Hard anodizing

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Hard anodizing , also known as hard anodizing, hard anodizing or hard coating, is an electrolytic oxidation of aluminum materials in order to create protective layers on the surface. These serve as protection against wear and / or corrosion , have good tribological properties and, depending on the counterpart partner, have very good sliding properties, which can be further improved by PTFE impregnation.

Procedure

The aluminum component is immersed in an electrolyte and connected as an anode . The surface is oxidized and a layer of aluminum oxide is formed on the metallic aluminum . In the process, there is an increase in volume on the component surface, i.e. an increase in dimension, usually by approx. 50% of the total layer thickness with standard processes. Layer thicknesses of approximately 25 to 50 µm are usually  produced.

In contrast to anodizing , which is carried out at room temperature and with relatively low currents, hard anodizing or hard anodizing is carried out with very high currents. The electrolyte must therefore be cooled so that the components do not overheat or even "burn". Usually, as in anodizing, sulfuric acid is used as the electrolyte, but other acids or mixtures are also possible and influence the layer properties.

All aluminum alloys can be hard anodized, the purer the alloy, the more oxides can be formed. Layer thicknesses of up to 200 µm can therefore be achieved on very pure alloys. Such alloys are almost never used, however, since in technical applications there are usually very high and specific demands on the base material in terms of hardness, corrosion resistance or other properties.

application

Hard anodic layers are much more dense and much harder than "normal" anodic layers. The aluminum oxides in the layer as well as the alloy components that are dissolved out during the coating or conversion process (e.g. Cu) or incorporated as insoluble components (e.g. Si) have a significant influence on the structure, the hardness and corrosion resistance of the layer. The layer hardnesses that can be achieved depend on the aluminum alloy used and range between 350 and 600 HV (Vickers hardness). The hardness of the layer is measured exclusively in the cross-section, because when measuring such thin and brittle layers, there is a risk of destroying the layer. this means that the hardness of the base material can also flow into the measurement, which would falsify the measurement as a whole. A mixed or pseudo hardness is obtained through the pores in the layer. The oxides of aluminum actually have a much higher hardness, from which one can then deduce that, under certain conditions, significantly better wear properties can be achieved than with metallic layers such as e.g. B. against a chrome layer, which has a Vickers hardness of approx. 1000 - 1100 HV. Hard anodized layers are used for technical purposes, for example for bolts, bearings, housings, guides, control valves, control pistons, gears, worms, molds and aluminum profiles. Since it acts as an insulating ceramic layer, the hard anodized aluminum is also used for electrical insulation. The respective insulation effect depends on the base material and the layer thickness. The advantage of these layers is that you can use lighter materials (unless a certain basic strength and toughness of the material is required) and still achieve extremely good wear and corrosion protection values.

Aftercare

Due to their porous structure, hard anodized layers can be post-treated to improve or change the properties. The layers "grow" vertically out of the base material and form a barrier layer on the base. Above this barrier layer there are channels through which the current flows to the base material and allows the layer to grow to the desired or maximum possible thickness. These pores can be through a recompression in about 96 ° C hot water VE closed. The closed pores make it very difficult for moisture and oxygen to reach the base material, which means that much improved protection against corrosion is achieved. However, this protection comes at the expense of good wear protection properties, since one in the recompression (Sealing) boehmite detaches from the layer and this weakens the top. The pores can also be "impregnated" with an aqueous PTFE solution. PTFE gives the hard anodized aluminum significantly improved sliding properties. However, only PTFE layers with a maximum thickness of 3 µm are achieved. The PTFE layer cannot withstand abrasive wear and tear for long.

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