How to define the hardness of the aluminum oxide film formed by hard anodizing

The term “Hard Coat” or Hard Anodizing gives the impression of an anodizing process which gives a very hard anodic layer. The values below show that the anodic layer formed by this process really is harder but it is still important to remember that saying "Hard Coat" to an anodizer doesn’t give him enough information to process the metal.

HARDNESS COMPARISON between different materials
  • Untreated Aluminum Alloy 6082 - HV 100 - 120
  • Hard Anodized Alloy 6082 - HV 400 – 460
  • Stainless Steel - HV 300 – 350
  • Mild Steel - HV 200 - 220
The values are measured in (VPN) = Vickers pyramid number, also referred to as the Vickers hardness number (HV or VHN).

The Vickers hardness is the amount of force applied to the diamond divided by the area of the indentation the diamond makes in the material; in practice the diagonal of the pyramidal indentation is measured and the result is read from a table and is stated as an empirical measurement, without units.

Other hardness measurement numbers are found, such as, Brinell, Rockwell and Knoops. The Vickers hardness is up to about HV 500 about 1.04 times the Brinell hardness but most of the time hardness of the anodic oxide layer is measure by Vickers.
Knoops hardness is almost identical with Vickers hardness except for the form of the diamond. In this testing method the diamond has a rhombic-based pyramidal shape. The form of this
indentation makes it possible to measure the hardness of aluminum oxide more accurate but it is still not widely used.

The anodic oxide layer is very brittle and to obtain the best reproducibility of the measurements the Knoops diamond should be used. Using Vickers hardness measurements causes cracks in the oxide layer, so only measurements in the middle of the oxide layer are possible.

HARDENSS TESTING on anodic coatings should be carried out on the edge of the film so that the effect of the underlying, soft, aluminum is eliminated but not to close to the edge so the
softness of the resin influence the results.

The image to the right shows a cross section of a hard anodic coating with Vickers indentations and thickness measurements.

The pyramid has to be square formed to be sure of hardness value measured.

The light blue to the left is the aluminum alloy and the dark to the right in the picture is the resin.

THE MILITARY SPECIFICATION, MIL-A-8625F, for “Anodic Coatings for Aluminum and Aluminum Alloys” says nothing about any requirements of the hardness of Type III – Hard Anodic Coatings, see earlier post about this subject here.

The European Standard EN 12373 ”Aluminium and aluminium alloys – Anodizing” has none either. Both of them have on the other hand requirements of a wear resistance of the coating.

The maximum wear index for coatings on aluminum alloys having a copper content of 2 % or higher is of 3.5 mg/1000 cycles and 1.5 mg/1000 cycles for all other alloys.

Next post will discuss the wear resistance versus the hardness of hard anodic oxide coatings.

For more information on how to define your hard coat for your product please contact me