Preliminary Study On The Mechanism Of Achieving Oil-Based Superlubricity Via Synergistic Effect Between Polyetheretherketone-GCr15 Tribopairs And Cu Nanoadditives

Oil-based superlubricity with a friction coefficient lower 0.01,provides a new pathway to meeting the requirements for saving energy and prolonging the services life of mechanical components.Up to now,oil-based superlubricity is facing many difficulties,such as a long running-in period,specific fric-tion pairs（ceramics,sapphires）etc.,which limits its industrial applications.In order to realize the rapid re-sponse of oil-based superlubricity and reduce the serious wear caused by running-in,dalkyldithiophosp-horic modified copper nanoparticles（Cu-DDP NPs）were added into base oil（i.e.,poly-alpha olefin oil,PAO40,a synthetic base oil with high viscosity）as lubricating additive,due to their superior dispersibility as well as excellent anti-friction and wear reduction properties.Polyetheretherketone（PEEK）with lower cohesive energy density（CED）formed a friction pair with GCr15,in which PEEK was easily transferred to the surface of GCr15 as a protective layer,also as a carbon source.Because of frictional shear force and pressure,Cu-DDP NPs can play a catalytic role to promote the decomposition of PAO40 or PEEK chains,developing a high-performance chemical reaction film based on carbon.The main research contents and conclusions of this paper are as follows:（1）Oil-based sueperlubricity mechanism of PEEK-GCr15 friction pairsUsing Cu-DDP NPs as additives and PAO40 as lubricating oil,the effects of Cu-DDP NPs concentration,applied load and friction speed on tribological properties were studied at the PEEK-GCr15interface by the MRH-3 high-speed ring-block friction and wear testing machine.Then the oil-based superlubricity mechanism of the system was deeply discussed.As shown in the results,the anti-friction and wear reduction of Cu-DDP NPs is obvious,resulting the oil-based superlubricity of PEEK-GCr15 system.When 0.25 wt.%Cu-DDP NPs was added,the friction coefficient was as low as 0.002,which was 97%lower than PAO40,and the wear rate was in the order of 10^-9 mm³/Nm.SEM showed that the surface of the GCr15 was covered with a carbon-based film.ATR-FTIR and TEM outcomes indicated that the modification chains on the surface of Cu nanoparticles were broken during the friction process and then were adsorbed on the wear surface of GCr15 which became components of the tribofilm.Moreover,as an ideal substrate for graphene,single or multilayer graphene was found on the Cu（111）crystal surface and its weak interlayer bonding force could significantly reduce the coefficient of friction and wear.The high-performance carbon-based tribofilm works synergistically with the excellent self-lubricating performance of PEEK,the filling and repair of Cu NPs to achieve oil-based superlubricity of the system.（2）Oil-based superlubrictity mechanism of PEEK composites and GCr15 friction pairsAlthough the pure PEEK and GCr15 can achieve oil-based superlubricity due to the friction catalysis of Cu-DDP NPs,modern industrial equipment often faces harsh working conditions such as high-speed and high-load,low-speed and heavy load,frequent start-stop conditions,etc.,which puts forward higher requirements for the mechanical and mechanical properties of friction pair materials,and how to achieve superlubricity under high load is an important problem.With the purpose of achieving oil-based superlubricity under higher load,commonly used reinforcement fibers（short carbon fiber,SCF）and solid lubricants（Poly tetra fluoroethylene,PTFE）were used as fillers,and the influence of fillers on the mechanical properties of PEEK was also discussed.The influence of load（400 N～1000 N）on the tribological properties of PEEK/10SCF/3PTFE-GCr15 was studied,and the morphology and structure of the worn surface were analyzed by SEM,TEM and Raman.The mechanical properties of PEEK/SCF were significantly enhanced,and the tribological properties were also improved.Compared with pure PAO40,the friction coefficient and wear rate were substantially reduced when PAO40+Cu was used as lubricant.Due to the poor adhesion between SCF and PEEK interface,although adding SCF could improve the wear resistance of PEEK,only the friction coefficient of PEEK/5SCF decreased significantly when PAO40 was lubricated,and the friction coefficient increased when the SCF was more.PEEK/10SCF did not achieve superlubricity when lubricated with PAO40+Cu.PTFE was added to PEEK/10SCF,and it was found that PEEK/10SCF/PTFE-GCr15 could achieve oil-based superlubricity in the range of 400 N～1000 N.The SEM consequences illustrated that when PAO40+Cu as lubricants,the fatigue wear phenomenon of the wear surface of PEEK/10SCF/3PTFE and GCr15 were greatly weakened,and the furrows and cracks were reduced.ATR-FTIR explained that PEEK/10SCF/3PTFE was transferred to the GCr15 surface and deposited into a film.Raman and TEM indicated that the GCr15 interface generated a carbon-based friction film containing graphene and Cu.