Parameters affecting TGO growth rate and the lifetime of TBC systems with MCrAlY‐bondcoats

Abstract

In the present work different parameters which affect the oxide growth on MCrAlY-bondcoats have been studied, in an attempt to find a reliable way to define the critical oxide thickness to failure of an EB-PVD thermal barrier coating (TBC). It was found that the variation of selected parameters such as oxidation temperature, surface roughness, and bondcoat thickness certainly alters the thermally grown oxide (TGO) growth rate. However, simultaneously the morphology, composition, and/or microstructure of the oxide are changed, which can affect the critical TGO thickness to failure in a TBC system. In contrast, the variation of the oxygen partial pressure of the oxidizing atmosphere led to different TGO growth rates without significantly changing the oxide morphology and composition. Comparing the TGOs grown at different rates in the specimens exposed to high pO 2 and low pO 2 atmospheres, it was estimated that at failure the oxide scales in both specimens have reached a similar critical thickness.

Modelling and experimental study on β-phase depletion behaviour of HVOF sprayed free-standing CoNiCrAlY coatings during oxidation

“…This agrees with other work reported in the literature [12,19,58]. It has also been reported that surface roughness can affect the yttrium distribution by forming yttrium aluminides in concave regions of the coating [59], but this has not been observed in this study.…”

“…The -NiAl acts as an aluminium source and promotes the stable and continuous formation of aluminium oxides, predominantly Al 2 O 3 , during oxidation. This Al 2 O 3 scale, also referred to as the TGO, forms between the top coat and the MCrAlY bond coat [10,[17][18][19]. It is widely recognised that the growth of the TGO during service causes progressive build-up of stress in the system, leading to the spallation of the ceramic top coat and eventually causing detrimental effects to the turbine engine components [20][21][22].…”

Some aspects on modelling of the β-phase depletion behaviour under different oxide growth kinetics in HVOF CoNiCrAlY coatings

“…The oxidation models used may not be able to fully represent the actual oxidation kinetics that occurred to the coating, especially at longer exposure time. It has been reported that a critical oxide thickness may exist during oxidation for TBCs and the oxide layer that beyond this critical value will start to spall off [9,11]. It is usually found that the critical thickness is around 6 µm, which is also the case seen from Fig.…”

“…MCrAlY coatings exhibit their protective effect owing to the fact that aluminium forms a continuous oxide layer, predominantly alumina, on the coating surface [9][10][11][12]. The aluminium consumed by forming the protective oxides is mainly from the Al-rich β phase which acts as the Al reservoir in the coating.…”

An analytical approach to the β-phase coarsening behaviour in a thermally sprayed CoNiCrAlY bond coat alloy

“…The mechanical properties of the MCrAlY coatings largely depend on the volume fraction and morphology of the β-phase precipitates [15][16][17]. During service, the coating oxidises and forms an outer layer of thermally grown oxides (TGO) [18][19][20]. MCrAlY coatings exhibit their protective effect at elevated temperatures owing to the fact that aluminium from the Al-rich β-phase promotes a continuous oxide layer, predominantly alumina, to form at the coating surface [21][22][23][24][25][26].…”