Background In aero-engines, the blade of the high pressure turbine was for a long time the highest of the high technology in the aero gas turbine, and despite the complexity of the modern fan blade, the challenge it provides does not reduce. The ability to run at increasingly high gas temperatures has resulted from a combination of material improvements and the development of more sophisticated arrangements for internal and external cooling (figure 1).
Figure 1. Schematic of a gas turbine engine.
Modern AlloysA modern turbine blade alloy is complex in that it contains up to ten significant alloying elements, but its microstructure is very simple. The structure is analogous to an `Inca wall', which consisted of rectangular blocks of stone stacked in a regular array with narrow bands of cement to hold them together.
In the alloy case the `blocks' are an intermetallic compound with the approximate composition Ni3(Al,Ta), whereas the `cement' is a nickel solid solution containing chromium, tungsten and rhenium.
SuperalloysSuperalloys have always contained phases of this type, but in recent years the titanium in the original intermetallic has been replaced by tantalum. This change gave improved high temperature strength, and also improved oxidation resistance. However, the biggest change has occurred in the nickel, where high levels of tungsten and rhenium are present. These elements are very effective in solution strengthening.
Since the 1950’s, the evolution from wrought to conventionally cast to directionally solidified to single crystal turbine blades has yielded a 250°C increase in allowable metal temperatures, and cooling developments have nearly doubled this in terms of turbine entry gas temperature. An important recent contribution has come from the alignment of the alloy grain in the single crystal blade, which has allowed the elastic properties of the material to be controlled more closely. These properties in turn control the natural vibration frequencies of the blade.
If metallurgical development can be exploited by reducing the cooling air quantity this is a potentially important performance enhancer, as for example, the Rolls-Royce Trent 800 engine uses 5% of compressor air to cool its row of high pressure turbine blades. The single crystal alloy, RR3000, is able to run about 35°C hotter than its predecessor. This may seem a small increase, but it has allowed the Trent intermediate pressure turbine blade to remain uncooled. Continuing Developments It is estimated that over the next twenty years a 200°C increase in turbine entry gas temperature will be required to meet the airlines' demand for improved performance. Some of this increase will be made possible by the further adoption of thermal barrier coatings. These coatings are produced from ceramic pre-cursors and have the potential to contribute about 100°C through the protection they provide.
Thermal Barrier CoatingsThermal barrier coatings have been used for some years on static parts, initially using magnesium zirconate but more recently yttria-stabilised zirconia. On rotating parts, the possibility of ceramic spalling is particularly dangerous, and strain‑tolerant coatings are employed with an effective bond coat system to ensure mechanical reliability.
Ceramic Matrix CompositesFurther increases in temperature are likely to require the development of ceramic matrix composites. A number of simply shaped static components for military and civil applications are in the engine development phase and guide vanes have been manufactured to demonstrate process capability, such techniques involve advanced textile handling and chemical vapour infiltration.
However, it is the composite. ceramic rotor blade that provides the ultimate challenge. It will eventually appear because the rewards are so high, but it will take much longer to bring it to a satisfactory standard than was anticipated in the 1980’s. Research work has concentrated for some years on fibre reinforced ceramics for this application, as opposed to monolithic materials which possess adequate strength at high temperatures but the handicap of poor impact resistance.
Today's commercially available ceramic composites employ silicon carbide fibres in a ceramic matrix such as silicon carbide or alumina. These materials are capable of uncooled operation at temperatures up to 1200°C, barely beyond the capability of the current best coated nickel alloy systems. Uncooled turbine applications will require an all oxide ceramic material system, to ensure the long term stability at the very highest temperatures in an oxidising atmosphere. An early example of such a system is alumina fibres in an alumina matrix. To realise the ultimate load carrying capabilities at high temperatures, single crystal oxide fibres may be used. Operating temperatures of 1400°C are thought possible with these systems.