Adhesion

With coatings adhesion has natural practical interest, but also academic for many. When adhesion fails there is likely not much to see by eye, but likely a lot for one who is interested on surface sciences and can turn those into practise.

FEM model of practical adhesion test

Surface state

Surface state inherently varies from materials internal structure but one can say that adhesion correlates with the time that surface has experienced in between its formation and coating. Often this can be true as surface contaminates fastly, already in a minute with layers of water molecules if nothing else.

Sometimes adhesion failure can be more of a cohesive in its nature, eg. native copper oxide does not bond very strongly with copper due to lattice miscmatch 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.

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.

In-situ cleaning in vacuum prior coating by glow discharge and sputter cleaning is efficient low temperature cleaning method to remove certain organic residues and alter surface chemistry and -energy into direction that promotes adhesion. Selection of applied gass atoms and powering systems provides a window to play within.

Adhesion force

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 plastic substrate. Gold does not really like to bond others than with itself but can accept another metals into metallic bond, and another metal with different electron structure can bond with polymer surface.

Adhesion testing

Once surface is suffiently clean and materials combatibility to handle exprerienced loads assured, future reliability of system is ensured when adhesion is validated and verified with simple practical adhesion tests as tape peel test and/or thermal shocks.

Concluding words

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