Environmental Barrier Coating for Aerospace Engineering Applications

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Introduction

In the hot sections of most aerospace engineering applications, especially in gas turbines, metals have been used as the structural components. These structural components are the blades, nozzles and combustor lines. This has been in use for a very long time. A major drawback to the use of metallic structural components in gas turbines is their temperature capability is lower than what is expected for the efficiency needed now in gas turbines. This directly affects the performance of gas turbines. Even with the use of thermal barrier coatings (TBCs), they still fall short in terms of performance in comparison to what is needed now. With the need to increase the efficiency of the gas turbine (I am taking a gas turbine as a case study), it is necessary that the structures withstand the very high temperature requirements. As we know, the aviation industry try as much as possible to be efficient and a good way of getting efficiency is with increasing temperature limits. The temperature limit in aerospace engineering applications for metals has been reached and so it became necessary to use ceramics and composites.

To successfully use ceramics for components in the hot section of gas turbine engines, it is important to protect these components from the effects of the environmental conditions. This led to the development of Environmental Barrier Coatings (EBCs).

Environmental Barrier Coatings

As the name implies, Environmental Barrier coatings are used to protect the ceramic based matrix from the effect of the environment. In this case, the effect of the environment is the reaction of silica scale with water vapor and/or molten salts. The major reason why EBCs are needed is because of the effect of water vapor. Usually in the case of hot sections, when combustion occurs, one of the products of the reaction is water vapor, which accounts for about 10% of the reaction products.

Environmental barrier coatings usually consist of three layers. These layers are:

  • A bond coat layer
  • An intermediate coat layer
  • A top coat layer
aerospace engineering barrier coating

aerospace engineering barrier coating

 

Bond Coat Layer

The bond coat layer is used to aid adhesion between the substrate (the ceramic component) and the intermediate and top coats. It is meant to have very similar Coefficient of thermal expansion when compared to the substrate. This is to avoid failure by lamination.

Intermediate Coat Layer

The intermediate coat layer is used to as a chemical barrier to protect the substrate from attack. In the case of Silicon based ceramics, they block oxygen from reaching the substrate through diffusion.

Top Coat Layer

The top coat layer is also known as the steam barrier. It protects the substrate from the effect of water vapor. This top coat is meant to be good enough not to be ‘washed away’ by vapor.

Properties to be achieved by EBC

The major properties to be achieved by an Environmental Barrier coating are as follows:

  • Damage tolerance at high temperature
  • High Strength and durability
  • Good Coefficient of thermal expansion (CTE) match with the substrate

EBCs can be deposited by a number of coating methods:

  • Air Plasma Spray (APS)
  • Electron Beam Assisted Physical Vapor Deposition (EB-PVD)
  • Sputtering
  • Slurry Deposition

Presently, in Aerospace Engineering, Plasma Spraying is the best and most used procedure in applying EBCs.

Advantages of using EBCs
The use of EBCs, as can be seen, is very important in determining how well a gas turbine will perform. Some advantages are derived from their use:

  1.  Saves cost of replacing parts
  2.  Reduces the risk of catastrophic failure of the structural component
  3.  It allows for the use of composites that can accommodate very high temperatures. This, in turn, increases the efficiency of the system.

Disadvantages of using EBCs
The use of EBCs have some drawbacks in their application. These are:

  1.  Increase in manufacturing costs
  2.  Difficulty in effectively coating the structure without pores formed

Application of EBCs
EBCs are extensively used in the aerospace engineering industry. They are used primarily in the hot sections of the gas turbines. The components protected using EBCs include

  1.  Nozzles
  2.  Blades
  3.  Combustor lines

With more use of ceramic based composites being used in aerospace engineering applications, Environmental barrier coatings are going to be more and more important especially in hot sections.

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Authors

I am an aerospace engineering graduate student at Concordia University, Montreal.
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