Microstructure and diametral fracture strength of spark plasma sintered WC-6Co-0.2B and WC-6Co-0.5B ceramic composites from nanosized and metastable WC powders

The present work aimed at evaluating the microstructures and mechanical properties of spark plasma sintered WC-6 wt% Co ceramic composites doped with boron (0.2 and 0.5 wt%). W-50 at% C (W-6.13 wt% C) powders produced under different milling times (20, 60, 180, and 600 min) were used as starting materials and subsequently mixed with Co (6 wt%) and B (0.2 or 0.5 wt%) powders for 5 min. The resultant WC-Co-B powder mixtures were consolidated by spark plasma sintering at 1450 °C for 10 min at 40 MPa under vacuum in order to obtain the samples with 10 mm diameter and 3 mm thickness. The starting powders and the sintered samples were characterized by laser diffraction particle size analysis, X-ray diffraction with Rietveld refinement, relative density, scanning electron microscopy, energy dispersive spectrometry, Vickers hardness test, and diametral compression test. Supersaturated W solid solution (WSS) with a crystallite size of <50 nm was formed in W-50C (at%) powder mixtures. The relative density of the sintered composites increased with the increasing milling time up to 180 min from 79.7%±0.6% to 86.7%±0.5% for WC-6Co-0.2B and from 86.7%±0.6% to 90.9%±0.4% for WC-6Co-0.5B. Furthermore, the WC phase appeared as the matrix in the sintered samples, and W2CoB2 precipitated homogeneously around WC grain boundaries, mainly to those containing 0.5% B. The Vickers hardness values of the sintered WC-6Co-0.2B and WC-6Co-0.5B ranged from 1790±39 to 2158±25 HV and 1858±31 to 2182±28 HV, respectively. The fracture strength (determined by diametral compression test) varied between 144-353 MPa due to the porosity and precipitates in thin-thickness cylinder samples.