Excimer laser surface modification of coated steel for enhancement of adhesive bonding

Zinc coated sheet steel in the form of temper rolled galvanize and galvanneal are used extensively in the automotive industry. Through a process of excimer laser surface treatment, we have developed a procedure to significantly enhance the adhesion characteristics of these coated steels. We report here results of processing trials using both XeCl (308 nm) and KrF (248 nm) excimer lasers and a two-part epoxy adhesive (3M DP-460) with a range of processing conditions. Bond strengths are measured by T-peel and shear test methods. Using T-peel tests, bond strength improvements greater than five times than for untreated surfaces have been observed. With the improved surface condition, the bond strength becomes limited by the cohesive strength of the adhesive. Detailed measurements of the physical structure and chemical composition of the excimer laser processed surfaces are presented. The enhancement in bond strength is correlated with the observed changes in physical and chemical structure of the laser processed surfaces. Surface structure is observed using SEM and physical characteristics are quantified using a Talysurf profilometer. The chemical composition of the treated surface has been analysed using XPS and time-of-flight mass spectroscopy.