Development of New Abradable/Abrasive Sealing Systems for Clearance Control in Gas Turbines

Abstract

Abradable/abrasive sealing systems are currently used in gas turbines to reduce the blade tip gas leakage and consequently improve the turbine efficiency. The coatings selection is directly related to the section in which they are used. Seal systems for hot gas paths are primarily required to withstand high temperature. The abradable coating should be easily removed by the tip blade without causing significant blade wear, whereas the blades should have sufficient cutting capabilities. Durability properties, such as erosion resistance, are also required. Owing to their temperature capabilities, porous ceramic coatings are successfully used as abradable coatings. Although they are characterized by good abradability properties, their resistance to environmental attacks, such as solid particle erosion, is limited by the porous microstructure which negatively affects their service life. It is apparent that durability and abradability are the main targets to be simultaneously achieved for ensuring longer service life and improved efficiency. The present work is aimed at developing new abradable/abrasive coatings pairs able to ensure both the durability performances of the coatings and good abradability properties. Three ceramic abradable coatings with DVC and porous microstructure have been studied. The down-selection process has been carried out by considering the microstructure, the hardness, the tensile adhesion strength, the erosion resistance, and the furnace cycle test resistance. A composite coating made by NiCoCrAlY matrix containing abrasive grits applied by electrolytic process was selected as abrasive material system. The abrasive grits (patent application in process by GE Oil&Gas) consists of a mixture of ceramic particles. These grits ensure both short-term cutting capability and thermal stability, assuring the clearance maintenance over time. The abradability of the seal system was assessed by a properly designed test, namely Rub Rig test, which simulates the blade incursion in the abradable coating. Surface patterns on abradable coating were also considered to further enhance the abradability. Engine tests are foreseen for assessing the service behavior of this seal system.