Research On Tribochemical Mechanisms Of AZ31B Magnesium Alloy During The Fretting Wear

With the progress of science and technology,Energy Conservation and Emission Reduction become a social hot spot.Nowadays,magnesium alloy has been widely used in automotive,aerospace and electronics industries.While some work focused on sliding wear,corrosion wear and friction and wear of surface modification,less effort devoted to investigate fretting wear under high temperature.The magnesium element is vivaciously active and readily oxidizable,thus,its tribo-chemical behavior will have a significate influence on fretting behavior during the fretting wear process.In this paper,fretting wear tests of AZ31B magnesium alloy plate against GCr15 steel ball were carried out using a tangential fretting wear tester under different temperatures.It will not only enrich the research data of fretting behavior of magnesium alloy at high temperature,but also develop the basic theory of the tribo-chemical mechanism.After testing,the worn surfaces were observed through T optical microscope,scan electrical microscope,electron spectroscopy,electron probe micro-analyzer,X-ray photoelectron spectroscopy and white light interference surface morphology in order to obtain morphological and chemical information.The analysis led to the following conclusions:?1?Running characteristics of AZ31B magnesium alloy during fretting wear processAt room temperature RT,the running condition fretting map of AZ31B magnesium alloy was established.And the influence of temperature on fretting behavior was systematically studied.As the increase of the displacement amplitude or the decrease of the normal load,the tangential fretting run state changed from the partial slip regime to the mixed regime and then the slip regime.The results indicated that the temperature had a slight effect on the fretting running regime.When the fretting running regime was different,the evolution of friction coefficient with the number of cycles was different.In the slip regime,as temperature increase,the growth rate of friction coefficient increased during ascending stage,and the peak value of friction coefficient increased,as well as the number of cycles required to reach the maximum and stable stages were decreased.?2?Damage mechanisms of AZ31B magnesium alloy during fretting wear processThe damage features for AZ31B magnesium alloy in the different fretting regimes were outlined as below:in the partial slip regime,the temperature almost had no effect on damage mechanisms The main wear mechanisms were abrasive wear and oxidation wear.In the mixed regime,the damage mechanisms were mainly abrasive wear,delamination and oxidation wear.The main wear mechanism changed from abrasive wear into adhesion wear when temperature ranged from RT to 300°C.In the slip regime,the main wear mechanisms were delamination,abrasive wear and oxidation wear.Abrasive wear and material transferred were observed at 300°C.When the number of cycles was constant,the wear volume decreased first and then increased as the temperature increased.When the number of cycles was 103 and 104,the minimum value of wear volume was measured at 200°C.?3?The evolution process of debris layer at the contact interfaceIn the different temperature and cycles,the evolution process of debris layer and composition of debris were investigated.The research results showed that:the size of the wear debris layer expanded,and the oxygen content increased with the increase of the displacement amplitude.In the slip regime,the increase of temperature accelerated the formation of debris layer,but resulted in the increase of oxygen content in the debris layer.The debris layer was initially compacted at the center,then extended toward the edge,and adhered to the entire contact area.During the fretting wear process,the water vapor and CO2 in the air were involved in the chemical reaction between the contact interfaces.At RT,the compositions of loose debris layer were Mg?OH?₂.MgO and MgCO₃.The increase of testing cycles or temperature would accelerate the decomposition of MgCO₃ into MgO.After 104 cycles,there was no MgCO₃ observed in the debris layer.The oxide glaze layer,composited with Mg?OH?₂?MgO and Fe₂O₃,formed wear scar at 200°C.It acted as a solid lubricating effect and resulted in sight fretting damage.