Study On Duplex Surface Techniques Enhancing Reducing-friction And Anti-wear Performances Of Titanium Via Nanocrystallization And Plasma Nitriding

With the development of geoscience,complex and changeable working conditions raise a higher demand on the safety and reliability of the geological drilling device.Lightweight of geological drilling device is an effective method to cope with the working conditions in outer space.Comparing with conventional steel geological drilling devices,the poor vacuum tribological performances of the devices made of light weight alloys,for example,titanium?Ti?and its alloys limit its application.Accordingly,an effective surface treatment method for enhancing the vacuum tribological performances of light weight alloys has been pursued in this paper.The material considered in this investigation is commercial titanium TA2.ultrasonic surface rolling processing?USRP?,an efficient surface nanocrystallization approach,has been introduced as pretreatment of plasma nitriding;and the following results can be obtained.?1?After USRP treatment,a gradient hardening layer with thickness of 200?m was formed on the surface of TA2.Surface average grain size was refined to 38 nm from 35?m.Due to fine-grain strengthening,surface hardness was enhanced to 320HV_0.05 from 140 HV_0.05.The enhancement of hardness leads to the improvements of wear resistance under vacuum and atmospherical conditions.USRP treatment plays positive roles on oxidation wear under atmospherical condition.Nevertheless,it plays passive roles on adhesive wear and plastic deformation occurred on the worn surface,leading to smooth and clean worn surface,and finally resulted in low friction coefficient under vacuum condition.?2?After plasma nitriding,a nitriding layer consist of Ti N?Ti₂N and?-Ti?N?was formed on the surface of TA2.The formation of nitriding layer improves surface hardness.The surface roughness,thickness and surface hardness of nitriding layer keep rising with the increase of nitriding temperature.Under atompsherical condition,the hard nitriding layer can retain the occurrence of oxidation and abrasion wear;and hence improves the wear resistance of TA2.With the increase of nitriding temperature,the plasma-nitrided TA2 exhibits more and more excellent wear resistance.Under vacuum condition,nitriding layer impedes the direct contact between metal and its sliders,restrains the occurrence of adhesive wear,causes a flat and smooth worn surface,and hence enhance the anti-wear and friction reducing performances.Unfortunately,higher nitriding temperature results in poor load bearing?cracking resistance?capacity.To sum up,plasma-nitriding TA2 exhibit the optimized the anti-wear and friction reducing performances,when the nitriding temperature is850?.?3?USRP pretreatment is able to lift the efficiency of nitriding and decrease nitriding temperature.It can obviously enhance the surface roughness,hardness and thickness of nitride layer on TA2.Nevertheless,USRP pretreatment also results in the formation of columnar crystal zone with large-sized grains on the surface of plasma-nitrided Ti,adverse to the enhancement of load bearing capacity.Therefore,USRP pretreatment harms vacuum reducing-friction and anti-wear performances of Ti nitriding under low temperature?above 750??.Optimized nitriding temperature for improving the vacuum tribological performances can be effectively decreased from850 to 750?by USRP pretreatment.