multiple layers over many years or as one thick layer. Overbaking or multiple bakes can lead to cracking if the glass transition and degree of crosslinking are driven up. Such increases tend to make a coating more brittle and allow it to freeze in more stress. Some of the cracking that I have seen in automotive coatings was related to overbakes (excessively hot ovens), but most involved in-plant repairs that added extra layers and additional bakes. A common test for cracking is to apply multiple coats to a panel, then expose the specimen to a number of hot-cold cycles.

Cracking can be prevented by applying thin coats, controlling oven temperatures, avoiding brittle coatings formulations (such as very high melamine levels) and preventing surface defects so that recoating is unnecessary.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].