Studies on the role of HVOF coatings to combat erosion in turbine alloys

Abstract

Erosive wear is caused by the action of sliding or impact of solids, liquids, gases or the combination of these. Solid particle erosion is an important material degradation mechanism encountered in a number of engineering systems such as gas turbine engines, thermal power plants, coal slurry pipe lines. Erosive action results from the impact of particulates, such as coal ash, dolomite and un-burnt carbon particles on the surface of turbine. Super alloys developed for high temperature applications suffer from the drawback that the strength and the erosion-corrosion resistance at high temperature exhibited are poor. To improve the resistance property, one way would be the use of coatings on the super alloy component. Coatings take care of the problems related to erosion whereas the super alloys take care of the requirement of strength at elevated temperature. Various coatings have been highly attractive; however, High Velocity Oxy-fuel (HVOF) process, a family of thermal spray techniques uses kinetic energy of the burnt gases to soften and to propel the spray powder producing dense very low porosity, good inter-particle cohesion and well bonded coatings. In the present study successful attempts have been made to spray Stellite-6, 10%Al2O3+90%CoCrAlTaY and 25%Cr3C2-20(Ni-Cr)+75%NiCrAlY on three kinds of turbine alloys, namely Ti-6Al-4V, Co-based super alloy (Super co 605) and Fe-based special steel (MDN121).Microstructure and mechanical properties of the coatings have been characterized. Erosion tests for different conditions using Air-jet erosion test rig for different impingement angles have been studied in detail. From these studies it is observed that satellite-6 coating exhibits lower erosive rate compared to other two coating materials. The morphology of the eroded surface shows craters, groove formation in the binder matrix and carbide pull-out as the existing erosion mechanism. Higher erosion loss is observed for Al2O3+CoCrAlTaY coating. � 2018 Elsevier Ltd.