High Performance Thermal Barrier Coatings for Aerospace Applications
Thermal Barrier Coatings (TBCs) can significantly improve the lifetime and efficiency of aero engines. TBCs can be deposited via different methods – but for high-performance applications, Electron Beam Physical Vapor Deposition (EB-PVD) is preferred. In this article, we examine the benefits offered by EB-PVD technology for aerospace applications.
The Importance of Thermal Barrier Coatings for Aerospace Components
The rotor blades and stator vanes that make up high-pressure gas turbines are among the most highly loaded engineering components in the world.1 Under operating conditions, gas turbines reach tens of thousands of revolutions per minute: Each blade is subjected to enormous forces, and extracts a colossal amount of energy from the pressurized gas stream. The mechanical demands on these components are such that, typically, among the metallic materials only high-temperature Ni-based "superalloys" can be considered.
Nickel superalloys are, by definition, capable of operating at a high fraction of their melting temperature.2 The Nickel-based superalloys used for aerospace applications are, structurally, some of the toughest materials available, capable of withstanding tremendously high loads at elevated temperatures and withstanding corrosion.
However, in modern gas turbines, hot gas temperatures can approach the melting points of Ni-base alloys. This leaves components prone to creep over extended use, as well as drastically increasing oxidation and thermal fatigue. The most widespread approach to prevent Ni-based superalloy gas turbine components from thermally induced failure is to use Thermal Barrier Coatings (TBCs). As their name suggests, TBCs are coatings that are capable of preventing components from reaching extremely high temperatures.
TBCs for aerospace typically comprise an yttria-stabilized zirconia ceramic layer, which can range in thickness from 100μm to 2 mm. In between the ceramic and the underlying alloy, a layer of metallic bond coat (often an MCrAlY-type alloy) is deposited to further protect the substrate from oxidation and corrosion and to bond the ceramic to the component. The extremely low coefficients of thermal conductivity offered by yttria/zirconia ceramics mean that they are capable of maintaining a large temperature gradient between the surface of, for example, a Ni-based superalloy airfoil in a gas turbine and the heated gas surrounding it.
By protecting the underlying Ni-based superalloy from exposure to high temperatures and oxidation, TBCs play an essential role in increasing the lifetime of gas turbine components.
Demand for ever higher thrust-to-weight ratios requires the development of turbines with even higher working fluid temperatures. As a result, in some modern engines, gas temperatures can actually exceed by hundreds of degrees Celsius the melting points of the Ni-based superalloys from which their components are made.
The laws of thermodynamics tell us that any component – even if it's coated with a high-performance insulator – will eventually reach thermal equilibrium with its surroundings. This is, obviously, bad news for turbine blades surrounded by gases that exceed their melting point. In order to prevent them from ever reaching the same temperature as the heated gas within the turbines over sustained operation, TBCs can be assisted by cooling air channels inside of the parts. This cooling however increases the thermal gradient and thermal shock the TBCs must withstand.
Increasing demand for more efficient and powerful engines which run at higher temperatures can only be facilitated by high performance TBCs that can provide better durability, longer lifetime and lower mass for rotor components.
Electron Beam Physical Vapor Deposition
Thermal barrier coatings can be produced via several methods; including plasma spraying, High Velocity Oxygen-Fuel (HVOF), laser chemical vapor deposition and Electron Beam Physical Vapor Deposition (EB-PVD). The latter, in particular, offers many advantages for TBCs – not least providing a longer lifetime – and as a result it is preferred for high-performance applications.
EB-PVD is a type of physical vapor deposition process whereby an electron beam is used to sublimate or vaporize atoms from an ingot of coating material.3 Ejected atoms, which are transformed into a gaseous phase by the beam's energy, form a coating on any material held within line-of-sight, depositing into a thin solid layer.
The advantages of EB-PVD for producing TBCs come down to the properties of the resulting coating itself, which vary from those produced by other methods. TBCs produced by EB-PVD have a columnar crystal microstructure, which imparts a certain pseudo-plasticity to the material.1,4 This in turn translates into better tolerance to spalling, strain and thermal shock, yielding a significant increase in lifetime.5,6
Reducing Defects in Thermal Barrier Coatings Produced by EB-PVD
The quality of TBCs produced by EB-PVD is highly dependent on the quality of the ingot used.7,8 Inconsistencies or variation in ingots can lead to coating thickness deviations or "spits and pits". Spits occur when small droplets of liquid material eject from the molten pool and remain on the ingot while pits are voids created by these droplets. These defects produce an inconsistent coating which can be very costly to repair.
By listening to our customers' challenges, Saint Gobain developed Magma ingots, a high-performance ingot for EB-PVD processes that minimizes eruptions with stable vaporization leading to a 30-50% reduction in pits and spits. Magma ingots also have highly homogeneous chemistry and morphology, giving a highly consistent coating structure and thickness.
Saint Gobain Magma ingots are available in a variety of formulations and dimensions to suit all applications. To find out more about Saint Gobain's ingots for thermal barrier coatings, visit our website or get in touch with us today.
References and Further Reading
- 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).
- 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.
- Principles of Vapor Deposition of Thin Films. (Elsevier, 2006). doi:10.1016/B978-0-08-044699-8.X5000-1.
- 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.
- Materials for advanced power engineering 1998. (Forschungszentrum Jülich, 1998).
- Nicholls, J. R., Jaslier, Y. & Rickerby, D. S. Erosion of EB-PVD thermal barrier coatings. Materials at High Temperatures 15, 15–22 (1998).
- 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).
- 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).