PD Dr.-Ing. habil. Stefan Rosiwal
Research Group Leader, Friedrich Alexander Universität Erlangen-Nürnberg
Aluminum Casting CVD Diamond as Protective Layer in Aluminum Casting
Crystalline diamond layers applied by chemical vapor deposition (CVD) prevent direct contact between the molten aluminum and the steel tool. In a „New Materials in Bavaria“ project, the Chair of Materials Science and Technology of Metals at the Friedrich-Alexander University Erlangen-Nuremberg has succeeded for the first time in adhesively coating various steel alloys with crystalline diamond.
The strong reactivity of aluminum with iron and other metals leads to the alloying of aluminum to the tool surface when an aluminum melt comes into direct contact with a steel mold. This reduces the dimensional accuracy of the castings and the demoldability and service life of the mold. Elaborate countermeasures such as spray treatments to increase the demoldability or restrictions in the geometry are currently necessary both technically and economically. Crystalline diamond layers applied by chemical vapour deposition (CVD) prevent direct contact between the molten aluminum and the steel tool. At the usual casting temperatures, the crystalline diamond does not react with the aluminum.
In a project funded by the Bavarian Research Foundation, the Chair of Materials Science and Technology of Metals (WTM) at the Friedrich-Alexander University of Erlangen-Nuremberg (FAU) has succeeded for the first time in coating various steel alloys with crystalline diamond in an adhesive manner. The worldwide first industrial testing of CVD-diamond-coated steel tools in cooperation with two Franconian industrial partners (Schulte & Schmidt - Nuremberg, Telsonic - Erlangen) has now taken place within the "New Materials in Bavaria" project "DiAlum - CVD diamond against aluminum wear" funded by the Project Management Jülich and the Bavarian State Ministry for Economic Affairs, Regional Development and Energy.
Crystalline Diamond Coatings
Crystalline diamond coatings are produced by hot wire activated chemical vapour deposition (HF-CVD) under vacuum from an activated hydrogen-methane atmosphere. A direct adhesive diamond coating of steel substrates is not possible due to the high affinity of iron and carbon. At the present deposition parameters (hydrocarbon-containing atmosphere and temperatures of 750 °C to 950 °C), metastable iron carbide would first form on the steel surface, which would then decompose to graphite and iron. To prevent this, a barrier layer must be applied before the diamond coating.
Boron-doped titanium nitride (TiNB) fulfils the requirement profile as a barrier layer very well, as it strongly slows down both carbon diffusion into the steel substrate and iron diffusion out of the work piece. The interlayer system also ensures excellent mechanical anchoring of the diamond layer due to its rough and plate-like morphology. Boron doping enables excellent chemical bonding by creating free bonding sites for carbon. The high-temperature TiNB coating is produced at over 1000 °C by hot-wall CVD (HW-CVD). Alternatively, chromium carbide or tantalum based intermediate layers are available as barrier layers for a CVD diamond coating.
The CVD barrier coatings are applied in a hot-wall CVD coating plant from Surmetall. For CVD diamond coating, high-performance (50 kW) diamond coating systems with coating volumes of up to 30,000 cm³ (40 cm x 25 cm x 30 cm) are constantly being adapted and further developed in the course of optimizing the coating processes to suit the increasingly complex and larger tool geometries (max. unit weight previously 40 kg). This enables a homogeneous coating of voluminous tools at the upper temperature end of the diamond process window (up to approx. 950°C). An integrated helium quenching unit ensures high cooling rates. This allows heat treatment of the steel substrate to achieve a hardening structure and the associated high strength characteristics directly in the diamond coating process. By adding boron-containing gases, the diamond can be doped and the electrical conductivity can be adapted to the requirements via the boron content. In contrast to amorphous carbon (Diamond Like Carbon - DLC), the carbon-carbon bonds are almost completely sp3-hybridized, resulting in maximum hardness and wear resistance.
A reduction of the thermal cooling residual stresses between diamond and the steel tool after the diamond coating is decisive for reliable adhesion of the diamond coating. These are caused by the very different thermal expansion coefficients of the ceramic diamond coating and the metallic steel substrate. The differences in strain at room temperature can be significantly reduced by making targeted use of the volume expansion of the steel during cooling from the diamond coating temperature, which occurs during the phase transition from austenite to ferrite/bainite/martensite. Very high diamond coating temperatures above 900°C are therefore necessary to achieve sufficient austenitisation (e.g. chromium carbide dissolution). The measurement of the thermomechanical parameters, such as the temperature-dependent coefficients of thermal expansion, the phase transformation points as well as the time-dependent phase transformations by means of dilatometry in a DFG research project running in parallel led to a deeper understanding of the processes taking place and enabled a further adjustment of the diamond coating parameters.
Coating and Testing
Within the DiAlum project the CVD diamond coating of different steel tools for aluminum die casting was carried out as described above. The service life of core pins (Fig. 2) could be quintupled by a diamond coating as shown in Fig. 3. The crystalline diamond coating also performs better than a commercially available PVD coating. Facilitated demolding reduces the mechanical stress on the mold. The diamond coating was able to increase the service life of center cores from 2,800 shots to at least 5,800 shots (Fig. 4). The component and surface quality was excellent over the entire series, as no damage to the insert occurred during the test period.
Uncoated slide valve cores showed aluminum adhesions after only about 500 shots. A diamond coating (Fig. 5) was able to prevent sticking over the entire series (19,000 shots). The project also successfully coated mold inserts with large dimensions (Fig. 6). Industrial testing was not limited to mold tools. Electrically conductive diamond coatings were applied to fill level electrodes. The non-stick effect of the diamond has the advantage that no drops are formed and thus the filling level of the crucible can be determined exactly. Thus a constant filling level of the ladle is achieved and empty shots are effectively prevented. An enormous extension of the tool life (Fig. 9) was achieved by CVD diamond coating of punching knives (Fig. 8). The diamond coating reliably prevents the formation of built-up edges, so that instead of a two-week tool change, the punching knives are in use for months.
Diamond coatings can be used in aluminum die casting in many different ways. The service life of the molds can be significantly increased, since both the chemical reactions of the aluminum melt with the mold and abrasive processes are effectively prevented by the extremely hard and inert layer. Plant damage due to insufficiently filled casting chambers, for example, can be avoided by using diamond-coated casting electrodes. By increasing the service life, the economic efficiency of the diamond coating is guaranteed, since the coating costs must be balanced against the additional mold costs as well as the set-up costs and downtimes. This could be demonstrated on an industrial scale in the project "DiAlum - CVD diamond against aluminum wear" with a tool life of several years.
Contact: PD Dr.-Ing. habil. Stefan M. Rosiwal (Email: firstname.lastname@example.org)