Direct Metallization of PMMA with Aluminum Films Using HIPIMS

摘要

Metallization of plastic parts today is mainly realized using electroplating. Within the European Union the use of chromium Ⅵ will be restricted by 2017 following the REACH directive. This besides other aspects like environmental friendliness is driving the development of alternatives. Evaporation as representative of physical vapor deposition PVD processing is the primarily used alternative. Nevertheless there are restrictions with respect to part size and throughput. The major challenge in evaporation is the realization of satisfying adhesion. In many cases chromate (Cr Ⅵ) based etch solutions, additional lacquers, or adhesion improving interface coatings have to be applied before metallization. Using ionized sputtering like high power impulse magnetron sputtering HIPIMS opens new horizons for cost effective, environmental friendly plastic metallization with excellent adhesion. First investigation using titanium as metal on different untreated plastics (PPSU, PEI, PEEK, PESU, PSU) showed significant adhesion improvement using a simple tape test for evaluation when comparing mid-frequency sputtering and HIPIMS. Further investigation on aluminum deposition on Plexiglas PMMA showed exciting results. Since PMMA is very sensitive to the UV radiation of technical plasmas, direct metallization of the surface by sputtering is conventionally not possible. Using ionized sputtering it is shown that the adhesion can be enhanced to excellent level passing a combined cross cut and tape test without any failure. The study of the interface reveals some insight in the responsible mechanisms. With increasing peak current in the HIPIMS discharge, i.e. increasing degree of ionized species forming the film, the adhesion is significantly improved. The failure mechanism changes from adhesive failure and poor adhesion to a cohesive failure and excellent adhesion. Furthermore a surface modification of the polymer is observed with increasing ionization. The PMMA surface reorganizes and roughens due to ions forming the film and additionally electrons providing local thermal annealing by recombination.