Application of fracture mechanics to the adhesion of metal coatings on CFRP

Carbon fibre reinforced polymers (CFRP) have to be plated with wear resistant metal coatings for many applications. The mechanisms of adhesion between the CFRP and metal phases are primarily mechanical interlocking and van der Waals bonding. The adhesive failure of the metal coatings is greatly influenced by the condition of the original CFRP surface. Therefore, a mechanical roughening process, which produces interlocking surface sites, and in addition the change of the chemical nature of the CFRP surface by various processes, has to be applied in order to promote adhesion of the plating. In this case, it is important to understand the mechanisms involved, and to develop testing methods to measure the adhesive strength. Instead of pull-off tests, a fracture mechanics test is a more promising approach, as it directly determines the ability of the CFRP-metal interface to resist the growth of a crack. To carry out this test procedure, a modified CT specimen according to ASTM Standard was used. The crack length was controlled by a prior preparation of one side of the CFRP specimen, so that the metal adhesion was weakened there, causing this part of the interface to serve as a crack. This adhesion test method has been tried on Cu and Ni plated CFRP specimens and resulted in maximum critical free energy release rate values G(c) of 10-30 N m(-1). The testing method described here is also applicable to adhesion measurements of other coated polymers.