Thixoforming : Semi-solid Metal Processing

272j 8 Tool Technologies for Forming of Semi-solid Metals
Table 8.9 Experimental conditions during the deposition
experiments: peak voltage (VP), deposition temperature (Ts),
pressure, precursors ratio (P R) and precursor content (P C).
Set of experiments 1:
variation of VP and Ts
Set of experiments 2:
variation of Pc
Set of experiments 3:
variation of PR and Ts
coated sample at temperatures of 1170 30 C for 5 s. The sample was then cooled
under an air stream for 15 s. This procedure was repeated 1000 times. After this
treatment, a cross-section of the sample was evaluated by SEM and EDS.
8.5.2
Results and Discussion
Peak
voltage (V)
Temperature
( C)
Pressure
(Pa)
AlCl 3/O 2
ratio
720 to 905 500–600 175 1 2
AlCl 3 þ O 2
content (%)
900 600 175 1.15 0.8–5.3
900 540–600 175 0.5–1.15 2–3.45
8.5.2.1 Chemical Composition and Constitution
The influence of the power density (PD) on the films constitution was studied by
varying the peak voltage from 720 to 905 V (set of experiments 1, Table 8.9). P D
was calculated by multiplying the measured current by the voltage, which was then
normalized with respect to the cathode surface area. The influence of PD on the
chemical composition of the films was evaluated by EDS. No dependence of the Al/O
ratio which was measured to be 0.59 0.02 at.% was observed. Nevertheless, it was
found that all films contain chlorine at levels from 0.9 0.01 to 2.3 0.01 at.%
depending on their constitution. This will be discussed later. The phase-formation
data from 12 experiments were compiled into one diagram correlating TS and PD with
the film constitution (Figure 8.28).
The data in the diagram can be grouped into three regions according to the
constitution of the films, namely region I, phase pure g-alumina; region II, mixture of
g- and a-alumina; and region III, phase pure a-alumina. In this diagram, it is clearly
observed that, on increasing the energy supplied to the growing films (thermal or
through particle bombardment), a-alumina is preferentially grown. It is important to
note that by increasing PD to 6.6 Wcm 2 and TS at about 560 C it is possible to grow
phase pure a-alumina.
According to theory, the mobility and dwell time of adsorbed particles on the
surface are important for the film constitution. These parameters are mainly
controlled by the energy provided to the growing film. In PECVD processes, this
energy can be provided not only thermally by substrate heating as in conventional
CVD processes, but also by collision of adsorbed particles with the ions bombarding
the surface. Due to the latter phenomena, the adsorbed particles received additional
energy which led to higher mobility and therefore to modification of the film