Hi Awalini and John
Metallography invariably will show a fault line at the interface between old and new coating. How much this weakens the coating integrity is anybodies guess without conducting thorough coating evaluation or from experience of these coating systems working in practise.
Ideally, the best coatings are applied in a continuous manner at a constant coating/substrate temperature. I have seen metallography of coatings where the coating process was paused overnight and restarted next day. Just to say a good metallographer will probably be able to pin point this pause in spraying in the coating microstructure, even though the effect may be a subtle one and may not significantly effect the purpose of the coating. Re-coating of old coatings that are likely to have been contaminated or have been machined finished and grit blasted may even fail duing the spraying process or later during finish machining which i must say is better scenario then failure in service. In conclusion I would say strip back to the original sound substrate and start again. If you have to re-coat on top of a coating, make sure you test/evaluate the coating or are very confident from past experience that the coating will be fit for service.
Two successful jobs that I can think of, that were sprayed on top of existing coatings:
An old massive yankee dryer roll which had seen many thermal spray repairs, had a very thick arc sprayed 13 Cr steel coating. This was cleaned and ground (dry) true, then very carefully inspected and any holes or imperfections in the coating skilfully patch repaired using arc spray Ni5Al. This was grit blasted and re-inspected. The surface was then bond-coated with arc spray Ni5Al and then HVOF sprayed with a chromium carbide/nickel chromium. The coating was then finish ground. (note yankee roll was under internal steam heating 100 C+ throughout long process). At the time I thought this was a very risky procedure, but these guys were very experienced and skilful in their art and confident of success (and so they were). The procedure probably cut time and cost to less than half. I still would not recommend the procedure unless you really know what you doing.
Another example being large gate valves which had fused stellite like coatings. Many failed attempts at HVOF coating these parts before they were given to me for a last try. Can't remember what the HVOF coating was, but trying to coat these using recommended techniques resulted in the coating being rejected half way through the build-up. Anyway, knowing the existing coating had good heat resisting qualities, I preheated well above the normal 100 C to around 350 ? 400 C and then sprayed the coating without problems. It was nice to see the faces of many people involved that claimed it to be impossible to coat
So yes it is possible, but you must take account that it will compromise the integrity of the coating system and you need to be sure that the coating will be fit for service.
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don't re-coat on top of old or failed coatings covertly (aim at cowboys/rogue traders) you only need a few bad/failed coatings to tar all thermal spray coatings with a bad name.
