Investigation of the bond coat microstructure and interface topography importance on thermal barrier coatings

Abstract

MCrAlY bond coatings typically used in thermal barrier coatings (TBC) systems for gas-turbine engines are studied in this thesis, with special focus on microstructural, topographical and oxidation behaviour. The effect of bond coat/thermal barrier coating (BC/TBC) interface topography on the lifetime of air-plasma sprayed (APS) TBCs has been comprehensively investigated in the present work. A quantitative description of the interface topography was achieved by utilising multiple surface texture parameters obtained from confocal microscopy, including a newly formulated parameter, denominated as total thresholded summit area, Ssth. Thermal cycling fatigue (TCF) testing showed a clear correlation between the TBC lifetime and the interface topography, especially for the newly proposed Ssth parameter. The topographical and microstructural analysis revealed that deposition of a TBC over tortuous BC topographies leads to a highly curved TBC splat morphology that influences the crack path configuration. Microindentation and three-point bending conducted on as-deposited and heat-treated specimens have showed that the aforementioned microstructural changes lead to a reduced local and global elastic moduli. These features were later associated with more beneficial fracture mechanics behaviours and higher TCF lifetimes. A modified four-point bending experiment is subsequently conducted on air-plasma sprayed (APS) TBC to gain further insight into the impact of bond coat (BC) topography tortuosity on the TBC fracture mechanics. The degree of BC tortuosity was quantified by the Ssth metric and was found to influence the crack path configuration, critical energy release rate (Gc) and crack propagation velocity (νprop). Specimens with higher BC interface tortuosity and more compliant TBC microstructures ere associated with reduced strain energy release rates (G_c≤79.9 J.m^(-2)) and crack propagation velocities (v_prop≤0.9 mm.s^(-1)). These observations seem to indicate that TBC failure is primarily governed by the degree of inter-splat cohesion and intra-splat segmentation, although the topography-induced inter-splat tortuosity still had a relevant impact on fracture mechanics. These findings suggest that despite TBC systems with higher tortuosity specimens and more compliant microstructures tending to show a premature propensity for crack nucleation, the lower strain energy release rates observed induce a slower crack propagation rate and may delay failure. The correlation observed with the lifetime of analogous thermally cycled specimens tested herein seemed to confirm this, as more compliant TBC microstructures were correlated with longer TBC lifetimes. The oxidation behaviour of CoNiCrAlY coatings deposited by high velocity oxygen fuel are studied via a combination of correlative microscopy and spectroscopy techniques. The dual-phase BC microstructure was found to promote a site-specific oxidation behaviour that had significant implications in the formation and composition of the oxide scale during the early stages. Oxide growth over the β-phase grains was faster due to accelerated diffusion of cations along the γ/β grain boundaries. It was found that this effect was more prominent in smaller grains with reduced γ/β grain boundary distance (0.8 ≤x_PB ≤ 1.2 µm), as the resulting interaction between fluxes resulted in the undesirable nucleation of mixed oxide formations in the outer scale. The evolution of the oxide scale with oxidation time further revealed that the outer scale undergoes a notable thermo-chemical transformation via outward diffusion of non-Al cations across the inner scale. The high residual stresses and premature failure suggest that the outer scale has an adverse effect on the durability of the oxide scale. Furthermore, the incorporation of large quantities of Cr in the oxide scale (~4 at.%) and subsequent segregation to the outer scale is detrimental for the early oxidation behaviour, overall stress state

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Ping Xiao (Supervisor)