In the chemically synthesized WC powder individual particle are practically amorphous and thereby spherical. During sintering, the tungsten carbide crystallized and the crystals grow the larger, the higher the sintering temperature.

This is an all-natural process that can be explained by thermodynamic (Gibbs energy of the crystalline tungsten carbide is smaller than that of amorphous and with the grain growth decreases the Gibbs energy in addition). It is thus impossible to avoid the growth of crystals during the high-temperature sintering.

In the chemically synthesized WC powder individual particle are practically amorphous and thereby spherical. During sintering, the tungsten carbide crystallized and the crystals grow the larger, the higher the sintering temperature.

This is an all-natural process that can be explained by thermodynamic (Gibbs energy of the crystalline tungsten carbide is smaller than that of amorphous and with the grain growth decreases the Gibbs energy in addition). It is thus impossible to avoid the growth of crystals during the high-temperature sintering.

Totally agree with Vadim's earlier answer to your question and apart from precipitation by low temperature chemical synthesis, there's no (known) way around it.  

I agree with both of the scientists...but I guess if you use the process with higher heating and cooling rate (like microwave assistance sintering), round particle (or at least fine structure with low differences in all dimensions) will be achieve.

thank you all 

WC powder particles are generally crystalline and form as faceted crystals. However, the generally used milling process (attriting) damages theses crystals and may make them more uniform. The spherical "powder" used in pressing compacts is typically spherical aggregates of WC and Co powders manufactured by spray drying after wet milling as an aid to flowability during pressing.

During sintering, WC dissolves in the molten Co binder (hence "liquid-phase sintering"), the finest particles dissolving first and therefore disappearing. However, on cooling, most of the dissolved WC comes out of solution and precipitates preferentially on the largest particles, which sometimes grow to giant size. The crystalline structure of the solid particles (NOT amorphous) means that all can nucleate grain growth, but the nucleated grains are generally of a higher degree of perfection than the damaged crystals in the original mix. At the end of sintering, the binder is not near-pure cobalt but a pseudo-eutectic of WC and Co.

There are a number of variables in this process, such as grain-growth-inhibiting additives, milling parameters, sintering time and temperature, and so on. And at least one company claims a process for making WC/Co with spheroidal grains - without great commercial success.

Notwithstanding all of the above, I agree that spheroidal carbide grains are to be preferred, because the facets and corners of typical WC grains act as microscopic stress-raisers and lower the strength, though not the hardness, of WC-based hardmetal. In my own research I helped to develop improved milling methods and additives to reduce grain growth (large grains are very serious stress-raisers) and also WC-based spheroidal carbide grains. This is achieved by adding appropriate amounts of TiC, TaC and NbC to the WC, then carburising at very high temperature so that all the carbides are in solid solution with one another. The result is a two-phase WC-based hardmetal with spheroidal carbide particles, which is what the enquirer requires.

Unfortunately there is little commercial interest in such products nowadays. Almost all the research is on WC/Co materials, which is like rssearching plain carbon steel for the steel insustry, ignoring tool steels, stainless steels and all the other types of alloy steels. And of course research into coatings, since much of industry think the coating does 99% of machining and therefore the underlying hardmetal is of little importance. Incidentally, I can confirm that the carbide described above indeed has greater strength than WC/Co of equivalent hardness, and also greater resistance to heat and corrosion.

thanks sir ...your information was really helpful...If we try to cool the wc-co after sintering at higher rates of cooling...Is it possible to reduce the reprecipitation process and arrest grain growth so that we might possibly get spheroidal Wc?

Lots of researchers have tried this (for example Kennametal at Latrobe) though I'm not sure exactly what has been published. But the main reason was to investigate the reactions above and just below the liquidus temperature. Depending on the speed of cooling, it is possible to reduce grain growth and to retain much more WC in solution in the Co-based binder, as examples. Retaining the shapes of damaged WC crystals (what you call "spheroidal") would not be very interesting by comparison and the effect would be minor. Any increase in toughness due to this would be more than balanced by the thermai microstresses induced by the rapid cooling.

Hope that's a help. Obviously I'm not going into more detail since, as you probably know, I'm about the only consultant to the hardmetals industry around and my wife likes to eat occasionally. But you probably have some of my books in your university library.

Regards

thanks sir...