SUBSTRATE COATING INTERFACE STRUCTURE OF WC-CO HIGH-VELOCITY OXYGEN FUEL SPRAYED COATING ON LOW-ALLOY STEEL/

The microstructures developing at the interface of a high velocity oxy gen fuel (HVOF) splayed WC-Co coating on a medium C 1%Cr steel substra te have been studied by preparing thin foils from the interfacial legi on and examining them by transmission electron microscopy. A scanning transmission electron microscopy system has also been used to determin e chemical changes adjacent to the interface. It has been found that o nly partial melting of the steel occurs to a depth of similar to 3-5 m u m, but this enables good bonding to develop with the liquid phase in the coating and also with the solid WC. Nd transfer of Co to the Fe o r Fe to the Co has been found. At a depth of 3-15 mu m below the Inter face, low C martensite develops and structural changes are still visib le some 50 mu m into the substrate when partial dissolution of cementi te occurs. The results obtained are discussed in relation to factors l ikely to account for the known good adhesion between HVOF splayed WC-C o coatings and steel substrates. Parallel modelling studies on the hea r transfer from an impinging splat to a steel substrate have been used to verify the validity of the modelling results in relation to the mi crostructural observations. Good agreement has been found.