Composite surface pre-treatments: Improvement on adhesion mechanisms and mechanical performance of metal–composite friction spot joints with additional film interlayer

Friction spot joints of aluminum alloy 2024-T3 and carbon-fiber-reinforced polyphenylene sulfide (CF-PPS) were produced with the PPS film interlayer. Mechanical grinding, sandblasting, and sandblasting combined with plasma activation were performed on the composite part to enhance the interface adhesion. The surface features–roughness, wettability, and chemical activation–were correlated with the ultimate lap shear force of the joints. The composite surface with the highest surface roughness (sandblasting: 5.3 ± 0.6 µm) led to joints approximately 95% stronger (3068 ± 192 N) than the joints with the lowest surface roughness (mechanical grinding: 0.6 ± 0.1 µm, 1573 ± 84 N). The increase in surface roughness enlarged the effective contact surface area, leading to a better micro-mechanical interlocking between the PPS film and composite. Although functional groups were identified in the plasma-treated specimens using X-ray photoelectron spectroscopy, no contribution to the mechanical strength of the joints was observed. The fracture surface analysis supported the conclusion that sandblasting was the most effective treatment, maximizing the mechanical performance of the joints. Impressions containing pieces of carbon fibers were identified on the interlayer surface. It indicates effective micro-mechanical interlocking at the interface of interlayer-composite achieved with the sandblasted specimens.