Effect of MoS₂ Coating Types on the Tribological Behavior of Copper-Based Wet Friction Materials under High-Speed and Heavy-Load Conditions

The metal-based powder metallurgy friction material is a crucial component for high-speed and heavy-duty clutches, primarily consisting of matrix components and functional components (including lubrication components and friction components). Among these, the matrix components determine the physical and mechanical properties of the material such as strength, hardness, heat conduction, and heat resistance. The lubrication components are employed to eliminate vibration, reduce noise, adjust the coefficient of friction, and stabilize the friction process. On the other hand, the friction components predominantly influence the friction coefficient as well as wear resistance and adhesion resistance of the material. The surface coating treatment of functional components is an effective method for modifying surfaces, which can maintain the friction material's function while enhancing its strength and toughness. This approach contributes to improving the reliability of friction materials under high-speed and heavy load conditions. The aim of this study was to investigate the impact of various surface coating techniques on the performance of MoS₂, as an important lubricating component in copper-based friction materials. A series of analysis and detection techniques, including SEM, EDS and XRD, were employed to investigate the morphology and intrinsic performance of MoS₂ lubricating components with different coatings, the MoS₂ and matrix interface characteristics. The micro-friction test was carried out to evaluate the interface bonding properties and micro-friction properties of Cu-matrix/MoS₂ lubricating components with different coatings. The tribological behaviors of Cu-based friction materials (Cu-BFM) with different MoS₂ were overall compared using the MM-3000 friction test. The result showed that both Ni and Cu coatings contributed to enhancing the interface performance between MoS₂ and the matrix. However, it should be noted that Cu coating fails to prevent the decomposition of MoS₂, leading to the formation of a limited amount of sulfide phase at the matrix/MoS₂ interface with inherent defects in the diffusion-reaction interface. On the other hand, Ni coating optimized the interface performance by effectively inhibiting MoS₂ decomposition during sintering and facilitating the formation of a good diffusion bonding interface between nickel-plated MoS₂ (MoS₂@Ni) and the matrix. The Cu-based friction material (Cu-BFM) containing copper-coated MoS₂ (MoS₂@Cu) exhibited a higher friction coefficient at lower rotation speeds (3 000 r/min) due to the decomposition of MoS₂. However, at higher rotation speeds, MoS₂@Ni demonstrated a higher friction coefficient and a greater stability factor owing to its stabilizing effect on the friction process. In terms of wear performance, MoS₂@Ni exhibited a significantly superior effect on the wear resistance of friction materials compared to MoS₂@Cu. Particularly at high rotating speeds, MoS₂@Ni effectively suppressed the occurrence of fatigue wear in friction materials, resulting in a more than 30% reduction in the wear rate of friction materials containing MoS₂@Ni when compared to those containing MoS₂@Cu. Regarding the wear mechanism, friction materials containing both types of MoS₂ exhibited ploughing wear as the dominant mechanism at low rotational speeds, while at high rotational speeds, the presence of MoS₂@Cu in friction materials led to a transition from ploughing wear to a combination of ploughing and fatigue-induced delamination wear.