Microstructural characterisation of NiWCrBSiC alloy coating produced by HVOF thermal spraying

Nickel based self-fluxing thermal sprayed alloys have been used in many applications to protect machinery parts against wear and corrosion. Although the mechanical and chemical properties of nickel based self-fluxing coatings are well known, the constituents of the microstructure have not been fully analysed. In the present work, the coatings were thermal sprayed on a 1020 steel substrate using a JP-5000 gun. Scanning electron microscopy and transmission electron microscopy techniques were employed for microstructural characterisation of the coatings, as complementary techniques to the X-ray diffraction studies already published by the authors. It has been determined that the as sprayed coatings are formed by a multiphase structure with an inhomogeneous distribution of soluble elements in an FCC gamma Ni matrix. However, a well distributed network of hard phases in the matrix has been found and these were identified as: silicides (Ni2Si), borides (W2B, B4Si, CrB41,W-5(Si,B)(3), Cr3B5) and carbides (M23C6 type carbides where metals are Cr, W, Si and Ni). Also, some other complex phases such as C0.17Fe0.81Si0.02 and Ni2.9Cr0.7Fe0.36 were found. In particular, the existence of the borides had not been reported in the literature before possibly owing to their small size, of similar to 0.31 mu m.