Stress Development In Coatings Research Articles

АНАЛІЗ ІСНУЮЧИХ МОДЕЛЕЙ ВИНИКНЕННЯ НАПРУЖЕНЬ В ТОНКИХ ПЛІВКАХ І ПОКРИТТЯХ

The analysis of existing models of stress in thin films and coatings was carried out. While reaching critical value, stress can lead to defects, cracks, delamination of coating from substrate, etc. The task of prediction and controlling of the direction and magnitude of the stress of coating-substrate system is relevant nowadays regardless of coating and thin films deposition methods. Different types of coatings and thin films are widely used in almost all industries: optics, mechanical engineering, measuring technology, medicine, micro- and nanoelectronics, etc. Development and investigation of new promising methods for the formation of nanostructures, such as nanowires in a plasma environment, requires a sufficient theoretical basis for the origin and growing of stresses. Depending on the mechanism, the causes of stress in thin films and coatings can be: chemical reactions, phase transformations, inclusions and impurities, particle bombardment (the cause of internal stress during coating growing); temperature changes (the cause of thermal stress due to different values of coefficients of thermal expansion of coating and substrate materials); deformation of coating-substrate system, etc. Models of stress development in coatings and thin films can be divided into the following groups: stress that occur at the coating-substrate interface, internal coating stress, and stress at the coating-environment interface. The study presents methods of stress measuring in thin films and coatings. Based on the results of the current research, it can be concluded that the existing models of stress in the process of growth of coatings and films, as well as stress arising under the action of external forces, describe only the causes of the stress and unfortunately do not give an understanding of their complex effect on stress-strain state of coating-substrate system and need further development and improvement. Stress relaxation is also important to obtain new structures and certain properties of coatings. The development of stress management tools can be considered as one of the ways to increase the lifetime of products with coatings and thin films.

Stress Development in Hard Particle Coatings in the Absence of Lateral Drying

Stress development in coatings prepared from aqueous suspensions of hard particles was monitored using a modified cantilever deflection technique that suppresses lateral drying fronts. Without complications from lateral drying, stress development was found to be negligible until a critical amount of drying when stress increases dramatically to a maximum and then falls more slowly as drying completes. In addition, the maximum stress measured for coatings prepared on the modified cantilevers was more than double the maximum stress for coatings measured on standard cantilevers where lateral drying fronts were present. By achieving drying uniformity, direct correlations were made between the microstructure of the coating and the measured coating stress. Cryogenic scanning electron microscopy (cryoSEM) was used to image the microstructure of aqueous alumina and silica particle coatings during the intermediate stages of drying. CryoSEM revealed that stress increases dramatically once the particles form a saturated close‐packed network, and that stress decays as water evaporates from the pore structure.

Modeling Stress and Failure in Shrinking Coatings

AbstractDrying or curing of a coating after vitrification or gelation is accompanied by stress development. Evaporation of solvent, polymerization, cross-linking, and cooling all cause shrinkage, but adhesion of the coating to the substrate prevents shrinkage to a stress-free state. The interaction of shrinkage and restraint creates strain and stress. If the local stress grows above the local strength of the coating, it can produce cracking, delamination, or other defects.A large deformation elastic model based on the Galerkin/finite element method is developed to analyze stress development in coatings subject to uniform shrinkage. The model is used to analyze effects of delamination and surface cracks. The strain energy release rates in both delamination and surface cracking are computed at different crack lengths. In both cases, results show that thicker coatings have larger energy release rates and are more vulnerable to cracking. A key conclusion of this modeling is that a crack can propagate only from an inherent flaw greater than a certain size. If the coating is thin enough so that the maximum energy release rate is less than the crack growth resistance, then no inherent flaws in the coating can grow into a crack, and so the coating remains crack-free. The model also shows how to calculate a critical coating thickness, i.e., the maximum thickness of a coating that can remain crack-free.

In situ stress measurement apparatus for liquid applied coatings

An apparatus for measuring stress development in polymer and ceramic coatings is described. The device combines the coating, processing (chilling, curing, drying), and stress measurement steps necessary to accurately monitor stress development in situ. The measurement system was developed to study the effects of processing variables (e.g., temperature, air flow, relative humidity, etc.) on stress development in coatings ranging from solvent-cast polymers and particulate films to radiation-cured materials. The combination of coating, processing, and stress measurement allows us to mimic conditions used in an industrial setting. Results are reported for polyisobutyl methacrylate cast in toluene and aqueous gelatin coatings that demonstrate the utility of the apparatus in studying how stress evolution and magnitude relate to different process controls.