## 3 Results and Discussion
### Ceramic-Metal Interface Analysis  
The preparation of ceramic coatings for metal substrates refers to the formation of ceramic coatings with certain functionality on the surface of metal substrates by means of technical deposition, or high-temperature oxidation, or chemical adsorption, as shown in Figure 8. The metal-ceramic interface structure is a typical heterogeneous interface. Due to the large differences in the chemical properties of heterogeneous material surfaces and the complexity of the interface formation process, the metal-ceramic interfacial structure presents a variety of combination types. According to whether a chemical reaction occurs at the metal-ceramic interface, the interfacial bonding can be categorized into three types: mechanical bonding, reaction bonding, and diffusion bonding. Combined with the cross-sectional morphology and XRD analysis of the nano-TiO\({}_{2}\) ceramic coating shown in Figure 5 in the previous section, it can be observed that the interface between the nano-TiO\({}_{2}\) ceramic coating and the Q235 steel has a clear boundary and is mainly dominated by mechanical bonding. Mechanical bonding refers to the physical bonding mode formed by metal and ceramic relying on the mechanical interlocking effect of surface rough fibers. In this bonding mode, there is neither a chemical reaction nor atomic diffusion between the matrix and ceramic reinforced phase, and the interface structure is clear and flat. However, due to the low strength of the interfacial connection, this structure is prone to interfacial fracture under external forces. This phenomenon has been fully verified in previous bonding performance test results [60]. The mechanical interlocking mechanism suggests that at the metal-ceramic interface, the irregularity of the substrate surface can significantly increase the contact area between the adhesive or ceramic particles and the substrate, thus enhancing the interfacial bond strength [61]. The higher the roughness of the surface, the stronger the mechanical interlocking effect. This theory can explain why the adhesive force of the coating interface is strongly influenced by the surface finish of the substrate. In addition, physisorption (intermolecular forces such as van der Waals forces and hydrogen bonding) plays a key role in the adhesion process of ceramic coatings. For example, phosphate adhesives can effectively adhere to a wide range of materials due to their intermolecular forces [62]. Copper oxide (CuO) in the  
Figure 8: Schematic structure and function of metal matrix-ceramic composites. cramic component reacts chemically with the phosphate adhesive to form an inorganic polymer network structure that stabilizes the metal oxide particles in the ceramic coating. In addition, the macromolecules of the adhesive can be bonded to each other through hydrogen and ionic bonds, further enhancing the interfacial bonding strength. It is worth noting that the adhesive not only binds to the substrate through physical adsorption but may also react chemically with the metal surface to form chemical bonds. For example, when the adhesive is used for the bonding of metal workpieces such as iron and aluminum, metal surface substitution reactions may occur, so that the cured adhesive and the metal substrate penetrate into each other, thus enhancing the adhesion [63]. Therefore, in order to ensure the effective bonding between the adhesive and the substrate, it is necessary to ensure that it is in full contact with the surface of the substrate, which requires the improvement of the interfacial bonding quality through appropriate surface treatment processes (e.g., solvent cleaning, polishing, and abrasion roughening, etc.) [64]. Nanoparticles are susceptible to chemisorption with metal surfaces, which is mainly attributed to the high surface energy of the nanoparticles, the high activity of the surface atoms, and the surface defects of the metal substrate [65].