Thixoforming : Semi-solid Metal Processing

Figure 8.8 Coating concept for avoiding the softening of the steel substrate.
the corrosion tests were carried out at the GHI (Institute of Mineral Engineering) and
showed that only the oxides withstand the corrosion attacks from the semi-solid steel.
Also, thermocycle tests were carried out at the MCh (Institute of Materials Chemistry)
using HS 6-5-2 as a counterpart material. These tests showed no counterpart
interaction with the bulk material Al2O3,Al2O3/SiO2 (mullite) and MgAl2O4.MgOand
Y2O3-stabilized ZrO2 bulk materials were also tested and showed limited
applicability for the thixoforming process. It is concluded that only oxide materials
are suitable as potential coating systems. From the load profile shown in Figure 8.1, it
is evident that the coatings need to protect the die s surface from abrasive and
chemical wear. Oxidic ceramic coating materials often exhibit superior chemical
resistance to other hard coatings [12].
8.3
PVD and PECVD Coating Technologies
8.3 PVD and PECVD Coating Technologiesj249
There are different methods for the application of thin-film coatings. PVD and
PECVD offer different advantages and disadvantages. Hence both technologies were
investigated within the Collaborative Research Centre (SFB) 289.
PVD methods cover a range of thin-film deposition techniques, which include
evaporation, laser ablation, vacuum arc-based deposition and many different modes
of physical sputter deposition [13]. The PVD method used in the present work is
magnetron sputtering (Figure 8.9), in which an electric field is applied between two
electrodes immersed in an inert gas (argon) to create a plasma. One of these
electrodes (the cathode) is the material source, the so-called target, and the chamber
walls commonly act as the anode. The argon ions created in the plasma are attracted to
the target (e.g. aluminium, zirconium) and sputter the latter, dislodging atoms [14].
These atoms are subsequently transferred in a controlled manner in an evacuated
chamber to the substrate where the film growth proceeds atomistically. The typical
operating pressure for magnetron sputtering ranges between 0.1 and 3 Pa, which
means there is a very low probability of chemical reactions in the plasma. The
deposition and properties of the films made by magnetron sputtering depend on a
variety of parameters, such as the energy and direction of the incident particles, the