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THE DEVELOPMENT OF ENVIRONMENTALLY TOLLERANT PVD THERMAL BARRIER COATINGS

Objective

To develop second generation physical vapour deposited (PVD) thermal barrier coatings. This will be achieved through the development of coating microstructures, coating compositions and PVD process technology.
Overall the project made significant progress in all four objectives, broadly meeting two of the four objectives set; establishing whether or not alternative ceramic compositions hold any benefit over ZrO2 - 7 wt.% Y2O3 and evaluating lifing methods; and partially achieving the other two; a new bond coat composition with matched alpha was identified, but could not be successfully deposited by PVD methods and ion-plating was demonstrated to improved the erosion performance of the ceramic. However, segmentation of the ceramic implied that the ceramic deposition process must be carried out at 1000 C. More importantly perhaps, the project highlighted the position of processing technology in the science-base in Europe relative to that practiced commercially in the USA, namely evaporation of ceramic at 1000 C.

The project was also successful in developing a high-temperature erosion facility at Cranfield University with the capability of testing thermal barrier coatings at 950 C with particle velocities of 300 m/s. The project also accelerated the development of a high-temperature elastic modulus measurement facility at AEA-Technology which can measure the moduli of coatings to 1200 C. Both of these are unique facilities. The work undertaken within this project also allowed the development of a thermo-mechanical fatigue (TMF) test for thermal barrier coatings based on burner-rig technology which demonstrated the need to test thermal barriers under TMF conditions if the behaviour in engines is to be understood properly.

Several important conclusions about EB-PVD thermal barrier technology were made, and these naturally point the way to areas of further work in this field which are necessary to enable complete exploitation.

(1) The performance of the thermal barrier system was found to be heavily related to the quality of the bond coats, particularly to the cleanliness and finishing of these coatings. The bond coats evaluated in the project required further optimisation to enable the maximum benefit of PVD thermal barriers to be exploited.

(2) The work undertaken in this project has shown that there is a potential benefit in using ZrO2 - CeO2 based thermal barrier coatings, especially encouraging was the overall reduction in the oxidation rate of the bond coat under the latter ceramic compared to that under the ZrO2-Y2O3 system. To establish fully this potential benefit, further developments in the bond coat technology (item 1 above) will have to be undertaken).

(3) The results generated under this project show that ion-plating at low temperatures is not a replacement for the evaporation of the ceramic at temperatures of 1000 C as currently commercially practiced. If any potential benefit from ion-plating of thermal barrier coatings based on ZrO2 is to be exploited, such as better erosion resistance, then the deposition of the ceramic will have to be undertaken at around 1000 C. Although this was achieved in the science-base within the project, this will have to be transferred to a commercial scale plant before the technology can be used on aeroengines.

(4) The erosion resistance of EB-PVD thermal barrier coatings is approximately 7 times that of their plasma sprayed counterparts. The work has also shown that the EB-PVD ZrO2 - based systems behave in a more ductile manner in high-temperature erosion than was expected, a behaviour not seen in the plasma sprayed counterpart.

(5) The moduli of the EB-PVD thermal barrier coatings is heavily influenced by their columnar microstructure, and the data generated demonstrates that the moduli are typically 50% of that of what one would expect of a solid block of similar material. The moduli of the development coatings did not significantly differ from that of the reference RT33 coating.

(6) The thermal conductivity of the development coatings was comparable to that of the RT33 reference coating at approximately 1.4 W/m.K. The lowest thermal conductivity was shown by the high-temperature evaporated coating from Cranfield with a mean conductivity of 1.2 W/m.K.
The programme is structured in four phases :
-Phase 1: Trialling of thermally evaporated PVD datum coating and the study of ion plating and its effects on structure property relationships.
-Phase 2: Thermal cyclic testing of candidate coatings.
-Phase 3: Corrosion, fatigue and erosion testing of datum and most promising development coating.
-Phase 4: Development of lifing methodology.

Coordinator

Rolls Royce plc
Address
Alfreton Road
DE2 8BJ Derby
United Kingdom

Participants (5)

Advanced Surface Engineering Technologies Ltd
United Kingdom
Address
Camben Street
NE6 EJ Newcastle Upon Tyne
CRANFIELD UNIVERSITY
United Kingdom
Address
Road Wharley End
MK43 0AL Cranfield,bedford
GERMAN AEROSPACE CENTRE
Germany
Address
Linder Höhe
51147 Koeln
MTU AERO ENGINES GMBH
Germany
Address
Dachauer Strasse, 665
Muenchen
Stichting Geavanceerde Metaalkunde
Netherlands
Address
8039
7550 KA Hengelo