Gordon England Surface Engineering Forum

Re: adhesion

Posted by Hong Wei Wang ( on 15:14:11 27/01/05

In Reply to:adhesion posted by Arturas

: Is it possible to measure thermal spray coating (WC-12Co) adhesion by using scratch method or this method is suitable only for very thin coatings
: Thank you


I suppose you are right that scratch test is useful to measure the adhesion of thin films such as those by PVD, etc, and would not be useful on thick coatings.

As you may well know, during the scratch test, a stylus in constant contact under increasing normal load F(N) with the tested surface undergoes relative motion. The lateral driving force F(L) needed to maintain such a constant movement has to be increased correspondingly as the F(N) increases. A real-time record or F(L) as a fucntion of F(N) curve is taken.

Depending on the thickness of the film, the initial F(L)-F(N) curve is more or less linearas a result of the increasing contact area with the normal force until when a sudden fluctuation of the F(L) is observed - this marking the onset of the film de-hesion ( the loss of adhesion).

The reason the scratch test can successfully probe the coating to the de-hesion point is that the contact stress field in the subsurface (using the traditional Herzian elastic contact theory is a good estimate)can only reach/maximise at a limited depth - a depth good enough to destroy a thin film.

For a thick coating over 100-250 um say of a thermal spray coating, the shear stress transmitted to this depth would be virtually zero, thus such a coating can not be failed by adhesion under normal loading range of a typical scrach test machine.

Hope this would be helpful.

Hong Wei Wang
Greenhey Engineering Services, UK

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