When investigating the AlOx coated samples, the roughness data obtained was very similar to the plain films and no impact of pre- or post-treatment on the roughness of the AlOx layer was observed. For each film type, plain and AlOx coated film exhibited virtually identical surface roughness values, which again indicate that the thin AlOx layer reproduces the underlying surface roughness and structure as seen in the SEM investigation. Deng et al., who used electron beam evaporation to deposit AlOx layers onto PET and polypropylene, found that this was only the case for PET, whilst for polypropylene the 10 nm AlOx layer showed an increased roughness compared to the plain film. Therefore, they hypothesized an island growth mechanism for the AlOx layer on polypropylene and a layer-by-layer growth mechanism on PET