Coating of Parts

In this project we focus on the high-rate deposition of thick molybdenum layers. The extent to which this can be used as an alternative production method for filigree components made from this thermally and mechanically highly resistant metal is being investigated.

Molybdenum as a material and as a coating

The production of fine sheets or tubes from molybdenum is technically difficult, given that molybdenum has to be processed at high temperatures. Forming takes place at a temperature of 700°C, often followed by annealing steps at 1000°C or at even higher temperature. Because the metal exhibits brittle behaviour when more than 100 ppm of oxygen are included, all steps must be carried out under inert gas atmosphere. Altogether, this results in high energy consumption and high production costs. Due to its properties, the metal is also not suitable for powder-based additive manufacturing.

An alternative way to produce filigree components made of molybdenum could be to deposit thick layers of the material on a supporting structure made of easy-to-machine base material with subsequent separation of the support and the layer. In order to achieve components with sufficient stability, comparatively thick layers need to be deposited. 

The Coating Components Group has investigated the deposition of thick molybdenum layers, facing the challenge of creating a dense, pore-free structure. At the same time, the stresses in the coating, or in the coating-substrate composite, must not become too big in order to avoid stress-induced coating failure or substrate damage.

In pulse magnetron sputtering, in addition to the working pressure and the deposition temperature, the choice of pulse parameters represents an additional way of influencing the microstructure and the internal stresses of the resulting layers. If these are changed in the course of the layer build-up, a stress profile can be generated that ensures the integrity of even thick layers. With these measures, molybdenum layers up to 50 µm thick have been deposited to date. In order to minimize thermally induced stresses caused by the different expansion coefficients of the substrate and the coating, a suitable substrate material must also be selected. So far, glass has proven to be a good substrate to which molybdenum adheres very well. So well, in fact, that a thin intermediate layer must be created, which specifically weakens the layer-substrate interface, to separate the filigree components. The non-destructive separation of component and support structure is currently in the focus of the project work.