Improved Adhesion of Different Environmental Barrier Coatings on Al₂O₃/Al₂O₃‐Ceramic Matrix Composites

Abstract

Among other things, the efficiency of a gas turbine is limited by its maximum service temperature, which is determined by the thermomechanical capabilities of the components in the hightemperature section of the turbine. Decades of research are therefore aiming for solutions that enable a further increase in the turbine inlet temperature. In the past years, the maximum service temperature could be drastically increased by the use of thermal barrier coatings (TBCs) and complex cooling systems. Unfortunately, the extra work that is needed for cooling causes high-efficiency losses. Calculations showed that operation at 1300 C without cooling could increase the output power by almost 50%. [1,2] Another factor, especially relevant to applications in aviation, is the reduction of weight. Any weight reduction reduces fuel consumption not only by the amount of the original mass which does not need to be accelerated, but also by the mass of the saved fuel. Therefore, the desire to further optimize the process generates the demand for lighter materials with a higher thermomechanical stability compared with the singlecrystal superalloys that are frequently used up to now. Ceramic matrix composites (CMCs) based on oxides, for example, alumina, are suitable materials to meet the increasing requirements for high-temperature applications in gas turbine environment. This is due to their excellent temperature stability, [3,4] as well as due to their reduced weight compared with current state-of-theart nickel-based alloys. Although alumina is quite inert, water vapor that is present in the aggressive turbine atmosphere (%10%) [5,6] can cause severe corrosion, as it leads to the formation of volatile hydroxides at temperatures above 1200 C. [7] Therefore, the application of suitable environmental barrier coatings (EBC) to mitigate this corrosion is inevitable for long-term use. Al 2 O 3ðsÞ þ 3H 2 O ðgÞ 2AlðOHÞ 3 ðgÞ (1) Obviously, the ideal EBC should have high corrosion resistance and should be stable at high temperatures. To identify potential EBC candidates, corrosion rates of several materials have been studied. [8-14] Water vapor corrosion rates of an Al 2 O 3 / Al 2 O 3-CMC and possible coating materials are shown in Table 1. In addition to a high water vapor resistance, a dense microstructure is preferred to suppress the diffusion of water through the coating. However, a dense coating can cause high stresses during thermal cycling, leading to premature failure. To achieve a long service life, a low coefficient of thermal expansion (CTE) mismatch between coating and substrate is advantageous. Considering that, materials such as Y 2 O 3 and yttrium aluminates appear to be more suitable than yttria-stabilized zirconia (YSZ) and Gd 2 Zr 2 O 7. The suitability of Y 2 O 3 as EBC for oxide-based WHIPOX CMCs has been studied by Mechnich et al. and was also demonstrated on real combustor components. [23,24] Prior to coating by atmospheric plasma spraying (APS), a reaction-bonded aluminium oxide (RBAO) bond coat was applied to increas...

A comprehensive review of modern engineered ceramics coatings for optimised resistance to wear and corrosion

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