Study On The Tribological Behaviour Of Surface Gradient Nanostructure In TB8 Titanium Alloy

In recent years,the number of patients suffering from joint diseases has been increasing.Artificial joint replacement,one of the most effective treatments available,can rebuild joint function and restore the ability to perform daily activities.However,after implantation into the body,wear and tear of the artificial joint is inevitable.Over time,the abrasive debris generated at the friction interface can induce a bioimmune response,causing bone dissolution and leading to artificial joint failure.Therefore,improving the wear resistance of artificial joints has received special attention.Given that frictional wear occurs at the surface,surface strengthening of artificial joints becomes an important way to improve their wear resistance.In this paper,we use ultrasonic surface rolling process technique to strengthen TB8 titanium alloy,prepare stable gradient nanostructures on its surface layer,and analyze its surface properties.On this basis,the frictional wear properties of the gradient nanostructured surface layer of TB8 titanium alloy are investigated in depth to reveal the wear mechanism under different working conditions and the effect of biomimetic environment biological media on the microstructure and wear resistance of TB8 titanium alloy wear surface.The ultrasonic surface rolling process induced intense plastic deformation in the surface layer of titanium alloy,and the deformation layer was about 90μm thick.the grains of the surface layer were refined and distributed in a gradient.The hardness of the surface layer increases significantly,and the hardness value of the surface is the largest.As the depth increases,the hardness gradually decreases to be consistent with the substrate.When the static pressure used for ultrasonic surface rolling was 200 N,400 N and 600 N,the hardness values of the surface were 420 HV,475 HV and 522HV,respectively,which increased about 32.1%,49.1%and 64.19%compared with the untreated original state（317.93 HV）.At the same time,the depth of the surface hardening layer also increases with the increase of static pressure.From the Hall-Petch relationship,it is clear that the surface gradient nanostructure will show smaller grain size and deeper thickness as the static pressure increases.The trend of tensile strength is consistent with the hardness,i.e.,the higher the static pressure,the higher the tensile strength of the surface layer.Conversely,the higher the static pressure,the lower the surface roughness.This also implies that the best overall performance of TB8 surface structure can be obtained by 600 N static pressure.Tribological behavior of TB8 titanium alloy prepared by ultrasonic surface rolling process was studied,and tribological wear experiments were performed under four lubrication conditions.The results showed that the highest friction coefficient and wear were obtained under dry friction conditions,followed by 0.9%Na Cl conditions,15%hyaluronic acid conditions,and finally 25%fetal bovine serum conditions.With the increase of ultrasonic surface rolling static pressure,the average friction coefficient and wear amount showed a decreasing trend.At the same time,the decrease in wear caused by biomimetic environmental biomedia became more and more obvious with the increase of static pressure.The detailed classification of lubrication state by film thickness ratio yielded that the friction under 0.9%Na Cl condition belonged to partial fluid lubrication state,while the friction under 15%hyaluronic acid condition and 25%fetal bovine serum condition belonged to full film elastic flow lubrication.Based on the results of the lubrication state analysis,15%hyaluronic acid solution and 25%fetal bovine serum solution were selected as the frictional wear test conditions to further analyze the tribological behavior of ultrasonic surface rolling strengthened TB8 titanium alloy.The biohistological properties of the surface gradient nanostructures of TB8 titanium alloy were analyzed to investigate the wear mechanism under the full-film elastic flow lubrication condition.The wear surface morphology and chemical composition were characterized,followed by a discussion of the effects of ultrasonic surface rolling reinforcement on the material transfer at the friction interface,the triple-layer film structure and the wear mechanism.The results show that the best frictional wear properties appear in the 600 N ultrasonic surface rolling process specimens under 15%hyaluronic acid conditions.The smoothest and densest oxide layer consisting of Ti O₂,Ti C and Ti C_xO_ywas found on the wear surface of these specimens,which protects the TB8 substrate from further oxidation and wear.Similarly,the best frictional wear performance was found in 600N ultrasonic surface rolling process specimens under 25%fetal bovine serum condition,and the wear resistance was better than that of 15%hyaluronic acid condition.The full-film elastomeric lubrication state depends mainly on the proteins in the bionic solution.EDS analysis shows that the wear surface is characterized by oxidation reactions,and titanium combines with C elements from macromolecular proteins to form more Ti（OC₃H₇）₄similar to the deposited material,which can effectively avoid direct contact with the wear surface.Fetal bovine serum has more macromolecular proteins,peptides and fats than hyaluronic acid,so the best frictional wear performance can be obtained under 25%fetal bovine serum condition.Finally,the correlation between the mechanical properties of ultrasonic surface rolling process-reinforced TB8 titanium alloy surfaces and the specific wear rate and friction coefficient was analyzed,and a neural network-based wear resistance prediction model was developed using correlation statistical analysis.The results showed that the mechanical properties of the gradient nanostructured surface layer were significantly correlated with the specific wear rate and friction coefficient,whether considered individually or in combination with multiple factors,and also proved once again that ultrasonic surface tumbling strengthening can effectively improve the frictional wear performance of TB8 titanium alloy.