How Aerospace Applications Use Thermal Barrier Coatings

Thermal Barrier Coatings (TBCs) offer the potential for substantial improvements in aero engine lifetime and efficiency.

TBCs are typically deposited via a number of different methods, but Electron Beam Physical Vapor Deposition (EB-PVD) is preferred for high-performance applications. This article outlines the benefits of leveraging EB-PVD technology in aerospace applications.

Thermal Barrier Coatings for Aerospace Components

The stator vanes and rotor blades used in high-pressure gas turbines represent some of the world’s most highly loaded engineering components^1. Gas turbines reach tens of thousands of revolutions per minute under normal operating conditions.

Each blade is subjected to huge forces, extracting a vast amount of energy from the pressurized gas stream. The extremely high mechanical demands placed on these components mean that only high-temperature Ni-based ‘superalloys’ are usually considered for metallic components.

By definition, nickel superalloys can operate at a high percentage of their melting temperature.² Structurally, the nickel-based superalloys employed in aerospace applications represent some of the most robust materials available, able to resist corrosion and withstand extremely high loads at high temperatures.

The hot gas temperatures found in modern gas turbines may approach the melting points of Ni-based alloys, however, leaving components at risk of creep over extended periods while significantly increasing thermal fatigue and oxidation.

Thermal Barrier Coatings (TBCs) remain the most common approach to mitigating the risks to Ni-based superalloy gas turbine components from thermally induced failure. TBCs are highly specialized coatings able to prevent components from reaching exceedingly high temperatures.

TBCs for aerospace applications are frequently comprised of an yttria-stabilized zirconia ceramic layer ranging in thickness from 100 μm to 2 mm. A layer of metallic bond coat (often an MCrAlY-type alloy) is deposited in between the ceramic and the underlying alloy to help protect the substrate from corrosion and oxidation while bonding the ceramic to the component.

Yttria/zirconia ceramics offer extremely low coefficients of thermal conductivity, allowing them to maintain a sizeable temperature gradient; for example, between the surface of a Ni-based superalloy airfoil in a gas turbine and the heated gas that surrounds this.

TBCs are instrumental in extending the lifetime of gas turbine components due to their role in protecting the underlying Ni-based superalloy from exposure to high temperatures and oxidation.

Demand for increasingly high thrust-to-weight ratios has necessitated the development of turbines with higher working fluid temperatures. Therefore, gas temperatures in some modern engines can exceed the melting points of the Ni-based superalloys (from which their components are made) by hundreds of degrees Celsius.

The laws of thermodynamics state that eventually, every component will reach thermal equilibrium with its surroundings, even if this is coated with a high-performance insulator. This is a major issue where turbine blades are in operation surrounded by gases exceeding their melting point.

Use of TBCs can be supplemented by cooling air channels inside parts, in order to prevent these from reaching thermal equilibrium with the heated gas within the turbines during continuous operation. This cooling will increase the thermal gradient and thermal shock, however, increasing strain on the TBCs.

High performance TBCs are the only solution to meet the ongoing and increasing demand for more powerful, efficient engines suitable for operating higher temperatures. These TBCs ensure increased durability, extended lifetime and lower mass for rotor components.

Electron Beam Physical Vapor Deposition

Several methods can be employed to produce thermal barrier coatings; for example, High Velocity Oxygen-Fuel (HVOF), plasma spraying, laser chemical vapor deposition and Electron Beam Physical Vapor Deposition (EB-PVD).

EB-PVD in particular offers a number of advantages for TBCs. This method ensures a longer lifetime, making it the preferred option for high-performance applications.

EB-PVD is a physical vapor deposition process that sees an electron beam used to sublimate or vaporize atoms from an ingot of coating material.³ The beam’s energy converts ejected atoms into a gaseous phase, forming a coating on any material within line-of-sight and depositing into a thin solid layer.

EB-PVD’s key advantages when employed in the production of TBCs are related to the resulting coating’s properties, which differ from those produced by other methods. TBCs produced via EB-PVD exhibit a columnar crystal microstructure, conveying a degree of pseudo-plasticity to the material.^1,4

This pseudo-plasticity translates into improved tolerance to spalling, strain and thermal shock, ultimately ensuring a notable increase in lifetime.^5,6

Reducing Defects in Thermal Barrier Coatings Produced by EB-PVD

The quality of TBCs produced via EB-PVD will be significantly impacted by the quality of the ingot used.^7,8 Variation or inconsistencies in ingots can result in problematic coating thickness deviations known as ‘spits and pits’. 

Spits appear in cases where small droplets of liquid material are ejected from the molten pool, ultimately remaining on the ingot. Pits are the voids formed by these droplets. These defects result in an inconsistent coating which frequently incurs substantial repair costs.

Saint Gobain has developed Magma ingots after extensive consultation around customers’ challenges. These high-performance ingots are ideally suited for EB-PVD processes. They can minimize eruptions with stable vaporization, resulting in a 30-50% reduction in pits and spits.

Magma ingots exhibit extremely homogeneous morphology and chemistry, ensuring a consistent coating structure and thickness.

Magma ingots from Saint Gobain are available in a range of formulations and dimensions to accommodate any application needs.

References

1. Peters, M., Leyens, C., Schulz, U. & Kaysser, W. A. EB-PVD Thermal Barrier Coatings for Aeroengines and Gas Turbines. Advanced Engineering Materials 3, 193–204 (2001).
2. Sims, C. T. A History of Superalloy Metallurgy for Superalloy Metallurgists. in Superalloys 1984 (Fifth International Symposium) 399–419 (TMS, 1984). doi:10.7449/1984/Superalloys_1984_399_419.
3. Principles of Vapor Deposition of Thin Films. (Elsevier, 2006). doi:10.1016/B978-0-08-044699-8.X5000-1.
4. Zhang, D. Thermal barrier coatings prepared by electron beam physical vapor deposition (EB–PVD). in Thermal Barrier Coatings 3–24 (Elsevier, 2011). doi:10.1533/9780857090829.1.3.
5. Materials for advanced power engineering 1998. (Forschungszentrum Jülich, 1998).
6. Nicholls, J. R., Jaslier, Y. & Rickerby, D. S. Erosion of EB-PVD thermal barrier coatings. Materials at High Temperatures 15, 15–22 (1998).
7. Feuerstein, A. et al. Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD: A Review. J Therm Spray Tech 17, 199–213 (2008).
8. Panjan, P., Drnovšek, A., Gselman, P., Čekada, M. & Panjan, M. Review of Growth Defects in Thin Films Prepared by PVD Techniques. Coatings 10, 447 (2020).

This information has been sourced, reviewed and adapted from materials provided by Saint-Gobain Specialty Grains and Powders.

For more information on this source, please visit Saint-Gobain Specialty Grains and Powders.