During the liquid-phase sintering, tungsten, carbon gradually dissolve in the liquid phase, at a certain temperature they can achieve homeostasis during dissolution- precipitation .The solubility of tungsten carbide particles in the liquid phase is closely related to the particle size, the degree of surface energy and temperature. The greater size difference between the small particles and large particle, the greater difference between the saturated solubility in the liquid phase, so that the driving force of the tungsten carbide grains grows larger. With the refinement of the initial tungsten carbide, cobalt powder, especially for the tungsten carbide particle size less than 0.2 microns, tungsten carbide grains, during the sintering process, will grow rapidly to form coarse particles discontinuously, which significantly reduces the mechanical properties of carbide. Therefore, when the sintering of ultra-fine carbide, often we need to inhibit grain growth of tungsten carbide, to minimize and eliminate the non-continuous grain growth.
The mechanism of inhibitor: As inhibitors can fuse in the binder phase, whereby the liquid can be absorbed on the surface of tungsten carbide, hereby to diminish the area of contact with the liquid, and reduce the solid – liquid interfacial energy , so that the solubility of tungsten carbide particles mitigates, at the same time the differences of solubility among varying grains is reduced, the recrystallization grains of tungsten carbide is hindered. If under the sintering temperature of the WC-Co carbide, the solubility of tungsten carbide in a cobalt binder phase is 40%, while by the aid of inhibitor, the solubility of tungsten is reduced to 10%. The reduced solubility impact directly on the dissolution and precipitation of tungsten carbide, at the same time, the inhibitors may be deposited on the tungsten-cobalt, tungsten carbide-tungsten carbide interface during precipitation, which further prevents the grain growth of tungsten carbide. Among the various grain growth inhibitors, the most effective inhibitors are vanadium carbide and chromium carbide,which can reduce the grain size of tungsten carbide to 0.15 microns.
Changsha Langfeng Metallic Material Co., Ltd. is one of very few leading manufacturer for metallic carbide powders:
carbide powder: Chromium Carbide Cr3C2 , Hafnium Carbide HfC, Molybdenum Carbide Mo2C, Niobium Carbide NbC, Tantalum Carbide Tac, Titanium Carbide TiC, Vanadium Carbide VC, Zirconium Carbide ZrC, Titanium Carbide Nitrides Ti (C,N); (Ta,Nb)C powder, (W,Ti)C powder, (W,Ta)C powder, (Ti,Ta)C powder,(W,Ti ,Ta)C powder, (W,Ti,Ta,Nb)C powder
Different types of grain inhibitors have different effects on the carbide performance, while adding more than one grain inhibitors is more favorable to enhance the overall performance of carbide. Adding vanadium carbide alone can significantly improve the hardness of the alloy, but will decline its strength, while adding chromium carbide alone can significantly improve the strength and corrosion resistance of ultra-fine cemented carbide and keep the hardness, therefore, vanadium carbide and chromium carbide are the most popular inhibitors to produce of ultra-fine carbide, and the combination of the two can play the best, generally the adding amount is of 1% or less.