Research On The Impact Wear Mechanism Of Several Typical Metals And Coatings

The material degradation and safty problems induced by impact wear is widely existed in all kinds of engineering equipment and high-tech fields in the condition of clearance vibration or dynamic contact.The movement of impact wear is characterized by repeated nonlinear contact-separation,resulting in thinning and cracking of contact area,which seriously threatens the safe operation of equipment and ecological environment.At present,the research and analysis methods of impact wear are relatively simple,and the influence of material properties and impact parameters on the impact dynamic behavior and impact wear performance has not systematically studied.Based on the characterization of the impact dynamic behavior and energy dissipation,the impact wear tests and finite element analysis have been carried out in this paper.This work not only enriches the theory system of impact wear,but also provides reference basis and research guideline for protection problems in engineering applications.Several metals and coatings were selected,and the effects of mechanical properties,clamping schemes,constitution of kinetic energy and surface engineering on impact dynamic behavior were systematically studied.Meanwhile,the mechanisms of impact damage and energy dissipation have been revealed by the microscopic damage morphology.The relationships between physical parameters and impact dynamics behavior and characteristics were simulated through finite element calculation.Besides,the simulation of multiple impacts was achieved,which deepened the understanding of the impact damage mechanisms.The main accomplished research contents and the chief conclusions are as followed.1.Effects of kinetic energy on dynamic behavior and wear mechanism(1)The residual stress was increased with impact velocity,resulting in aggravation of work hardening effect,and leading to the decrease of contact time indirectly.When the kinetic energy was changed by mass,the ability to maintain the motion state of the punch has been affected.The greater the mass of punch,the harder the change of motion states,therefore the contact time becomes longer.Under the same impact kinetic energy,the synergistic changes of velocity and mass has almost no effect on impact force,indicating that the resistance strength of material to impact is depend on the values of kinetic energy instead of its consitution.(2)The energy absorption ratio was increased with impact velocity and decreased when impact mass was increased.The wear results also show that the impact damage was distinctly different even in the case of the same kinetic energy.Therefore,the kinetic energy can not be used as an independent control parameter for impact wear test.The basic physical parameters of kinetic energy,velocity and mass,should be clearly defined.(3)The impact damage area can be divided into two regions according to the characteristics of micromorphology.There were black oxide layers and light-colored matrix in the center of scars,and the main damage mechanism was delamination wear.The ring area was smooth and the plastic deformation is the main damage form.When the kinetic energy is the same,the size of the characteristic regions is dependent on the constitution of kinetic energy: the greater the velocity,the larger the wear area;and the greater the mass,the larger the plastic deformation area.2.Impact wear mechanisms of several metals(1)The material with low elastic modulus had poor resistance to deformation and was prone to severe plastic damage.While the material with low hardness was poor in wear resistance,which made the material easily worn and caused serious material removal.With the improvement of material mechanical properties,the impact wear mechanism was transformed from abrasive wear to delamination wear,and the main energy consumption way was also transformed from plastic deformation to the initiation and propagation of micro cracks beneath the contact surface.(2)When the specimen was clamped in the vertical direction,the impact dynamic behavior was actually the response of threaded connection.Therefore,although the selected five metals have great differences in hardness and elastic modulus,the results of impact force,contact time and energy absorption were very similar.In order to avoid the influence of clamping,the specimen should be fixed in parallel to the impact direction,so that the specimen can respond to impact wear with the properties of the material itself.3.The impact wear mechanisms of two typical hard coatings(1)Based on the results such as impact force,contact time and energy absorptivity,the application of coatings effectively improved the overall impact dynamics behavior and greatly reduced the impact damage.Due to the high brittleness and high hardness,the Cr-DLC composite coating maked the material removed mainly in fatigue spalling.Meanwhile,the diamond like structure has a relatively stable covalent bond,and friction oxidation was not easily to occur.Moreover,the impact wear mechanism of Mo S2/Pb nanocomposite coating was delamination wear.(2)The evolution of the energy absorption ratio indicated the changes of impact damage forms and mechanisms.In the previous cycles of Mo S2/Pb composite coating,the plastic deformation was the main impact damage,and the associated work hardening restrained the further development of plastic behavior.Subsequently,the ability to absorb energy was increased,making more energy used for the initiation and expansion of the subsurface cracks.Besides,the increase of impact velocity resulted in fragmentation of delamination on the edge of the wear scars;and the increase of impact mass led obvious delamination and spalling in the center of wear scars,resulting in serious material removal.4.Finite element simulation of impact contact behavior(1)Changes in impact velocity or mass affected the the resistance of the material under impact,but the ways of changing caused diferences in dynamic behavior,which is mainly due to the physical properties of velocity and mass.Therefore,the specific impact dynamic behavior and response rate were dependent on the velocity and mass instead of kinetic energy.The plastic strain was concave in the contact area,and the plastic strain and residual stress in the center were decreased with the increasing of the velocity/mass ratio.(2)The radius of impact ball greatly affected the resistance to the impact.The smaller the radius,the plastic behavior is more intense.At the same time,the friction dissipation energy increased with the friction coefficient.Although the order of magnitude is low,serious material wear and removal will happen,taking into account the the cumulative effect of time in actual conditions.(3)The effect of the plastic deformation and the working hardening on the local properties of the contact area maked the dynamic behavior of the first impact greatly differed from the subsequent impacts.After the strength of the contact area was increased,the further yield of the material was more difficult,until the contact stress and the yield strength were balanced.Although the increase of impact cycles led to less energy used in impact damage,it was predicable that the impact damage in later stage will be wear degradtion,according to the increase of friction dissipation energy.