Practical adhesion of electroless Ni thin films on polymer surfaces. A novel approach combining fragmentation test with electrical resistance measurements

摘要

This paper deals with the practical adhesion of electroless Ni thin films deposited on flexible polymer substrates (polyimide (PI), polypropylene (PP), polycarbonate (PC) and poly(tetrafluoroethylene) (PTFE)) via a method previously developed in our laboratory, involving the plasma treatment of the polymer surfaces in an ammonia atmosphere (NH3) and their direct "activation" by immersion in an acidic PdCl2 solution. The so-grafted Pd(+2) species are then chemically reduced to the Pd(0) oxidation state (substrate immersion in a hypophosphite solution) and used to initiate the electroless plating process. Practical adhesion of the Ni films to their substrates is investigated using a fragmentation test (also termed a stretch deformation or a tensile shear test) according to a novel approach which consists in measuring the change in the electrical resistance R of the metal film when the metal-polymer composite system is submitted to an increasing uniaxial strain. Results show that correlations can be established between the changes in the relative electrical resistance 8R/R and the development of the metal crack pattern. In addition, the proposed approach allows to evidence differences in adhesion performances as a function of the plasma treatment time and the nature of the polymer substrate.