In Situ XPS Analysis of Fretting-induced Tribo-chemical Behavior in a Pure Iron
Fretting causes severe damage to the material surfaces which leads to the failure of the mechanical components in the fields of railways,mining,and aerospace industries.Many researches have shown that in addition to the fretting parameters such as load,frequency and displacement amplitude,the service environment also has an important impact on fretting.Pure iron has the advantage of high magnetic permeability,high saturated magnetization,low coercivity,low hardness,and high plasticity.It is widely used for manufacturing highly precise electronic devices.Especially in the aerospace industry,fretting damage is more severe under certain conditions such as a vacuum environment and high temperature.The tribochemical reaction occurring on the contact surface during the fretting wear process has a significant influence on determining the associated wear mechanism and fretting wear resistance.To study the tribochemical state and its effect on the fretting wear behavior of pure iron,systematical fretting wear experiments at different displacement amplitudes have been conducted under the vacuum(p=4 mPa)and air atmosphere(p=100kPa)by using an in-situ XPS analysis test combined with a self-designed high precision fretting wear device.3D white light interferometer and scanning electron microscope were utilized for quantitative characterization of wear volume and morphological observations of the worn surface,respectively.The results show that pure iron presents significantly different fretting wear characteristics between the vacuum and air atmosphere.With the increase of displacement in the air atmosphere,the fretting regime enters into slip regime(SR)directly from partial slip regime(PSR)without mixed fretting regime(MFR).Under vacuum,it is relatively hard to enter into SR due to the strong interface adhesion,hence resulting in a relatively wide mixed fretting region.Furthermore,in general the contact interface displays a higher friction factor in vacuum than that in air atmosphere at the same displacements.XPS results show that with the increase of the displacement,in vacuum atmosphere there is more exposure of Fe on the worn scar,and the formation of FeO is dominant while no Fe2O3 is produced.In contrast,for the air atmosphere,the tribochemical production of worn scars mainly consists of FeO and Fe2O3,and in SR the formation of Fe2O3 is dominant.The fretting wear volume increases with the increase of displacement amplitude for both vacuum and air atmosphere,but the magnitude of the increase is significantly different.In PSR(D=1μm),the initial oxide(FeO and Fe2O3)is still observed on the worn surface,thus representing a very slight amount of wear.When the displacement increases to 5μm,in vacuum,the disappearance of Fe2O3 and the exposure of Fe enhance the interface adhesion,which leads to a significant increase in wear volume.In contrast,for the air atmosphere,the production of Fe2O3 and FeO on the worn surface has better lubrication protection,resulting in relatively low wear volume.In SR(D=20 μm),the wear volume in vacuum rises rapidly and the worn surface consists of more Fe and less FeO,which makes the interface adhesion even stronger and hence leads to more serious wear.However,in the air atmosphere the worn surface is almost covered by Fe2O3 and FeO,and lower wear volume can be observed than that in vacuum,which indicates that the FeO and Fe2O3 have good protection with a lubricant role against fretting wear,especially for the Fe2O3.The in-situ XPS technique can characterize the real tribochemical state of the contact surface and reveal more accurately the effect of the tribochemical reaction on the fretting wear behavior,which is of great scientific significance to enrich and develop the basic theory of fretting wear.
pure ironin-situ XPS analysisfretting wearwear oxidationfretting wear mechanism
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