Ultrafine-grained hardmetals with improved mechanical properties

Abstract

Mechanical and physical properties of harmetals containing a cobalt- and an iron-based binder matrix were studied. Three WC powders with different median grain size were tested. Two different cobalt powders, an extrafine elongated one and a nano spherical one, were selected. Based on a literature review, a so-called alternative binder system was also investigated: Fe/Co/Ni. Sintering was performed at 1360°C for the Co-based alloys and at 1440°C for the Fe/Co/Ni alloys.<br />The hardness of the WC/Co alloys at a specific binder content is directly linked with the average grain size of the WC powder used. The finer the WC powder, the harder the alloys get.<br />The finer the Co powder grade used for sintering, the more homogeneous the binder and the WC grain size distribution appeared to be in the sintered alloy. The more homogeneous carbide microstructure resulted in a higher alloy hardness and fracture toughness, but not in a higher coercivity.<br />During the sintering cycle up to 95% of the total densification occurs during the heating period. In this stage of intensive shrinkage in the solid state the densification is clearly promoted by a smaller WC particle size, a smaller Co particle size, a more uniform dispersion of the binder during WC/Co powder milling. It is likely that the high densification rates are possible due to the presence of a low viscosity contact zone substance between adjacent particles which enhances particle rearrangements and the flow WC/binder areas.<br />The addition of stable oxides and nitride led to mixed results. These additions did not improve the mechanical properties at room temperature.<br />But the addition of MO1 improved significantly the transverse rupture strength (TRS) at 750°C. The reduction in strength was reduced from 40 to 23 % for a grade.<br />Excellent hardness/fracture combinations were obtained in case of Fe/Co/Ni hardmetals with optimised compositions, actually comparable to WC/Co alloys. The effects of grain growth inhibitors such as Cr3C2 and VC were underscored. For example, the hardness of an ultrafine WC/9 wt% Fe/Co/Ni grade containing 0.5 wt% Cr3C2 and 0.3 wt% VC as growth inhibitors is 2020 HV30, whereas the hardness of such a grade without growth inhibitors is about 1750 HV30. The addition of growth inhibitors also increased the fracture toughness, though only little, from 8.9 to 9.1 MPa*m1/2. An ultrafine and uniform microstructure was formed as a result of the strong growth inhibition effect of Cr3C2 and VC. Almost no local WC grain growth occurred and pore free microstructure was attained.<br />The addition of a stable oxide did not improve the mechanical properties. In fact, the negative effects observed in Co, such as grain coarsening, were accentuated in this system. This grain coarsening led to a reduction of the mechanical properties.<br />The sintering dwell was either extended or performed at a higher temperature but the grain growth could not be controlled in any case and local grain coarsening was observed.<br />