Polymer-Derived Ceramic Coatings for Stress and Corrosion Resistance in Stainless Steel: Optimization of Thermal Stability and Structural Integrity

Abstract

This dissertation focuses on the development and characterization of polymer-derived ceramic (PDC) coatings, particularly SiON and SiOCN, aimed at enhancing corrosion resistance, thermal stability, and structural integrity of stainless steel in harsh environments. Using perhydropolysilazane (PHPS) and its derivatives, various coating compositions and pyrolysis conditions were optimized to tailor their microstructure and performance. The first part of the study evaluates the compatibility of SiOCN coatings on stainless steel, highlighting their ability to mitigate corrosion under aqueous and high-temperature conditions. The coatings demonstrate excellent adhesion and mechanical properties, making them suitable for demanding applications. Next, the corrosion resistance of PHPS-derived SiON coatings on welded stainless steel was investigated, revealing their effectiveness in reducing localized corrosion while identifying the trade-offs between pyrolysis temperature and coating brittleness. To address stress corrosion cracking (SCC) in chloride-rich environments, SiON coatings were anlayzed for their performance on U-bend stainless steel specimens. Coatings pyrolyzed at lower temperatures exhibited superior SCC resistance due to their flexible and defect-resistant structure, while higher-temperature coatings, despite their improved hardness, were prone to cracking under mechanical stress. Finally, the influence of carbon content on the thermal stability of SiOCN coatings was studied under different atmospheres (Ar, Air, and Ar+H2O). Results indicate that carbon plays a critical role in determining coating stability, with excessive carbon leading to microstructural degradation in oxygen-containing environments. This research provides a comprehensive understanding of PDC coatings' structural and chemical evolution under various stress and environmental conditions. It offers valuable insights into optimizing coating composition and processing parameters for applications in nuclear waste storage, aerospace, and other high-performance industries.