胶体颗粒涂层干燥过程中的开裂行为:机理、抑制及应用

As one of the simple and convenient methods in preparing coatings, drying of colloidal suspension is widely used in protective and functional coatings. If not properly controlled, however, colloidal coatings are prone to cracking during the drying process, leading to the failure of coatings. Therefore, the mechanism of drying⁃induced cracking, as well as the methods to reduce cracking have been the focus of recent studies in both industrial and academic research of colloidal coatings. Although empirical solutions for the crack suppression have been accumulated after many years of practice in preparing colloidal coating, the deep understanding of the drying⁃induced cracking is still lacking due to the complex film formation process in colloidal coatings. Conventional methods such as adding chemical binders in colloidal coatings to prevent cracking become unfeasiblefor coatings that need to meet higher environmental regulations and for the preparation of functional nanoparticle coatings. Therefore, extensive theoretical and experimental studies on the fundamental mechanism of drying⁃induced cracking have been carried out in academia and industry in recent years. Based on those findings, novel techniques for reducing and controlling crackings in drying colloidal coatings have been developed. Theoretical models on the driving force and dynamics of drying⁃induced cracking have been proposed, aiming to reveal the mechanism of cracking from the perspectivesof the interaction between colloidal particles and the equilibrium between strain energy and surface energy. Many processing factors affecting the crack formation, including the colloidal particle network structure, thickness of coatings, the moduli of colloidal particles and the properties of substrates, were studied in order to understand how they affect the crack formation. Novel processing methods, including adding non⁃spherical particles, mixing of soft and hard particles, and multi⁃layer deposition, are proposed to suppress drying⁃induced cracking and improve the performance of coatings. On the other hand, recent studies proposed the utilization of coatings with cracks as the template to fabricate the micro⁃structure and nano⁃structure of metals and micro⁃channels and nano⁃channels by accurately controlling the morphology of drying⁃induced cracks of colloidal coatings. This article reviewed recent studies on cracking in drying colloidal coatings. First, different crack morphologies are introduced and the mechanism and dynamics of cracking are discussed. Four main factors affecting the cracking of colloidal coatings and the corresponding methods for crack reduction are symmetrically summarized. In addition, the utilization of cracking in colloidal coating for alternative micro⁃fabrication and nano⁃fabrication is also explored. Further research on the cracking mechanism of colloidal particle coatings can provide design principles for the development of environment⁃friendly coatings and the design of high⁃quality functional nanoparticle coatings.