Posted by Gordon England (62.252.0.8) 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.
Regards Gordon
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?
Thank you.
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.
Regards Gordon
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).
Regards Gordon
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