Unveiling Wear and Failure Mechanisms of Protective Coatings

Coatings are incredibly important as a protective barrier in a number of engineering industries, including, but not limited to, automotive and nuclear. Due to their wide variety of uses and operating conditions, there are many different, specialized properties that may be necessary for an effective coating. This dissertation focuses on two types of coatings: tribofilms and chromium (Cr) cold spray (CS) coatings. Although tribofilms have been extensively studied, there are still many important questions that remain unanswered, and regulation changes and strives towards improved performance drive continued evolution in additive composition. Likewise, increased attention on accident tolerant fuel (ATF) in response to the Fukushima nuclear accident has triggered the development of next generation nuclear fuel claddings, with Cr CS coatings being identified as superior to other protective coatings for near term applications. Before implementation in nuclear reactors, however, the coatings must first be systematically studied in terms of mechanical and tribological properties under operation conditions.
For the tribofilm study, the goal was to determine how additives in the lubricating fluid affect the mechanical and tribological behaviors of the tribofilms, and further, how they may be optimized to achieve high performance. Combined chemical and mechanical characterization methodologies, such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), atomic force microscopy (AFM), and nanoindentation, were applied to a variety of tribofilms to elucidate the functionality of the additives within the tribofilms. Key findings were made in establishing systemic correlations between tribofilm chemical and mechanical properties and friction and wear performance for a wide range of commercial axle oils, individual antiwear and detergent oils, and metal-based engine oils. The sum of the tribofilm work is significant in informing additive choices of lubricants that can contribute to improved efficiency and reduced emissions.
For the Cr CS cladding study, in situ three-dimensional digital image correlation (3D-DIC) and acoustic emissions (AE) sensing uncovered coating cracking mechanisms associated with the microstructural differences in CS coatings deposited by helium (He) and nitrogen (N2) for light water reactor (LWR) operating conditions, establishing potential non-contact methodology for real-time crack detection. Additionally, scratch and fretting wear experiments further elucidated the damage mechanisms of the different CS types under tribological contact in ways that simulate realistic conditions within a reactor environment. The findings here provide a comprehensive understanding of the coating’s protective qualities and establish design guidelines for robust, safe next generation claddings.