title: Rapid Curing and Boron Modification of Polysilazane-Derived Functional Coatings
Creators: name: Zhan, Ying
Publisher: name: UNSPECIFIED
Year: 2022

description: The growing demand for high-performance and long-lasting materials in our society has promoted the robust development of coating systems. Polysilazane is among the most widely investigated types of coating materials due to its strong adhesion to almost all kinds of substrate materials and its great potential to be modified to obtain tailored properties for a variety of applications. This dissertation presents the simple and cost-effective preparation of polysilazane-based coatings intended to be applied as protective, easy-to-clean, and anti-corrosion coatings in the field of automotive and transportation, architecture and construction, and industrial facilities. A typical spin coating process followed by moisture curing under ambient conditions is employed to obtain polysilazane coatings with industrial favorable thicknesses (5–7 μm). A systematic study of six polysilazane coatings provides insight into the relationship between moisture curing behavior, film formation, and final coating properties. The molecular structure of polysilazanes is found to play a key role in the crosslinking rate and degree, thereby affecting the coating performance. An intuitive guide to selecting suitable polysilazanes for specific applications is provided. Though moisture curing is of advantage to real-life applications, it often takes a longer time than other curing methods to crosslink organopolysilazane (OPSZ) films at room temperature. Two approaches have been developed to facilitate the moisture curing of OPSZ coatings. In the first approach, the curing time of OPSZ coating is shortened from 19 h to 1 h with the assistance of a nucleophilic catalyst, tetrabutylammonium chloride (TBAC). It is found that 1.6 times more of the Si–H and Si–NH groups are hydrolyzed, and 2 times more of the Si–O–Si groups are formed in the TBAC-catalyzed OPSZ coating. The significantly increased crosslinking rate and degree result in a dense and smooth film with double value of hardness and quadruple value of elastic modulus compared to the uncatalyzed coating. In the second approach OPSZ is reacted with borane dimethyl sulfide complex to obtain highly crosslinked boron-modified OPSZ (BOPSZ) prior to the coating process. The resultant BOPSZ coating exhibits not only improved moisture curing rate and degree, but also higher hydrophobicity (contact angle = 101.1°) as opposed to the boron-free OPSZ coating with a contact angle of 88.7°. Enhanced corrosion resistance of the BOPSZ coating against HCl is thus achieved. So far, polysilazane coatings with outstanding surface hardness, high hydrophobicity, and remarkable chemical stability are successfully prepared at room temperature with greatly reduced processing time. Moreover, the thermal behavior and polymer-to-ceramic conversion of boron-modified perhydropolysilazanes are experimentally and theoretically studied. Boron incorporation induces the formation of rigid B–N bonds, which restricts the atomic motion of nitrogen in the ternary Si–B–N network. Therefore, the crystallization of the amorphous silicon nitride is suppressed, and the thermal stability is improved. A preliminary groundbreaking work for the development of SiBN ceramic coatings as e.g. thermal barrier coating is laid.