Crack initiation and propagation in coating systems for super alloys under high temperature low cycle fatigue

Kurzfassung

Components made from superalloys are employed in hot engines such as gas turbines. For increasing the turbine inlet temperature or the lifetime of the component, respectively, high temperature coating systems are applied. In the first stages of a turbine complex coating systems, consisting of a metallic oxidation protection layer and a ceramic thermal barrier coating (TBC), are used. In the subsequent stages only metallic protective coatings are applied. The coating systems are operated at temperatures close to 1000°C and have to sustain fatigue due to mechanical and thermal loading. During long term operation, diffusion controlled changes in chemical composition and microstructure occur, which interact with fatigue processes. In this work, results of low cycle fatigue (LCF) experiments performed at 950°C on specimens made from cast nickel-based superalloy IN 100 with two different coating systems are presented. The coatings were a NiCoCrAlY oxidation protection layer only and a coating system with an additional yttria stabilized zirconia TBC. The specimens were systematically pre-aged, and the crack initiation and propagation process was investigated on specimens after interrupted tests.Due to pre-ageing a thermally grown oxide (TGO) layer formed on the NiCoCrAlY surface, and at the interface between substrate and coating a diffusion zone with intermetallic phases evolved. The fatigue crack initiation was different in the coating system with and without TBC. Without TBC, fatigue cracks were in most cases initiated at the oxidized surface, predominantly at rumpling instabilities of the TGO. With TBC, rumpling of the TGO was suppressed and fatigue cracks initiated at small undulations at the interface between TGO and NiCoCrAlY due to selected oxidation. In both systems, crack propagation was influenced by the spatial distribution and morphology of intermetallic phases in the diffusion zone between NiCoCrAlY coating and substrate. In general, crack propagation appeared to decelerate at or within the diffusion zone, respectively, revealing crack deviation at intermetallic precipitates, such as shown for example in Figure 1. Depending on pre-ageing time the density and type of precipitates and their influence on crack propagation changed.The obtained results reveal complex non-linear interaction of mechanisms due to ageing and LCF loading. The observed effects on crack initiation and growth will be discussed with respect to strategies for accelerated testing by separation of pre-ageing and fatigue and prospects to life time extension of components.