**1. Introduction**

Natural organisms live in harsh and complex environments for long periods, so their body surfaces must possess a strong ability to resist external damage. A non-smooth morphology, non-smooth structure, and di fferent material compositions on the surface of the organism are typical biological coupling characteristics [1–5]. Various non-smooth features provide an organism with an appropriate combination of strength and toughness and play a unique role in resisting external alternating stress or direct damage. For example, dragonfly wings [6,7] are often in a vibrational state without failing, which is the result of the collaborative e ffect of the wing surface's grid shape and the wing vein's sandwich structure composed of multiphase materials. The desert scorpion [8] has been subjected to the erosion by sandstone permanently without damage, which is the result of the collaborative e ffect of its non-smooth surface and multiple layers of soft connective tissue under the skin; shellfish [9–11] have been eroded by seawater frequently without crack initiation, which is the result of their non-smooth surface composed of an alternating soft and hard structure and the coupling e ffect of the composite materials. Thus, inspired by the coupling characteristics of the surfaces of organisms, constructing a bionic non-smooth structure on the surface of mechanical parts to obtain specific properties has become a new surface modification method.

Laser remelting treatment [12–15], as a new technology, can be combined with coupled bionics to offer a new approach for manufacturing bionic non-smooth structures. In this process, a high energy laser beam irradiates the surface of the workpiece, making the laser radiation region of the workpiece rapidly melt. Then, the region is quickly solidified via heat conduction from the substrate, so the obtained microstructure is di fferent from the matrix, and when the selected laser processing parameters are appropriate, the grain will be refined. The material's hardness is also much improved based on the pattern [16–18]. In this way, the non-smooth bionic structure with soft and hard phases is processed on the surface. Among these structures, the hard structure processed by laser fusion is called the bionic unit [19].

Previous studies [20–23] have shown that the improvement of material properties is closely related to the biomimetic unit. Materials with non-smooth structural biomimetic units have good wear resistance and fatigue resistance. The properties of the biomimetic unit are closely related to their composition, the microstructure of their matrix material, and their laser processing parameters. For example, Meng [24] studied the unit microstructure and thermal fatigue resistance of di fferent kinds of die steel after laser biomimetic treatment. Zang [25] compared the microstructure and mechanical properties of H13 steel with di fferent microstructures after laser biomimetic treatment. At present, the e ffect of the material composition on laser biomimetic treatment is rarely discussed in the literature. Therefore, in this paper, the low and medium carbon steels commonly used in industry were chosen as the research object, and we studied the influence of carbon content on the characteristics, microstructure, and mechanical properties of the bionic unit by machining the strip bionic unit on its surface. Finally, the wear experiments of samples with di fferent carbon content were demonstrated. The purpose of this study is to provide a reliable theoretical reference for laser remelting strengthening of di fferent carbon steel parts in actual industrial production and to serve industrial production practices.
