Multicomponent equimolar oxides for high-performance materials for future application in thermal barrier coatings (MEO-TBCs)
Research Associates: Jonas Johannes Pflug and Manuel Schenker
Regarding the energy supply and the efficient usage of resources, the minimization of fossil fuel consumption and the maximal exploitation of energy-generating processes moves more and more into the focus or current research topics. For the increase in efficiency of gas-turbines an elevation of the operating temperature of the fuel combustion leads to an increase of the overall performance of the gas turbine. Because of the melting point of the metallic turbine parts being the main limitation for the operating temperature, the application of thermal barrier coatings (TBCs) started in the 1980’s. This enabled process temperatures above the melting point of metallic turbine parts. TBCs are applied to the metallic parts in the hot zones in the inside of the turbine and the rotor blades, so various requirements of the material need to be fulfilled: chemical compliance with the bond coat, similar coefficient of thermal expansion, as well as low thermal conductivity and resistance against sintering. One of the most frequently used materials for TBCs is zirconia, which is stabilized with 8 wt% of yttria (8YSZ), and can be used up to 1200 °C.
For further improvements, TBC materials are needed that have good mechanic and thermocyclic stability, as well as high corrosion resistance. These characteristics are investigated within two projects focusing on multicomponent equimolar oxides (MEOs). In this case high-entropy zirconates are used with the general formula A2B2O7. In the sub-lattice of the A-cation, four or more rare earth metal cations in equal amounts are used, which are spread stoichiometrically in the crystal lattice. The resulting high configurational entropy stabilises the materials phases at high temperatures despite positive enthalpy. This leads to new, interesting properties that broaden the TBC application.
Thermocyclic behaviour
Since a turbine is used in a cyclic thermal environment, the stability of the MEOs themselves as well as of the material composite in the TBC must be investigated in such a setting. To determine these materials properties, samples are often heated in a furnace, subsequently cooled to room temperature, and heated again. In this setup no temperature gradient is generated inside the material which occurs in a real aircraft turbine. To enable realistic investigations, thermal barrier coatings are therefore heated in burner-rigs. In addition, active cooling of the back surface is possible, which is also used in an aircraft turbine. This setup enables the investigation of the behaviour of the TBC with a temperature gradient and allows additionally to analyse the performance of the TBC material composite in combination with the other components of an aircraft turbine. Alternatively, the sample can be heated with a laser instead of a burner. The method using a laser is advantageous over a burner-rig because the surface temperature can be set more precisely, and a wider temperature range can be investigated. Such rigs are rare in public facilities worldwide and laser-rigs are not widely used.
Such a laser-rig will be built and validated with well-studied materials in this project. The HEO‑TBCs will then be investigated with varying surface temperatures and temperature gradients.
Corrosion resistance
During the operation of airplane turbines, small particles from the air are deposited in the combustion chamber and the rotor blades. Those particles are, especially in sandy deserts and in the vicinity of vulcanic eruption, composed of calcium-, magnesium-, aluminium- and siliciumoxide (CMAS). At high temperatures the CMAS debris melts and infiltrates the TBC material and because of chemical interactions, various damaging mechanisms are initiated. The dissolution and recrystallization of the TBCs components can lead to the formation of new phases, which can delaminated from the bond coat due to differences in the thermal expansion. Irreparable damage to the turbine causes the underlying coatings to be directly exposed to the CMAS corrosion. Thus, the turbine can no longer be used due to a significant safety risk. Because of this, die characteristics of the thermal barrier coating need to be adapted to be resistant against environmental influence and are thus also referred to as environmental barrier coating (EBC).
In this project the properties of multicomponent equimolar oxides are investigated regarding their corrosive resistance against CMAS. The reaction products and damaging mechanisms are to be identified, and possible strategies for the minimization of corrosion will be developed.
In cooperation of both projects, the mechanic properties, e.g. hardness and fracture toughness will be investigated.
This Project is funded as part of the NanoMatFutur junior research group by the Bundesministerium für Bildung und Forschung (FKZ: 03XP0301A).
