Silicon oxide films deposited by PECVD are widely used for corrosion resistance or as nickel diffusion or gas and water vapour permeation barrier on metals and plastic substrates. In fact, silica layers are inert in acid or low basic environments and on a plastic surface can decrease oxygen permeation even of two orders of magnitude and reduce Ni diffusion in products decorated by galvanic coatings. However, dense and tough oxide films are brittle and in flexible devices such as polymer film for packaging or flexible thin metal products such as foils or glasses temples the protective performances are reduced due to cracks propagation or coating delamination during the elastic or plastic deformation.
Therefore, in order to avoid cracking, to accommodate surface defects and to achieve a higher mechanical compatibility between substrate and coating, a multilayer structure has been developed that alternates deformable layers and dense "blocking" layers in the nanometre scale. The RF-PECVD deposition process has been developed as easy as possible choosing single layer properties just changing the oxygen flow in the chamber. The process has been developed in a 1m3 industrial deposition chamber, using HMDSO as silica precursor. The oxygen and precursor flow controls the methyl groups content and the reticulation degree of silica leading to different mechanical and structural properties.
The coatings have been deposited with different number of periods and layers thickness with a total film thickness of 100nm for permeation barrier purposes and of about 2µm for diffusion barrier and corrosion resistance aims. The coatings composition, morphology and topography and layers thickness have been characterised by ellipsometry, FT-IR and GDOES, SEM and optical profilometry. Mechanical properties have been measured by nanoindentation finding out the possibility to design the coating properties by the multilayer structure. Moreover in temperature nanoindentation measurements up to 300°C have been performed to check the temperature stability of the coating. The multilayer design influence on permeation behaviour, on nickel diffusion and on corrosion resistance performances has been also verified. |