Tungsten carbide tools with polycrystalline diamond coatings are known to have high wear-resistance. However, it’s not clearly understood why sometimes the adhesion between the cutting tool and the diamond coating fails. It seems that the cobalt (Co) binder phase in tungsten carbide (WC) plays an important role for layer adhesion. Therefore, an appropriate sample preparation method is absolutely important to avoid any misinterpretation of microanalysis results. Our initial goal during this study was to prepare a large surface area for Electron Backscatter Diffraction (EBSD) characterization to obtain a good statistical representation of the samples. This also means that samples do not need to undergo several dimension reduction processes before microscopic preparation. Nevertheless, large surface area preparation for WC-Co is tedious, due to the high hardness of the material, and difficult due to cobalt leaching during conventional mechanical preparation processes. Therefore, we applied several preparation methods varying from large areas (in mm order of magnitude) to small areas (tens of microns) to decide on which method is most suitable for WC-Co materials. The methods studied include: mechanical polishing (MP) [1], waterless mechanical polishing (WMP), WMP with corrosion inhibitors, WMP with ion beam etching (IBE) [2], broad ion beam (BIB) and Focused Ion Beam (FIB) – Scanning Electron Microscope (SEM) preparation [3], and standard Transmission Electron Microscope (TEM) preparation with Ar+ ion polishing [4]. Our characterization criteria are based on the following: Cobalt content indexed using EBSD, magnetic saturation, and cobalt signal detected by X-ray Diffraction (XRD). We use BIB as a reference method, as it provides the best sample surface quality, preparation reliability, and most accurate presentation for WC-Co samples [3]. Our samples are DK120, and they have 6% cobalt content with a mean WC grain size of 1.2 microns supplied by G-Elit-Präzisionswerkzeug-GmbH. We found a strong dependence on the preparation method concerning the cobalt content and the amounts of cubic and hexagonal Co. For all mechanically prepared samples the Co content drastically decreased, and the samples that underwent IBE showed a significant phase transformation from cubic to hexagonal Co; this means the aforementioned methods will result in misrepresentation of phase analysis.

References
[1] E. Bennet et al; Measurement Good Price Guide, 1997, 39-40.
[2] J. Marshall, G. Sweetman; PM2010 World Congress – Hardmetals I, 2010.
[3] B. Winiarski et al; Ultramicroscopy, 2017, 172, 52-63.
[4] M. Rettenmayr, H. E. Exner, W. Mader; Materials Science and Technology, 1988, 4:11, 984-990.