Local repair by LPPR process of damaged thermal barrier coatings on turbine components – RepCoat

Thermal Barrier Coatings (TBCs) are thermal insulating, oxide ceramic coatings protecting metallic components in gas turbines. They are designed to increase the gas inlet temperature and therefore engine efficiency and/or to prolong the components lifetimes leading to substantial economic benefits. Besides the search for new systems with increased reliability for longer times, the repair of the current system is a technological and economical issue for both civilian and military engine end users. The present project aims to explore a new ceramic deposition process, the Low Power Plasma Reactor (LPPR) process developed by the ENSCP-LGPPTS lab, as a possible solution for repairing locally damaged TBCs.

A previous work was performed within a collaboration project including ENSCP and ONERA (REI financed by DGA). This work has led to the development of a new plasma process at a laboratory scale, the LPPR process. The LPPR permits to elaborate micro/nanostructured TBCs from aqueous precursors. The precursors consist of aqueous solutions containing nitrate salts which are sprayed in an Ar/O2/N2 plasma discharge at low power and transformed into oxide coatings. Thus it has been possible to obtain highly porous Yttria partially Stabilized Zirconia (YpSZ) coatings exhibiting a thermal diffusivity 2 to 3 times lower as compared with standard industrial TBCs elaborated by thermal plasma spraying or EB-PVD.

In this project, it is proposed firstly to investigate the laboratory scale LPPR for repairing partially spalled TBCs on small flat coupons. In a second step it is planned to develop an up-scaled LPPR reactor able to cover larger areas (~50 cm2) with complex shape and finally to repair locally damaged TBCs onto parts of real components (for example one single blade). The microstructure of the repaired coatings obtained in the two LPPR reactors will be characterized by several techniques (Scanning Electron Microscopy, X-Ray Diffraction) in order to establish relationships between process parameters and coating morphology. The efficiency and the stability of the repaired zone will be checked, mainly by SEM observations, during both ageing tests at various time/temperature conditions and interaction tests with synthetic atmospheric dusts (calcium-magnesium alumino-silicates – CMAS). This research work is expected to validate the LPPR process as a simple, efficient and cheap way to repair locally damaged TBCs.

Within the project, the academic partner LGPPTS will engineer, develop and study the properties of the up-scaled LPPR reactor in order to coat complex shaped samples with YpSZ TBCs. The data coming from the laboratory scale LPPR will be helpful to quickly perform the up scaling. The team will also proceed in the local repair of damaged TBCs by using the two LPPR processes (laboratory and up-scaled ones). The research institute ONERA will proceed in the complete microstructure characterization of TBCs, in the as-deposited and as-repaired states, after ageing at high temperature and after reaction with CMAS. Two aeronautics overhaul and repair industrial companies, AIA from the French Air Force for military engines, and CRMA (subsidiary of Air France-KLM) for civilian aircrafts, are part of the project. They will report their technological and economical expectations regarding TBC repair, bring their industrial experience and expertise in this field to the consortium and supply parts of real turbine components (blades, parts of combustors) with damaged TBCs.