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Cemented carbides, or hard metals, are ceramic–metal composites usually consisting of tungsten carbide particles bound by a cobalt-based alloy. They are the backbone materials for the tooling industry, as a direct consequence of the outstanding range of property combinations, depending on their effective microstructural assemblage, i.e., the physical dimensions and relative content of their constitutive phases. Hence, reliable microstructural characterization becomes key for hard metal grade selection and quality control. This work aimed to assess the practical two-dimensional characterization methods for the most important one- and two-phase properties of cemented carbides, i.e., the carbide grain size, phase fraction, carbide contiguity, and binder mean free path. Three different methods—point, line, and area analysis—were implemented to characterize four microstructurally distinct grades. The images were acquired by optical and scanning electron microscopy, with the latter through both secondary and backscattered electrons. Results were critically discussed by comparing the obtained values of properties and the different characterization methodology. Inspection technique combinations were finally ranked based on accuracy, accessibility, and operability considerations. The line method was used to analyze all the properties, the area method, for the one-phase properties, and the point method, for only the phase fraction. It was found that the combination of optical microscopy and the line analysis method was suitable for a direct inspection and rapid estimation for carbides above fine grain size. The most precise results were achieved using line analysis of the images obtained by the backscattered electrons of the scanning electron microscope.

Different studies have emphasized the technological relevance of residual stresses in engineered surfaces, such as thermally sprayed coatings, and their effect on the fracture and fatigue behavior. In arc-sprayed coatings, the microstructural characteristics and resulting residual stresses are determined primarily by the inherent process characteristics and feedstock material. With the scope of this work, a study on the residual stress field in coatings formed by an arc spraying process for different electric parameter settings has been carried out using different wire configurations. Thus, iron-based cored wires with different grain-sized tungsten carbides as filler material were used as feedstock. The coatings are mainly composed of eutectic carbides, and eta carbides such as M6C, M12C, or M23C and some iron-rich oxide phases, as well as characterized by an inhomogeneous, lamellar microstructure. The results demonstrated that the magnitude of the residual stresses in the coating depends on the carbide grain size fraction used as filling for cored wires. A smaller carbide grain size leads to a more pronounced dissolution of eutectic carbides, resulting in the formation of eta carbides, which in turn is accompanied by decreased tensile residual stresses across the coating. With respect to the spray parameter settings, reduced tensile residual stresses are observed when an increased voltage is applied, which can be attributed to phase evolution phenomena during spraying and thermal effects on the substrate-coating system. [ABSTRACT FROM AUTHOR]

Duplex coatings are an innovative approach to further improve cemented carbide surfaces in terms of tribo-mechanical properties by depositing a thin Physical Vapor Deposition (PVD) layer on top of the High Velocity Oxygen Fuel (HVOF) sprayed coating. Within this investigation, AISI M3 steel substrates were primarily coated with three different WC-12Co feedstock powders by means of HVOF spraying. The agglomerate size of all powders was constant (− 45 + 15 μm), the WC grain size was varied from conventional (carbide size 2.5 μm) over fine (carbide size 1 μm) to nanosized (carbide size 0.1 μm) carbides. Subsequently, the samples were polished prior to the deposition of a CrAlN PVD coating by means of magnetron sputtering. The aim is to evaluate the influence of the WC grain size on the CrAlN/WC-12Co duplex coating properties. It was shown that a smaller carbide grain size influences the microstructure of the CrAlN coating and causes a finer crystalline structure. Furthermore, higher compressive residual stresses occur in the duplex interlayer and the top layer when using nanosized carbides. Moreover, nanosized carbides in the HVOF coating are favorable in terms of layer adhesion, which was analyzed by means of HRC indent test and scratch test. Further enhancements are the increase in hardness and the reduction in roughness which was directly influenced by the WC grain size. Considering the obtained results, it is to conclude that a reduction of the interlayer WC grain size improves the performance of CrAlN/WC-12Co duplex coatings. [ABSTRACT FROM AUTHOR]

Cemented carbide materials develop very large micro-stresses due to the coefficient of thermal expansion (CTE) mismatch between the metallic carbide phase and the binder phase. While micro-stresses developed in WC-Co cemented carbides have been investigated, there is very little to no information on the micro-stresses developed in NbC-Ni materials. In this study the thermal stresses developed in NbC-Ni materials as a function of the carbide grain size (from 2.5 μm to 12.8 μm) and the binder content (from 8 vol% to 12 vol%) were characterized by neutron diffraction. It was found that the residual stresses decreased with the increase of the binder content and the increase of the carbides grain size. For both binder and carbide phases, stresses were higher for smaller NbC particles, reaching 4 GPa in Ni at the lowest binder content. In general, stresses were higher than the ones measured in WC-Co or WC-Ni composites. This unexpected finding was attributed to the very different shapes presented by the carbide particles; NbC particles are almost spherical, and WC particles are triangular plates with truncated corners. These different shapes create different strain/stress distributions on the microscopic level around the carbide particles. • Quantitative thermal residual stresses measurements in NbC-Ni materials. • Stresses decrease with the increase of binder content/carbide grain size. • Stresses in NbC-Ni materials higher than in WC-Co ones. • Carbide particle shape affects the thermal residual stresses. [ABSTRACT FROM AUTHOR]

Linear intercept on scanning electron microscopy micrographs is the most commonly used measurement method to determine carbide grain size and contiguity in WC–Co cemented carbides (hardmetals). However, it involves manual time-consuming measurements and is critically dependent on the quality of the micrographs as well as on the identification and definition of grain boundaries. In this study a two-stage methodology for microstructural characterization of hardmetals is presented. First, a digital semi-automatic image analysis procedure for grain size determination of the carbide phase is presented. It involves an experimental assessment of grain size on processed images corresponding to a series of WC–Co and WC–Ni cemented carbide grades with different microstructural characteristics. Obtained results are then compared to the values obtained by means of the linear intercept technique. A good correlation between the mean grain sizes determined following both measurement techniques was attained. Based on experimental findings, a series of empirical relations were found to correlate grain size distributions obtained following both methods. Second, an empirical relation for estimating carbide contiguity in WC–Co cemented carbides is proposed. This relation considers simultaneously the influence of the binder content and the experimentally determined mean grain size on contiguity. The proposed equation for contiguity estimation is based on extensive data collection from open literature. An excellent agreement was attained between contiguity values estimated from such equation and those obtained using the linear intercept technique. This validates the two-stage procedure as an effective time-saving methodology for microstructural characterization of WC–Co cemented carbides. [ABSTRACT FROM AUTHOR]