NDE for verifying hard coat on aluminum (not decorative anodizing)

[ccw]

.
Is there a handy verificaton for Hard Coat on aluminum. (I think the original process was patented "Martin Coat") without getting too exotic?

What about a micropoint hardness test?

Seems to be some confusion in the industry, with some decorative anodizing being a poor substitute for Hard Coat.

 

[kenvlach]

Hardness testing is difficult due to the thinness of the anodize, its brittleness & the relatively soft aluminum substrate. Microhardness testing can be done by cross-sectioning, mounting in Bakelite, polishing and then Knoop or Vickers microhardness testing.

Easy method: Rub with a piece of known hard anodized aluminum. The hard anodize will scratch the conventional anodize. Note, however, that hard anodize on some alloys such as 2024 is considerably softer than on 6061.

Another method is thickness testing, as hard anodize is typically 0.002" thick on wrought alloys, while Type II (per MIL-A-8625F) is often about 0.0008". But, easiest if you have an eddy current thickness tester.

Also, if non-dyed, alloys develop particular colors when anodized, e.g., 7075 becomes olive-bronze color. Also, hard anodize is less bright and due more difficult to dye lighter colors. See 'Anodizing Reference Guide'

http://www.anodizing.org/reference_guide.html

Re "Seems to be some confusion in the industry, with some decorative anodizing being a poor substitute for Hard Coat."
-- What's your QA system? Don't you order hard anodized parts from an approved supplier per MIL-A-8625F, Type III (or AMS 2469) and get certs? Initial survey the supplier, then every once in a while drop in to check their lab analyses, especially Taber abrasion test results.
Lockheed Martin approved suppliers for hard anodize per MIL-A-8625F (note: they have more than one list):

http://supplier.external.lmco.com/supplier/specproc_certs/suppliers_for_c03.html

MIL-A-8625F (& Amendment):

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=7074

 

[CoryPad]

Ken mentioned an eddy current thickness tester. I have found this one to be a good unit, and it should meet your needs.

http://www.defelsko.com/pt6kFR.html

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ccw]

Ken,
Thanks for the help with the Specs. and list of approved vendors.

Cory,
Thanks for the tip on the eddy current thickness tester. I assume one needs cal. standards or "GO" samples for a specific coating and base material.

PS:
As far as QA, I am away from Nuc. world into Commercial world now. Certs are worthless pieces of paper, or not available at all. Back in the day, we were not even allowed to use the words "hard anodize", had to say "hard coat". The culture avoided confusion in the fab. process that way. Of course we had our own plating labs. and analytical metallurgy labs. to provide verification. "Hard Coat" (.002 tk.) had its own Mfg. Spec. The only time we used "hard coat" was when we wanted abrasion resistance. And, there was only one color. I notice the MilSpec. author slips up and still uses the term "hard coat" in a few places.

Now I see from the MilSpec that "Hard anodize" doesn't really mean anything, without Class II, Class III, subpart B, sealed, not sealed, dyed, not dyed, etc., tacked on, and one can get results from .0008 to .002 if not careful with full definition.

 

[kenvlach]

The eddy current thickness standards are just plastic films. The actual materials cost ~$0; you pay for the certification of the thickness.
For commercial work not requiring NIST-traceability, make your own standards: find an assortment of plastic bags & saranwrap, cut to convenient size, say 1x4" and then measure the thickness by micrometer.* Label with a permanent marker.
These also work for measuring paint or zinc thickness on steel using magnetic pull-off gauges.
*Possibly, with a calibrated micrometer, can create calibration paperwork.

For the eddy current measurement, also will need some very flat, clean aluminum panels to zero the meter & then place standards upon for calibration. You can get Al 2024, 6061 & 7074 panels in peel-off protective plastic from laboratories that do salt spray, thickness & wear testing for anodized Al, plated Al, painted Al, etc. Must use a panel of the same alloy (or very similar) as the part being measured.

For more info, DeFelsko mentioned above is good. Also
Elcometer

http://www.elcometer.com/international index pages/international/Coatings/index.htm

FISCHER

http://www.helmut-fischer.com/Product_Group_overview_1.asp?PSG=0120&wg=11&CountryID=91&LanguageID=2

Oxford (includes CMI)

http://www.oicm.com/applications/anodizing.php

An into article on 'Measuring Coating Thickness'

http://www.pfonline.com/articles/pfd0027.html