Thixoforming : Semi-solid Metal Processing

8.4 Multifunctional PVD Composites for Thixoforming Mouldsj255
applications [25]. The major problem with the common DC sputtering technology is
the so-called target poisoning depending on the oxygen ratio. The chemical reaction
takes place not only on the substrate, but also on the chamber walls and on the target.
When a certain oxygen partial pressure is exceeded, the target is totally poisoned and
the deposition rate decreases. On the other hand, a low oxygen partial pressure leads
to the deposition of metallic, conductive material. For this reason, the partial pressure
has to be controlled closely. Another problem is the formation of an insulating layer
on the shielding and the targets, which leads to potential displacement and an
increasing tendency for arcing, respectively. This makes the reactive DC sputtering
unattractive for industrial applications. To solve these problems, Schiller et al. used
pulsed power supplies [27, 28]. A further beneficial effect of the pulse technology is
the increase in the plasma density [28]. In 1998, Belkind et al. examined the influence
of different pulse parameters on the reactive sputtering of alumina by applying the
unipolar pulse technique [29]. The critical frequency where arcing occurs depends on
cathode current, pressure and reverse time (duration of the positive discharging
time) and lies in the range 1–80 kHz. It increases with decreasing pressure and
reverse time.
8.4.1.1 Experimental Details for the Development of g-Al 2O 3
The development of crystalline g-Al2O3 for application in tool inserts was done using
a Model Z400 laboratory sputtering PVD coating machine (Leybold Heraeus, Hanau,
Germany). For deposition of the bilayer system (TiAlN/g-Al2O3, a Melec SPIK 2000 A
pulsed power supply was used. For the target material, an aluminium metal target of
99.2% purity and a titanium–aluminium metal target (50:50 at.%) with a purity of
99.9% was used. Prior to deposition, the vacuum chamber is evacuated to a base
pressure of 7.8 10 5 mbar. The process gas argon and the reactive gases nitrogen
and oxygen possess a purity of 99.999%. The Ar, N2 and O2 flow rates are controlled
independently by mass flow controllers. For the TiAlN interlayer the N2 concentration
was kept constant at 30%. while the oxygen flow was varied between 2 and 16%.
The variation of the deposition parameters is reported in Table 8.2. The substrates
were also precleaned by r.f. sputtering for 30 min using the parameters listed in
Table 8.2.
8.4.1.2 Results and Discussion
The challenge for the development of crystalline Al2O3 coatings lies in avoiding the
formation of a dielectric reactive layer at the targets. A strong O 2 flow-dependent
hysteresis effect is observed and needs to be stabilized within a small process window.
At a significant oxygen flow rate, the effect of target poisoning is observed by a drop in
voltage and deposition rate (coating time ¼ constant) (see Figure 8.14). This effect
marks the transition region of the target s surface from a metallic (zone 1) to a
crystalline (zone 2) and finally to an amorphous zone (zone 3). By forming a
nanocrystalline structure, an increase in mechanical properties and hardness was
achieved for an oxygen flow rate of 2.25 sccm (Figure 8.14) [30].
To investigate the high-temperature stability during the thixoforming process,
high-temperature X-ray diffraction (XRD) measurements were performed with the