Design and Properties of Thermal Barrier Coatings for advanced turbine engines

Abstract

Dedicated to Prof. Wolfgang Bunk on the occasion of his 70th birthdayThe efficiency and performance of advanced aircraft turbines can be markedly increased if higher gas temperatures are used. Although the highly loaded blades and vanes in the high pressure turbine are heavily cooled, today's substrate materials are unable to provide sufficient strength in the temperature range up to 1500 "C and above. If thermal barrier coatings (TBCs) are applied on superalloy turbine blades a substantial temperature drop of the parts can be achieved. The resulting increase in efficiency comes from reduced cooling andlor increased gas turbine inlet temperatures of up to 150°C. TBCs are either processed by plasma spraying (PS) or electron beam physical vapour deposition (EB-PVD). While PS is lower in cost EB-PVD leads to superior strain and thermoshock tolerant coatings. Furthermore, cooling hole closure of turbine blades and vanes is prevented and aerodynamic design maintained. Finally, future research and development needs in TBC technology are stressed.Die Verbesserung des Wirkungsgrades stationarer wie auch fliegender Gasturbinen erfordert eine Erhohung der ProzeRtemperaturen und damit auch der Werkstoffeinsatztemperaturen. Trotz aufwendiger Kuhlung und einkristalliner Bauweise stoRen die hochstbeanspruchten rotierenden Teile in der Turbinedie Turbinenschaufelnheute an Einsatzgrenzen, die durch den Schmelzpunkt der Ni-Superlegierungen vorgegeben sind. Hier haben Warmedammschichten eine zentrale Bedeutung erlangt. Aufgrund der im Vergleich zum Substrat aul3erst geringen Warmeleitfahigkeit dieser keramischen Schichten lassen sich bei innengekuhlten Turbinenschaufeln Gastemperaturerhohungen von bis zu 150 "C erzielen. Warmedammschichten werden primar durch thermisches Spritzen (PS) oder Elektronenstrahlverdampfung (EB -PVD) hergestellt. Wahrend PS kostengunstiger ist, weisen EB-PVD-Warmedammschichten ein uberlegenes Schadenstoleranzverhalten auf. Zudem werden durch das Aufdampfen die Kuhlluftbohrungen nicht verstopft, und aerodynamisch glatte Oberflachen bleiben erhalten. Zum SchluR wird ein Ausblick auf Forschungs-und Entwicklungstrends in der Warmedammschichttechnologie gegeben.

Advanced Ceramic Materials for High Temperature Applications

“…APS and EB-PVD, great differences in the properties of the YSZ topcoatings can be found due to the distinct coating morphologies. While APS leads to the orientation of the pores between the splats parallel to the substrate surface, reducing the YSZ bulk theoretical thermal conductivity values from 2.2-2.6 W·m -1 ·K -1 to 0.8-1.7 W·m -1 ·K -1 , [36] the columnar grain microstructure created by EB-PVD process contains channels between the columns and pores within the grains which are oriented perpendicular to the substrate surface, provoking a lower reduction in the thermal conductivity than APS (1.5-2.0 W·m -1 ·K -1 [37] ). Conversely, the columnar distribution of the grains allows an increase on the strain tolerance of the TBC and, hence, EB-PVD coatings present greater durability than APS ones.…”

EB-PVD Thermal Barrier Coatings for Aeroengines and Gas Turbines

“…Figure 7 shows a GE Aircraft Engines CF6 high pressure turbine blade with an EB-PVD ceramic coating, TBCs were manufactured in a semi-commercial 150 kW dual-source jumping beam von Ardenne Anlagentechnik EB-PVD coater shown in Figure 8. [19,20] State-of-the-art commercial production facilities are aimed at reducing the overall coating costs per coated article by increasing the number of coated components per coating charge, by extending the production cycle between maintenance stops, by simultaneously minimising the latter and by optimising the processing steps per coating run. Recently, some process improvements have been introduced such as electron beam pre-heating for large industrial components or in-line trolly installation for higher flexibility and throughput [15,21,22] or dual-source jumping beam evaporation for generation of new types of TBC.…”

“…Potential benefits in gas turbines for the use of TBCs: depending on operating conditions and requirements, TBCs can improve engine performance and thrust as well as component lifetime[19].…”

Oxidation Behaviour of TBC Systems on γ-TiAl Based Alloy Ti–45Al–8Nb

“…The segmented columnar morphology of the latter top coat imparts strain tolerance, accommodating thermal expansion mismatch stresses. Furthermore, the EB-PVD process provides a smooth surface with better aerodynamic quality and less interference to cooling holes in comparison to plasma deposition [35]. Failure of the TBC systems deposited on Ni-based superalloys occurs in the vicinity of the TGO, generally at the TBC/TGO or TGO/bond coat interfaces [31,36].…”