Influence on Thermal Barrier Coating Delamination Behaviour of Edge Geometry

Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150μm and 350μm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB2 BO B3B partially stabilised ZrO B2B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.