On the Anelastic Behavior of Plasma Sprayed Ceramic Coatings: Observations, Characterizations and Applications

Abstract

Plasma sprayed ceramic materials contain an assortment of microstructural defects, including pores, cracks, and interfaces arising from the droplet based assemblage of the spray deposition technique. The defective architecture of the deposits introduces a novel anelastic response in the coatings comprising of their non-linear and hysteretic stress-strain relationship under mechanical loading. It has been established that this anelasticity can be attributed to the relative movement of the embedded defects under varying stresses; while the non-linear response of the coatings arises from the opening/closure of defects, hysteresis is produced by the frictional sliding among defect surfaces. Recent studies have indicated that anelastic behavior of coatings can be a unique descriptor of their mechanical behavior and related to the defect configuration. In this dissertation, a multi-variable study employing systematic processing strategies was conducted to augment the understanding on various aspects of the reported anelastic behavior. Enhancements to bi-layer curvature measurement technique allowed for reliable and repeatable quantification of the anelastic response, enabling extraction of three anelastic parameters; elastic modulus, non-linear degree and hysteresis degree. This allowed for further exploration of the process space enabling controlled introduction of anelasticity in thermal sprayed ceramic coatings. This dissertation reports on these findings by first describing the experimental advancements in bilayer curvature measurements via thermal cycling of a coated beam. This experimental development allowed assessment of sensitivity and repeatability of the obtained anelastic parameters to varying microstructures imposed by processing excursions. Subsequently, controlled modification of anelasticity was achieved through material and process parameters as well as through extrinsic modification of the defects within the microstructure. The results suggest that anelasticity can be tuned by manipulation of the material as well as processing conditions, and the presence of foreign materials in a coating is seen to have significant influence on the coating response. The anelastic response was also verified through purely mechanical (four-point-bend) loading of ceramic coatings on substrates at room temperature in order to avoid any temperature effects in anelasticity measurements. The implication of this work is significant as it provides a comprehensive and quantitative description of the properties of layered, high defect density ceramic coatings produced from complex deposition processes such as plasma spray. These quantitative descriptors will not only provide opportunities to generate/produce/create enhanced design of thermo-structural coatings but also a robust methodology for process-structure-property-performance relations.