Mohs Scale for HardCoat Anodizing MIL-A-8625A, Type III 1

[LONDONDERRY]

We have a 6061-T651 aluminum piece that is hard coat and they PTFE sprayed. Our customer is highly concerned in their application how hard this material is. From my research everything points to Rockwell C 60-70 but what would that be in Mohs? what is the conversion factor? I had one place tell me its Mohs 4 and someone else tell be that because its an Aluminum Oxide finish it will be 9.5 on the Mohs chart.

 

[tbuelna]

LONDONDERRY,

If you are processing in accordance with MIL-A-8625A type III, there is no specific requirement for coating hardness. If you refer to MIL-A-8625 table II, there are only process control test requirements for type III coating weight, coating thickness, and abrasion resistance. I believe the A rev of MIL-A-8625 you specify states the abrasion resistance test shall be performed in accordance with FED-STD-141 method 6192.1. The most recent rev D of FED-STD-141 states method 6192.1 has been replaced with ASTM D4060, which I don't have a copy of.

Basically, MIL-A-8625 allows use of any process that conforms to the specification requirements. In your case, the coating can have any hardness as long as it passes the process control test for abrasion resistance. So if you simply specify "anodize per MIL-A-8625, type III", then you have no way of knowing precisely what the coating hardness will actually be. If you need to ensure the coating will have a certain minimum hardness, then you should include a supplementary QA requirement to verify coating hardness in addition to the basic MIL-A-8625A, type III process. Of course your anodize vendor will not like this, since it will require them to either perform NDI verifying the coating hardness of one (or possibly every) part in each batch processed, or develop a controlled/validated process for coating the parts that ensures a consistent/conforming result.

I'm sure this seems like a big hassle. But if your customer is "highly concerned" about the anodize coating on the product you deliver having a certain minimum hardness, then this is what you need to do.

Hope that helps.

 

[WKTaylor]

Londonderry... Tbuelna hit the nail-on-the head...

IF Your customer is 'highly concerned', then THEY must have a 'target' hardness range MIL-A-8625 Ty III 'unsealed' anodize for 6061-T6 ... including how they want that hardness measured [over/above the base specification]... and it should have been part of the base contract.

As I recall 6061 [high silicon content] is difficult to consistently get a hard/deep Type III anodize finish.

I worked a similar problem a few years ago. A VERY long, large OD, thick wall 6061-T6 tube was ground [16-microinch RA machined finish] to pre-anodize Dia; it was then Type III anodized; it was then then reground to final dimensions [16-RA, Dia]; the anodize finish was then over-coated with a [baked-on] solid film lubricant; then the SFL coat was 'polished'. What a nightmare... and the single-source vendor charged us an arm-and-a-leg for each finished tube.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[LONDONDERRY]

Okay some updates:
The TuffCoat finishing will be in contact with a Silicon Nitrate wafer. So my concern is which material harder on the Moh's scale or scratch resistance the Silicon Nitrate wafer or the Tuffcoar finishing (which is alumina oxide)


BR
Frank

 

[WKTaylor]

Londonderry... I got to thinking...

This is more of a 'finishes and/or tribology question... may get more informed responses in either of the following forums... but be careful going crazy with multiple 'posts'.

Paint/coatings engineering Forum

http://www.eng-tips.com/threadminder.cfm?pid=332

Tribology Forum

http://www.eng-tips.com/threadminder.cfm?pid=342

Also, one of the old standbys in engineering: when-in-the-dark 'do a test'! There are standard friction/wear tests to compare finishes V finishes or finishes V standards.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

LONDONDERRY,

MIL-A-63576 is a specification for aluminum oxide anodic coatings on aluminum substrates with post-anodize PTFE impregnation or application of a thermoplastic resin containing PTFE. Might be worth taking a look at since this is the process you are actually using.

 

[WKTaylor]

Londonderry... what specification(s) are You working with, RE: PTFE coating process???

Tbuelna... MIL-A-63536 was canceled WO replacement ... However for aerospace use I would suggest that the following AMS, which mostly replaced it...

AMS2482 Hard Anodic Coating on Aluminum Alloys Polytetrafluoroethylene (PTFE)-Impregnated or Codeposited

Same problem exists... no definition of Mohs hardness... probably because each alloy creates a unique oxide hardness layer + the PTFE coating changes that surface substantially.

Is the PTFE coating thick enough so that the [machined smooth?] raw anodic coating NEVER contacts the Silicon Nitrate wafer???

NOTE.
Within AMS2482 there are [2] abrasion test specifications listed.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[MintJulep]

Moh's scale applies to rocks.

Neither of your materials of interest are rocks.

If you want to know which will scratch the other rub them together, then you'll know.

 

[berkshire]

The Mohs hardness scale is also used for Abrasive materials .
B.E.

You are judged not by what you know, but by what you can do.

 

[WKTaylor]

One thing that troubles me about long-term use of PTFE against a soft critical surface is micro-debris accumulation embedded within the PTFE surface film.

Years ago we discovered that band-clamps with PTFE cushions can accumulate embedded particles on/in the PTFE surface against the wire or tubing [etc] that it was supporting... resulting in an amazing abrasion-rate on the wire jacket or tubing surfaces.

I had emergency actions on several F-15s with this phenomena: (a) abraded hydraulic/fuel/oil tubing ODs and (b) some abraded wire/wire-harness jackets [W/WO exposed wire].

The Tubing OD abrasion was only discovered when the camp was loosened and there was a sharp-edge wear pattern matching the cushion.

The wiring jacket damage was reveled after extensive maintenance troubleshooting on intermittent defaults without finding obvious problems [CND or NFF however You categorize it]. The damage was discovered when a 'steely-eyed crew chief' loosened a wire-harness to adjust the harness and found an abraded jacket with bare exposed wire against the dirty PTFE cushion. This 'remove/inspect under the clamp' practice became routine when subsequent CND's or NFFs were maddeningly persistent in the face of obvious intermittent electrical faults.

OK OK OK... this problem was really bad with acft deployed to the middle east and exposed to ultra-fine desert dust. IF in a 'cleanroom' environment may never happen... but I am still wary of PTFE for this reason.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

The coating described appears to be hard anodize (TuffCoat process) with a .001" thick PTFE overspray. One problem with using thin film plain PTFE coatings for surface contacts subject to relative sliding is the very low shear strength of PTFE (<800 psi). Unless the localized contact pressures at the sliding interface are kept very low the thin PTFE film will wear away fairly quickly, allowing contact to occur at the asperity tips of the anodize coating and nitrate wafer surfaces. The continuous process of adhesion/shearing at the asperity tip contacts will generate very hard debris particles that become embedded in the remaining PTFE film, similar to what WKTaylor described above. Once that occurs, the abrasive wear process becomes self-perpetuating and will rapidly accelerate.

Thin PTFE coatings are no magic cure for eliminating wear and reducing friction between surfaces having sliding contact. However, there are some things you can do to improve the coating's performance. The most important thing is to modify your part designs to ensure the localized surface pressures at the contact are kept as low as practical. This would include careful control of the finished contact surfaces profile and texture characteristics. Another thing that can be very effective for reducing scuffing is finishing the surfaces using a progressive honing process that produces "plateaued" asperity tips. The larger surface area of the truncated asperity tips reduces the local contact pressure and inhibits mechanical adhesion.