Anodizing: Typical Hardness of Type III (Hardcoat) Anodized Coatings

Q. What is the typical hardness for MIL-A-8625, Type III, Class 2 coatings? Is wear resistance proportional to hardness? We have a problem with hard anodize wearing through on a 7075-T6 aluminum part that gets heavy wear. The hardness of the coating is 350–360 HV. Would increasing the hardness of the coating show a significant increase in wear resistance?

A. There is no “typical” hardness for Type III anodic coatings. Vickers Hardness 360 is roughly equivalent to Rockwell C of 37. The hardness of the coating depends on alloy and the anodizing conditions. Hardness can be compared to wear resistance only under like conditions. Just as a reminder, reference to “wear resistance” applies only to “flat,” or at least conforming, surfaces rubbing together. Sharp, pointed surfaces and “sharp” edges have very little wear resistance. This is because the anodic coating is brittle and the substrate is relatively soft. A sharp or pointed object can score the coating relatively easily. Sharp cut or machined edges and corners do not wear well because the anodic coating builds perpendicularly to the substrate, so there is actually little, or no, coating where the substrate profile is sharp, pointed or thin.

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If you want to compare wear resistance of Type III coatings produced under differing conditions, but on the same product, you can get an idea of the relative wear resistance of different coating hardnesses and coating thicknesses. Each manufacturer should conduct wear testing that is geared to the application conditions of their product to determine what the acceptable wear characteristics are.

Under certain conditions of process and alloy, a Rockwell C hardness of 50 to 60 (520–700 HV) can be produced for Type III anodic coatings. A harder coating, and perhaps a thicker coating, might show better wear resistance than a less hard and/or thinner coating under the same test or application conditions.

It is possible that the Type III anodic coating is not suitable for your product. This can be determined by testing or field performance evaluation. If the wear resistance of the coating used to be acceptable, but has changed abruptly or even gradually over some period of time, perhaps something in the part design and/or the finishing process has changed. This is often the case, though, at first glance, it can easily be assumed that nothing has changed. You stated that the anodizing is being done by an outside supplier. I’m sure you have reviewed the conditions under which your Type III coatings are produced with the anodizer. Pertinent items for discussion might include finish specification details, the anodizing quality and process plans, actual processing records, routing sheets and inspection data, as well as changes that could be made to both the product design and the process that might result in improved performance of the product.

Originally published in the December 2015 issue.