The High-temperature Friction And Wear Properties Of The Nitrided Layer On Al_0.25CoCrFeNi High-entropy Alloy

High-entropy alloys?HEAs?are defined as solid solution alloys which contain more than five principal elements in equimolar or near-equimolar ratios.HEAs have attracted wide attention due to their unique structure and excellent performance,and the tribological properties of HEAs have also been a popular topic.However,at present,much attention has been paid on the room temperature tribological behaviors of HEAs,while few studies are available in the literatures on the high-temperature friction and wear properties of HEAs.In this paper,Al_0.25CoCrFeNi HEA with good mechanical properties was used as the research object,and the effect of the temperature on tribological behaviors of Al_0.25CoCrFeNi HEA before and after nitriding was mainly investigated.The high-entropy alloys used in this experiments were synthesized by WK-type tungsten non-consumable arc furnace,and the high-entropy alloys were nitrided by ldmc-100a plasma nitriding furnace.The hardness of alloys was measured by HVS-1000 type Vickers Microhardness tester?Shanghai?and Nano Indenter G200 type Nano indentation tester?USA?.Microstructure and phase analysis were carried out with metallographic microscope,X-ray diffractometer?XRD,D/MAX-2400?,scanning electron microscope?SEM,JEOLJSM-6390?and X-ray photoelectron spectroscopy analysis?XPS,ESCALAB250?.The tribological behaviors of Al_0.25CoCrFeNi HEA before and after nitriding was mainly investigated by using a universal wear testing machine?HT-4001,China?at different temperatures,such as room temperature,100,200,300,400,500,and 600?.The main results of this paper are as follows.?1?The microstructures of as-cast and roll-annealed Al_0.25CoCrFeNi HEA are single-phase FCC.The hardness of the as-case Al_0.25CoCrFeNi HEA is about150 HV 0.3/15,and that of the roll-annealed HEA is about 260 HV 0.3/15.?2?The nitrided layer of Al_0.25CoCrFeNi HEA mainly consists of AlN,CrN and some iron-nitrogen compounds.The thickness of the nitrided layer is about9?m,and the hardness is 13.9 GPa.The composite hardness of the nitriding HEA reaches 720 HV0.5/15,which is about 3 times that before nitriding.?3?The friction coefficient of Al_0.25CoCrFeNi high-entropy alloy before and after nitriding is negatively correlated with temperature,which means that the friction coefficient of both decreases with the increase of temperature.On one hand,the higher temperature,the more obvious the phenomenon of oxidation,leading to the friction coefficient decreases.On the other hand,when the contact interface temperature was higher than a certain value,the adhesive forces decreased with increasing temperature.?4?The tribological properties of Al_0.25CoCrFeNi high-entropy alloy are stable at high temperature.Meanwhile,the nitriding Al_0.25CoCrFeNi high-entropy alloy shows better wear resistance.The wear resistance of high entropy alloy after nitriding at room temperature is 3 times of that before nitriding.When the test temperature higher than 300?,the wear resistance of high entropy alloys after nitriding is 7 times of that before nitriding.?5?The wear mechanisms of Al_0.25CoCrFeNi high-entropy alloy before and after nitriding are different at different temperatures.The wear mechanism of the alloy changes from the abrasive wear at room temperature to the delamination wear at 200?,and then transforms to the delamination and oxidative wear at300?.Finally it fully becomes oxidative above 300?.Moreover,the adhesive wear exists concomitantly below 300?.However,from room temperature to200?,the wear mechanisms of nitrided Al_0.25CoCrFeNi high-entropy alloy are abrasive wear and adhesive wear,and then,the wear mechanism is dominated by oxidative wear above 300?.