Unveiling the influence of elevated temperatures on enamel coating: Viscosity, yield strength, stress-strain behavior and structure

Enamel coatings are crucial for safeguarding key engineering components against high-temperature oxidation. An understanding of their mechanical properties, such as viscosity, yield strength, stress-strain behavior, and modulus, is essential for their high-temperature applications. Prior research typically explored these properties separately, lacking an understanding of their combined impacts. Our study uses an integrated high-temperature nanoindentation and finite-element approach to precisely characterize the viscosity, yield strength, and stress-strain of coatings under high temperatures. By considering material crushing during tip compression and adding a plastic yield function to the enamel's governing equations, our simulations match indentation data well in force-depth and creep displacement. We discovered non-monotonic changes in stress-strain with temperature. For example, their yield strength decreased from 3.6 to 1.7 GPa between 24 ˚C and 600 ˚C, then increased to 2 GPa at 700 ˚C. Additionally, there was an abnormal increase in modulus. XRD analysis shows a strong connection between internal quasi-crystalline structure changes and the unusual behavior of the stress-strain. Moreover, the elastic and viscous parameters, which we characterized using the proposed method, are independent of plastic properties. As a result, the plastic yield function can be removed from the constitutive equations, facilitating the establishment of a viscoelastic continuum model and determination of related material strength for the enamel coating at elevated temperatures. This research offers a more complete view of enamel coating mechanics at high temperatures, beneficial for enhancing their engineering applications.