Innovative surface treatment of turbojet and turbine engine blades

Heat engines operating on the principle of Joule-Brayton circulation are used particularly in aviation (turbojet engines) and power engineering (turbine engines) to drive electrical energy generators. Cooperation between BUT scientists and TnUAD, along with experts from PBS Velká Bíteš, has resulted in the discovery of a technical solution which will contribute to increasing engine output and service life.

Stator (guide) and rotor (circulating) blades of turbojet and turbine engines are exposed to extreme operational conditions during operation. A special protective layer spread on the outer surface of blades can minimize the effects of oxidation and corrosion. 

rotorové lopatky staré

Fig. 1: Rotor (circulating) blades of a combustion turbine after 3 years of operation.

rotorové lopatky nové

Fig. 2: New rotor blades of a combustion turbine provided with protective coat.

Both stator and rotor blades are manufactured using the technology of precise casting in shell moulds. One material frequently used is the nickel super-alloy Inconel 713LC. This is a high-strength material which tolerates high temperatures well. Unfortunately, these construction materials are less resistant to oxidation and corrosion. One of the ways of providing sufficient resistance against such effects is the application of a protective layer on the outer surface of single parts. This layer separates the materials of the flow parts from direct contact with hot combustion residues formed in the motor combustion chamber. The protective layer is consumed due to abrasive properties of hot gases; however, it protects the basic materials of the flow part. This enables the service life of relatively expensive components to be extended.

The applied layer (coat) must not influence the mechanical properties of the protected material. There exist a range of technologies for applying protective coats and here we find considerable potential for optimization. The most widely recognized coating technology is based on aluminium modified with chromium or platinum elements. The alloys are most often in the form of granulate. At high temperatures vapours develop and react, and the protective alloy (e.g. CrAl) diffuses into the basic material which results in the protective layer. Using additional heat treatment, a more advantageous phase composition of the coat and a better connection with the basic material can be achieved. The advantage of the method is the formation of a relatively even layer and thus the possibility to coat the quite complex shape components of turbines. By adjusting the parameters during coating, the thickness of the protective barrier and depth of saturation of the material with the coat can be regulated. Coat thickness ranges between 30 and 80μm, where the coat is joined to the substrate up to 2/3 of the total thickness.

The cooperating scientists Doc. Ing. Marta Kianicová, Ph.D. from the Alexander Dubček Trenčianská University in Trenčín, the scientific team at the Institute of Physical Engineering at BUT under the guidance of Prof. RNDr. Jaroslav Pokluda, CSc., alongside a further team from the Institute of Material Sciences and Engineering at BUT led by Prof. Ing. Tomáš Podrábský, CSc. working with experts from PBS Velká Bíteš have discovered and tested a technical and technological solution consisting of the use of a protective surface of nickel super-alloy which is more resistant to corrosion and oxidation. The coat structure is optimized with regard to the maximum effectiveness of the turbine. The surface of the cast is now covered in an aluminide coating with a thickness of 30 to 90μm and consists of an inner and outer sub-layer. The aluminide surface is formed through a low-active deposition process in which a chemical compound based on aluminium is transported to the surface and reacts with the base material of the cast, thus creating a protective coating. The outer layer consists in particular of a NiAl matrix, and creates a reserve of aluminium needed for the formation of protective oxide. The inner layer is also based on a NiAl matrix, but it also contains a large amount of heat-resistant elements. Elements of the flow parts of turbojet and turbine engines which are coated with this protective layer (coating) provide a higher degree of protection from aggressive combustion products.

This method of protection from aggressive combustion products is protected by the IPO CZ as a utility model: “Structure of Flow Parts of Turbojet and Turbine Engines”.

Source of main photo: Nabonaco, Wikimedia Commons, 2008

Photo in the text: Prof. Ing. Tomáš Podrábský, CSc., 2013

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