Thermal barrier coatings adherence and spallation: Interfacial indentation resistance and cyclic oxidation behaviour under thermal gradient

Abstract

One of the most critical damage of Thermal barrier coatings (TBC) results from the spallation of the yttria stabilized zirconia (YSZ) layer. The toughness of the interface between the bond coat (BC) and the top coat (TC), measured using indentation technique, is the main parameter that controls the adherence of the zirconia layer. Cyclic oxidation tests, with and without through thickness thermal gradient, are performed and monitored to investigate spallation mechanisms and overall TBC durability.

Influence of isothermal and cyclic oxidation on the apparent interfacial toughness in thermal barrier coating systems

“…Of course, correlatively to the thermally activated decrease of the critical force discussed above, toughness also decreases as the oxidation temperature increases. Besides, it was shown elsewhere [7] that for a given oxidation temperature, the interfacial degradation was similarly time-dependent too. This unambiguously shows that the propensity of the coating to detach from the substrate results from complex solid-state diffusion processes that impair the mechan- Beyond the mechanistic approach, the fractographic analysis gives interesting information on the mechanisms of crack initiation and further propagation, which can both vary depending on the aging conditions.…”

“…The latter, further developed in [14], employs a pyramidal Vickers indenter and can be applied for a large range of coating thicknesses (greater than ∼100 m). Typically, it is specifically used for investigating adhesion of TBC systems [7,8,14,15]. The principle of interfacial indentation is to accurately align one diagonal of the Vickers pyramid with the interface between the substrate and the coating while loading the system to hopefully generate the local delamination of the coating.…”

“…3). As a consequence, the influence of the TGO in terms of mechanistic issue is deliberately neglected [7]. However, it will be shown later that the thickness of the TGO has an influence on the location of the crack initiation and subsequently the propagation path.…”

“…The resistance to spallation is intimately related to the capacity of interfaces of the complex TBC system to sustain crack initiation and propagation, which can be evaluated by measuring the interfacial toughness. Several methods have been proposed to achieve interfacial fracture toughness measurement for various substrate/coating systems, including "four point bending test" [3], "barb test" [4], "buckling test" [5], "micro-bending test" [6] and various indentation techniques [7][8][9][10][11]. This paper proposes to implement the Vickers hardness technique to estimate the interfacial toughness in EB-PVD TBCs as well as its evolution upon various isothermal and cyclic aging conditions.…”

Quantitative assessment of the interfacial roughness in multi-layered materials using image analysis: Application to oxidation in ceramic-based materials

“…It is then determined graphically by seeking the intercept, for each aging condition, between the two curves. 40 In turn, the apparent toughness of the interface is calculated using the following expression:…”