Insight into the Process-Microstructure-Property Relationship: Single Splat Analysis, Adhesion Testing, and Thermal Cycling of Inner Diameter (ID) Thermal Barrier Coatings

Although inner-diameter (ID) thermal barrier coating (TBC) processes are well-established for large aero/land-based turbine liners, the relationships between the processes, microstructures, and properties of coatings applied to smaller, highly confined passages (ID < 200 mm), such as combustor liners, exhaust manifolds, and pipes that face comparable thermal loads, remain largely undocumented. This study examines 8 wt.% YSZ topcoats, which are deposited using an ID-atmospheric plasma spray (APS) torch, as well as bond coats, which are deposited using an ID-high velocity oxyfuel (HVOF) torch, inside 200 mm diameter, 8 mm wall tubes. The results are then compared with those of flat substrates sprayed under otherwise identical conditions. Single splats of YSZ demonstrate that oblique impact angles imposed by the tube create splashed lamellae, resulting in slightly higher local porosity than on flat substrates. Pull-off tests reveal the adhesion strengths of ID TBCs when sprayed using the two ID spraying systems. Two types of topcoat microstructures were developed using a lower-power ID-APS torch: porous variants with different porosities ranging from 9% to 17 vol% and a dense, vertically cracked (DVC) variant. The microstructures were subjected to (i) burner-rig thermal-gradient cycling and (ii) 1100°C furnace cycling. In the burner rig, lifetime scales with the through-thickness temperature gradient and bond-coat thickness. The best porous ID coating endured 246 cycles, while the specimen with a thin bond coat failed abruptly after 101 cycles. Furnace tests impose uniform oxidation; all porous coatings spalled between 60 and 100 cycles, and the DVC cracked after 40 cycles. The failure modes indicate, respectively, oxidation-driven delamination of the ceramic from the bond coat and cracking through the ceramic topcoat. In conclusion, disk-type splat formation, adequate bond coat thickness, and sufficient top coat porosity are microstructural prerequisites for durable ID-TBCs, providing a process map for future ID applications.