Elaboration and characterization of composite coatings in NiFeCrBSi- WC, applied to drilling tools

Abstract

Low alloy steel is commonly used in the manufacture of drilling bits and this to ensure good toughness to these parts. Since drilling bits are exposed to harsh working conditions, they are often coated to reduce wear and corrosion. This work was carried out with the aim of examining the behavior of two types of nickel-based cermets (ie NiFeCrBSi-WC), using them as coatings, to further improve the surface properties of an ordinary X18 steel namely the electrochemical and anti-wear properties.The choice of the coating process was focused on the flame thermal projection process taking into consideration all parameters affecting the morphology and microstructure of deposits such as preheating temperature, roughness, gas flow rate, distance between the torch and the substrate and the speed of movement of the torch. The thicknesse of the coatings was of the order of 3 mm. The preliminary determination of the chemical composition of the used materials was made by X-ray fluorescence spectrometry. The characterization of the deposits was performed by means of electron microscopy, hardness, microhardness and nanoindentation measurements, as well as by X-ray diffraction. Electrochemical tests (dynamic polarization and EIS impedance spectrometry) were carried out in NaCl and Na2SO4 solutions. The microstructure of the first NiFeCrBSi-WC coating was marked by the presence of a nickel dendritic phase surrounded by the eutectic and a dispersion of chromium carbides. This microstructure additionally contains a porosity level of about 2%. The results show that the microstructure of the second coating NiFeCrBSi-WC is dendritic and contains a distribution of chromium carbides alongside with the tungsten carbides. Two distinct types of Ni-γ dendrites have been observed. One is more enriched in nickel than the other. The microscopic analyzes have shown that tungsten carbides were subject to decarburization The hardness of the coatings depends on the nature of the phases present and the porosity in the indented areas. Microhardness measurements have revealed that the hardness of these coatings can reach a maximum value of around 3000 HV. Moreover, the EIS curves proved that the increase in ion concentration augments the charge transfer at the coating/electrolyte interface and accelerates corrosion. In the sulfuric solutions, the first coatings form a thin and compact passive film layer that makes the charge transfer permanently constant at high ion concentrations (35 g/l).