Mechanical And Tribological Properties Of Polyetheretherketon Composites Filled With Liquid Crystal Poly(p-oxybenzoate) And Functionalized Carbon Nanotubes

Polyetheretherketon(PEEK) has great potential for developing high-performance structural and tribological material,due to its excellent comprehensive mechanical properties,high resistance to heat,good sealing property and high resistance to wear.However,pure PEEK can not meet the demand of operating in harsh conditions such as high temperature and high pressure,owing to its low glass transition temperature,low thermal conductivity,high creep rate and wear rate at high temperature,and low mechanical retaining rate at high temperature.Liquid crystal poly(p-oxybenzoate)(POB)with high resistance to heat,high reducing friction and resistance to wear is an ideal reinforcement at high temperature.Carbon nanotubes(CNTs)are widely used as filler for polymers,due to their excellent mechanical,thermal and electrical properties.However,carbon nanotubes are apt to entanglement and have surfaces with chemical inertness.The poor dispersion in polymer and weak interfacial bonding between carbon nanotubes with polymer limit the full potential development of CNTs.In order to improve the dispersion of CNTs in matrix and interfacial adhesion to matrix,CNTs were functionalized first and then filled synergistically in PEEK matrix with POB,expecting for enhancing the resulting mechanical,thermal and tribological properties.Firstly,carbon nanotubes were treated by mixed acids and poly(ether sulfone)(PES),respectively,and then the dispersion,structure,kinds and amounts of functional group(modifier)as well as content on CNTs surface were investigated by transmission electron microscopy(TEM),Fourier transform infrared(FT-IR)spectra,Raman spectra(Raman)and thermogravimetric analysis(TGA),respectively.These results show that PES-treated CNTs have better dispersion than acid-oxidized CNTs,and PES treatment of CNTs did not destroy the perfect graphence structure any more after acid-oxidation.A layer of PES was grafted to the surface of CNTs,decreasing the Van de walles forces between CNTs and leading to better dispersion.Secondly,various treated CNTs were filled in PEEK matrix,yielding PEEK nanocomposites.The effect of various surface treatments of CNTs on the resulting mechanical properties of nanocomposites was comparatively studied by tensile,creep,and impact test and morphological observation.Based on it,the mechanical properties of PEEK nanocomposites filled with PES treated CNTs and POB was further studied.The results indicate that PES treated CNTs have better dispersion in matrix and stronger interfacial adhesion to matrix than acid-oxidized CNTs,resulting in higher mechanical properties.The addition of POB into PEEK nanocomposites filled with PES treated CNTs leads to higher mechanical properties due to the bridging of CNTs between neighboring POB and the three dimensional network structure.Thirdly,Thermo-gravimetric analysis(TGA),Differential scanning calorimetry(DSC),dynamic thermo-mechanical analysis(DMA)and thermal conductivity of various treated CNTs/PEEK nanocomposites and PES-treated CNTs/POB/PEEK composites were carried out.These results indicate that PES treatment is superior to acid treatment to improve the thermal stability,melting temperature,crystallinity and storage modulus of CNTs/PEEK nanocomposites,and multiscale composites has the highest thermal properties.This result can be explained by the fact that PES treatment of CNTs has better dispersion and stronger interfacial adhesion to matrix,thus hindering the mobility of matrix molecular chains and enhancing the nucleation of CNTs.Moreover,the incorporation of POB into CNTs/PEEK composites further inhibited the mobility of molecules and improved the thermal properties.In particular,the thermal conductivity increased by 3.5 times due to the formation of three dimensional pathway.Fourthly,the tribological properties and PV limiting value of PEEK-based composites were systematically studied under different load,velocity and temperature.The results show that the coefficients of friction(COFs)of composites are all decreasing with the increasing load,velocity and temperature,respectively,while the wear rate of composites decreases with the increase in load,and increases with increasing velocity and temperature.The addition of POB filler into PEEK matrix leads to significant improvement in resistance to wear.PES treated CNTs/PEEK nanocomposites has better tribological performance than acid-oxidized CNTs/PEEK nanocomposites.In addition,the multiscale composites has the highest wear resistance and PV limiting value.Fifthly,the mechanism on the effect of PES treatment of CNTs on the tribological performance of CNTs/PEEK composites was discussed,and the synergistic mechanism of POB and PES-treated CNTs reinforcement was also explored.PES treatment of CNTs led to improved strength and stiffness of PEEK matrix,preventing the detaching of PEEK crystal from the worn surface,and thus improving the wear resistance.The synergistic effect of POB and PES treated CNTs on the tribological perfomance is as followed:(1)micro-needle bearing consisted of POB and CNTs extruded from the wear surface was formed;(2)3-dimensional network structure composed of POB and CNTs in matrix further improved the stiffness of matrix and reduced the plastic deformation during the sliding test;(3)PES-treated CNTs lowered the stress concentration between POB and matrix interphase,improving the capacity-load;(4)three dimensional pathway conducting heat composed of POB and CNTs greatly increased the thermal conductivity,decreased the sliding surface temperature and thus significantly enhanced reducing-friction and anti-wear properties.This study focused on PES-treatment of CNTs,characterized the modifying effect,and investigated the mechanical,thermal and tribological properties of PES-treated CNTs/PEEK nanocomposites and POB/CNTs/PEEK composites.In addition,the mechanism of PES treatment of CNTs was discussed,and a multifunctional composite material with excellent mechanical,thermal and tribological properties was obtained.This study developed new method of PES treatment,and gave a guide to develop PEEK-based structural and tribological material used under high temperature and high pressure conditions.