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Letter 34033
Hardness of type II vs type III anodize
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What is the differance Rockwell hardness for
Mil-A-8625 [link is to spec at TechStreet] Type II and
Type III.
Tom Provost
- Santa FE, NM, USA
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Neither of these can be measured directly by the Rockwell method -
the coatings are too thin for that. If they could be measured by
Rockwell, they would be about Rc 70, type II, perhaps slightly less.
The main difference is thickness, with type III being more than twice
as thick, hence the superior wear resistance of T III.
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Jeffrey Holmes,
CEF
- Spartanburg, SC, USA
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You cannot measure hard aluminum oxide over soft aluminum
substrate with Rockwell, it will crush through and give meaningless
numbers.
Hardness on anodizing is measured on the crosssection with a Knoop
diamond. Reynolds has published some hardness numbers in their MAE
papers.
The Military gives no verbage to hardness, they only address
thickness. "Type III" being around 0.002.
There is very little hardness difference, if any, between aluminum
oxide thin as Type II versus Aluminum Oxide thick as Type III. Again
"Hard Coat" really means "thick coat". It is all very hard and you
cannot indent it from a perpendicular angle without crushing through
to the soft substrate.
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Actually, hardness is a misnomer. Abrasion resistance would be a
more-correct term. And although type III coatings can be thicker, it
is the DENSITY of the coating that offers the abrasion resistance. As
far as hardness is concerned, the hardness is directly related to the
material composition, such that a "harder" alloy like 7075 would be
"harder" than a 6000 etc etc
Bill Grayson
- San Jose, CA
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