Coatings Critical to High-Performance Engines
State of the Aerospace IndustryThere is a strong trend to upgrade the current engines used on existing airframes. Some engine designs have seen efficiencies improve anywhere from 10-40% in recent years with a great benefit of higher thrust and reduced emissions. If less fuel is needed for the same flight, weight will be reduced and efficiencies improved. The need for higher profitability for the airlines is forcing jet-engine change.
Today’s push for higher inlet blade temperature is outstripping the capability of these uncoated higher-temperature-alloy materials. As an example, a coated and uncoated blade run for 2,500 hours of service in low-altitude ocean operation shows substantial blade oxidation (Fig. 1). Even with the improved high-performance alloy, the blades will see surface oxidation and eventual failure.
Continuous ImprovementThe real question that needs asking is how can we make such substantial increases in efficiency and power output with reduced emissions while still using the same engine mounting frame? The answer is simple. Raise the turbine-gas firing temperature and gas velocity to improve the amount of power released. Today’s high-performance aerospace jet engines typically reach gas inlet temperatures up to 3000oF, and land-based turbines usually go up to 2700oF.1
The limitation in raising the engine firing temperature above these current operating temperatures is basic metallurgy. When we increase the temperature and the force on the blades and vanes, the internal stresses of the blades and the potential for creep and oxidation will substantially increase. These higher stresses cause the traditional jet-aircraft engine components to fail sooner than the design life of the original blades at these higher firing temperatures. This increase in firing temperature is good for the industry, but without improvements in metallurgy, cooling and coating technology, a significant reduction in engine life would result. Some of the newest engine designs have target turbine-inlet temperatures as high as 3600oF (Fig. 2).
Blade Surface CoatingsWhat is the solution that allows blades to withstand these higher gas temperatures? There are two commonly used platinum aluminide ceramic coatings primarily made up of Nickel/Aluminide materials (NiAl or Ni2Al3) as a base coat and a TBC (Thermal Barrier Coating), or top coat, which provide the oxidation and corrosion resistance needed to withstand the severe jet-engine operational temperatures. This platinum aluminide ceramic coating is commonly called a bond coat, and it must be uniformly applied to the blade’s external surface and the internal-cooling gas paths to maximize the protection the coating provides.
The key to this uniform coating is very precise control of the gas flow and timing at various temperatures. Optimizing the coating requires correct gas flow with mass-flow control valves and a computerized control system to repeatably achieve the required coating thickness and uniformity. It is critical to optimize coating uniformity and bonding properties to meet or exceed the required quality measurements. When this process is properly completed, the quality measured routinely exceeds the quality standards as set by the engine manufacturer.
Surface Coating 101Any uncoated engine blade surface exposed to the hot combustion gas will oxidize/corrode rapidly. An additional thermal-protection coating is applied on top of the platinum aluminide coating for maximum thermal protection. This top coat – TBC as it is commonly called – is a thin layer of metal oxide. It is very critical to properly bond all of these layers together to the high-temperature, nickel-based-alloy turbine blade.
The platinum aluminide process is accomplished in a number of ways by different engine manufacturers. One method involves the following steps:
- Platinum plating of the nickel-based turbine blades
- Post-plating – Diffusion-bond heat-treat cycle in a vacuum furnace
- CVD platinum aluminide coating process in a retort argon-atmosphere furnace
- Post platinum aluminide – Diffusion-bond heat-treat cycle in a vacuum furnace
- Multi-layer/pass PVD coating process in an EB/PVD (Electron Beam Physical Vapor Deposition) vacuum top coater
- Post top coat – PVD diffusion-bond heat-treat cycle in a vacuum furnace
Surface-Coating EquipmentThe equipment used to apply each of these plating/coating processes is first the cleaning/plating line that plates the nickel turbine blades (Fig. 4). The next step in the platinum aluminide process is the use of a high-temperature/purity, heavy-duty, 2-bar vacuum furnace frequently used in the aerospace industry such as that manufactured by G-M Enterprises (Fig. 5). This is followed by the use of a CVD (Chemical Vapor Deposition) electrically heated retort (model SAR) furnace with high-performance argon-gas mass-flow control, computerized temperature and process control (Fig. 6). The last piece of manufacturing equipment needed to complete the platinum aluminide/top coat process is a EB/PVD coater as manufactured by ALD of Dusseldorf, Germany (Fig. 7).
ConclusionThe aerospace industry is in a major growth mode, and with the more recent extensive improvements in platinum aluminide coating technologies, we expect continued long-term growth. The trend of increasing turbine inlet temperatures is expected to continue since the benefits of increased thrust, efficiency and reduced emissions are so great to the aircraft industry and their customers. You should expect continued success of the industry and the heat treating and coating of these high-performance jet-engine blades. IH
For more information: Robert M. Huckins is national sales manager for G-M Enterprises, 525 Klug Cir., Corona CA 92880; tel: 951-340-4646; fax: 951-340-9090; web: gmenterprises.com. He can be reached at: tel: 630-762-1750; fax: 630-762-1752
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrial heating.com: surface oxidation, creep, thermal barrier coating, electron beam physical vapor deposition, chemical vapor deposition