Posted by Gordon England (62.252.0.12) on 19:14:13 08/01/06
In Reply to: Re: Cluster oxide in Inco718 posted by Hong Wang
I will try to explain my theory on how these oxide cluster
formations occur (see rough sketch). The effect is
certainly not confined to Inco 718 alloy coatings and I believe it
may effect many different thermal spray coatings. Also, the problem
is not confined just to the plasma spray process, but HVOF and
possibly others.
I think the problem stems from fine material being ejected sideways
(parallel to substrate surface)from the spray stream at or
around the point of primary particle impact with the substrate.
These ejected particles then form secondary deposits on protrusions
on the coating/substrate surface. As the coating builds up these
protrusions become magnified due the additional secondary deposits,
resulting in a tube like column (horseshoe shaped as viewed
from coating surface) rising up through the coating, usually at
an angle of about 60-70 degrees from the substrate surface plane.
It is important to note that these structures are three dimensional
and will appear different when cross-sectioned from different
angles.
These column like structures usually consist mainly of oxide phases
and sometimes excessive porosity.
The fine material that produce the secondary deposits probably
originates from two sources:
1. Fines in the original spray powder, which over heat and
oxidise.
2. Splash from impacting particles with substrate/coating.
I have observed this problem with plasma sprayed 80/20 NiCr, CoCrNi
alloys, chromium carbide/NiCr,
and possibly a similar effect with chromium oxide.
Problems with HVOF coatings include chromium carbide/NiCr, Inconel
625, Hasteloy C.
Predominantly, the effect occurs on flat substrate surfaces and
large external diameters, smaller diameters seem less effected. It
is probably reasonable to assume internal diameters would be
effected the most, but I have no observations to confirm this.
The as-spray coating texture is usually effected when the problem
is severe. Wart or pimple like protrusions being evident. When the
surface is ground, the surface may show dark spots about 1mm in
diameter. when view closer, a horse shoe shape may be seen.
I would be interest to hear of any alternative theories, or cases
which involve coatings not containing chromium.
Regards Gordon
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