Crack formation and its prevention in PVD films on epoxy coatings

Severe cracking was found to occur in PVD titanium films on epoxy powder coatings. After all baking treatments, the epoxy coating had smooth, crack-free surfaces and the cracking of both the titanium film and the epoxy only took place as a result of physical vapour deposition. Tensile cracks were observed in the titanium film and not the compressive cracks expected from the conventional two-layered theoretical model. An alternative model has been developed for the prediction of thermal stress in a three-layered film-epoxy-substrate system. The model is consistent with the experimental trials and showed that cracking originated from thermal stresses developed in the titanium-epoxy-aluminum system due to the PVD process. Tensile instability and cracking were initiated where pores intersected the film-coating interface. The results showed that crack formation could be prevented by increasing the baking temperature to 210 °C. This critical temperature activates full crosslinking in the epoxy structure and raises its strength sufficiently to avoid tensile instability due to residual stress. Crack-free and high-gloss sputtered titanium films could therefore be produced on organic coatings. This offers the potential of a combined in-line PVD-powder coating technology as an alternative to electroplating.