Substrate/coating interface structure of we-co high velocity oxygen fuel sprayed coating on low alloy steel

The microstructures developing at the interface of a high velocity oxygen fuel (HVOF) sprayed WC–Co coating on a medium C 1%Cr steel substrate have been studied by preparing thinfoilsfrom the interfacial region and examining them by transmission electron microscopy. A scanning transmission electron microscopy system has also been used to determine chemical changes adjacent to the interface. It has beenfound that only partial melting of the steel occurs to a depth of ∼3–5 μm, but this enables good bonding to develop with the liquid phase in the coating and also with the solid WC. No transfer of Co to the Fe or Fe to the Co has beenfound. At a depth of 3–15 μm below the interface, low C martensite develops and structural changes are still visible some 50 flm into the substrate when partial dissolution of cementite occurs. The results obtained are discussed in relation to factors likely to accountfor the known good adhesion between HVOF sprayed WC–Co coatings and steel substrates. Parallel modelling studies on the heat transfer from an impinging splat to a steel substrate have been used to verify the validity of the modelling results in relation to the microstructural observations. Good agreement has beenfound.

MST/3038