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Development of advanced surface technology for extended resistance in extreme environment

Final Report Summary - ASTERIXE (Development of Advanced Surface Technology for Extended Resistance in Extreme Environment)

The project ASTERIXE aimed to develop innovative combined surface treatments to promote a technological breakthrough for high performance in the field of Environmental barrier coatings (EBC's). These coatings play a key role in high temperature (> 600 degrees Celsius), highly corrosive environments and high mechanical (and cumulative) stress conditions. The goal of the project was to open new routes of surface engineering by combining coating deposition and advanced post treatment technologies by pulsed e-beam (PEB). The understanding of the phenomena interacting on the coating composition and structure were developed throughout the project with the creation and optimisation of a model predicting the coating composition and structure after post treatment.

The corrosion resistant coatings are deposited by a range of different processes, electrochemical, spray or vapour deposition, the process choice being mainly dependent on the elemental composition needed to fulfil the application requirements. The pulsed e-beam treatment is used to give the coating its final structure and morphology. This may for some applications involve interface melting promoting coating to base material alloying. As a result, the coating process is optimised by using the results of the predictive model, and by testing new combinations with respect to the chemical composition (new chemical systems, hybrid layers, self-repairing coatings).

The Thermal barrier coating (TBC) is deposited by EBPVD. This technology allows the development of luminescent phosphorus (lanthanides) insertion into the YSZ. The project aimed to validate both material and diagnosis technology.

Project deliverables included:
- a new concept of interface treatment leading to enhanced coating adhesion strength by simultaneously melting of the coating and the substrate surface;
- a new process of extremely fast surface quenching from the melt leading to values of density and toughness of the surface layer close to the theoretical 'bulk' values without affecting the substrate material;
- a new route to obtain still unavailable chemical compositions by combining multilayer coating and surface alloying;
- aA new route for a self diagnostic coating or sensor coating designed to allow 'in service' temperature measurements.

The project was structured into work packages (WPs), as follows:

WP1: Theoretical requirements of extreme conditions coatings and new measurement methodology
The WP1 aimed to define precisely the extreme service condition envisaged from a diverse range of industrial requirements as well as carry out a theoretical assessment of available, existing coating solutions and an analysis of coating requirements for pulsed electron beam processing. In addition, the application of thermographic phosphor technologies to thermal barrier coating (TBC) systems was studied.

WP2: Development of the coupled coating / PEB process for existing coating
No promising results were found after pulsed electron beam post treatment of low melting point PVD coatings. In the same way, there is no interest of a pulsed electron beam post treatment of a high melting point material PVD coating (TiN for example) on a low melting point material substrate (aluminium for example). Concerning titanium deposited on steel, very good results were achieved after pulsed electron beam post treatment. Whatever the process and deposition parameters, it leads to higher hardness, lower friction coefficient and smoother surfaces.

Significant improvements of HVOF (or VPS) as deposited coatings were achieved, after pulsed electron beam post treatment. Smoothening and densification of HVOF MCrAlY coatings after the post treatment were observed and, in a general way, there is an improvement of interface bonding of both HVOF and VPS coatings. In the same way, bonding to the substrate can be achieved for coatings deposited by means of thermal evaporation.

Effects of a pulsed electron beam post treatment on simple as deposited coatings depend strongly on the composite film / substrate. In some cases, it has been shown that the quality (smoothening, densification, adhesion improvement…) of this composite can be increased significantly.

WP3: Development of the coupled coating / PEB process for new material coating by surface alloying
Two post treatment methods were explored for their use to create new surface layers; Pulsed electron beams (PEB) and lasers. Lasers were explored to investigate the alloying of AlCr layers deposited by ionic liquids and the re-melting of CrC-NiCr layers sprayed using HVOF technique. Both different tasks could be fulfilled more or less. The alloying operated only at relative high pre-heating temperatures (660 degrees Celsius, the melting temperature of Al) of the sample. Nevertheless, an almost perfect intermixing of coating and substrate could be achieved. The other even more promising result concerns the re-melting of CrC-NiCr coatings. Applying adequate preheating a 25 micrometres thick layer of this coating could be re-melted und by that densified entirely. Especially, the absence of any cracks within this modified layer is very promising. Such laser treated coatings might have a great potential to be used in corrosive seawater environment.

PEB treatment was explored for several different principal architectures, single layers varying in thermo-physical properties like for example melting point, multilayer coatings and single layer multi-element coatings. The alloying of most material combinations worked properly. However, alloying of high melting point materials like W into low melting point materials like Al failed. The idea to overcome such difficulties by introduction of an intermediate layer having medium melting temperature still has to be proven finally. The idea of multilayer coating was explored using AlCr coating with different number of layers. After PEB treatment the starting architecture was not preserved and no influence of the number of layers on the result after PEB treatment could be found. The chosen system AlCr periodically changing, leads in case of thick coatings to highly brittle phases and cracking of such layers after PEB treatment was expected and occurred.

The third point multi-element single layer coating was investigated using a Noricrom target. Beside all difficulties in deposition such layers could be remelted and alloyed to the substrate. The potential of such coating to be used to substitute massive Noricrom parts is one of the most promising outcomes of this WP.

WP4: Characterise at the lab scale the coating functional properties
The characterisation at the lab scale of the coating systems obtained by the consortium produced many interesting results which were crucial for the project development. It allowed a continuous feedback among the characterisation, deposition and post treatment phases allowing the optimisation of the processes. Further, the quality of the characterisation work produced was excellent covering several topics for the dissemination of the technical and scientific work in referred journals. A precise determination of the coatings' properties has been fully achieved in agreement with original objectives.

The efficiency of the PEB treatment provided by FZK on the coatings deposited by Turbo has been demonstrated as it was found that the lifetime of the HVOF MCrAlY's bond coats dramatically increases.

WP5: Concept of industrial equipment capable to integrate both processes
The original aim to design an integrated facility that means a facility combining the coating and the PEB process in one process chamber was given up, due to the uncontrollable interference of the magnetic fields of the coating and the PEB facility.

Two configurations, a batch and a roll to roll equipment concept are possible. Both require, due to the above mentioned problems by the magnetic fields, two individual process chambers with a transfer chamber in between these two chambers. In the case of roll to roll equipment, the main problem will be to evaluate and adjust the efficiency and the time needed for the coating process and the PEB treatment.

Finally, a standalone PEB facility for the treatment especially of HVOF sprayed MCrAlY due to the extremely positive obtained with such treated coatings was designed. The integration into the production process was positive evaluated. The design of the new facility is that flexible that it can be adapted via any kind of transfer system to other coating processes and devices. Even for a roll to roll equipment the electronic part, cathode, accelerator, marx and control unit can be used without any major changes.

WP6: Knowledge dissemination and exploitation of the results
The main objectives of the WP were:
i. the organisation of the knowledge management and strategies (including continuous benchmarking) and
ii. the dissemination of project results to the scientific community and to a wider range of potential users.

WP7: Management of the project
The work package included the carrying out the technical, administrative, financial and strategic coordination of the project and the animation of the consortium.

Increasing the working temperature of turbines is of key importance in both energy production and aeronautic fields and ASTERIXE provides definitive breakthrough concerning the TBC systems including bond coat and oxide layer.

Concerning the bond coat, the PEB process allows a coating densification providing a factor two life time improvement. Moreover, the surface alloying with Zr could increase also the corrosion resistance.

Concerning the YSZ layers, lanthanides insertion allows on line temperature monitoring up to 1400 degrees Celsius instead of 800 degrees Celsius initially planned. Moreover, the temperature stability of the YSZ is increased by this dopant insertion.

These very promising results concern both bond coat and YSZ layers and open the way for a new generation thermal barrier coating system of advanced functionality.