The Substrate is non metal (ceramic for e.g). The coating was achieved by Plasma thermal spraying but the coating was not adherent. It could be easily wiped off. What could be the reason for this and any suggestions to improve this. (When coating was done using sputtering the coating was reasonably good)
Sputtering and plasma spraying have quite different mechanisms for layer formation. In the case of sputtering a very thin layer is formed at the automar level: single atoms of the evaporated material are deposited on the substrate. In this case, one needs a polished surface and bonding layer-substrate arises through the interatomic interaction (electrostatic attraction between atoms).
In the case of plasma spraying, the layer is not formed from individual atoms but from droplets of molten material. If these droplets can wet the substrate surface, for example metallic melt on metallic substrate, good adhesion can also form (not always but theoretically possible). If, however, no wetting can take place, for example metallic melt on a ceramic substrate, only a very weak adhesion by clamping is possible. For this, purely mechanical adhesion, you need a rough (sand-blasted) surface of the substrate.
Sputtering and plasma spraying have quite different mechanisms for layer formation. In the case of sputtering a very thin layer is formed at the automar level: single atoms of the evaporated material are deposited on the substrate. In this case, one needs a polished surface and bonding layer-substrate arises through the interatomic interaction (electrostatic attraction between atoms).
In the case of plasma spraying, the layer is not formed from individual atoms but from droplets of molten material. If these droplets can wet the substrate surface, for example metallic melt on metallic substrate, good adhesion can also form (not always but theoretically possible). If, however, no wetting can take place, for example metallic melt on a ceramic substrate, only a very weak adhesion by clamping is possible. For this, purely mechanical adhesion, you need a rough (sand-blasted) surface of the substrate.
In addition to the above information I would like to add that one can think of an intermediate layer to improve bonding. For example alumina - steel bonding is not good, but if one puts a metallic Al intermediate layer, the adhesion improves dramatically.
So, if one is looking for metallic coating on graphite, then putting a low temperature carbide forming intermediate metallic layer could improve the final bonding. Because the inner side of the layer will be carbide but outerside will still be metallic on which heavy metals can be deposited.
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