Inhaltsverzeichnis
5.2.1 Trends in material and production technologies
New technologies promise improvements and schould improve the safety. But, do they keep this promise?
They are indeed exorbitant expensive, but give absolute functional reliability even under highest loads, the new brake discs made from fibre-reinforced ceramics.
Extreme pressure fuel injecton pumps can very appealing reduce the fuel consumption. But, what about the abrasion when even a water jet can cut hard materials?
Headlights that can illuminate around the corner helping us to avoid accidents in the night. Do they justify a new car?
Tyres that can not get flat, but what about the driving comfort and durability?
The sensor for the tyre pressure, a super feature. Will it suffer in splash water under contamination and corrosion? Doesn’t this pre-programme an early failing? Where there not enough malfunctions with the already usual monitoring probes?
The gas turbine presents the owner/operator with comparable questions. The satisfactory answer should be expected from adequate operation experience. But can we trust the OEM’s information and holy oaths? How about the fine Swabian maxim: ”…never do something as the first off the mark…”. Unfortunately it’s so not possible to profit at the highest possible benefits by new developments.
The proof of positive operation relevant experience decides on the acceptance of a new technology. This include for stationary gas turbins the following examples. In order to reduce the cooling air to increase the overall efficiency of the engine, thermal barrier coatings on the hot gas side are applied ( "Ill. 3.2.3-4"). New engines show these coatings not only on the combustor wall and the guide vane shrouds, but also on the airfoils of the high pressure turbines rotor blades. If these coatings are damaged (e.g., through erosion, "Ill. 3.2.3-8") or if it comes to blistering (e.g., because of oxidation underneath), such activities lead understandably to an early component failure. Here also, the proof of positive relevant operation experience decides the acceptance.
Hot parts are produced out of permanently more heat resistant materials. One attains them through new alloys and /or micro structures, created through especial manufacturing processes. This includes so called directional solidified structures, by which the failure sensitive grain boundaries are parallel and related to the main load, as well as the grain boundary free single crystals ( "Ill. 3.3-4"). Components with directional solidified structure, with bigger thermal strain directed across the grain boundaries, tend to a wooden type of failure picture where the grain boundaries split ( "Ill. 3.3-17"). Technical single crystals are not to be mistaken for the single crystals from the chip production. They can show not harming inner structural defects and inhomogenity, typical for a grain. The component has merely no grain boundaries. Such single crystals comprise especially suitable material variations and have the advantage of high creep stability, good oxidation resistance and good thermal fatigue behavior. Disadvantages against blades of conventional materials could be their high price and a limited repair aptitude (regeneration, heat treatment). To the high-temperature materials belong also monolithic ceramics which are already used today in combustor linings and tiles ( "Ill. 3.2.1-4" and "Ill. 5.2-2", Lit. 5.2-4).
In the following, examples for new technologies are described whose applications still wait.
Even more exotic materials are in the development stage. This includes dispersion hardened materials, where the plastic deformation under stress (creep) is hindered through fine, dispersed ceramic particles. In these materials, the processing as well as the machining creates special problems. One can presume a distinct brittleness compared to conventional materials.
There is a strengthened interest in recent years for the material family of the inter metallic phases. Such metallic materials have a special, strong composition, defined by the micro structure. For technically interesting combinations, Ni Al and Ti Al this means, e.g., that one half each of nickel and aluminum atoms respectively, titan and aluminum atoms are involved. In engineering, they have emerged until now mostly as undesired concomitants like, e.g., the embrittling sigma phase in high alloyed steels. Now one has recognized their, nevertheless, practical usable features like high thermal resistance. Problematic is, however, the distinct, alloy dependent brittleness up to temperatures of some 100°C. Experience must still show how far technical usage is limited through that.
Completing, one has to point to the usage of monolithic (compact) ceramic materials and fiber strengthened ceramics. One deals in the first place with Si3 N4 and SiC. Precisely in industry gas turbines, advantages through saving of cooling air, through high erosion - and good chemical resistance are expected. A problem is the proverbial brittleness that leads to catastrophic failure through overstress or shock load (Lit. 5-1, "Ill. 5.2-1"). Reservations against the use of such materials can only be reduced through sufficient experience in the development stage. Efforts in Japan are to be seen in this light, for instance. Over there, ceramic turbine guide vanes are being developed for a larger industrial gas turbine. Work on monolithic, (one piece and homogeneous) ceramic guide vanes and rotor blades is well known in the U.S.A. Further components in development are combustor linings and tiles ( "Ill. 3.2.1-4") made out of fiber strengthened and /or monolithic ceramic, as well as gas ducts and heat shields. Fiber strengthened ceramic of a pseudo ductility, i.e., a crack does not lead to rapid crack propagation and spontaneous failure. To demonstrate this, a nail can be driven through such material without damage. The disadvantage of these materials is, however, the strength and the oxidation damage. Indeed there are effective surface coatings, however, when there is a damage there follows inner oxidation. Long life use under temperatures clearly over 800° C, such as industrial gas turbines demand are, currently at least, problematic.
"Illustration 5.2-1": The implementation of so called monolithic ceramics (homogenous ceramic material like sintered silicon nitride or silicon carbide) is intensively examined in near service tests on high pressure turbine guide vanes. An advantage of this material as opposed to metal alloys is the heat resistance of relatively high material temperatures. Operation temperatures of around 1400° C over long periods of service, necessary for employment in industrial gas turbines appear possible. One tries similarly focused use of high oxidation resistance and endurance against some typical aggressive gas impurities.
The high pressure turbine guide vane as stationary component is subject to particularly high operation temperatures ( "Ill. 3.2.3-2" and "Ill. 3.3-9") and needs an extremely large amount of cooling air
The first developments took place in Germany by W.Krueger et.al. Today one pours forth intensive efforts in Japan, regarding the use of a 20 MW industrial gas turbine. A standard type of operational use does not seem to be imminent. The assembly of this type of turbine guide vanes, according to the hybrid principle, consists, in contrast to the full ceramic components in engines of small performance, of the combination of cooled metal parts (e.g., car turbines) with ceramic components (C). The metal structure takes the operation stresses (gas bending loads) and thermal expansion differences of the ceramic blade foils (A). They balance out thermal expansions and lead the forces into the casing structure. Between the cooled metal center (C) and the blade foil (A) one more radiation protection (B) can be placed.
"Illustration 5.2-2": (Lit 5-4): The high power density, respectively thermal load of modern combustion chambers of heavy frame gas turbines ( "Ill. 2.1-7"), need highly heat resisting materials if an increased consumption of cooling air should be avoided. This picture shows an ‘open combustion chamber concept’ in which the cooling air/ sealing air exits between the supporting metallic wall of the combustion chamber and heat shield (tiles, detail above).
The tiles consist of alumina and silica and are fixed with spring elements at the supporting metallic wall ( "Ill. 3.2.1-4"). So the thermal expansion can adjust. The operation temperatures of the tile surface reach 1500°C. Such combustion chambers are obviously in service.
Literatur of chapter 5.2
5.2-1 A.Rossmann, „Schadenuntersuchung und Schadenverhütung an Bauteilen der Ingenieurkeramik“, Expert Verlag, Schadenskunde im Maschinenbau, AE Kontakt & Studium Band 308, Page 76-96.
5.2-2 N.Bolt, „Kosteneffiziente Forschungsergebnisse für Gasturbinenbetreiber“, VGB Kraftwerkstechnik 76 (1996), Heft 6, Page 471-475.
5.2-3 T.Hansen, R.Smock, „Gas turbines aim at world power market dominance“, Zeitschrift „Power Engineering International“, http://pe.articles.clickability.com, 12.08.2008, Page 1-9.
5.2-4 J.Hellat, A.Eroglu, W.Krebs, „Technische Verbrennungssysteme“, Kapitel in C.Lechner, J.Seume, „Stationäre Gasturbinen“, Springer Verlag Berlin Heidelberg New York, ISBN 3- 540-42831-3, 2003, Seite 466-480.Literature to Chapter 5.2