Research On Torsional Fretting Corrosion Behaviors Of Titanium Alloy

Torsional fretting can be defined as a relative angular motion which is induced by reciprocating torsion in an oscillatory vibratory environment. It often occurs in varied industrial fields, and frequently runs in corrosive media. As one of the four basic motion modes of fretting, the torsional fretting has been rarely studied up to now. The existing researches on torsional fretting were carried out mostly in the dry environment, and the researches on torsional fretting corrosion in fluid media have not been reported yet. Therefore, this research has scientific significance to explore unknown, and will not only enrich and develop the basic theory of fretting tribology, but also has some practical values for understanding the failure mechanism of the artificial joints.In this paper, a new-style device for torsional corrosion wear test rig in liquid media at constant temperature was successfully developed on a low speed reciprocating rotary system with a constant temperature circulating water system and an electrochemical analysis system. The test device can actually simulate torsional fretting corrosion process in the liquid media at constant temperature, and its test results presented better comparability and repeatability.In this paper, Ti6A14V titanium alloy and ZrCO2 ceramic, which are commonly used as artificial joint materials, were selected as the counter-pair. The studies on torsional fretting wear and torsional fretting corrosion in varied environments (dry, pure water, Saline solution and Hank's solution) were carried out systemically. The running and damage behaviors of the torsional fretting wear and torsional fretting corrosion were investigated systematically, based on the analyses of torsional dynamics and electrochemical corrosion behaviors and combined with many micro-analytical means such as surface profile-meter, optical microscope (OM), laser confocal scanning microscopy (LCSM), scanning electron microscopy (SEM), electron energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy and so on. The qualitative and quantitative analyses for the interaction of wear and corrosion during the torsional fretting corrosion were conducted. The conclusions obtained in this thesis are as follows:(1) Electrochemical corrosion behaviors of titanium alloy in simulated body fluidThe obvious passive phenomenon of titanium alloy presented in simulated body fluids (Saline and Hank's solutions), and their corrosion rates were very low. The corrosion rates of the samples with crevice were slightly higher than that of the samples without crevice, but no significant crevice corrosion or pitting corrosion appeared on all samples. The existence of the crevice impeded the formation of passive state of the samples to reduce the passive stability. Thus, the corrosion rate of titanium alloy slightly accelerated. Under the same conditions, the corrosion potential of titanium alloy in Hank's solution was slightly higher than that of in the Saline solution, and the corrosion rate was slightly lower than that of in the Saline solution.(2) Running and damage mechanisms of titanium alloy during torsional fretting wear processThe running condition fretting maps (RCFMs) of torsional fretting for titanium alloy in two environments of air and pure water were established, respectively. The friction torque time-varying evolution curves presented different principles in the three running regimes. As pure water was almost no corrosiveness on titanium alloy, the torsional fretting wear mechanisms of titanium alloy in the both environments were very similar, but some differences existed because of the lubrication and debris removal action in pure water.(a) In the partial slip regime (PSR):No damage was observed in the contact center due to the sticking. The micro-slip and slight wear occurred in the ring area of the contact boundary. So, the wear scar appeared in shape of annularity and the width of scar unchanged with the increase of the number of cycles. In the PSR, the damage mechanisms of torsional fretting for titanium alloy in air and pure water mainly were slight abrasive wear and scuffing at the contact edge zone.(b) The mixed fretting regime (MFR):With the increase of the number of cycles, the sticking zone in the contact center gradually reduced, and the damage zone spread to the contact center until all the contact zone was covered. The profile of the wear scar presented the type of "W". Under dry conditions, the wear debris was difficult to remove from the wear scar, and the debris layer covered on the wear scar surface. In the MFR, the damage mechanisms for titanium alloy in air and pure water mainly were abrasive wear, oxidation wear and delamination, and companied with some slight materials transfer.(c) In the slip regime (SR):The much severer wear and the typical "U"-type profile of wear scar appeared on titanium alloy samples. The depth of wear scar in pure water was greater than that of in air. Obvious material transfer occurred on damage areas of test samples in the both environments. In the SR, damage mechanisms of titanium alloy mainly were severe abrasive wear, delamination and oxidation wear.(3) Running and damage mechanisms of torsional fretting corrosion for titanium alloyElectrochemical corrosion behaviors of titanium alloy during the torsional fretting corrosion in the both simulated body fluids were closely related to the torsional angular displacement amplitudes. When the angular displacement amplitude increased to a certain degree, the corrosion potential shifted negatively and the corrosion current increased at the beginning of torsional motion. The requisite minimum torsional angular displacement amplitude for the corrosion potential negatively shifting in the Hank's solution was lower than that of in the Saline solution. The torsional fretting had a little effect on the cathodic reaction but a significant impact on the anode reaction in the electrochemical corrosion of titanium alloy. The passive film on surface of titanium alloy samples damaged due to the torsional fretting wear, and the exposed fresh metal surface on the worn zone became the active sites of corrosion, which induced serious crevice corrosion on the titanium alloy sample surface.The running condition fretting maps (RCFMs) of torsional fretting corrosion for titanium alloy in the both simulated body fluids were established, respectively. To compare with the Saline solution, the MFR of titanium alloy in Hank's solution enlarged to the PSR, and its width was greater than that of in the Saline solution. The friction torque curves in the three running regimes showed different evolutions.The normal load, angular displacement amplitude and number of cycles had a significant effect on torsional fretting corrosion damage behaviors of titanium alloy. In the both simulated body fluids, the damage volumes increased with the increase of the angular displacement amplitudes and normal loads. Under the same test conditions, the damage volumes in Hank's solution were higher than that of in the Saline solution. In the different torsional fretting running regimes, there were some differences on torsional fretting corrosion damage mechanisms of titanium alloy:(a) In the partial slip regime (PSR):The damages were main wear that was slight, and the corrosion was hardly observed. The wear scars appeared in shape of annularity and the damage mechanisms were similar to that in dry environment and pure water.(b) In the mixed fretting regime (MFR):With the increase of the cycles, the center sticking zones gradually shrank, and the damage zones extended to the contact center. The profile of the wear scar presented the type of "W", and some obvious traces of corrosion were observed. In the MFR, the damage mechanisms of titanium alloy in the both simulated body fluids mainly were abrasive wear, oxidation wear and delamination, and companied with slight material transfer and electrochemical corrosion in the Hank's solution,(c) In the slip regime (SR):The gross slip state presented on entire contact zones all time, and much severer wear occurred. The typical "U"-type profile of wear scar appeared, and a thick layer of debris covered the contact surface. In the SR, the damage mechanisms of titanium alloy of torsional fretting corrosion mainly were abrasive wear, oxidation wear and delamination, accompanied with severer material transfer and electrochemical corrosion.(4) Interaction between wear and corrosion of torsional fretting corrosion of titanium alloyA large number of quantitative analysis results indicated that the accelerated effect of torsional fretting on corrosion was closely related to the angular displacement amplitudes, normal loads and the medium types. When the angular displacement amplitudes were lower, the torsional fretting had almost no effect on the corrosion. Under the larger angular displacement amplitudes, the torsional fretting significantly accelerated the corrosion, and the corrosion increment induced by the torsional fretting increased with the increase of the angular displacement amplitudes and normal loads. Under the same test conditions, the accelerated effect of torsional fretting on corrosion in the Hank's solution was greater than that of in the Saline solution.The accelerated effect of corrosion on torsional fretting was controlled by the normal load. When the normal load was lower, the corrosive effect of the simulated body fluids did not speed up the wear of titanium alloy, inversely slowed it down, and the increment of wear volume was negative. However, under the higher normal loads, a reverse result can be obtained.In the simulated body fluids, the damage of titanium alloy of the torsional fretting corrosion mainly was wear, and there was a similar law between the interaction volume and the wear volume increment. Under the lower normal loads, the significant negative interaction occurred, and the negative interaction in the Hank's solution was stronger than that of in the Saline solution. Under the high normal loads, much obvious positive interaction appeared, and the interaction in the Saline solution was stronger than that of in the Hank's solution.