Theory for Cluster oxide Formation

Posted by Gordon England ( 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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