Thixoforming : Semi-solid Metal Processing

252j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.11 Schematic drawing of the segmented test die (a) and coated inserts (b).
behaviour during the corrosion tests [5]. It was concluded from these basic investigations
that these materials are promising coating materials for tool protection.
The first investigations using PVD processes for thixoforming moulds for steel
were carried out by Hornig [19]. During his investigations, he performed a material
screening with different stable oxides deposited on hot working steel and TZM dies.
To gain an understanding of which coating materials and coating concepts work for
the forming process, different segments were coated and then tested at the Institute
of Metal Forming (IBF), RWTH Aachen (Figure 8.11).
Hornig investigated the need for an interlayer and also the process parameters for
the deposition of partially stabilized zirconia (PSZ)–ZrO 2,MgAl 2O 4 and Al 2O 3
(Table 8.1). This fundamental characterization combines the mechanical stability
and the adhesion properties of the substrate–coating system. The depositions of the
Table 8.1 Deposition parameters for coated segments of the test die.
Process parameters
Etching
phase
Interlayer
TiN/CrN
Top layer
PSZ–ZrO 2–5%Y 2O 3 MgAl 2O 4 Al 2O 3
Process pressure (Pa) 1 0.75 1 1 1
Process time (s) 1200 600–1800 14 400 14 400 14 400
R.f. power (W) 100 — — — —
R.f. frequency (MHz) 13.56 — 13.56 13.56 13.56
Ar:N2 ratio (%) — 33.4 — — —
Coating parameters
Power supply — DC r.f. r.f. r.f.
Power (W) — 400 200 200 200
Deposition rate (mmh 1 ) — 6 0.3 0.125 0.125
Heating — Varied Varied Varied Varied