Research On Wear Behavior And Mechanism Of Titanium Alloys Under Various Sliding Velocities And Environmental Media

Titanium alloys are generally considered to have poor tribological properties, which have been greatly limited their applications. With the extension of applications for titanium alloys, the study on their friction and wear properties has attracted more and more attentions.The wear behaviours and wear mechanisms of TC4 and TC11 alloys were systematically studied under different sliding speeds and environmental media in this study. Affecting factors of wear behaviors at room temperature for titanium alloys as well as their wear mechanisms and transitions in different conditons were explored. Effects and mechanisms of tribo-layers and tribo-oxides on the wear process for titanium alloys were focused on to be investigated and the effect of tribo-oxides was verified by double wear test. The research in this field has important theoretical significance and actual industrial application value.The study found that two typical titanium alloys, TC4 and TC11 shared the commonness of wear behaviours and characteristics. Titanium alloys presented a marked variation of wear rate as a function of sliding velocity. When the sliding velocity increased from 0.5 to 4 m/s, the wear rate slightly decreased at first, then rapidly increased to reach the climax at 2.68 m/s, and finally substantially decreased to the lowest point at 4 m/s. The wear rate of TC4 alloy was always higher than that of TC11 alloy under the same sliding condition. Both of the two titanium alloys had the optimal wear resistance at 4 m/s, the suboptimal at 0.75 m/s, and the worst at 2.68 m/s. Titanium alloys also presented a marked variation of wear mechanism as a function of sliding velociety. With the rising of sliding veocity, titanium alloys underwent the transitions of wear mechanisms from the combination of abrasive wear, adhesive wear and oxidative wear at 0.75 m/s to delamination wear at 2.68 m/s, then to typical mild-oxidative wear at 4 m/s.The results showed that titanium alloys didn’t always have poor wear resistance. Two kinds of wear could be found under different Experimental Conditions: mild wear and severe wear. Mild wear and severe wear occured alternately at sliding velocities of 0.5-4 m/s. At 0.5-2.68 m/s, with an increase of the load, the transition from mild to severe wear occured abruptly when a certain level of load was exceeded. Additionally, a severe to mild wear transition arised with an increase of the sliding velociety from 2.68 m/s to 4 m/s in titanium alloys. The latter indicated that titanium alloys did not conformed to the surface-temperature criterion of mild-to-severe wear transition, put forwards by Wilson and Apas. This meant that titanium alloy would present some peculiar wear behaviors and characteristics, different from other metal alloys.The environmental media had a significant influence on wear behaviors and mechanisms of titanium alloys. Corrosion accelerated wear and wear accelerated corrosion. The wear rate of titanium alloys in simulated seawater was obviously higher than that in pure water and atmosphere. The wear mechanism in atmosphere was abrasive and adhesive wear accompanied with some degree of oxidative wear. The wear in pure water was typical abrasive wear, whereas in simulated seawater, the wear mechanism changed into the combination of abrasive wear and fatigue wear.Tribo-layers of titanium alloys formed at room temperature in the air could be divided into two categories: mechanical mixed layer under the general speed and in situ oxide-layer at extremely high speed. It was noticed that the mechanical layer which contained some tribo-oxides was thought to be protective. Mild wear and severe wear were supposed to correspond to appearing and disappearing of tribo-oxides induced by sliding, which was entirely contrary to the traditional view of no protective tribo-oxides of titanium alloys. Tribo-oxide layers of titanium alloys formed at 4 m/s was confirmed to possess obvious protective effects by double sliding wear tests.Based on the comprehensive analysis of previous studies and our experimental results, the influence factors of wear behaviors for titanium alloys at room temperature were initially clarified and divided into three mainly affected zones: adiabatic shear band(ASB) and mechanical mixed layer(MML) afftected zone at low speed, plastic deformation and MML afftected zone at moderate speed, and plastic deformation and in stiu oxide layer afftected zone at high speed. The wear mechanism map for titanium alloys as a function of sliding speed and applied load at room temperature was presented. Along with the increase of sliding speed, the wear mechanism of titanium alloys varied in sequence of adhesive, abrasive and oxidative wear, delaminative wear, mild-oxidative wear, severe-oxidative wear, plastic extrusive wear. The affected zone of wear varied with sliding velociety, which resulted in the wear mechanism transitions, and ultimately induced the wear behavior to change.