Thixoforming : Semi-solid Metal Processing

276j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.32 Bright-field TEM images of alumina films deposited
at normalized ion flux values of (a) 50, (b) 100, (c) 140 and (d)
480 [64].
Mesopores are those pores with diameters varying between approximately 10 and
30 nm observable on bright-field TEM images (Figure 8.32).
Finally, in Figure 8.33, the bright-field TEM images show pores belonging to the
third population, the nanopores, which have diameters of about 5 nm or smaller.
Several mechanisms could explain the formation of these porous structures. The
first is the production of point defects due to ion bombardment and subsequent
agglomeration at grain boundaries, dislocations and free surfaces resulting in the
formation of pores and voids. A second mechanism which may contribute to the
observed porous structure is void formation parallel to the growth direction due to selfshadowing
in competitive growth. Finally, the formation of the porous structure under
ion bombardment may also be understood based on ion bombardment-induced
bubble formation [58, 59]. The incorporation of Ar and N in reactive bias-sputtered TiN
films [58] and the incorporation of Cl during PECVD growth of alumina have been