Some effects of structure and composition on the properties of electron beam vapor-deposited coatings for gas turbine superalloys II. The origin of defects

Abstract

The advent of the gas turbine engine has imposed serious materials problems with respect to oxidation and hot corrosion. One solution has been the application of multi-element electron beam overlay coatings of the MCrAlY type. However, state-of-the-art overlay coatings inherently exhibit such structural imperfections as spits, flakes and linear defects (leaders). This paper presents the results of the continued investigation to determine the origin of these imperfections. Microprobe and laboratory data are presented which characterize these defects. Spits were found to consist of ejected pool material, exhibiting enrichment in impurity elements of low vapor pressure. Investigation indicates spitting may be the result of non-metallic (carbide or oxide) particles within the melt, which upon superheating initiate ejection of pool material. Flakes, generally cone shaped, were found to originate at non-metallic particles loosely attached to the surface. Leader formation was found to be weakly dependent upon surface finish (to 100 AA) and strongly dependent upon the angle of incidence of the arriving vapor flux. Both flakes and leaders seem to be enhanced by preferential growth and shadowing phenomena.