Abstract:
Aluminium alloys are materials of huge practical importance. However their use is
dependent on surface oxides and hydroxides which are critical in protecting the highly
reactive underlying metal. The stability and integrity of the oxides and hydroxides are
also crucial in finishing and bonding applications. Better understanding of these
surfaces has significant implications in enhancing their application.
LM6 and LM25 aluminium-silicon casting alloys were studied as these materials
show a particularly inhomogeneous phase structure and complex surface behaviour.
This complexity is of fundamental interest and leads to considerable practical
difficulties, especially in surface finishing. The surfaces were characterised, subjected
to thermal treatments and modified with Ion Assisted Deposition coatings of TiN. A
characterisation method for these surfaces was also developed based around the
layered structure of aluminium hydroxides.
It was observed that the thermally induced surface segregation of minor elements,
such as Mg and Na, is availability-limited. Surface concentrations of these elements
are determined by the net effect of enriching via surface segregation and depleting
through surface evaporation. The inhomogeneous phase structure of the alloys used
in this study enables the observation of two migration processes driven by different
forces. Below the oxide dominated surface layer, the migration of Mg is driven by
chemical potential gradient and is primarily perpendicular to the surface. Closer to
the surface, concentration driven horizontal diffusion of the element occurs.
The deposition of a thin TiN layer has been used to probe the interface. While the
deposition conditions of TiN coatings affect the chemistry of the coatings, substrate
surface conditions determine how well bonding is achieved between the film and
substrate. The height difference between silicon particles in the eutectic phase and the
primary aluminium phase of the casting alloys provides surface roughening and
optimal adhesion through mechanical interlocking with the coating. This height
difference is achieved by preferentially removing the surface exposed primary
aluminium phases with ion bombardment.