Thixoforming : Semi-solid Metal Processing

Table 8.6 Mechanical properties of the deposited coatings.
Property
Metallic
ZrO 2
8.4 Multifunctional PVD Composites for Thixoforming Mouldsj263
Monoclinic
ZrO 2
Tetragonal þ monoclinic
ZrO 2
Tetragonal
ZrO 2
Hardness, HUniv (GPa) 16.5 5.7 15.1 21.4
Young s modulus (GPa) 238.7 146.8 241.9 292.7
The mechanical properties of all samples are given in Table 8.6. The t-ZrO 2 exhibits
the highest hardness and Young s modulus of all deposited ZrO2 modifications.
8.4.2.3 Summary for the Development of t-ZrO2
The investigations have shown that it is possible to find a process window where
stabilized t-ZrO2 can be synthesized without the use of dopands such as Y2O3, CaO or
MgO. The t-ZrO2 exhibits the highest hardness and Young s modulus of all deposited
ZrO2 modifications. The deposition of t-ZrO2 is limited to a narrow process window.
Further developments such as upscaling to an industrial coating unit are needed to
apply these promising coatings to tool inserts.
8.4.2.4 Experimental Details on the Development of t-ZrO 2 on an Industrial
Coating Unit
For upscaling, a co-sputtering setup with two TiAl targets and two zirconium targets
was used. The size of all targets was 88 500 mm. The purity of the zirconium targets
was 99.2%. For the Ti50Al50 targets, the purity was about 99.5% for the titanium and
99.9% for the cylindrical aluminium inserts within the sputter track. During deposition,
the samples were moved in a planetary motion to ensure a constant film thickness
distribution at the substrates. Prior to deposition, the samples were ion etched in
an argon atmosphere. For the etching process, an m.f. power source was used.
The coatings were deposited by using a MELEC pulse power supply (SPIK 2000 A).
For the TiAlZr N interlayer a total pressure of 500 mPa by reactive sputtering in a
mixed atmosphere of argon and nitrogen was used. To enhance adhesion, the coating
process started first with a pure TiAlN interlayer and after a coating time 1000 s
zirconium was added to the process. Pre-examination showed that the adhesions with
a TiAlZrN interlayer suffer in comparison with a graded one. The deposition of a ZrO2
top layer was performed by using two zirconium targets (purity 99.2%; target size
88 500 mm) and the dual cathode arrangement (power 3500 W, pulse frequency
18.5 kHz). The process was flow controlled: the argon flow was kept constant at
250 sccm while the oxygen flow was varied. The oxygen partial pressure was set by
adjusting a certain voltage at the cathodes. The process parameters are listed in
Table 8.7.
8.4.2.5 Results of the Deposition of t-ZrO 2 on an Industrial Coating Unit
The investigations of the deposition process of the ZrO2 top layer showed nearly the
same behaviour as the g-Al2O3 deposition. The properties of the deposited coatings