On The Potential Of Metal And Ceramic Based Abradables In Turbine Seal Applications.
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Abradable seals are employed in turbomachinery to reduce leakage gaps between stationary and rotating parts to improve efficiency and stall margin. Thermally sprayed abradable seals have been used in the compressor section of jet engines since the late 1960s. These seals are predominantly coatings made from composite material that derive their abradability from the use of low shear strength materials or form a porous, friable coating structure. Due to the extreme temperatures on the turbine side of gas turbine engines, brazed or welded metallic structures made from nickel (Ni) based alloys have typically been used to provide gas path sealing. When thermal shock resistant ceramics because available, oxide ceramic, mostly zirconia based high temperature abradable seals for the high pressure turbine stages were developed. For the highest temperature stages, thermally sprayed oxide ceramics with or without metallic strain isolators are frequently used. On stages with reduced temperatures both ceramic and metallic coatings of the McrA1Y type can be used. This paper reviews the state-of-the-art in compressor and turbine clearance control materials and systems, whereby a focus is placed on novel thermally sprayed ceramic turbine seals with encouraging property combinations, which are achieved by the introduction of alternative stabilizers. Normally ceramic seal surfaces require hard tipping of the rotor blades to allow them to cut properly and suitable hard tipping solutions will be presented. However, examples will be given of recent developments aimed at developing ceramic abradables that can be but by untipped blades. Examples will be given. Finally, the paper gives an overview of cuttability of thermally sprayed metallic turbine seal coatings by tipped and untipped blades at high temperature.
Sporer, Dieter; Wilson, Scott; Giovannetti, Iacopo; Refke, Arno; Giannozzi, Massimo (2007). On The Potential Of Metal And Ceramic Based Abradables In Turbine Seal Applications.. Texas A&M University. Turbomachinery Laboratories. Available electronically from