Metallic honeycombs are widely used in gas turbine engines as inner and outer abradable gas path seals. The ever increasing gas temperatures encountered in the high and low pressure turbine modules of modern engine designs challenge the durability of the thin foil metals used to fabricate seal type honeycomb. In this paper the performance of a number of alloys in turbine seal applications is reviewed. Emphasis is placed on resistance to hot gas corrosion attack and microstructural integrity after exposure to elevated temperatures. The abradability of fabricated seal structures under two distinctly different rub test conditions is reviewed. Among the alloys considered, the Fe-Cr-Al-Y alloy MI 2100 offers a potentially superior combination of oxidation resistance, abradability, fabricability and material cost for seal honeycomb applications.
Three abradable gas path seal material systems based on a sintered NiCrAlY fibermetal structure were evaluated under a range of wear conditions representative of those likely to be encountered in various knife-edge seal (labyrinth or shrouded turbine) applications. Conditions leading to undesirable wear of the rotating knife were identified and a model proposed based on thermal effects arising under different rub conditions. It was found, and predicted by the model, that low incursion (plunge) rates tended to promote smearing of the low density sintered material with consequent wear to the knife-edge. Trade-off benefits between baseline 19 percent dense material, a similar material of increased density, and a self-lubricating coating applied to the 19 percent dense material were identified based on relative rub tolerance and erosion resistance.
Abradable seals are typically used in compressors of aircraft and industrial gas turbines to decrease clearance between the stator casing and rotor blade tips, and hence, to increase compressor efficiency and decrease fuel consumption. The abradable seal concept originally developed for aircraft gas turbine applications is presently applied to other types of rotary equipment such as steam turbines and turbochargers. Among the most important variables influencing the choice of abradables for a particular section of a gas turbine are: application temperature, material of the rubbing element (titanium, nickel or iron alloy), the rubbing element geometry (blade, knife edge), seal durability (aircraft engines are refurbished more often than land based turbines), and of course the installed cost. In order to satisfy all the seal requirements, the chemical composition and structure of an abradable seal have to be carefully designed and controlled. The main challenge is to optimize the contradictory requirements such as durability and abradability. The process for choosing the best material starts with identifying the main variables and choosing the most suitable systems to fulfill all the requirements. This choice must be confirmed by a series of laboratory tests that simulate the actual application. The tests, test equipment and test results on a new seal system will be described to demonstrate the influence of all important parameters on seal performance. A new generation of abradable seals that, for example, can rub against titanium alloy blades at any temperature in the titanium alloy application range, will be presented. Emphasis will be put on the seal performance in “aged” conditions, i.e. after exposure to air at high temperatures for prolonged periods of time. Such results are necessary to evaluate the seal performance after extended service in the gas turbine when rubbing can still occur.
Analytical and experimental research was conducted to evaluate a ceramic seal system for a high-pressure turbine employing plasma-sprayed graded metallic ceramic, yttria-stabilized zirconium oxide (YSZ). The performance characteristics of several YSZ configurations were determined through rig testing for thermal shock resistance, abradability and erosion resistance. Results and test data acquired from this work indicate that this type of sealing system offers the potential to meet the operating requirements of future gas-turbine engines. However, continued development and refinement of this technology, particularly in the area of improving cyclic thermal stress tolerance, is necessary.
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