Preparation And Tribological Behavior Of MXene Based Coating As Solid Lubricant And Multi-phase Reinforced Copper Matrix Composites

Benefitting from desired electrical and thermal conductivity,copper and its alloy have been widely utilized in industrial field.However,poor mechanical and tribological performances restrict their further application.Therefore,preparation of copper materials with enhanced mechanical and tribological performance attracts the attention of scholars.With good strength,anti-oxidation and anti-corrosion performance,ceramic particles become good candidate of reinforcement phase for copper composites.Besides,carbonaceous materials could also be ideal reinforcement,for their low density,high electrical and thermal conductivities.Up to now,fabricating ceramic particles or carbonaceous nano-materials reinforced copper composites become the decent alternatives to solve the problem.Moreover,deposition of self-lubricating coating on the copper matrix provides desired tribological performance without damage of thermal and electrical conductivities.However,the enhancement of brittleness caused by the strengthening phase of ceramic materials and the agglomeration of nano-carbon materials affect the properties of the materials.The weakness of copper matrix composites strengthened by single strengthening phase were overcome by the preparation of multi-reinforced copper matrix composites,and a new type of solid lubricant with self-lubricating properties under various atmosphere conditions is explored.(1)Graphene oxide(GO)-Ti3AlC2 copper matrix composites were successfully prepared by the design of ceramic-carbon composite strengthening system followed by hot-pressing sintering of core-shell Cu/GO/Cu composite powder and electroless plating copper particle decorated Ti3AlC2 particles.The composites exhibit lower coefficient of friction(COF)and wear rate than those reinforced with single GO or Ti3AlC2 ceramic particles.The anti-wear mechanism is mild fatigue wear.Whereas,the wear mechanism of GO reinforced copper matrix composites is adhesion wear and wear wear,while the wear mechanism of Ti3AlC2 ceramic reinforced copper composites is abrasive wear and fatigue wear.This improvement in tribological properties is attributed to the synergistic strengthening of GO and Ti3AlC2.GO and Ti3AlC2 ceramic particles jointly bear the load and enhance the strength of the composites,effectively preventing the occurrence of adhesive wear and abrasive wear of the composites,thus reducing the wear rate.In addition,GO and Ti3AlC2 work together to form dense,continuous,easy-shear layers on the worn surface by combining the GO with tribological oxidation products from Ti3AlC2 during the sliding process,thereby reducing the COF.Therefore,with the special design of core-shell and copper decorated strengthening phases,the distribution of strengthening phase in the matrix space is controlled,avoiding the problems of easy agglomeration of graphene strengthening phase and interface bonding between cermet strengthening phase and copper matrix.The synergistic reinforcing effect of the two strengthening phases significantly improves the tribologoical performance of copper composites during the sliding process.(2)By etching Ti3AlC2 followed with subsequent intercalation,graphene-like Ti3C2 nano materials is successfully fabricated and have great potential in the field of tribology.Therefore,Ti3C2 coating was prepared on the surface of copper substrate via facile spray coating as a solid lubricant.By means of XRD,SEM,metallographic microscope and Raman spectrum,the tribological behavior and structure of Ti3C2 coatings were characterized,and the influence of phase change and properties of Ti3C2 transferred film during the sliding process in different atmospheric environment was explored.The friction and wear tests of Ti3C2 coatings were carried out at both the ambient and vacuum environment.The results show that in atmospheric environment,the COF of Ti3C2 coating modified samples is very low,about 1/6 of that of the blank control group,and the COF curve is very stable,while the wear rate is as low as 1/100 of the bare copper.In the atmospheric environment,the wear morphology of bare copper and Ti3C2 coating modified sampleswere characterized by SEM.Raman spectra was used to analyze the change of transferred film in each sliding stage.The results show that the characteristic peaks of Ti-C in the original Raman spectrum gradually disappear during the sliding process,and the characteristic peaks of D,G and 2D of carbon materials appear.According to the analysis of Raman spectra,lubricating mechanism could be attributed to graphitization of amorphous carbon.And Ti3C2 will generate graphitization during sliding process under ambient atmospheric environment,while carbon-rich transferred film with lubricating effect will be generated.At the same time,it can effectively block the direct contact between the counter ball and the copper substrate.In the vacuum environment,Ti3C2 coating modified samples still have good anti-friction and wear resistance.Although the friction factor was slightly higher than in the atmospheric environment,rising to 0.25,the friction factor was 60%lower than that of the blank control group.The wear track and the transferred film were analyzed by optical microscope and SEM,and the wear mechanism was mild fatigue wear and abrasive wear.And in the vacuum environment,the carbon-rich transferred film can still be found on the counter ball.This work shows that Ti3C2 coatings has good anti-friction and wear resistance in both vacuum and ambient atmosphere.However,its stability during the sliding process still needs to be further improved.(3)In order to improve the stability of Ti3C2 in the process of sliding friction,and can adapt to more working environment.The Ti3C2/GO heterostructure coating with higher stability was successfully prepared by constructing a heterogeneous structure based on Ti3C2.Under normal atmospheric conditions,the heterostructure coating has comparable self-lubricating and anti-friction properties as GO.The friction factor of the Ti3C2/GO heterostructure coating was 25%lower and the wear rate was 1/100 of that of the GO coating,comparing with the GO coating under dry nitrogen atmosphere.The improvement of tribological properties can be attributed to the synergistic lubrication of GO and Ti3C2 and the higher stability of the heterogeneous structure.Therefore,during the sliding process at dry nitrogen atmosphere,Ti3C2/GO heterostructure coating remains stable to generate carbon-rich transferred film and continuous,uniformly distributingtribo-layer with easy-shear to disperse shear stress and inhibit the direct contact between counter ball and substrate,thus limiting the friction produces in the process of adhesive and abrasive wear,thereby reducing the wear rate and COF.(4)The Ti3C2/GO heterogeneous structure coating exhibits excellent stability and self-lubricating performance.By designing the spatial distribution of Ti3C2 and GO in the copper matrix,good interface bonding and avoiding agglomeration can be achieved.Ti3C2/GO heterostructure strengthening phase was prepared in situ on copper powder by the combination between negatively charged hydroxyl group on Ti3C2/GO and copper ion.The 10%Ti3C2/1%GO/Cu composites were prepared by vacuum hot-pressing sintering.During the sliding process,the multi phase reinforced copper matrix composites show excellent tribological properties.Compared with the pure copper control group,the COF of 10%Ti3C2/1%GO/Cu composites was 40%lower,and the COF curve was stable,and the main wear mechanism was mild fatigue spalling.The improvement of tribological properties can be attributed to the synergistic lubrication from GO and Ti3C2 along with higher stability of the heterogeneous structure.Therefore,wear track of the 10%Ti3C2/1%GO/Cu composites is found to be covered by continuous and uniform distributing tribo-layer with easy shearing,effectively inhibit the direct contact of the sliding interface between counter ball and copper matrix,and thus limit the adhesive wear and abrasive wear during sliding process,thereby reducing the COF.Although no transfer film was found on the counter ball.The wear surface was relatively smooth,indicating that the hardness of 10%Ti3C2/1%GO/Cu composite material was relatively high.Therefore,it indicates that 10%Ti3C2/1%GO/Cu composites not only have good self-lubricating properties,but also have improved mechanical properties.