Onset and Propagation of Segmentation Cracks in Air Plasma Sprayed Ceramics: In-situ Observations and Mechanistic Descriptions

The industrial drive for more efficient Thermal Barrier Coatings (TBCs) have concurrently ushered in the need for innovation in materials and their processing. It is widely recognized that periodic segmentation cracks in coatings can impart high in-plane strain tolerance, sintering resistance and fracture toughness which impacts durability of the coatings. The literature addressing the materials property criteria of formation and subsequent propagation of such cracks, and their relationship to processing conditions is still unresolved. However, by investigating the adhesion and micro-cracking patterns of splats deposited onto substrates of varying material and temperature while mimicking the in-situ deposition conditions, the criteria for coherent inter-splat cracking can be isolated. The consequential stress build-up from the deposition at such conditions can be monitored simultaneously using insitu beam curvature methods. In doing so, the relative extent of stress accommodation in different ceramics as a function of processing condition can be determined. These stress accommodation mechanisms have been associated with microcracking and pore formation and typically associate with a near-constant stress evolution layer-by-layer. However, when microcracking is suppressed and segmentation cracking is favored, the curvature response changes dramatically. The curvature phenomena observed can further be related to segmentation crack onset and propagation events.