Dear Fariba, I have some doubts about suspension plasma/flame spraying of WC-Co. If you want to reach nanostructured coatings of this carbide, you can use spray dried agglomerated powders and HVOF spray technique. There are many of such powders on the market. Where come my doubts from? I am afraid that the oxidation of Co and W will accompany usual decarburation of WC powder. But, to my knowledge, no one has tried to spray suspension and there is always chance that it works.

Dear Lech,

Thank you for your valid comment. However, I suppose it worths trying. Selection of the right spray parameters may help us alot. But, the first challenge is making a stable suspension. My present powder is loosely agglomerated (non-sintered) .

Hi, I had referred an article by J. Oberste Berghaus, B. Marple, C. Moreau, "Suspension plasma spraying of nanostructured WC-12Co coatings", in 2006 Proc. International Thermal Spray Conf. I suppose they used some amine + ethanol as carrier medium. I do not have that article right now, but i'm sure you must be having in the university. There was some effort from Prof. Kear in Rutgers University, making WC-Co powders in-situ using the Co, W-salts, from which we can find a way to make a deposit. As Prof. Pawlowski said we also deposited spray-dried nanostructured WC-12Co using D-gun, which was not so attractive in terms of its properties, except for marginal increase in erosion resistance and fracture toughness values. Other tribological properties were either equal or poorer than the coarse grade WC-12Co. we had also sprayed nano, Bi-modal WC-12Co from Inframat, and NEI corporation, respectively. The whole issue is the control of parameters in avoiding the decarburization. Even if we do that, what will be the value-addition that we get compared to coarse grade powders? It was not quiet substantial!

However, for suspension spray processing, it might be worth trying some kind of surface capping, which we tried some years back. Though we didn't succeed greatly, you can have a try with a reduced solid loading. 30 wt% appears to be high and reduced loading might help in reducing the decarburization.

BTW, what is the perceived application? Why it has to be specifically done through Suspension routes? Any specific objective to this?

Thank you for you response. Yes, I have this reference. it seems that Jorg Berghaus whom I know pretty well, was the only one spraying this suspension. But may I ask more detail about surface capping? I am also not clear why higher solid loading should result in more decarburization.

We had some success in doing surface capping, but it was done by my colleagues working in the area of nanomaterials for some commercial applications. The main advantage is that it disallows the decarburization to some extent and if any decarb takes place, could also be possibly compensated with free carbon derived from the surface capping agent. We did some efforts, but not published.

Our trials with suspension based studies used solid loading of about 4-5% for ceramic particles. Obviously, the density being higher for WC, it might need more weight%. With high surface area, the general tendency for WC-Co materials is to dissolution in Co and also formation of W2C/W. We had seen higher dissolution of WC with reduced WC grain size. When we operate thru suspension spraying, a general tendency is to provide an higher plasma power to allow the removal of solvents along with the melting of suspensions. In the process, if we have higher solid loading, i foresee possibilities of higher decarburization with higher loading...Prof. Pawlowski can throw a better light into this, since he had done lots of suspension spraying of BMG related materials, etc.

Dear Fariba, in our experiments, we observed that the WC-Co raw powder plays a major effect in the formulation and characteristics of the suspensions (solid load, particle sizes distribution, grain sizes,sedimentation rate), as well as on the long-term stability of the spray process. Some decarburation in the coatings was also observed. In my opinion, the WC-Co powders for suspension should be obtain in a different manner as the current WC-Co feedstock powders. I am still looking for the suitable WC-Co powder for suspension spraying.