**1. Introduction**

Results have shown that the base metal on the brake drum surface of gray cast iron may be melted rapidly by laser technology. This can change the structure of the brake drum base metal, refine the grain, and greatly improve hardness, strength, and toughness [1–3]. By imitating the biological characteristics of organisms that exist in nature that show excellent wear and fatigue resistance [4,5], a bionic functional surface similar to the surface of such organisms can be fabricated by applying a laser to the inner wall of a gray cast iron brake drum [6,7]. The structure of the base metal of the brake drum can be irradiated by a laser [8,9], causing it to melt rapidly and then solidify again instantaneously [10]. The new structure obtained has a strength and toughness far beyond that of the gray cast iron base metal [11,12]. The locus of the melted structure is distributed on the inner surface of the brake drum with a certain shape [13,14]. Thus, the hard unit and the base metal are combined to form a soft/hard interphase bionic surface composed of di fferent structures and shapes. Previous test results showed that the hard element embedded in the matrix has a "dike" and "nail pile" e ffect [15,16]. This hard element can e ffectively prevent the growth of cracks in the brake drum, reduce the growth rate of surface cracks in the gray cast iron, and improve the service life of the brake drum.

The experiments conducted in this study attempted to address human needs by learning and applying the mechanisms and laws of the biological world that have been discovered by the application of the bionics principle [17,18]. The phenomenon of biological coupling is an inherent property of living things that has increased the vitality of organisms throughout their evolutionary history. The researchers in this study found that the ability of natural organisms to adapt to their environment did not simply involve changes in a single factor; instead, this adaptability resulted from the synergy of two or more di fferent parts or the coupling of di fferent factors. Some examples of this adaptability include the self-cleaning function of the leaves of plants such as the lotus leaf and reed and the anti-sticking property exhibited by the wings of insects such as the night moth [19,20]. These functionalities are all realized by the coupling of various factors such as the non-smooth shape of a surface or the micro/nanocomposite structure of low-energy materials. For example, the non-smooth hard scales on the backs of lizards, rock lizards, and scorpions are coupled with multiple layers of flexible connective tissue that lie just under the skin, which allows these species to have excellent resistance to erosion in desert environments. The excellent wear resistance of conch and other seashells depends on the coupling of non-smooth composite morphology, multilayer structures, and special materials [21,22].

Similarity science points out that the principles of certain biological structures and functions can be used to construct technical systems and make the characteristics of these technical systems like those of biological systems. The systems formed have functions that are like those of the original system. The surface structures of organisms with excellent wear resistance and fatigue resistance share many similar characteristics [23]. First, they all have alternating structures made up of hard and soft elements. The distribution of hard elements can take various forms. The relatively high degree of hardness of the hard elements comes from the di fference in structure or material between the hard and soft elements. The coupling of morphology, structure, and material gives such surfaces excellent wear resistance and fatigue resistance. The properties of a surface comprising soft phases and hard phases can be brought fully into play in biology [24]. The hard phase structure can play a supporting role by, for example, preventing crack initiation and propagation; in addition, this structure can improve the wear resistance of materials. According to the principle of biological coupling, the bionic coupling wear-resistant and thermal fatigue-resistant model is a type of hard element with certain shapes distributed on the soft base metal. This hard element derives di fferent microstructural or constituent materials from the soft base metal, and the two constitute a structure with soft and hard intersections [25]. Coupling bionic wear-resistance and anti-fatigue properties has been proposed as a way to theoretically solve the problem of part failure caused by fatigue and wear on the surface of materials and engineering application problems such as surface adhesion and drag reduction.

Much research and achievements in applications of bionic coupling wear-resistance and fatigue resistance have been made using laser treatment [26]. Laser technology also has a lot of applications in the surface strengthening of alloy parts. Qiuyue Su et al. studied the influence of nano layer depth etched by femtosecond and nanosecond laser on the precision of resistance modulation [27]. This method is applied to wear-resistant parts under various working conditions. The hard phases are processed on their parent bodies by laser melting or cladding. These hard phases and the parent bodies form di fferent types of soft and hard interphase structures on the surface of the parts, replacing the original surface and greatly improving their thermal fatigue resistance, wear-resistant performance, and the service life of the parts. According to the principle of biological coupling, di fferent functions can be obtained by di fferent coupling and factor combinations, and di fferent functional requirements can be obtained by changing the parameters of each coupling element to form corresponding models [28]. All these studies are based on the experimental optimization design of various bionic coupling models, which are composed of coupling elements that include various shapes, structures, and materials. After the effects of di fferent coupling bionic treatments on the thermal fatigue resistance and wear resistance of materials are determined, they can be applied to each wear-resistant component to improve the service life of components [29].

The application of coupled biomimetic theory has achieved many breakthroughs in terms of wear resistance and fatigue resistance. However, in the process of biomimetic model design, research has primarily been based on a single type of spacing [30]. Although the performance achieved by the biomimetic

model depends on the characteristic parameters of the coupling element, the spacing distribution of the unit always maintains an average distribution [31]. In nature, the non-uniform distribution of various bio-coupling elements on the surface of organisms can further improve the performance of organisms by enhancing their ability to cope with more complex biological environments. In the process of braking, the surface of the brake drum is constantly subjected to tension and compression stress in the process of brake pad wear [32]. At the same time, because of the large amount of heat generated in the process of friction, the interior of the brake drum is constantly subjected to the internal stress transformations caused by the alternation of cold and hot temperatures. Therefore, the fatigue failure and crack growth mode of the brake drum represent locally concentrated non-uniform sudden changes in growth. This study designed a coupling bionic model of the surface of a non-uniform gray cast iron brake drum to further enhance the reliability of the wear-resistance and anti-fatigue characteristics of the bionic brake drum. In this paper, based on the above research, the uniform distribution of the bionic surface was designed as a non-uniform distribution. The microstructure and internal stress of laser melting changed from discontinuous processing to continuous processing of the local area. Therefore, the wear resistance and fatigue resistance of the non-uniform distribution bionic surface also change correspondingly. This paper hopes to further optimize the wear resistance and fatigue resistance of the gray iron coupling bionic surface and provide a more practical model. The model that we designed used a di fferent combination of multiple units rather than a single unit to enhance the wear-resistance and anti-fatigue properties [33].
