Microstructure-Property Relationships and Cross-Property Correlations of Thermal Sprayed Ni-Alloy Coatings

Abstract

Abstract Relationships between the properties of thermally sprayed nickel based alloys and their microstructure (density, porosity, oxide phase content) are investigated. Cross-property- correlation of physical properties such as electrical conductivity and elasticity were examined. The experimental results of the structures and properties of the different coatings are discussed with respect to their pore surface area obtained by small angle neutron scattering (SANS) measurements. As wide as possible range of thermally sprayed microstructures of NiCr and NiCrAlY was produced by vacuum - and atmospheric plasma spraying, flame spraying, HVOF and water stabilized plasma spraying. Commercially available powders with process specific grain size distributions were used as feedstock materials resulting in a wide range of microstructures of the coatings depending on the spraying technique and, to much less extent, on the variation of the process parameters. In this work the examination of the pore structure was carried out by optical microscopy on metallographic cross sections. Phase composition and distribution were investigated by hot gas extraction for oxygen and nitrogen determination and by Scanning Auger Microscopy on polished cross sections and fracture surfaces. The properties of the coatings were characterized by their wear (ASTM G75) behavior, reflecting application-oriented properties. Significant and varying amount of anisotropy of the coating properties - electrical conductivity and elastic modulus - was found in the sprayed microstructures. This anisotropy could be directly linked to microstructure anisotropy as characterized by Small-Angle Neutron Scattering.