Adhesion of Coating

With coatings adhesion has natural practical interest, but also academic for many. In vacuum coating, adhesion is where surface science meets practical reality. When a coating fails, the cause is rarely visible to the naked eye; it usually lies in the atomic-scale "surface state" at the moment of deposition.

FEM model of stresses in practical adhesion test

The Race Against Contamination

Adhesion of the film to surface is best accomplished by having some degree of solubility or chemical reaction. Surface state inherently varies from materials internal structure and one can also say that adhesion correlates with the time that surface has experienced in between its formation and coating. A surface is a living entity. From the moment it is created, it begins to react with its environment, often covering itself in layers of water molecules or hydrocarbons within minutes.

Sometimes adhesion failure can be more of a cohesive in its nature, eg. copper oxide does not bond very strongly with copper due to lattice mismatch or polymer surface can contain broken polymer chains, or weakly bonded filler materials. If adhesion fails problem solving can require rather extensive practical testing and utilization of analytical methods.

Surface cleaning

Number of available surface cleaning and -modification methods is vast. Factors to be considered, beside the physical limitations, are type of contamination and size of particles one would like to remove.

At NextGen Surfaces Finland, we mitigate this through in-situ cleaning. By using glow discharge or sputter etching inside the vacuum chamber, one can strip away organic residues and activate the surface immediately before the coating begins.

Adhesion force

With rough surfaces mechanical interlocking in between substrate and coating may take place but assuming surface is clean, adhesion force is determined by the type of bond in between materials. Sometimes additional adhesion layer is needed, eg. if one would like to deposit a layer of gold onto polymer substrate or onto a metal that already has oxide. Gold is chemically inert and has a low affinity for bonding with non-metallic substrates.

Coating adhesion testing

Once surface is sufficiently clean and materials compatibility to handle experienced loads assured, future reliability of system is ensured when adhesion is validated and verified with simple practical adhesion tests, such as tape peel- and/or thermal shocks, while the evaluation of structural strenght would require scratch- or other destructive method.

Concluding words

Applied deposition method, possible internal stresses in coating and it's cohesive forces has it's role in determining resulting practical adhesion but when things are done in vacuum, in where very little anything extra is present, it's possible to prepare also a very clean surfaces, and alter surface state to without changing surface topography.