Details on HVOF

[rik]

Hello everyone!

I need some details related to HVOF Sprayed Tungsten Carbide Coatings.

- would the oxide content in the coating affect the wear resistance of the coating?

- what could be the reasons for increase in oxidation levels in HVOF sprayed coatings?

If you can provide any reading material/links associated to the same, It would be very helpful!

Thanks is advance

RIK

[Gordon]

Hi Rik

to the Surface Engineering Forum.

Quote:- would the oxide content in the coating affect the wear resistance of the coating?

Yes, it would. If we are talking about WC/Co type coatings, an observable presence of metallic oxides - tungsten and cobalt oxides would indicate really extreme deviation from ideal spraying conditions. In these materials carbon tends to be the prime target of oxidation.

Again, if we are talking sintered type (not cast and crushed or fused) WC/Co type powder starting material - essentially a pure soft and ductile cobalt matrix containing WC crystallites. Ideally, the coating produced will be of the same chemistry and phase distribution, basically unchanged except for compaction into a coating. Problem here is that at a certain level of heating WC can start to dissolve into the Co matrix. Under equilibrium conditions - slow cooling, this would tend to revert back to WC and Co, but under spray coating conditions - rapid cooling, metastable phases containing various proportions of WCoC are produced. These metastable phases tend to make the coating matrix harder and brittle, resulting in a coating showing less ductility and poorer abrasion resistance. Further increasing heating/oxidation conditions during spraying will lead to loss of carbon and in extreme conditions oxidation of the metallic components left.

Quote:- what could be the reasons for increase in oxidation levels in HVOF sprayed coatings?

Factors are particle temperature, time and oxidation potential of spray environment. Increasing any of these will lead to increased oxidation and loss of primary WC phase in coating. A difficult balancing act between providing the right particle heating, dwell time and velocity. Advances in high velocity spraying helps us here. Coating compaction relying more on particle kinetic energy than thermal, We can get away with heating particles much less reducing WC degradation and oxidation.

Not sure on level of reading you are after, purely academic, more practical or layman?
One good source of reading would be past papers from ASM thermal spray conference proceedings.

[rik]

Hello Gordon,

Thank you very much! The inputs provided by you were very informative.

Can you please tell me, where can i find the ASM papers. Is it readily available on the ASM web portal?

If there are more references and reading material related to practical application and research study, do let me know.

Currently, I am an operator of HVOF System but I would like to learn more about the finer details and characterestics of HVOF Coatings. you can also recommend good books on HVOF Coating and Characterestics. Although, i am not very fond of reading, I can make an exception for this subject..[/font]

Regards

RIK

[Gordon]

Hi Rik

Yes, the ASM Thermal Spray Society should be a good source of reading material. Though, much of this comes at a cost. If that's a problem, first check with your own company to make sure they don't already have a little library hidden away somewhere Searching the WWW can sometimes provide good free reading, just takes a bit of time and effort to find it and to sort the chaff from the wheat. Also don't forget the manuals/books that may of come with your thermal spray equipment.