Thixoforming : Semi-solid Metal Processing

Figure 8.33 Bright-field TEM images of alumina films showing
typical distribution of nanopores. The film was deposited at a
normalized ion flux of 50 [60].
8.5 Developing Al 2O 3 PECVD Coatings for Thixoforming Mouldsj277
reported [53, 60]. Hence it is conceivable here that during growth with Cl2 gas, the
supersaturated alumina matrix precipitates the incorporated Cl, which may then form
bubbles that give rise to the observed porosity. Incomplete decomposition of the AlCl3
molecule in the plasma may be one of the origins of the incorporation of Cl into the
films, as has been proposed based on optical emission measurements [53].
8.5.2.3 Mechanical Properties
The mechanical properties of the coatings were determined by nanoindentation.
Increasing the NIF from 50 to 100 caused a decrease in the elastic modulus from
227 7 to 152 8 GPa for g-alumina. A further increase in the NIF to 140 resulted in
the growth of a-alumina with an elastic modulus of 220 5 GPa. A detrimental
decrease in the elastic modulus to 179 7 GPa was observed with a final increase in
the NIF to 480. Hence the elastic modulus values of both the a- and g-alumina films
decrease with increase in NIF. The highest elastic-modulus values were measured for
the g- and a-alumina films grown at minimal NIF, which is necessary for the
formation of the corresponding phase. This behaviour may be understood by
considering the formation of porous microstructure observed by TEM, which may
influence the mechanical properties of the films.
An empirical correlation between measured elastic modulus and density values of
bulk alumina samples was developed by Knudsen [61] based on a proposal by
Spriggs [62]. The latter has been compared with literature reports [63–65] that discuss
the correlation between elastic properties and density, and strong agreement has
been observed [60]. The porosity of the bulk samples can be calculated by using the
following expression:
E ¼ E0expð bPÞ ð8:1Þ
where E is the elastic modulus of the porous body, E0 is the elastic modulus of a nonporous
body, b is an empirical constant and P is the pore volume fraction of the body.
Equation 8.1 was compared with literature reports that discuss the correlation