Study On Torsional Fretting Behaviors Of Pure Titanium And Titanium Alloys And Theirs Nitrogen Ion Implantation Layers

Torsional fretting can be defined as a relative angular motion which is induced by reciprocating torsion in an oscillatory vibratory environment. It is very common in industrial, medical, and other fields. Surface modification has advantages of low cost, easy implementation and excellent performance ect. Besides, it can effectively palliate fretting damage and improve the wear resistance of materials. Currently, there are a few researches on technology of surface engineering in resistance of torsional fretting abrasion. This research selects the typical biological alloy as the research object, and inspects the damage mechanism and the mechanism of ion implanted layer in torsional fretting to enrich tribology research.In this paper, the substrate and the ion implanted layer of pure titanium (TA2), titanium alloy Ti6Al7Nb and Ti6A14V were selected as the objects of study. The torsional fretting experiment is based on a rotary device with low speed and high precision and a six dimensional sensor (forces and torques of x, y and z directions), under a contact of ball-on-flat in the serum solution. Adopt micro-analytical tools such as micro hardness tester, optical microscopy (OM), scanning electron microscopy (SEM), electron spectrometer (EDX), X-ray diffraction (XRD), Raman spectroscopy (Raman) and surface topography instrument, combined with micro wear dynamics analysis, inspects the damage mechanism and the mechanism of pure titanium, ion implantation modified layer systematically. The main conclusions obtained are as follows:1. The pure titanium and its alloys after nitrogen ion implanted, a new phase of titanium nitride is formed on the surface of the materials. The surface roughness has a significant increase. With the increase of the nitrogen ion implantation dose, the hardness and roughness of the modified layer gradually becomes larger.2. Torsional fretting experiments showed that nitrogen ion implantation can well improve the torsional fretting wear properties of pure titanium and titanium alloys. Under the small angular displacement(θ=0.5°), T-θ curves of the substrates of pure titanium and titanium alloys are elliptical, combining with OM morphology analyses of wear scars, the torsional fretting run in the mixed fretting regime(MFR); The T-θ curves of ion-modified layer change to linear, indicating the nitrogen ion implantation delayed the arrival of the slip regime(SR); Under the angular displacement(θ=5°and15°), T-θ curves of the substrate and the modified layer of titanium and titanium alloys are parallelogram, and the torsional fretting wear run in the slip regime(SR). So the nitrogen ion implantation modified does not change the running area of the torsional fretting of the material; The torque values accompanying with the variation of cycle numbers could be divided into three stages as running-in, increasing and relatively stable stages; Under small angular displacement, the torque values of substrate and the modified layer of titanium and titanium alloys are smooth and low. Under large angular displacement, the torque values of ion-modified layers run in the stable stage faster comparing to the substrates. 3. In the PSR:the damage mechanisms of the substrates of pure titanium and titanium alloys in the serum solution were slight abrasive wear, but the nitrogen ion modified layer was mainly no damage; In the MFR:the surface of the substrate has a large number of debris, flaking and delamination, the damage mechanism was abrasive wear. In the SR:the wear occurred at the whole contact zone, same to the MFR the wear mechanisms of the substrates of pure titanium and titanium alloys mainly the flaking of the center of wear scar, and the large number of debris of outer edge, and delamiantion, the nitrogen ion modified layer was mainly abrasive wear.