Objective Wear is one of the common failure modes of mechanical parts, and the preparation of anti-friction and wear-resistant coatings on the surfaces of parts susceptible to wear is an effective method to relieve wear. Molybdenum (Mo) is considered one of the best materials in achieving high resistance to wear. Although thermal sprayed Mo coating has excellent wear resistance and self-lubricating properties, the mechanical bond between thermal spray coating and substrates is difficult to meet the strength requirements of some key components. Due to its metallurgical bonding feature, laser cladding has received considerable attention in recent years. For steel substrates, iron-based, cobalt-based, and nickel-based self-fluxing alloy powders are the most commonly used cladding materials. A small amount of Mo element (mass fraction <10%) is typically added to the laser cladding powder, mainly to refine the grain, improve toughness and plasticity, and reduce the crack sensitivity of the coating. However, in previous studies, Mo elements in the coating are in the form of carbides or intermetallic compounds, and it is difficult to have a similar lubricating effect to Mo in the thermal spray coating, and the effect of Mo, as the main component added to the laser cladding powder, on the coating performance is unclear. Therefore, in this study, five kinds of powders with Mo mass fraction of 0--70% were used to prepare coatings with different compositions on the surface of EA4T steel by laser cladding to study the effects of Mo content on the coating structure, mechanical properties, and friction and wear properties.

Methods Pure Mo powder was mixed with Fe-Cr powder using a YXQM-2L planetary ball mill, and five kinds of cladding powders (Fe-Cr, Fe-Cr+10%Mo, Fe-Cr+30%Mo, Fe-Cr+50% Mo, and Fe-Cr+70%Mo) were obtained. Multi-channel cladding layers with different Mo contents were prepared on the EA4T steel by laser cladding using a Nd∶YAG IPG-4000 solid-state fiber laser. The phase and microstructure of the coating were characterized by X-ray diffractometer (XRD), optical microscope (OM), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The mechanical properties of the coating were tested by the microhardness and the micro-shear tests. The wear-resisting property of the coating was tested by the friction and wear test.

Results and Discussions According to the XRD and EDS results of the coating (Fig.5, Table 5 and Table 6), the main phases of the Fe-Cr coating are the solid solution of Cr in α-Fe and the σ phase. When the mass fraction of Mo in cladding powder is 10%, Mo in the coating is mainly in the form of an intermetallic compound Mo₅Cr₆Fe₁₈. When the mass fraction of Mo is 30%, there are some Mo₅Cr₆Fe₁₈ and FeMo at the grain boundaries of the coating and few Mo atoms are dissolved in the crystal. The Mo content in the grains of the Fe-Cr+50%Mo coating is higher than that at the grain boundaries. There are more Mo simple substance phases in the coating and there are Mo₅Cr₆Fe₁₈ and FeMo phases at the grain boundaries. When the mass fraction of Mo in cladding powder is 70%, more Mo atoms are dissolved in α-Fe, and more Mo₅Cr₆Fe₁₈ and FeMo are formed at the grain boundaries. However, Fe-Cr+70%Mo coating cracks during cladding, so the performance of the Fe-Cr+70%Mo coating will not be discussed in the following. The hardness of the Fe-Cr cladding layer, Fe-Cr+10%Mo cladding layer, Fe-Cr+30%Mo cladding layer, and Fe-Cr+50%Mo cladding layer is higher than that of the substrate, and the addition of Mo reduces the hardness of the coating and the hardness gradient from the coating to the substrate (Fig.6). The micro-shear test results show that all coatings have higher strength and lower toughness and plasticity than other areas, and the toughness and plasticity of the coating first decrease and then increase with an increase in Mo content (Figs. 7 and 8). The results of the friction and wear test show that the four coatings can effectively reduce wear. Among them, the Fe-Cr coating has the lowest amount of wear, but its friction coefficient is higher than that of the substrate. The coefficient of friction of the coatings with Mo addition is lower than that of the substrate, and the coefficient of friction decreases with an increase in Mo content (Fig.12). The reason is that the addition of Mo reduces the hardness of the coating, and the oxide of Mo generated during friction exists in the surface and wear debris, which has a lubricating effect. According to the wear morphology, with an increase in Mo content, the wear form gradually changes from abrasive wear to adhesive wear (Fig.13).

Conclusions In this study, Fe-Cr cladding layer, Fe-Cr+10%Mo cladding layer, Fe-Cr+30%Mo cladding layer, and Fe-Cr+50%Mo cladding layer prepared under the selected laser cladding parameters are well formed, uniform, dense, and have good bonding performance with the substrate, where Fe-Cr+70%Mo cladding layer cracked. When the mass fractions of Mo in cladding powder are 10% and 30%, Mo in the coating mainly forms intermetallic compounds with Fe-Cr. When the mass fraction of Mo in cladding powder exceeds 50%, several Mo simple substances begin to appear in the coating. The hardness and shear strength of Fe-Cr cladding layer, Fe-Cr+10%Mo cladding layer, Fe-Cr+30%Mo cladding layer, and Fe-Cr+50%Mo cladding layer are higher than those of the substrate, and the addition of Mo reduces the hardness of the coating. Fe-Cr cladding layer, Fe-Cr+10%Mo cladding layer, Fe-Cr+30%Mo cladding layer, and Fe-Cr+50%Mo cladding layer can effectively reduce wear, but the friction coefficient of Fe-Cr cladding layer is higher than that of the substrate, and the friction coefficient of Mo-doped cladding layer is lower than it. Besides, the friction coefficient of the coating decreases with an increase in Mo content, and the wear form changes from abrasive wear to adhesive wear gradually. The Fe-Cr+50%Mo cladding layer has the best anti-friction and wear resistance, and the coefficient of friction in the test is 0.66.