Thixoforming : Semi-solid Metal Processing

8.5 Developing Al 2O 3 PECVD Coatings for Thixoforming Mouldsj271
Figure 8.27 Time-dependent evolution of substrate current and voltage [53].
substrates were mirror polished, ultrasonically cleaned in acetone and methanol and
positioned on the cathode, which served as substrate holder. The substrates were then
heated to the deposition temperature, Ts, (500–600 C) in an Ar-H2 plasma. Ts was
monitored by means of a Ni-Cr-Ni thermocouple attached to the substrate holder. The
deposition was carried out in an AlCl3–O2–H2–Ar mixture at a total pressure of 175 Pa
and a total gas flow rate of 22.3 slh (standard litres per hour). Except for AlCl3, all gases
had a purity of 99.999%. AlCl3 was generated in situ by flowing HCl over Al chips of
purity >99.99% at a temperature in the range 500–550 C. At this temperature, the
reaction is complete and the AlCl3 flow can be controlled by means of the HCl flow.
The precursor content (AlCl3 þ O2), PC, and the precursor ratio (AlCl3/O2), PR,were
varied keeping both the total flow rate and the pressure constant by adjusting the Ar
and H2 flows. The experimental conditions are summarized in Table 8.9.
The morphology and chemical composition of the deposited films were measured
by SEM and EDS, respectively. The crystallographic structure was determined by
grazing incidence X-ray diffraction (GI-XRD) using 900 W Cu Ka radiation with a
Siemens D500 diffractometer, which was operated at a grazing incidence of 3 . Planar
thin foils were examined in a FEI Tecnai G 2 TF 20 Ultra-Twin field emission gun
transmission electron microscope operated at 200 keV. Hardness and elastic modulus
were measured by nanoindentation using a triboindenter apparatus from Hysitron.
The tip was calibrated against fused quartz according to the method described by
Oliver and Pharr [54]. We assumed a Poisson s ratio of 0.3 and each sample was
characterized by at least 20 indents at a maximum penetration depth of