Fretting Wear Damage Of TC4 Titanium Alloy And Palliative Mechanism Of DLC Film

Titanium alloys are widely used in the applications of aerospace and marine because of their high strength,low density,fracture toughness and corrosion resistance.With the rapid development of aerospace technology,the fretting wear resistance of aerospace equipment are required.However,the disadvantages of titanium alloys such as low hardness,easy adhesion,and poor wear resistance make it extremely sensitive to fretting wear,which can easily cause failure and fracture of mechanical components.Therefore,there is an increasing demand to improve the fretting friction and wear properties of titanium alloys.Surface modification and lubrication technology are effective ways to improve the fretting resistance of titanium alloys.In order to improve the fretting resistance of TC4 titanium alloy,diamond-like carbon(DLC)films were deposited on TC4 titanium alloy by Unbalanced Magnetron Sputtering technology.In order to evaluate the performances,a series of fretting experiments were conducted under four lubrication conditions(dry,oil,DLC,DLC+oil).The wear debris,wear track surface and cross section,and wear scar were detected and analyzed by laser confocal microscope,scanning electron microscope,micro-area XRD,XPS,EDS electron spectroscopy,Raman and other methods.The fretting damage mechanism of TC4 and DLC were revealed.The main conclusions of the thesis are as follows:Under dry friction condition,the fretting wear mechanism of TC4 is adhesive wear,abrasive wear,fatigue peeling and oxidative wear,accompanied by plastic deformation under dry friction condition.Different load and displacement amplitude significantly affect the slip regime of fretting wear.Under lower normal loads or larger displacement amplitudes,gross slip regime(GSR)was dominant.Under higher normal load or smaller displacement amplitude,partial slip regime(PSR)and mixed slip regime(MSR)were dominant.In the partial slip regime,the friction coefficient was small and stable.The main wear types were oxidative wear,abrasive wear and fatigue cracks.In the mixed slip regime,the main damage type are oxidative wear,adhesive wear,and fatigue cracks.In the gross slip regime,the wear type was oxidative wear,abrasive wear,fatigue wear and slight adhesive wear.Based on the theory of friction dissipated energy,the result showed that under the same load,the wear volume had a linear relationship with the cumulative friction dissipated energy.The energy wear coefficient of TC4 titanium alloy increased with the increased normal load.Increasing frequency not only accelerated the wear debris ejection,but changed the fretting wear types from abrasive wear to adhesive wear.The increasing fretting frequency affects the development and evolution of TTS and the oxide wear debris layer,which can do harm effects at the fretting wear process.At low frequency,some oxidized wear debris layer and TTS were observed in the wear scar cross-section.At higher frequency,there was a competitive relationship between the friction oxidation time and the friction oxidation rate.The cross-section surface of wear scar had changed,including the severely refined oxidized wear debris layer and the plastically deformed TTS.As the number of cycles increases,friction dissipated energy gradually increases,and the fretting wear such as material friction oxidation and friction transfer behavior becomes more and more intense.Under oil lubrication condition,due to the loss of surface integrity and high roughness,it was difficult to form an elastohydrodynamic film on the surface of TC4,and the oil lubrication effect failed.In addition,under oil lubrication condition,increasing the rate of wear debris ejection led to increased wear rate and wear volume.The wear mechanism under oil lubrication is abrasive wear and plastic deformation.Under dry friction condition with different normal loads and displacement amplitudes,the slip regime and wear mechanism of DLC film were studied with the running condition fretting map(RCFM).The results show that the displacement amplitude and normal load had great effect on the fretting mode and damage mechanism.The mixed slip regime(MSR)was dominant at the displacement amplitude of 25?m.On the other hand,the slip regime was gross slip regime(GSR)at the displacement amplitude of 100?m.The abrasive wear was the main damage mechanism of DLC film under small displacement amplitude,whereas under larger displacement amplitude the wear mechanism of DLC film was mainly adhesive wear.In short,the good fretting wear performance of DLC film in dry friction lies in its excellent mechanical properties and self-lubricating properties.The fretting behavior of DLC film under oil lubrication conditions were investigated.The results show that under low load or large displacement amplitude,due to the good surface integrity and low roughness,it was easy to form an elastohydrodynamic film on the surface of DLC film,and the lubrication regime was elastohydrodynamic lubrication.Under large load or small displacement amplitude,the surface of the DLC film cannot form an elastohydrodynamic lubricating oil film,and the lubrication regime is boundary lubrication.At this time,the significant improvement in the tribological behavior of DLC film under fretting condition can be attributed to the synergy lubrication mechanism by the combination of the solid lubrication of and the boundary lubrication of the liquid oil film.Based on experimental research,this paper established a finite element model for fretting wear.Based on the theory of friction dissipated energy,and the wear depth of the TC4 titanium alloy was calculated through the software Abaqus.In the finite element numerical simulation calculation,the energy wear coefficient and the dynamic friction coefficient obtained from the fretting wear experiment were used to calculate the wear volume of the TC4 titanium alloy.By comparing with the experimental results,it is believed that the finite element method can provide reliable results for the fretting wear prediction of TC4 titanium alloy.