Development And Application Research Of The Modularized Multifunctional Fretting Wear Test System

Fretting damage,originating from the mechanical structure with immovable fit or clearance fit,has strong concealment and widely exists in various fields of modern industry.With the continuous development of major projects in the fields of aerospace,high-speed railway,nuclear power and advanced weapon systems,multiaxial stress,strong-impact vibration and extreme environment have become the routine service conditions of complex mechanical systems.Therefore,fretting damage has become one of the common key issues affecting the service safety and reliability of high-end equipment.A modularized multifunction fretting wear test system was developed in this work by integrating the most advanced precision motion drive technologies based on the development demand of basic scientific study and engineering application in the field of fretting wear.The developed test system realized the experimental simulation of all single and compound fretting wear modes through technological innovation of modular mechanical structure,high-precision displacement control and multifunctional human-machine interface.Meanwhile,the dynamic response and energy dissipation of the friction interface are detected in real-time by the test system.The motion mode decoupling and anti-fretting damage surface engineering design were conducted for a locking mechanism of the newly developed variable gauge high-speed train.Whereafter,the experimental studies under tangential fretting,impact wear,and compound fretting modes were systematically conducted based on the developed test system.The multimode fretting wear behavior and damage mechanism of 35Cr Mo steel and its various surface-treated layers were revealed.Finally,the surface treatment technology and protection method suitable for alleviating fretting damage to the locking mechanism was optimized,and some countermeasures for anti-fretting damage protection of clearance fit structure were put forward.The main conclusions of this work are as follows:1.Development of the modularized multifunction fretting wear test systemThe fretting wear test simulation of any basic（tangential,radial,torsional,and rotational）and compound（impact–tangential,radial–tangential,radial–torsional,and torsional–rotational）mode has been realized in the developed test system.The test system has been proved to have high-precision displacement control performance（linear displacement,0.1μm;angular displacement,0.01°）,large span motion range（0.1μm→25 mm or 0.01°→360°）,and normal load automatic compensation function（control accuracy,0.1 N）.2.The wear behavior and damage mechanism of 35Cr Mo alloy steel under various fretting modes were studiedIn tangential and torsional fretting modes,the fretting states change from the partial slip regime to the slip regime with the displacement amplitude increase or normal load decrease.Meanwhile,the damage mechanisms of the material change from slight abrasive wear to serious fatigue wear,abrasive wear,and oxidation wear.In impact wear mode,the accumulation of impact energy will lead to fatigue spalling as the final failure form of the materials in the contact interface.In addition,the applied tangential displacement intensifies the abrasive wear at the contact interface under the impact–tangential compound fretting mode.The effectiveness of the developed test system in revealing the fretting operation states and the wear mechanisms has been verified by various fretting test modes.3.The wear behaviors and damage mechanisms of various surface-treated layers in the tangential fretting mode were studied and revealedThe fretting damage of the substrate can be effectively alleviated（wear losses decreased by>31.5%）via surface treatments of the laser quenching（LQ）,plasma nitriding（PN）,and plasma nitriding+bonded Mo S₂（PN+Mo S₂）under dry condition.In the slip regime of the fretting wear,the main damage forms of LQ and PN samples are abrasive wear,fatigue spalling,and oxidative wear,while the wear mechanisms of PN+Mo S₂ composite coating are plastic deformation and oxidative wear.The boundary lubrication of the fretting interface is formed by adding grease,and the friction coefficient and wear amount of each sample are reduced to varying degrees.In particular,the material damage of the PN-treated surface is the slightest in the lubrication state,and its wear volume is 60%lower than that of the substrate.However,the grease accelerates the crack propagation and material spalling of the bonded coating.4.The impact wear properties and damage mechanisms of various surface-treated layers were studied and revealedFatigue spalling is the main failure mechanism of LQ and PN samples under the continuous impact load under dry conditions.The high-strength Fe N_x compound layer absorbs and moderates impact energy in the form of plastic collapse or spalling,resulting in slight material damage on the PN surface during impact wear.The plastic flow and lamellar structure on the PN+Mo S₂ coating are caused by its low surface tangential stiffness,and,finally,fail as flake particle fatigue spalling.Moreover,the addition of grease alleviates the impact stress and reduces the material damage of the substrate,LQ and PN samples.However,the penetration of grease has a great influence on the propagation of microcracks and spalling of materials,especially on the surface of PN+Mo S₂composite coating.The abrasion loss of the PN sample is about 70%lower than that of the substrate,showing the best impact wear resistance under the grease lubrication conditions.5.The wear behaviors and damage mechanisms of various surface-treated layers in the impact–tangential compound fretting mode were studied and revealedThe damage on the center of the wear scar is delamination controlled by contact fatigue in the bidirectional impact–tangential compound fretting mode,and the edge of the scar shows the material removal caused by abrasive wear.The damage mechanisms of LQ and PN specimens are fatigue spalling,abrasive wear,and oxidation wear.Moreover,plastic deformation,fatigue peeling,and oxidation wear on the surface of the bonded Mo S₂ coating are the main damage mechanism of the PN+Mo S₂ sample.Nevertheless,the microcrack propagation and material spalling in the fretting contact interface are aggravated by the addition of grease.Massive spalling appeared on the surface of the substrate,while the LQ sample showed surface flake spalling and microcutting.In addition,severe surface cracks and material spalling of PN+Mo S₂ samples lead to failure of the bonded Mo S₂ coating.6.Optimization of anti-fretting damage surface engineering technology and protection strategy for the locking mechanism in variable-gauge trainThe impact load in the test is borne by the plasma-sputtered high-strength Fe N_x particles on the PN-treated surface,the impact energy was offset by elastic–plastic deformation.Thus,the damage to the materials in the contact area is relieved.Furthermore,the surface texture of the PN sample provides suitable conditions for the storage and slow release of the grease,stable boundary lubrication,or collaborative lubrication effect is formed in the fretting interface,thus,energy dissipation and material wear are effectively reduced.Therefore,plasma nitriding is the preferred surface treatment method to relieve fretting damage to the locking mechanism.Moreover,adequate grease lubrication ensures that the PN-treated surface maximizes its fretting damage resistance.In addition,several protective suggestions are proposed for the clearance fit structure to alleviate fretting damage based on the study in this work.