Development of a fs-pulsed laser pretreatment process on thin-walled nickel to improve adhesion for adhesive bonding applications

Due to its inert surface, producing adhesive bonds on nickel is challenging and requires a surface pretreatment. A laser pretreatment process on nickel is investigated using a Yb:YAG slab laser at 780 fs pulse length. The process is varied in pulse density, pulse energy, and focus position to produce various surface structures on a nanometer scale. The surface structures are categorized topologically in scanning electron microscopy (SEM). Random nanostructures, laser-induced periodic surface structures (LIPSS), and process vapor depositions are observed. To quantify the adhesion properties of the produced surfaces, peel tests are conducted using a two-component epoxy adhesive. All investigated process parameters lead to a significant increase in peel strength; however, high pulse density and high pulse energy in focus lead to the highest peel strengths. These process parameters generally produce LIPSS and process vapor depositions on the surface, which might be linked to the high peel strengths. The analysis of the fracture pattern shows an adhesion failure, and in SEM, a partial failure of adhesive and adherent is visible. The surface structures are fully wetted by the adhesive on a sub-micrometer level. Nanoparticles deposited from the process plasma are broken out of the surface during the peel tests. A general trend to high accumulated fluence for best results in peel strength is shown.