Bond strength of Arc Sprayed Coating
07-21-2008, 03:00 PM,
Question  Bond strength of Arc Sprayed Coating

I would like to know the test method to test the bond strength (bend test) of wire arc spray coatings. It is 1.00 mm thk SS 316L coating on carbon steel. Is bond coat required? (In case of bend test, coating on inner face or outer face?)

07-22-2008, 03:47 PM,
RE: Bond strength of Arc Sprayed Coating
Hi sumant

Quote:I would like to know the test method to test the bond strength (bend test) of wire arc spray coatings.
For tensile bond strength testing you need to follow ASTM C633.

The simple bend test is not quantitative, nor is it a direct measure of bond strength. The results of a bend test are also effected by coating strength, ductility, thickness, shear strength etc. Example: flame sprayed zinc coating ~750 psi bond strength passes bend test, plasma sprayed WC/Co ~11000 psi bond strength fails very similar bend test (not to say WC/Co normally fails Happy0193 just in this particular case).

The simple bend test has an advantage in that it is quick and easy to perform. Ideal for thermal spray operators to perform to assess their own work and equipment set-up. It is a very qualitative test of general coating quality and to be of any meaning, needs to be set-up as a comparative test (against a known standard coating) for one coating type under very strict conditions of testing. Test piece thickness, coating thickness, angle of bend and coating type are factors that can dramatically effect results. These obviously need very tight control for the test to have any real qualitative meaning.

The simple bend test tends to be going out of favour, as it is seen to be superfluous, when other more quantitative testing is also employed.

A typical bend test set-up would be:
Test piece - 75 x 50 x 0.125 mm
Coating thickness - 0.15 (+/- 0.025) mm
Mandrel - 12.5 mm
Bend angle - 90 degrees (coating on outside of bend)

But really all the above can be changed to suit the circumstances. For instance the test piece could be bent to a point where a certain amount of failure is observed and the angle recorded. Thicker test piece, thicker coating, smaller mandrel, larger angle will all make the test more aggressive.

Quote:It is 1.00 mm thk SS 316L coating on carbon steel. Is bond coat required? (In case of bend test, coating on inner face or outer face?)

I would say a bond coat is advisable for the job. Bend test thin 316 coating only though, as this really can only be used to qualify equipment set-up/operator and not the coating on the job. Coating on outer face of bend during bend test.

Hope that helps Big Grin

Interesting topic. Does anybody use this test method? Any views on the value of using this test?
07-22-2008, 04:23 PM,
RE: Bond strength of Arc Sprayed Coating
Agreed with everything that Gordon says. Bend tests seem to be used as a quick check that would highlight severe defects in the coating/bond but are not accurate as so many different things can affect the result as Gordon mentioned.

I believe that there is an actual specification / process laid down by Rolls Royce but to my knowledge, this is the only 'standard' for this, e.g., there is no ASTM or ISO standard for bend tests.

We would always suggest a bond coat of 50-100 microns (a 'complete' coating) of Nickel Aluminium for best results on engineering coatings. If the bond is OK without, then don't bother but for the best results, use a bond coat.
Stuart Milton
Metallisation Ltd
07-31-2008, 12:54 PM,
RE: Bond strength of Arc Sprayed Coating
Sorry for the late reply...........

Depends really what level of work you're doing. The accepted bond test for Aerospace arc spray coatings is tensile bond bars: masked, blasted, sprayed, glued, cured and pulled apart in a a tensile tester - the same as Plasma sprayed coatings. However in most manuals there isn't a large list of arc spray coatings and subsequently not all required minimum psi (MPa) values are available. Therefore you would have to refer to the technical bulletin or material data sheet of the particularly material. I should mention that Rolls Royce manuals still specify the bend test and no procedure (or info) about Dual wire arc spraying is available. I do know that their manuals are seldom updated and not indicative of actual thermal spray practices. However I do find it ironic that the Americans embraced (and included) this process some time ago.

The bend test is really obsolete and only really indicates adequate grit blasting. We used to bend the panel into a U shape as a basic qualifying test on nightshift when the lab was closed! A bend test failure indicates that something is very, very wrong. GE have quite rightly dropped it from their Standard Practice Manual requirements. I expect all the others will follow as the major OEMs are looking to compile a universal Standard Practices Manual.


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