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

Abstract 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 top coats, 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 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 top coat 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 (1) burner-rig thermal gradient cycling and (2) 1100 °C furnace cycling. In the burner-rig tests, it was found that lifetime scaled with bond coat thickness and less distinctly with through-thickness temperature gradient. The best porous ID coating endured 246 cycles, while the specimen with a thin bond coat failed 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 top coat. In conclusion, disk-type splat formation and adequate bond and top coat thicknesses (>130 and > 300 µm, respectively) are microstructural prerequisites for durable ID-TBCs.