Study On Rotational Fretting Wear Of Coatings/modified Layers For Axle Steel

Rotational fretting can be defined as the relative motion which is induced by reciprocating rotation under the oscillatory vibratory environment. The damage of rotational fretting could greatly reduce the life of the parts, and this phenomenon widely exists in industrial areas. As an economic, convenient and excellent measure to slow down the fretting damage, the technology of surface engineering is well available to protect the substrate material. However, there is still no research report about resisting rotational fretting wear by engineering technology. Thus, in this paper, rotational fretting wear property and damage mechanism of bonded MoS2 solid lubricant coating, High Velocity Oxy-fuel Spraying (HVOF) WC-17Co, WC-27CrNi coating and laser transformation hardening layer have been systematically studied. And, not only does it have great value of the application of engineering technology on resisting rotational fretting, but also possesses important theoretical significance of revealing the rotational fretting wear mechanism of surface coatings/modified layers.In this dissertation, the rotational fretting tests of the substrate (LZ50 axle steel) and four coatings/modified layer (bonded MoS2 solid lubricant coating, HVOF sprayed WC-17Co, WC-27CrNi coating and laser hardened layer) have been carried out in the high-precision fretting wear test machine under different normal loads, angular displacement amplitudes and cycles by using the ball-on-flat contact configuration. Fretting wear dynamic analyses in combination with the microscopic examinations settled by micro-hardmeter, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), X-ray diffractometer (XRD), X-ray photoelectron spectrometer (XPS) and profilometer have been systematically utilized to study the fretting running behaviors and damage mechanisms of four coatings/modified layers. The main conclusions are as follows:1. Rotational fretting wear of MoS2 coating and its characteristicsBecause of the crystal structure of MoS2 coating, the layer of this coating could be quite prone to slip under tangential force. Thus, the fretting regime of the substrate would change, and slip regime (SR) was easier to occur under a small angular displacement amplitudes. Then, the operating range of partial slip regime (PSR) largely reduced, and subsequently, the range of slip regime expanded. The evolution law of curves of friction coefficient was related to the fretting regime. There were the four stages, up-peak-steady-down, as the friction dissipating energy changed with cycle numbers. When the fretting ran into PSR, the contact area of MoS2 coating still retained the feature of granular, besides some indentation. When it ran into SR, the transfer film and plastic flow layer formed between the contact pairs, while the working hardening and frictional oxidation of plastic flow layer played the main causes of the coating's failure during the wear process. In the state of gross slip, due to the coating's soft and well rheological properties, there was a phenomenon of the bulge, whereas, not remarkable.2. Rotational fretting wear of HVOF sprayed coating and its characteristicsBecause of the high hardness of this coating, when shear stiffness increased, the area of contact area reduced in the same condition simultaneously. And, it prompted the coating to slip under smaller angular displacement amplitudes, then, the running regimes of the substrate also changed. The evolution law of curves of friction coefficient was related to the fretting regime. There were five stages, start-up-peak-steady-down, as the friction dissipating energy changed with cycle number. When the fretting ran into PSR, the damage of WC-17Co coating was very slight. And, when the fretting ran into SR, because of the high hardness of the coating, the phenomenon of the bulge that might happen in central zone was restrained, and the damage was still very slight, also, the action of debris'removal ran well, while a lot of damages occurred on the GCr15 ball. Fretting wear behavior of HVOF WC-27CrNi coating was similar to that of WC-17Co coating, and their capacity of resisting fretting wear performed well. Nevertheless, the hardness of WC-27CrNi coating was lower, so, the damage was relatively severe. Under this circumstance, the damage mechanisms were abrasive wear, oxidation wear, and delamination.3. Rotational fretting wear of laser hardened layer and its characteristicsCompared with the substrate, SR of laser hardened layer was oriented to occur when angular displacement amplitudes were smaller. The evolution law of frictional coefficient and dissipating energy were certainly corresponding. When the fretting ran into PSR, the micro-slip occurring at the contact edge, the sticking occurring at the contact center and the damage of hardening layer being slight, the scar presented a typical morphology of annularity. And its mechanisms were abrasive wear and oxidation wear. When the fretting ran into SR, the mechanisms changed from abrasive wear and delamination to abrasive wear, oxidation wear and delamination, as normal load increased with cycle number. To some extent, after laser quenching, the hardness of the substrate increased and its plasticity decreased, the phenomenon of the bulge was not found. In rotational fretting process, plastic deformation was not easy to occur, while spalling happened and debris directly formed.4. The contrast of the rotational fretting damage behaviors and the damage mechanisms of the four coatings/modified layersFor the all coatings/modified layers in this serial research, their kinetic curves only presented three shapes of linear and parallelogram. MoS2 coating, HVOF coating, laser hardened layer changed the fretting running regime of the substrate by decreasing shear stiffness and the friction coefficient, altering the surface chemistry, increasing the surface hardness, and inducing a residual compressive stress, respectively. Under different angular displacement amplitudes, the order of four coatings/modified layers'friction coefficients was from low to high:MoS2 coating, laser hardening layer, WC-17Co and WC-27CrNi. As the normal load increased, the friction coefficients of coatings/modified layers gradually decreased, the decreasing rate was from low to high:MoS2 coating, WC-17Co, laser hardening layer and WC-27CrNi. The friction dissipating energy of four coatings/modified layers increased with cycle number. Summarily speaking, with the mechanism of delamination for the coating/modified layers, the damages detached like lamellate plates. However, only had bonded MoS2 coating have the phenomenon of the bulge. And, due to the high hardness of the other three coating/modified layers, plastic deformation was restrained, subsequently, particle shedding directly occurred and no bulge was found in the wear process.