Hard anodising layers generally have an inherent colour that is dependent on the alloy composition. This varies from grey and brown to black. The surface roughness usually increases during hard anodising. On edges and corners there is usually a distinct edge effect with the process. Therefore, one should always be careful to choose the largest possible radius of curvature in the design. Oxide layers formed by hard anodising have high abrasion and wear resistance. They are good thermal insulators because the layer has a thermal conductivity value about ten times lower than the base material. They also exhibit very good electrical insulation properties. The adhesion of the hard oxide layers is very good. In general, these coatings are also more corrosion resistant than Eloxal coatings.

The high wear resistance is due to the hardness and morphology of the aluminium oxide, which is alloy-dependent. The achievable hardness of the hard Eloxal layer is between 400 and 500 HV 0.025, depending on the composition and structure of the base material. In contrast to sulphuric or chromic acid anodised coatings, coatings produced by hard anodising have a uniform hardness over the entire thickness of the coating.

Hard Eloxal coatings provide optimum corrosion protection on aluminium. This can still be optimised by means of post-sealing.

The thermal conductivity is about 1/10 to 1/30 of the thermal conductivity of the base material. With the appropriate thickness, the layer forms a good thermal insulator and can be used for heat shielding.

Hard Eloxal coatings can withstand short-term temperature peaks of up to 2200 K.

The post-sealed hard anodising layer is electrically insulating. The breakdown voltage on the part is > 3000 V. For alloys without Cu content it is higher than for alloys with Cu content.

Depending on the semi-finished product, the aluminium oxide layer grows about half into the metal and about half out of the metal. The oxide layer is indissolubly anchored in the base metal and can only be removed mechanically by destroying the layer system. Usual layer thicknesses are in the range of 30 to 80 μm. In exceptional cases, up to 150 μm can be achieved.

Due to the growth characteristics of the oxide layer, a material shortage occurs at sharp edges. This can lead to cracks in the layer in the edge area. This makes the edges more sensitive to stress and can even cause them to flake off.

In hard anodising, the layer is built up by converting the base material and therefore forming a solid unit with it. Since the stretching capability of the aluminium oxide is practically zero, hairline cracks occur even with elastic deformation of the base material.

Due to the inherent colour of the layers (grey, brown, black), only adsorptive colouring with black is recommended. For other colour requests, consultation is necessary.

The following alloys are possible:

• Aluminium and aluminium alloys (however, the copper content of the alloys should preferably be below 2%)
• Cast alloys after consultation

Note: Various alloy components such as silicon, manganese, copper and zinc lead to a clouding and colouring (inherent colour) of the oxide layer.