Posted by Gordon England (188.8.131.52) on 11:54:22 20/02/06
In Reply to: Re: Microhardness of WC/Co coating and optimum gas flow parameters posted by vahidnematy
Please review the whole of this message thread below.
Posted by Arturas on 18:10:49 13/04/05
After X-ray analysis I got that amorphous phase of WC-12Co coating sprayed by HVOF increase with increasing fuel gas flow. How to explain it?
Posted by Gordon England on 23:10:37 13/04/05
Increasing amorphous phases is related to increasing particle temperature and melting. Ideally, tungsten carbide/cobalt particles should only be heated just enough to soften to provide deformation on impact and not to cause oxidation and reaction between cobalt and tungsten carbide phases. Sometimes you will find the density and hardness increasing with these amorphous phases, but generally they will adversely effect wear resistance and toughness of the coating.
Posted by Arturas on 05:38:58 14/04/05
It is very interesting , You said that with increasing fuel gas flow amorphous phases quantity of WC-12Co coating increase and decrease wear resistance , but at the same time coating porosity decrease. So, how to find optimum parameters of gas flow?
And I got that by increasing heat treatment temperature from 20C up to 600C microhardness of WC-12Co coating a little bit increase (5-10 percent) How to explain it?
Posted by Gordon England on 19:07:28 14/04/05
To clarify, increasing heat input into WC/Co particles will tend to increase oxidation and production of amorphous phases and other phases not found in original powder. Increasing fuel gas flows will only contribute to this if it is increasing the heating effect. Too little or too much fuel will reduce flame temperature (assuming oxygen is kept constant).
To achieve the best coating for your needs, requires more than just optimising gas flows. Other parameters such as powder feed rate, air flow (if used), spray distance, selecting best nozzle/air cap hardware etc.. Also using the right powder particle size and distribution is critical. Finer powders are more susceptible to these problems than courser powders.
Optimising parameters is down to conducting spraying trials, ideally only changing one variable at a time and evaluating the coatings. Choose your starting or base line parameters from a known good coating of similar chemistry and particle sizing. No easy task considering all the variables.
Heat treating these coatings is not common practice. Possibly, the increase in hardness may come from amorphous and metastable phases recrystallising producing some W2C and eta carbide phases (not sure if temperature is high enough though).
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