delamination in general

[epilson]

Dear SEF gurus, i'm doing a project on delamination on aircraft parts after repair by plasma spraying (APS). i just have a few qns, namely,
1. what are factors that affect delamination?
2. Is preheat needed ? the reason for the second qns is because i have come across instructions on NOT using preheat to prevent delamination, however, this is without references and goes against common instincts.
3.what can i do to improve the adhesion strength of the coating?

much help appreciated!

cheers
epilson

[Gordon]

Hi epilson

to the Surface Engineering Forum.

1. what are factors that affect delamination?

To be brief - bond strength and residual stress.

2. Is preheat needed ? the reason for the second qns is because i have come across instructions on NOT using preheat to prevent delamination, however, this is without references and goes against common instincts.

Again to be brief - In many cases it is recommended and beneficial. In some cases like aluminium, magnesium and delicate non-metal substrates direct preheating is not recommended.

Generally the hotter the substrate the higher the chances of coating/substrate interaction and less chance of water condensation. On the other hand the hotter the substrate the higher the chances of oxidation and unwanted degradation. Also considerations of differences in coefficient of expansion between coating and substrate.

3.what can i do to improve the adhesion strength of the coating?

Well first care and attention to substrate preparation, it needs to be clean and have a "sharp cut" (not peened) rough textured surface. Spraying should commence as soon as possible after surface preparation. As mentioned above preheating may be important. Many coatings will need or benefit from the use of a bond coat like NiAl. Selection of good spraying parameters, speeds and feeds and temperature control. Make sure coating material is well within its coating thickness limitation.