Investigations into the use of excimer laser irradiation as a titanium alloy surface treatment in a metal to composite adhesive bond

For an aerospace wing structure application, surface treatment of the metal substrate by excimer laser ablation was identified as a likely replacement for some of the more traditional, hazardous, chemical techniques. Three different types of excimer laser mixtures (KrF, ArF and XeCl) have been investigated in this study. The fluence levels of the lasers ranged from 0.3 to 5 J/cm2. This work is not a comparative study of the three named mixtures but instead presents a review of the critical property enhancements provided by these treatments (e.g. lap shear strength, contact angle and reduction in the amount of wicking by water). The dry, fresh bond strength of this system does correlate well with the predictions provided by the theoretical “work of adhesion” calculations. The issue of wet performance of the adhesive bond is addressed and mechanical testing does reveal that a durable bond can only be provided if an epoxy primer is applied after pretreating the titanium. Contact angles measured after surface treatment were shown to be significantly less than that for the untreated surfaces but were also shown to increase with prolonged storage of the treated sample under ambient conditions. Therefore, emphasising the importance of bonding or primer application within the immediate hours after the surface treatment. The amount of wicking by water along the modified titanium surface was measured and this was shown to be less than the amount measured on an untreated surface.