Critical Assessment of Two-Dimensional Methods for the Microstructural Characterization of Cemented Carbides

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 twodimensional 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 The work leading to this publication was supported by the Feodor Lynen Research Fellowship of the Alexander von Humboldt Foundation, by the individual research grant from the Deutsche Forschungsgemeinschaft (DFG-425923019), by the Spanish Ministerio de Ciencia e Innovación MICINN-FEDER (Spain) through grant PID2019-106631GB-C41 (AEI/10.13039/501100011033), and by the “Open Access Publication Funding” program of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and Saarland University. The work also obtained funding for the FIB/SEM instrument used for SEM imaging and EBSD by the Deutsche Forschungsgemeinschaft (INST 256/510-1 FUGG)