Three-body Abrasive Wear Behavior Of Titanium Alloy Via Surfacenanocrystallization Combined With Plasma Nitriding

TA2 titanium is widely used in aerospace devices,special geological drilling and other fields as a lightweight structural material.Adhesive wear is easy to occur in the application of friction because of the low alloy element content and low hardness.Therefore,it is necessary to strengthen the surface of titanium.The composite processing method of ultrasonic surface rolling process(USRP)combined with plasma nitriding technology was used to strengthen TA2 titanium: 750 ℃ nitrided,USRP +750 ℃ nitrided,850 ℃ nitrided,USRP + 850 ℃ nitrided.The tribological properties of titanium alloys was also studied by added simulated lunar regolith(SLR)particles as the external third body.Microhardness analysis,optical microscopy,XRD analysis,SEM,EDS analysis and microcosmic analysis methods was used to characterize the strengthened titanium alloy samples,and the third body tribological properties and damage mechanism of nitrided titanium alloy were investigated.The results are as follows:(1)‘USRP + plasma nitriding’ compound strengthening treatment was an effective process to improve the mechanical properties and tribological properties of TA2 titanium.USRP treatment improved the thickness of reinforcement layer and surface hardness of TA2,USRP + 850 ℃ plasma nitriding treatment of TA2 titanium improved the maximum hardness to 1097 HV0.05.The improvement of hardness improves the wear resistance of nitrided titanium.At the same time,USRP treatment could reduce the surface roughness after being plasma nitrided.The decline of surface roughness improved the surface properties of titanium and decreased the friction coefficient.(2)The simulated lunar regolith particles formed a continuous distribution of the third body on the surface of the nitrided titanium sample under condition of dry friction,which had a decisive influence on the tribological properties.The third body played an important role in the isolation of the friction pairs.The aggregation of thethird body particles on the wear scar increased the stress concentration on the friction surface,which made the friction process more stable.(3)In the third-body tribological system,the hard third body abrasive increased the wear rates of titanium alloy.The hard abrasive had a significant ‘plough’ effect on the low hardness of titanium alloy,the wear mechanism of 750 ℃ nitride titanium alloy was mainly plough wear.Aggregation of the third body wear particles in grinding marks caused stress concentration,so that more cracks were formed,which resulting in fatigue wear to the 850 ℃ nitrided titanium alloy.(4)The simulated lunar regolith particles with different particle sizes had an important effect on the tribological properties of nitrided layer.Larger size of the simulated lunar regolith particles(200-991 μm)increased the inhomogeneity of load distribution,resulting in larger friction coefficient,larger fluctuation and the largest wear rate of titanium alloy.The small particle size of distribution made the stress distribution more uniform and decreased the friction coefficient,but increased the fatigue wear of the nitrided titanium alloy.