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Surface Technology

Oxide Ceramic Coatings for Light Metals

| Editor: Janina Seit

The Berlin plant of AHC Oberflächentechnik GmbH is specialized in so-called plasma-chemical processes, which are marketed under the brand name Kepla-Coat (for aluminium and titanium) or Magoxid-Coat (for magnesium). These coating processes produce white, wear and corrosion resistant or black oxide ceramic coatings that absorb strong light.

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These coating processes produce white, wear and corrosion resistant or black oxide ceramic coatings that absorb strong light.
( Source: Pixabay / CC0 )

The Berlin facility of AHC Oberflächentechnik GmbH has a large-scale industrial plant for the complete pretreatment and post-treatment required during the plasma-chemical processes Kepla-Coat and Magoxid-Coat. The plant is equipped with a fully automatic process-control and has an electrolyte volume of 2.8 m3. It can coat components up to a length of 2 m in large series or as individual pieces. A new, fully automatic plant for Magoxid-Coat black is planned to be built for series production, especially for products demanded by the automotive industry.

Magnesium component: Left before and right after plasma chemical coating.
Magnesium component: Left before and right after plasma chemical coating.
( Source: AHC )

Layers Grow Partly into the Metal

The coating processes require salt solutions as electrolytes and an external power source. The workpiece to be coated, which consists of an aluminium, titanium or magnesium alloy, functions as anode. Plasma discharges in the electrolyte transform the surface of the workpiece into a largely crystalline layer formed by oxides of the corresponding base material. The layers grow partly into the metal when they are formed and therefore have a very high adhesive strength. According to the company, the coatings provide excellent fatigue strength. The strength of the base material is not or only slightly impaired. The Kepla-Coat layer can withstand temperatures of up to 2000 °C for a short period of time, and with titanium as a base material it can withstand temperatures of up to 700 °C for a long period.

The motivation for the use of light metals is to reduce the weight of the components. Magnesium alloys also offer advantages in the casting process, such as finer structures. The plasma chemical processes are designed to provide wear and corrosion resistant surfaces (white variants) or UV-resistant and light-absorbing layers (black processes). The light absorption coefficient of the black surfaces exceeds 95 %.

Corrosion Resistant or UV-Resistant

Kepla-Coat white is mainly used for machine components and is suitable for special applications where other coatings fail due to a lack of fatigue strength. Kepla-Coat black is used in cases that require very good UV resistance and high light absorption capabilities. A specialty of the Berlin-based company is the one-sided coating of 50 μm thin titanium foils with Kepla-Coat black and the subsequent gluing of components with these foils. Magoxid-Coat white is used to coat magnesium components in order to protect them from wear and corrosion. Magoxid-Coat white can also be painted later on (e. g. rims for sports cars, bicycles and wheelchairs). Magoxid-Coat black is used when magnesium components require very good light absorption capabilities and high UV resistance.

This article was first published by konstruktions praxis.

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