Thixoforming : Semi-solid Metal Processing

260j 8 Tool Technologies for Forming of Semi-solid Metals
of 460 V to 27.07 1.7 GPa at 560 V. The Young s modulus also increased from
246.7 24.3 GPa at 460 V to 368.6 10.16 GPa at 560 V. A result of these investigations
is that the deposition process window is small and for the properties of the
coatings it is necessary to operate in this window. Modern plasma diagnostic
equipment can be used to control the deposition process. First investigations using
optical emission spectroscopy (OES) have shown the ability to control this small
process window [31].
8.4.2
Deposition Process Development of Crystalline PVD ZrO2
The advantage of the ZrO 2-based coatings compared with the Al 2O 3 coatings is high
chemical inertness against liquid steel. These results are confirmed by the material
screening of Meyer-Rau [5]. Zirconium oxide occurs in three stable crystallographic
phases, depending on the temperature. The monoclinic phase is stable up to
1170 C, the tetragonal phase is stable within the range 1170–2370 Candathigher
temperatures the cubic phase is stable [32]. The thermal and mechanical properties
depend strongly on the phase structure [34]. In comparison with the other phases,
the tetragonal phase of ZrO2 offers high strength and toughness values. Various
methods have been investigated to stabilize the promising high-temperature
tetragonal phase at room temperature. The most common technology used is to
add dopants such as Y2O3, MgO and CaO to suppress the tetragonal-to-monoclinic
transformation.
8.4.2.1 Experimental Details for the Development of t-ZrO 2
The investigations on the deposition of crystalline ZrO2 were carried out on a Leybold
Heraeus Z400 laboratory sputtering PVD coating machine for the development of a
suitable deposition process window and a multilayer structure using tungsten as an
interlayer with a high thermal conductivity. For deposition, a zirconium metal target
with a purity of 99.5% and a tungsten metal target with a purity of a 99.9% were used.
The argon and oxygen flow rates were independently controlled by mass flow
controllers. The O2 gas concentration was determined by the ratio of O2 mass flow
rate to the total mass flow rate (Ar and O2). As a pulsed power supply, an Eifler bipolar
pulse generator was used. The variations of the deposition parameters are reported in
Table 8.4. The substrates were heated to up to 300 C. The substrates were also
precleaned by r.f. sputtering using the parameters listed in Table 8.4.
8.4.2.2 Results and Discussion
By using pulsed power supplies, it is possible to deposit fine crystalline ZrO2 with
different mechanical and tribological properties. The coated samples show different
properties that depend strongly on the chosen process working points. The variation
of the deposition parameters such as deposition pressure and oxygen flow rate allow
different phases of ZrO2 to be synthesized. It is also possible to stabilize t-ZrO2 at
room temperature without the use of dopands such as Y2O3, CaO or MgO. The
deposition parameters of these samples are presented in Table 8.5.