STRUCTURE AND WEAR RESISTANCE OF FeNiCrBSiC–MeB₂ ELECTROSPARK COATINGS

Abstract

The structurization of coatings produced by electrospark deposition (ESD) from the commercial self-fluxing FeNiCrBSiC alloy and FeNiCrBSiC-based composites, such as FTB20 (FeNiCrBSiC–20 wt.% TiB₂) and FCB20 (FeNiCrBSiC−20 wt.% CrB₂), on a steel 45 substrate was studied. The electrospark FeNiCrBSiC coating about 70 µm has a globular surface and the FTB20 and FCB20 coatings form a continuous layer up to 50 µm thick over the entire samples. The microhardness does not change across the deposited coating thickness and is 10−14 GPa.  The chemical compositions of the ESD coatings and the electrodes are the same, which indicates that the electrode material does not mix with the steel substrate. The structure of the FeNiCrBSiC, FTB20, and FCB20 electrodes and coatings differs significantly because chromium boride and/or titanium boride inclusions refine from 20−25 µm to 1 µm in the ESD process. The heterophase structure of the ESD coatings represents a nickel–iron-based matrix reinforced with fine boride and carboboride particles. The effect of speeds and loads on the wear rate of ESD coatings in dry friction conditions was examined. Electrospark coatings produced from the WC–6% Co hardmetal were tested to have a wear resistance reference. The wear rate of the FeNiCrBSiC, FTB20, and FCB20 coatings decreases and that of the WC−6% Co coating increases when the speed rises from 4 to 12 m/sec. The wear rate of the ESD coatings becomes one order of magnitude higher when the load increases from 0.1 to 0.4 MPa. Analysis of the friction surfaces showed that wear of the FeNiCrBSiC coating was caused by fracture of the globules and that of the FCB20 coating by brittle fracture of the deposited layer. The ESD FTB20 coating has two to three times higher wear resistance than the FeNiCrBSiС coating because of the oxidative wear mechanism whereby protective oxide films develop on the friction surfaces and act as a solid lubricant.