Thixoforming : Semi-solid Metal Processing

8.5 Developing Al 2O 3 PECVD Coatings for Thixoforming Mouldsj273
Figure 8.28 Phase-formation diagram of alumina films deposited by PECVD [44].
structure [55]. Based on the results presented here, it is proposed that both ion
flux and ion energy at the substrate surface play an important role in the formation
of a-alumina. Thus, a significant reduction in the deposition temperature can be
obtained by enhancing the surface mobility of the deposited atoms by increasing the
electrical power density at the substrate, which leads to an increase in both ion flux
and ion energy at the substrate surface [44].
Other process parameters than the peak voltage can be controlled in order to tune
the ion bombardment, for example, by varying the precursor content, PC, as has been
shown using the second set of experiments (see Table 8.9). In order to quantify the ion
bombardment, we determined the normalized ions flux (NIF) according to a method
suggested by Kester and Messier [56]. The NIF is a function of the ratio between the
measured ion flux and measured deposition flux. The contribution of secondary
electrons to the ion flux is assumed to be negligible and the sticking coefficient of the
deposited flux is assumed to be 1. The modification of PC leads to significant
modifications of the deposition rate (a) and, therefore, of the NIF. This is presented
in Figure 8.29, where one can observe a significant increase in a as a function of PC.
Conversely, the NIF decreases from 480 to 50, meaning a decrease in the energy
supplied to the growing film by ion bombardment. As expected, the constitution of
the deposited films has a strong dependence on NIF, as shown in Figure 8.29. As the
NIF is increased from 50 to 100, phase pure g-alumina is formed, whereas at NIF
140, phase pure a-alumina is observed (Figure 8.30).
Again,theimportanceoftheion bombardment onthe phaseconstitutionofalumina
films is clearly demonstrated and is explained by considering the additional energy
supplied to the growing films. For the discharge conditions employed within this set of
experiments, we have estimated, based on a simple model put forward previously, that
the number of collisions in the cathode sheath is approximately 16 [17], with an average