Laser shock peening enables 3D gradient metal structures: A case study on manufacturing self-armored hydrophobic surfaces

Gradient heterostructures typically exhibit excellent mechanical properties. The traditional laser shock method can produce only 1D or 2D gradient structures along the thickness of a material. In this study, we propose a technique called 3D gradient laser shock peening without coating (3LSPwoC) for manufacturing 3D gradient metal structures. An excellent application of this method is the manufacture of multi-scale hydrophobic surfaces with integrated enhanced armor (IE-armor) in a flexible, large-scale and low-cost manner. Hydrophobic surfaces of metals are of great importance, but are typically mechanically fragile and degrade quickly, as the surface nanostructures tend to break under mechanical forces. Current approaches either expose the functional large-aspect-ratio nanostructures directly to external forces or have unbalanced strength-ductility synergy for dynamic loads, resulting in degradation of the properties. A self-armored hydrophobic surface structure was obtained by a combination of laser shock and low surface energy treatment. An IE-armor structure with a well-designed strength-ductility synergy was considered to protect the rich nano-hydrophobic structures. The arrayed micro-pits and abundant micro-nano structures in the pits realized a stable Cassie-Baxter state, resulting in a superhydrophobic surface. The alternating regular distribution of hard and sub-hard domains on the metal surface, together with the soft domain in the core, formed a 3D gradient structure, which achieved excellent synergistic plastic deformation and provided superior mechanical robustness. The 3D gradient metal structure manufactured using the 3LSPwoC process is expected to play a crucial role in highly reliable functional surfaces in aerospace, locomotive manufacturing, and ocean engineering.