How Hard Anodising Improves Aluminium Wear Resistance
An interesting property of aluminium is that although it is light and easy to machine it has a very soft surface. This surface can be ‘treated’ to greatly improve its wear resistance.
What the Oxide Layer Actually Does
Hard Anodising converts the surface of the aluminium to aluminium oxide which is a much harder material than the original aluminium. Typically untreated aluminium has a Vickers hardness of around 60 to 100 HV. However, the hard anodic layer can be as hard as 400 HV to 600 HV depending on the alloy and process conditions. It is therefore a completely different material with different wear characteristics to untreated aluminium.
How Friction and Surface Texture Respond
The surface of the hard anodised layer is slightly porous and of low energy, thus not adhering to other surfaces. This makes the hard anodised layer particularly suitable for sliding applications and where the lubrication is light.
Coating Thickness and Dimensional Control
The typical coating thickness for wear resistance is in the region of 25 to 75 microns with 50 microns being the average. Approximately half of the coating thickness is built up within the parent metal, thus prior to anodising the part will have grown to approximately half the final size. This dimensional growth must be taken into account when calculating the pre-anodising machining tolerance.
How It Holds Up Against Abrasion
In comparison to the normal sulfuric acid anodizing process (5 to 25 microns thick, HV 200) the hard anodizing process offers much better wear resistance due to the higher hardness of the coating. For this reason, the hard anodizing process is frequently used in the hydraulic industry for components such as valve bodies and sliding guides. Aluminium surface treatment standards are specified for the anodizing process as well as for other surface treatments. The alloy to be treated and the process parameters must be chosen in accordance with these standards to guarantee a good and reproducible coating.
Which Alloys Respond Best
The 6000 series, particularly 6061 and 6082 are good alloys for hard anodising. The 7000 series can be anodised but it is important to control the electrolyte bath. The high copper 2000 series alloys are difficult to process and achieve high hardness, therefore it is recommended to check with your processor for suitability.
To create a wear resistant surface on aluminium parts, it is very important to match the hard anodising process with the correct alloy and application.