Wheels and rails of high-speed trains are prone to severe damage, fatigue, and fracture damage on the wheel surface owing to wear, corrosion, strength reduction, fatigue cracking, and other reasons, thus affecting the stability and safety of train operation. The commonly used repair process to eliminate wheel surface defects causes material wastage and economic losses. To improve the service life of a wheel, laser cladding technology is used to prepare a cladding layer on the surface of a wheel and rail to enhance their damage resistance. Therefore, in this study, Fe-, Ni-, and Co-based alloy coatings, widely used in the field of laser cladding, are prepared on the surface of the ER9 wheel material using laser cladding technology. The mechanical properties, damage mechanism, and corrosion behavior of the substrates and coatings are investigated.

The base material of the laser cladding experiment was taken from the ER9 wheel steel tread, and three types of self-fluxing alloy powders—Fe-, Ni-, and Co-based—were used as cladding materials. Laser cladding technology was used to prepare the powder coating with thickness of 15 mm on sample surface by coaxial powder feeding. All samples was cut using the wire-cutting method. First, after the prepared metallographic samples were corroded, a SU8010 scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to study the microstructure and phase of the cladding layer. The microhardness of the samples was measured with a Vickers hardness tester (Qness-Q60). The prepared tensile and impact specimens were then tested for mechanical properties using an MTS universal testing machine and a Charpy pendulum impact testing machine, respectively. Furthermore, the fracture morphologies of the tensile and impact specimens were observed by SEM. Next, the prepared friction and wear samples were characterized by an MFT-EC4000 tester, and the wear surface, wear debris morphology, and element content of the samples were characterized and analyzed using SEM and its accompanying EDS. An electronic balance scale with an accuracy of 0.1 mg was used to measure the average wear. Finally, potentiodynamic polarization curves (Tafel) and electrochemical impedance spectroscopy (EIS) of the samples were obtained using an electrochemical workstation in a 3.5% NaCl solution at room temperature.

As shown in Fig. 2, the coating surface is uniform and dense, without noticeable cracks, pores, and other defects. Furthermore, the microstructure is mainly composed of dendrites and eutectic structures. XRD spectrum analysis (Fig. 3) shows that the Fe-based coating is mainly composed of α-Fe, (Fe, Ni), Cr₇C₃, and other solid solutions. The Ni-based coating is mainly composed of solid solution γ-Ni, intermetallic compound FeNi₃ and hard Cr₂₃C₆ phase. The crystal phases of the Co-based coating are mainly the FeNi₃, γ-Co, and Cr₂₃C₆ phases. The investigation of mechanical properties indicates that the surface hardness after laser cladding treatment improves significantly (Fig. 4), and the Fe- and Ni-based alloy coatings have the highest microhardness (approximately 716.5 HV). The average hardness of the Ni-based alloy coating and Co-based alloy coating is approximately 384.2 HV and 456.1 HV, which are an increment 45.6% and 72.8%, respectively. The hardness of the coating structure is enhanced to achieve a strengthening effect. Figures 5 and 6 show that the elongation of the Fe-based tensile specimen is the lowest (1.34%), and the tensile fracture has cleavage steps. The tensile strength of the Co-based alloy coating is the highest (approximately 976.41 MPa), and the tensile fracture exhibits a river pattern feature. The tensile strength of the Ni-based alloy coating tensile specimen (approximately 813.95 MPa) decreases compared with the substrate, but the elongation reaches 34.5%, and the tensile fracture exhibits a dimple-like morphology. Figure 7 shows that the impact fractures of Fe- and Co-based coatings are brittle, while the Ni-based coating exhibits good ductility and an impact toughness considerably higher than that of the former two. In terms of friction and wear research (Figs. 9 and 11), the wear amount and wear rate of the coatings are significantly reduced, while those of the Co-based alloy coating are the lowest [4 mg and 0.4×10^-4 g/(N·m), respectively], which is 78.9% lower than that of the base material. Only furrows appeared on the wear surface. The wear mechanism is mainly abrasive wear. The wear rate of the Fe-based alloy coating was reduced by approximately 52.6% compared with the substrate, and the wear surface is slightly damaged. The wear mechanism is characterized by abrasive and adhesive wear. The Ni-based alloy coating has a rough grinding surface and a large amount of wear debris accumulation because of the coupling effect of abrasive and adhesive wear. In the electrochemical corrosion study, the Nyquist curves of the substrate and cladding layer in a 3.5% NaCl solution showed capacitive arc characteristics (Fig. 12). The maximum impedance of the cladding layer is two orders of magnitude higher than that of the substrate. According to the test parameters of the polarization curve (Table 4), the self-corrosion potentials of the Fe-, Ni-, and Co-based coatings are -0.475, -0.415, and -0.408 V, respectively, and the self-corrosion densities are 2.980, 0.249, and 0.172 μA/cm², respectively.

The microstructure of the laser cladding coating on the surface of the wheel material is mainly composed of dendritic and eutectic structures. The hardness of the coating is significantly improved. The Ni-based alloy coating has good tensile strength and impact toughness, and the fracture is characterized by toughness, whereas the Co- and Fe-based alloy coatings have a brittle fracture; however, the difference is marginal. Compared with the matrix, the cladding coatings have a lower friction factor, wear rate, and better corrosion resistance, and the Co-based alloy coating has higher hardness (the microhardness was increased by 72.8%). The wear resistance of the Co-based alloy coating is the best (the friction factor is 0.31, the wear amount is approximately 4 mg, and the wear scar depth is 10.70 μm). The corrosion resistance of the Co-based alloy coating is the best (the impedance value is two orders of magnitude higher than that of the substrate). A comparative analysis of the three coatings shows that the Ni-based coating has a rough surface, high wear rate, poor wear reduction effect, and weak hardness and strength. The wear and corrosion resistance of the Co-based coating is higher than that of the Fe-based coating, but the latter has lower engineering costs and also provides overall wheel protection.