We deposited Co based alloy on to steel substrate. Before to deposition, a buffer layer of NiCr was deposited. Then deposited coating was fused. Now after metallographic preparation we observed some defects (i.e. porosity, improper bonding) mainly in buffer layer region or surrounding. is due to insufficient fusing time or temperature? or what else can be the reason.
For reference, please have a look on attached optical image of coating.
Shrink holes during remelting are very typical for metals whose volume in the liquid state is much greater than in the solid state. NiCr is not the best material in this sense (no casting material). A possible additional source of pores could be Cr diffusion from the NiCr layer into the steel and into Co-layer (chromium diffuses into steel and into Co-layer leaving pores in the NiCr-layer). The diffusion zone and an enrichment of steel and Co with chromium must be well visible in the SEM analysis.
These pores highly look like the Kirkendall effect caused by different diffusion rates of elements .
Formation of this effect commonly depends on three main parameters: chemical compound of contacting materials, temperature and time.
Of course, it is better to identify the most movable element in your conditions using, for example, EDS analysis, as Dr. Verlotsky advised. After you'll find the element, you know what to do.
Additionally to the already mentioned Kirkendall effect and EDX analysis, are you sure that you did not remove intermetallic phases during preparation? On the other hand, already your optical image indicates a zoning perpendicular to the interface.
Dmitry Bogachov Gert Nolze Vadim Verlotski what could be a pore size in Kirkendall effect process?, here mean size around 40-50 micron. Even I prepared second sample very carefully and that one also showed same features. Anyway I'll go for SEM+EDS.
Thanks for you valuable comments:-
Did you only apply grinding and mechanical polishing with diamond, or did you use lubricants which also etch the surface (slightly)?
Kirkendall effects can produce pores, however it need some time for diffusion. How long did you do fusing?
Gert Nolze Sample cutting was done through alumina wheel and metallography polishing was done using diamond suspensions (9 and 3 microns) thereafter OP-S solution.
Thanks
Do it again without OP-S. This is "etching" and possibly removes phases. Kirkendall in two min...? I don't think so.
Shrinkage and cracks were some of defect during fused coating because of high temperature and usually applied with the aid of gas and oxygen application.
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