Lifetime investigations on novel thermal barrier coating systems for gas turbine applications

Abstract

In this study, relatively new materials for ceramic top coat, bond coat and substrate applications are studied and their effect on the lifetime of TBC systems is investigated with the aim of increasing the temperature capabilities of gas turbines and/ or improving the lifetime of current TBC systems. The standard TBC system tested for comparison is IN100/ NiCoCrAlY/ 7YSZ. During the study, each component of the standard system is replaced by an relatively advance material and the effect on lifetime is investigated. The material tested for ceramic top coat material is Gadolinium Zirconate (GdZ) which is well known for its lower thermal conductivity and higher thermal stability. Recently, studies demonstrated very good resistance of GdZ against CMAS and volcanic ash attack. In a gas turbine application, with the internal cooling present, GdZ will definitely reduce the temperature at the underlying metal surface due to its lower thermal conductivity. The aim of this study is however to investigate the effect of GdZ on the lifetime of TBC systems. For substrate effect, IN100 superalloy is compared with a more advance, CMSX-4 superalloy. CMSX-4 is a 2nd generation, single crystal superalloy manufactured by Cannon Muskegon Corporation and is well known for its improved high temperature properties. To investigate the bond coat effect, the performance of standard NiCoCrAlY has been compared with a Hf- doped version of NiCoCrAlY deposited on CMSX-4 substrate. Furthermore, the effect of higher Y-content has also been investigated by doping the NiCoCrAlY bond coats with extra Y and depositing on IN100 substrate. All the depositions have been carried out by Electron- beam physical vapor deposition (EBPVD) techniques. The lifetime investigations have been carried out by furnace cyclic testing (FCT) where the TBC samples are kept at 1100°C for 50mins and are cooled down to room temperature by forced air cooling for 10mins. The whole experimental research for this study, from the deposition of samples to the microstructural analysis, has been carried out at German Aerospace Center (DLR) in Cologne, Germany. This study showed that GdZ on NiCoCrAlY bond coat improves the lifetime of TBC systems considerably. This improvement of lifetime has been observed for both IN100 and CMSX-4 substrates. However, CMSX-4 and NiCoCrAlY based TBC systems show a lower lifetime than the IN100 based counterparts, irrespective of the ceramic top coat material. During thermal cycling, GdZ has been found to undergo a chemical reaction with the TGO on NiCoCrAlY bond coats and a new phase forms at the TBC- TGO interface. In the beginning, this new phase is formed in random patches at different locations, however, with increasing the number of thermal cycles; this new phase becomes continuous at the TBC- TGO interface. GdZ undergoes an ordering transformation from cubic fluorite to pyrochlore phase during thermal cycling. This pyrochlore phase has a very high thermal stability and is the main motivation behind using GdZ as ceramic top coat material. It has been observed that this fluorite to pyrochlore transformation is not detrimental for the lifetime of TBC systems as GdZ based TBCs show a very high lifetime despite this transformation. In case of NiCoCrAlY version with higher yttrium content, referred to as NiCoCrAlY-2 in this study, it was observed that Yttria forms at the TBC-TGO interface in the form of elongated islands even before the deposition of ceramic top coat. This yttria layer interacts with the TGO during thermal cycling and forms different Y- aluminates. When GdZ-NiCoCrAlY-2 TBC system is thermally cycled, the chemical reaction between GdZ and the TGO results in a reaction zone consisting of alternating Gd- and Y rich phases. GdZ-NiCoCrAlY-2 TBC systems show a significantly longer lifetimes, however, 7YSZ-NiCoCrAlY-2 TBC systems showed relatively shorter lifetime than the standard 7YSZ-NiCoCrAlY TBC systems. The effect of Hf has also been investigated on CMSX-4 substrate. It has been observed that doping NiCoCrAlY bond coats with 0.6 wt. % Hf improves the lifetime by around 10 times. This improved lifetime doesn’t change much when the ceramic top coat is replaced from 7YSZ to GdZ. In the CMSX-4 based TBC systems, diffusion of refractory elements has been found from the substrate towards the TGO. In the NiCoCrAlY-Hf systems, additional diffusion of Hf is also observed and Hf can be seen at the TBC-TGO interface, in the TGO and in the bond coat. In this study, an attempt has been made to understand the mechanisms which effect the lifetime of TBC systems by investigating diffusion of elements in TBC systems, phase changes, chemical reactions and sintering effects.