Thixoforming : Semi-solid Metal Processing

280j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.37 Friction coefficient measured during hightemperature
tribological tests of the deposited alumina films [56].
evaluation of the coatings after the treatment by cross-sectional SEM and EDS
analysis was aimed at identifying possible adhesive or cohesive failure and also signs
of possible interdiffusion taking place through cracks or other defects in the films.
The alumina films show very good resistance to thermal shock; as can be seen in
Figure 8.38, no crack formation or erosion of the films was observed. Moreover, after
contact with steel, the film surface was smooth without indication of damage or
delamination. Possible diffusion of the counterpart material into the substrate was
not observed based on EDS experiments.
The results of these performance-related tests confirmed the applicability of the
alumina films for die protection during semi-solid processing of steel.
8.5.3
Conclusions
Alumina thin films were deposited on tempered hot working steel by PECVD in an
AlCl3–O2–H2–Ar mixture. The constitution and mechanical properties of the films
were found to be strongly dependent on the deposition conditions such as substrate
temperature, gas mixture and peak voltage. It has been shown that a-alumina films can
be grown at substrate temperatures as low as 560 C. Finally, the constitution,
morphology, impurity incorporation and elastic properties of the alumina thin films
were found to depend on the normalized ion flux. The changes in structure and
properties may be understood by considering surface and bulk diffusion-related
mechanisms. The formation of porous structures is observed. The formation of the
porosity may be understood by considering vacancy generation, migration and subsequent
clustering and/or the incorporation of Cl during growth, migration and bubble
formation. The elastic properties of the films were strongly affected by the formation of
the pores in the structure and were intimately linked to the normalized ion flux.