The term tungsten carbide technically refers to a stoichiometric compound of tungsten, W, and carbon, The compound is an interstitial compound with a mixed bond: metallic, ionic and covalent.
The most important characteristic of WC is its toughness which is superior to any other equally hard material. The hardness of pure WC is around 2500-2600 HV. It is a good electrical (and consequently thermal) conductor and, second great characteristic, it has a great affinity and wettability with Cobalt, Nickel and Iron. The wettability is the ability to be easily covered by a liquid (see for example WETTABILITY OF POROUS ROCKS), in this case, of a transition metal.
Thanks to the high wettability it is possible to form a composite of WC with transition metals. The most famous and popular of such composites (usually called cemented tungsten carbides) is WC-Co. It has a percentage of cobalt from 4 to 28 %. This composite goes traditionally under the name of Widia, from the German Wie Diamant (like diamond), in Europe and Hard Metal, in the US as it was developed in parallel in both countries at the beginning of the 19th century.
Values around 4 to 14% are used for cutting tools employed in turning and milling operations. 12 to 18% are used for drilling tips in masonry. Above 18% are used for wear applications that involve high impacts. The hardness of the cemented tungsten carbides is from 855 to 2050 HV30 (that's Vickers hardness at 30 Kgf) whereas their toughness ranges from 4 to 25 MPam−−√
Grades with a higher hardness also have a finer microstructure and are used in smooth finishing operations. Microstructural control of cemented carbides is something that has been under heavy research since this material was developed and there has been a constant decrease in the grain size from relatively coarse 4-5 µm down to 0.5-0.8 µm of the finest commercial grades available today. Another interesting aspect is that hard metal for tooling is almost never pure, straight WC-Co. It always contains also: TiC (titanium carbide), TaC (tantalum carbide) and/or NbC (niobium carbide). The amount of cubic carbides (as they are called since WC is hexagonal) has increased progressively in time to increase tool life to the point that the extreme finishing tools have more TiC than WC and go under the name of cermets (ceramic-metals).