5. Conclusions
In this study the microhardness and wear resistance, as well
as the microstructure of porous anodic oxides, produced by
anodizing of 99.5% pure aluminium electrodes under controlled
conditions while varying the electrolyte temperature in the wide
range from 5 °C to 55 °C, were evaluated.
The microhardness of the anodized electrodes progressively
decreased with increasing electrolyte temperatures. The wear
resistance of the anodic oxide films, evaluated by means of
fretting wear tests, remained constant for the lower considered
temperatures from 5 °C to 25 °C, whereas from 25 °C on an
increasing electrolyte temperature led to the formation of anodic
films with reduced wear resistance. Both mechanical properties
displayed an important decrease when the electrolyte temperature
was raised from 45 °C to 55 °C.
FE-SEM analyses of the oxide films showed that under the
considered potentiostatic anodizing conditions initially porous
oxides with equal pore diameters were produced at the metaloxide
interface, though chemical dissolution of the oxide by the
electrolyte induced the formation of cone-shaped pores. Hence
in a specific electrolyte for a wide temperature range the pore
diameter at the pore base only depends on the anodizing voltage
that determines the electrical field distribution. On the other
hand, due to the thermal enhancement of the chemical
dissolution of the oxide by the electrolyte the increase of the
pore diameter towards the oxide surface becomes more
pronounced with increasing electrolyte temperature. On the
account of this effect the porosity of the oxides at the surface,
quantitatively determined by image analysis, increased from 4%
at 5 °C up to 32% at 55 °C.
The decrease of the microhardness of the anodic oxide with
increasing electrolyte temperature was in line with the increase
of the porosity with temperature. The wear resistance of anodic
oxide films also degraded with increasing electrolyte temperature,
though this tendency did not completely match to the
observed increase in porosity with temperature. Hence the
deterioration of the microhardness with increasing electrolyte
temperatures was mainly attributed to the increase of the
porosity in the outer region of the oxides. In contrast, the
observed variation of the wear resistance with increasing
electrolyte temperature could not solely be related to the oxide
porosity and was apparently also influenced by other characteristic(s)
of the oxide, as for instance the degree of hydration.
Overall this paper highlights the important influence of the
electrolyte temperature on the microstructure and mechanical
properties of the anodic film and further emphasizes the
significance that electrolyte convection and heat transfer of the
anodizing process have in terms of influence on the mechanical
properties of the anodic oxide film.
Acknowledgements
The authors acknowledge the support from the Instituut voor
de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract nr. SBO 040092). M. Peeters
(KULeuven, dept. MTM) is greatly acknowledged for the
performed wear tests.