The gas turbine
Figure 1: The Gas Turbine (Source: NPL Publications, issue 11)
For materials engineers, modern aircraft engine design and materials selection has been an extremely challenging area. Gas turbine engines, also called jet engines, work on the principle that air passes into the turbine, is compressed, mixed with fuel and is then ignited. The gas mixture is then ejected from the rear of the engine after passing though the turbine stage. The gas enters through a number of rows of blades called stages, which turn the gas stream and may also accelerate it. With heat engines, the efficiency is related to the maximum and minimum temperatures in the cycles. Currently today’s engines are running at temperatures of the order of 1350°C, which is achievable due to blade cooling introduced in the 1960s. However the increasing temperature requires new measures and technology to sustain reliability and safety.
Materials Used and Temperature Resistance
The majority of gas turbines are made from nickel-based alloys, however current running temperature of the gas turbine (1350°C) is often in excess of the melting point of these Nickel alloys (1200~1315°C)! In order to overcome this problem, two main advancements have been adopted. One is sophisticated cooling of the blades, using air that bypasses the combustion chamber after the compressor, and the second is low thermal conductivity coatings on the surface of the blade.
Gas turbines obviously require protection to prevent the nickel alloy melting at high temperature but also require protection to prevent corrosion at high temperature. Corrosion is defined as the unwanted reaction of a material that results in the dissolution or consumption of the material e.g. the rusting of iron. Corrosion is accelerated by high temperature and impurities present in the air due to the combustion of fuel in the engine. Therefore coating the gas turbine will protect against both kinds of attack i.e. high temperature and corrosion.
Coating Technology
The original means of protecting nickel-based alloys that are used in gas turbine blades against high temperature oxidation and corrosion has been aluminising. This involves diffusing aluminium on to the surface of the blades to form a protective oxide layer, alumina. Since 1970, most gas turbine blade coatings have been applied by pack cementation and more recently by chemical vapour deposition.
Nickel coatings containing chromium (Cr), aluminium (Al) and yttrium (Y) called “NiCrAlY Coatings” have emerged to combat high temperature corrosion and oxidation. A hot corrosion resistant MCrAlY (M = metal) based bond coating containing 18% chromium, 22% cobalt, 12% aluminium and 0.5% yttrium has been developed for use in gas turbine engines. These coatings exhibit maximum life in corrosive environments. The reason for the improved durability of the coatings is due to the formation of a thick, protective, and chemically stable alumina scale on the surface upon exposure to the environment.
The addition of yttrium to the coating is to increase the adherence of the oxide layer to the substrate (base Ni alloy). NiCrAlY coatings can be applied by a variety of techniques including vacuum plasma spraying, low pressure plasma spraying, air plasma spraying, argon shrouded plasma spraying, high velocity oxygen fuel spraying, vapour phase deposition and electron beam physical phase deposition.
Thermal Barrier Coatings (TBC) aim to prevent extreme high temperature. MCrAlY coatings are being used in combination with ceramic (zirconia, ZrO2) coatings, where the MCrAlY is acting as a bond coat for the ZrO2 coating.
These coatings are used to extend the life of metal components by creating a temperature drop across the coating, permitting the underlying metal to operate at a reduced temperature. Future gas turbines will use TBC technology to permit the simultaneous increase of turbine inlet temperature and the reduction of turbine cooling air, thereby increasing efficiency. The properties of zirconia most critical for TBCs are a very low thermal conductivity and a thermal expansion close to that of superalloys.
Research and development
Research in the area of extending life times of coatings, and stronger, more heat resistance coatings that can give longer lifetimes to failure are key areas of research. Electrodepositing or electroplating coatings have been developed over the last 20 years. Electroplating allows co-deposition of elements with improved properties and is extremely cost effective. This is one of the main driving forces for the technology. Co-deposition is a technique used to produce composite coatings by embedding small particles into the metal matrix during electrodeposition. Many different submicron powders like alumina, silicon carbide and chromia added to the metal have decreased the corrosion rate. The resistance of a coating to high temperature oxidation and corrosion depends on its structure and composition, and on its interaction with the metal matrix and the corrosion environment.
Advances made in the processing of coatings have been aimed at improving the corrosion resistance, mechanical properties and wear resistance of the materials. Wear resistance has been improved by including solid particles like carbides, oxides and diamond.
The incorporation of nanosized particles (<0.1μm), is another exciting field being investigated, to see the effect on grain size and microstructure and hence the properties of the materials.
It is continuing research and investigation that allows advances to be made and jet engines/gas turbines to run at higher temperature with better fuel efficiency.
Surface Science and Materials Science
Coating technology and the development of coatings comes under a general heading of surface science and is concerned with fundamental and applied research of the surface properties and processes on solid state materials. The Materials and Surface Institute (MSSI) within the University of Limerick is devoted to such research.
Coatings are important far beyond gas turbines. For example, projects that include the application of radar absorbing materials, thermal protection materials, IR and Sonar absorbing materials. Also high technical applications such as the Space Shuttle main engine fuel pumps, where an entire thermal protection coating is required. This involves surface preparation, application of new thermal protection barriers, curing the thermal materials, and testing.
A brief history of the gas turbine is available at NASA website.
Dr Lisa O’Donoghue
Dr. O’Donoghue is a Ph.D and holds a first class honours degree in Materials Science and Technology and a doctorate in High Temperature Technology in Aero Gas Turbines Engine applications. She is currently a Senior Research Fellow at the University of Limerick.
