Microstructural characterisation of NiWCrBSiC alloy coating produced by HVOF thermal spraying

Abstract

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 γ 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, W₅ (Si,B)₃, Cr₃B₅) and carbides (M₂₃C₆ type carbides where metals are Cr, W, Si and Ni). Also, some other complex phases such as C_0·17Fe_0·81Si_0·02 and Ni_2·9Cr_0·7Fe_0·36 were found. In particular, the existence of the borides had not been reported in the literature before possibly owing to their small size, of ∼0·31 μm.