Etude expérimentale à l'aide de couches minces de l'influence de l'oxydation sur l'adhésion entre un insert et une coulée en aluminium
Motor parts produced by casting are subjected to particularly high local stresses and must be reinforced by high-strength inserts. The insertion is normally made mechanically after casting, but, if good adhesion could be obtained, it would be preferable to place the inserts into the cast prior to production. The inert, insulating oxide layer which forms spontaneously on aluminium surfaces tends to prevent the insert and cast metals from interdiffusing, thereby rendering adhesion difficult. To solve the adhesion problem several different aspects of the casting process must be investigated. One important aspect concerns surface treatment, where it has been shown that deposition, onto the surface of the insert prior to casting, of a silver-zinc-silver multi-layer structure improves adhesion. The objective of the present work is to study the role that aluminium and zinc oxidation plays in adhesion between the insert and the casted part. The multi-layer structure is investigated by studying independently several single-layer systems, obtained by magnetron sputtering. The substrates are oxidized prior to deposition by anodic polarization or thermal treatment. The adhesion of the deposits is characterized by scratch tests, consisting of linear displacement with increasing normal load of an indenter along the surface of the samples. Normal and tangential forces are measured. Under certain conditions, adhesion can be associated with the applied normal force corresponding to the appearance of defects in the scratch. It can be shown that certain measured force transitions can be correlated with these defects and used to improve resolution and simplify interpretation of the measurements. The results obtained show that for zinc as well as for aluminium an increase in oxide-layer thickness results in a decrease in deposit adhesion. The relative decrease in adhesion as a function of the thickness of the oxide layer is the same for all of the systems studied. Nevertheless, thermal oxidation of the zinc is much faster than that of aluminium, and the zinc layers must be further coated with silver prior to casting.