A.G.P. da Silva

Synthesis of tungsten carbide through gas–solid reaction at low temperatures

Abstract Conventionally, WC powders are produced heating a mixture of W powder and carbon black. This method is based on solid state reaction controlled by diffusion and phase transformations. Thus high temperatures (>1400°C) and long times are necessary. This paper presents a method able to synthesize WC at low temperatures (∼850°C) in 2 h. It is based on a gas–solid reaction between a W source and a C-containing gas. The gas phase is a mixture of H2 and CH4. Ammonium paratungstate and tungsten blue oxide are used as the tungsten source. The higher reactivity of the precursor material and the better contact between the gas–solid reactants explain the lower temperatures and higher reaction rates of this method compared to the conventional one. Very fine WC crystallites (

A low temperature synthesized NbC as grain growth inhibitor for WC–Co composites

Abstract Niobium carbide can be used to inhibit WC grain growth in hardmetal. The performance of a NbC powder produced at low temperature by solid–gas reaction (an experimental powder) as WC grain growth inhibitor is compared with that of a commercial NbC powder. It is verified that NbC effectively inhibits heterogeneous WC coarsening. This results in an increase in hardness. The commercial and experimental NbC powders exhibit a comparable performance in inhibiting the WC grain coarsening, in spite of a significant difference in particle size and shape. The commercial NbC powder is very fine while the experimental one is coarse and porous, but its crystallites are finer than those of the commercial product. The milling procedure used to prepare the alloys is able to reduce the particle size of the experimental NbC, and thus guarantee a dispersion of the particles with a quality level comparable to that found for the alloy prepared with the commercial NbC.

The role of the binder phase in the WC-Co sintering

The sintering of hardmetal in the solid state is studied. The influence of the WC particle size on the sintering kinetics, the role of the binder phase in the densification process and how sintering depends on the heating conditions are investigated. It is observed that alloys with different WC particle size show quite different structural evolution during sintering, although the densification mechanisms are the same. This is explained by the formation of agglomerates of WC and Co. Hardmetal alloys can sinter very rapidly when high heating rates are used, since rapid heating accelerates the binder spreading and the formation of WC-Co agglomerates. The binder phase (Co) spreads on the WC particles initially as a thin layer. Subsequently, more Co spreads on this layer and WC-Co agglomerates are formed.

Synthesis of niobium carbide at low temperature and its use in hardmetal

Abstract A new method of synthesizing niobium monocarbide at a low temperature (950 °C) and in a short time (2 h) is described. Conventionally, niobium monocarbide (NbC) can only be produced at 1600–1800 °C for longer periods. The method consists of the carburization of a niobium complex by a gaseous (H2+CH4) mixture. The method of preparation of the precursor and its carburization are presented. Precursor and carbide are characterized by XRD, SEM, TG/DTG and granulometry. Shape, mean size and size distribution of the NbC particles are similar to those of the niobium precursor. Reactions during carburization do not alter these characteristics. NbC particles are very large and porous. NbC powder is used in a hardmetal alloy as a WC grain growth inhibitor. Its performance is compared to that of a commercial NbC powder. NbC synthesized by the new technique exhibited a slightly higher efficiency in inhibiting the WC coarsening, evidenced by the SEM of the hardmetal structures and hardness measurements.