Thixoforming : Semi-solid Metal Processing

8.5 Developing Al 2O 3 PECVD Coatings for Thixoforming Mouldsj269
g-Al2O3 and t-ZrO2, worked well in the forming experiments. A suitable interlayer
concept was developed to provide good adhesion in order to support the oxidic PVD
top layer. The combination of oxide coatings with a TiAlN interlayer showed good
results after SEM and EDS evaluation. A comparison between uncoated and coated
dies confirmed the stability of the oxide coating system and its potential for
technological application in the semi-solid forming process.
8.5
Developing Al2O3 PECVD Coatings for Thixoforming Moulds
Alumina (Al2O3) coatings are known to exhibit advantageous mechanical properties
and also chemical inertness and oxidation resistance at high temperatures [39]. Thin
alumina films act as diffusion barriers for Ni, Al, Ti and Ta even at 1100 C during
annealing for 100 h [40] and it has been shown that this material is a suitable
candidate for coatings used in wear and hot gas protection applications [41, 42].
Therefore, in this work, we investigated alumina coatings for protecting die surfaces
during the semi-solid processing of steel [43]. Despite the availability of tooling
concepts for the semi-solid processing of Al-based alloys [44], the increase in the tool
lifetime is an almost unaddressed challenge for the semi-solid processing of steel [43].
The degradation of the die is enabled through the interaction between the die and the
casting alloy at temperatures >1300 C [45] and may result in the formation of
intermetallic phases, chemical reaction products, thermal fatigue cracking and wear
by erosion and corrosion [46, 47]. Alumina, exhibiting the previously discussed
properties, is therefore a promising candidate to fulfil the requirements of semi-solid
processing of steel. Alumina as a polymorphous material exists in several modifications
(a-, k-, g-, d-, q-Al2O3). The alumina polymorphs presenting the highest
protective performances are the thermodynamically stable a-alumina and the
metastable k-alumina. Various deposition techniques including CVD [42] and
PECVD methods have also been employed to synthesize alumina coatings at
relatively low temperatures. Lin et al. reported the growth of amorphous alumina
for deposition temperatures ranging between 200 and 600 C [48], while T€aschner
et al. reported a mixture of a- and g-alumina and even phase pure a-alumina, utilizing
an unipolar pulsed DC plasma at a substrate temperature of 650 C [49].
Although the reduction of the deposition temperature allowed by these latter
methods is significant, it is not sufficient for coatings on steel since tempering of steel
tools occurs at approximately 550 C [50]. The only published studies reporting
deposition of a-alumina at temperatures permitting deposition on steel tools are
based on homoepitaxial [51] and localized epitaxial [52] growth on chromium
templates. However, the reported deposition rates (1 nm min 1 ) of both of these
processes are not compatible with the requirements of the die coating industry. The
major aim of our work within the SFB 289 project was to design a process to perform
the deposition of a-alumina at substrate temperatures allowing deposition on steel
dies, namely at 550 C. To achieve that goal, a bipolar DC PECVD method has been
developed. The systematic study of the relationship between the process conditions