Thixoforming : Semi-solid Metal Processing

Table 8.4 Deposition parameters for the multilayer system W–ZrO2.
Process parameters Etching phase Interlayer tungsten Top layer ZrO2
Process pressure (Pa) 1 0.5 0.3–1
Process time (s) 1800 3600 7200
R.f. power (W) 100 — —
R.f. frequency (MHz) 13.56 — —
N 2,O 2 concentration (%) — — 2–16
Pulse parameters
Pulse mode/pulse power (W) — DC/400 Unipolar/500
Pulse sequence (ton–/toff–) (ms) — 5/25 3.56/2.67
M.f. (kHz) — 0 33
Pulse voltage (V) — Power controlled 980–1022
Pulse current (A) — — 0.5
Table 8.5 Properties of four representative coatings.
Property
Metallic
ZrO2
8.4 Multifunctional PVD Composites for Thixoforming Mouldsj261
Monoclinic
ZrO2
Tetragonal þ monoclinic
ZrO2
Tetragonal
ZrO2
Depositing pressure (Pa) 1 1 0.5 0.3
Coating thickness (mm) 9.9 3.8 8.3 6
O2 (sccm) 1.76 3.96 2.07 1.7
O 2/Ar (%) 2 2 5 7
Phase composition Metallic Monoclinic Tetragonal þ monoclinic Tetragonal
To study the effect of the different crystalline phase on the properties of the films,
four characteristic samples were selected. By changing the O 2 to argon ratio, a drop in
the deposition rate in nanometres per minute can be observed. The effect of the
oxygen flow rate on the coating thickness is shown in Figure 8.18 This behaviour can
be explained by the hysteresis effect of the deposition process [33]. With increasing
oxygen flow rate, the coating structure changed from metallic to crystalline and finally
to amorphous (Figure 8.19). The phase composition of the deposited samples was
determined using XRD as described previously. By changing the oxygen to argon
ratio, the experiments revealed that with a deposition pressure of 1 and 0.5 Pa only a
small amount of tetragonal phase can be deposited. For the coating, the development
of t-ZrO2 coatings with very small crystallite sizes is needed to stabilize this phase [34].
Calculations using the Debye–Scherer equitation show that the domain size for the
t-ZrO2 coatings is in the range 4–5nm.
Figure 8.19 shows the cross-sections of the investigated samples. The cross-section
show in Figure 8.19c indicates a very fine-grained morphology for the tetragonal
phase compared with the other three SEM images.