Study On Mechanisms Of Anti-Torsional Fretting Wear Of Several Duplex Surface Treatments Base On Plasma Nitriding

Torsional fretting can be defined as a relative angular motion which was induced by reciprocating torsion in an oscillatory vibratory environment. It widely exists in industrial area and greatly reduces the service life of the parts. As an effective measure to alleviate the fretting damage, surface engineering technology is well available to protect the substrate matierial. However, there is still few research report on resisting torsional fretting by surface engineering technology. Thus, in this paper, torsional fretting wear properties and damage mechanisms of the plasma sulfurized/plasma nitrided layer(SN layer), the laser quenched/plasma nitrided layer(LN layer), DLC/plasma nitride layer(DN layer) have been studied, which has great important theoretical significance and engineering guidance value on the application of surface engineering technology on resisting torsional fretting.In this study, the torsional fretting tests of substrate(LZ50axle steel) and three duplex layers (the SN layer, the LN layer and DN layer) have been carried out on a high-precision torisional fretting wear test rig under different normal loads and angular displacement amplitudes with a ball-on-flat contact configuration. Base on analyzing the dynamic behavior in combination with mirco-observations by the optical microscope(OM), X-ray diffractmeter (XRD), scanning electrical microscope (SEM), energy dispersive spectroscope (EDX), Raman spectroscopy and surface profile-meter of2D/3Ddual modes, the fretting running behaviors and damage mechanisms of three duplex layers have been studied systematically. The obtained main conclusions are as follows:1. Torsional fretting wear of the plasma sulfurized/plasma nitrided layerBecause of the support from the nitriding layer and the close-packed hexagonal crystal structure of FeS in the sulphurizing layer, the duplex layer changed the fretting regimes of the substrate, which resulting in the slip regime (SR) shifted to the direction of lower angular displacement amplitudes, and lessening the mixed regime(MR), enlarging the SR. The value of friction torques was greatly influenced by the angular displacement amplitudes and normal loads. The evolution of curves of friction torque was related to the fretting regimes. The friction torque in the partial slip regime(PSR) was always at a low level during the whole cycles. There were four stages, i.e. running-in, ascent, peak and steady in MR. However the four stage feature disappeared in the SR. Similar to the friction torque, the evolution of curves of friction dissipating energy curve was lower than that of its substrate. When the fretting ran in PSR, the damage of SN layer was very slight; When it ran in MR, the wear mechanisms were abrasive wear, oxidation wear and delamiantion; When it ran in SR, the damage was sever and the wear mechanisms were still abrasive wear, oxidation wear and delamiantion. In conclusion, the damage of the SN layer was slighter under the same test conditions.2. Torsional fretting wear of the laser quenched/plasma nitrided layerDue to the high hardness and surface residual compressive stress, the running regimes of the substrate also were changed, which resulting in the SR shifted to the direction of lower angular displacement amplitudes, and lessening the mixed regime(MR), enlarging the SR. The evloutions of friction torque and friction dissipating energy for this duplex layer was similar to the SN layer. In the PSR, the damage was still very slight. When the fretting ran in the MR, the wear mechanisms were abrasive wear, oxidation wear and delamiantion. The damage was severe and the wear mechanisms were abrasive wear, oxidation wear and delamiantion in the SR. In word, the damage of the LN layer was slighter than that of its substrate under the same test conditions.3. Torsional fretting wear of DLC/plasma nitride layerOwing to the high hardness and low friction coefficients, it prompted the SR of the substrate shifted to the direction of lower angular displacement amplitudes, which resulting in the MR disappeared, and the SR enlarged greatly. The value of the friction torque was at a low level during the whole cycles in the PSR and slowly rising in the SR. The evolution of friction dissipating energy was similar to the friction torque and was lower than that of its substrate. In the PSR, the contact zone of the DN layer still retained the feature of granular, besides some indentation and slight damage occurred. In the SR, the wear mechanisms were abrasive wear, delamiantion and graphitization. The DN layer greatly reduced the wear of its substrate material and exhibited good capability of alleviating torsional fretting damage.4. Contrast of the three duplex layersThree duplex layers, to some extent, all changed the running condition fretting map of the substrate. From the effect of changing the fretting regime, the DN layer was the best in three duplex layers, then the SN layer and the LN layer was the worst. From the comparisons of friction torque, friction energy dissipation and wear depth of the three duplex layers in torsional fretting wear, the result showed that the rank of the anti-friction and ant-wear properties were as follows:the DN layer, the LN layer and the SN layer.