Letter 34033

Hardness of type II vs type III anodize [New Mexico] 

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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

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.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

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.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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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