Thixoforming : Semi-solid Metal Processing

Figure 8.10 Schematic PECVD coating concept.
Among the most obvious differences between reactive magnetron sputtering
and PECVD are the physical state of the primary source of the depositing species
(which is solid for sputtering and gaseous for PECVD) [13, 14] and the collision
probability (generally in magnetron sputtering collisionless transport is expected
across the substrate sheath, whereas in PECVD collisions have to be taken into
account) [17].
8.4
Multifunctional PVD Composites for Thixoforming Moulds
8.4 Multifunctional PVD Composites for Thixoforming Mouldsj251
One of the challenging aspects of steel thixoforming is the development of a die
concept which works up to a peak temperature of 1400 C. The investigations focused
on a PVD-coated metallic mould to withstand tribological loads such as thermal,
chemical and mechanical interactions between the die surface and the forming
material 100Cr6, HS6-5-2 or X210CrW12. Suitable tool material solutions are
described in Chapter 1 (Table 1.2). These possible solutions seem to be compromises.
Molybdenum and its alloys, for instance, suffer from their poor oxidation resistance
in air at temperatures exceeding 600 C. The following investigations focused on
TZM and hot working tool steel as die material with a protective PVD coating on top,
which seem to have the best combination of properties regarding the forming of steel.
The requirements for the coating chemistry and architecture can be derived from the
forming process and from the tool materials used. The challenge for a PVD protective
coating of the TZM mould is to ensure chemical resistance against the semi-solid
metal and oxygen. The degradation of the uncoated moulds results in the formation
of volatile molybdenum oxides above 700 C in air. There is also an internal demand
in a sufficient bonding within the coating structure, resulting in a mismatch of the
coefficient of thermal expansion and the Young s modulus. More than 90 different
bulk ceramic materials were investigated during material screening for suitable
coating materials, but only the Al2O3 and ZrO2 bulk materials showed promising