纳米颗粒作为涂层的抗磨效果

Materials Science and Engineering A426(2006)59–65Investigation of the tribological properties of polyfluo wax/polyurethane composite coatingfilled with nano-SiC or nano-ZrO2Hao-Jie Song a,b,Zhao-Zhu Zhang a,∗a State Key Laboratory of Solid Lubrication,Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,Lanzhou730000,PR Chinab Graduate School,Chinese Academy of Sciences,Beijing100039,PR ChinaReceived25October2005;accepted23March2006AbstractEffect of nano-SiC or nano-ZrO2on the friction and wear behaviors of the polyfluo wax(PFW)/polyurethane(PU)composite coating was studied using a ring-on-block wear tester under dry friction condition,and the worn surfaces of the PFW/PU composite coating and the transfer films formed on the surface of the counterpart ring were investigated by SEM and optical microscope,respectively.The structural changes of the PFW/PU coatingfilled with nano-ZrO2during the friction process were investigated with a Fourier transform infrared microscope.It was found that nano-SiC or nano-ZrO2as thefillers contributed to improve the friction-reducing and anti-wear abilities of the PFW/PU coating.Especially, the anti-wear ability of thefilled coating is the most effective when content of nano-ZrO2is5.0wt.%.Furthermore,we studied the effect of curing temperature,sliding speed and applied load on the friction and wear behaviors of the PFW/PU coatingfilled with5.0wt.%nano-SiC or5.0wt.% nano-ZrO2.The results showed that the friction and wear behaviors of all coatings were much better in lower curing temperature than those in higher curing temperature.The friction coefficient of the composite coatings reinforced with differentfillers decreased with increasing sliding speed and applied load,and the anti-wear behavior of the coatingfilled with5.0wt.%ZrO2was the best under420N and at a speed of2.56m/s. The investigations of the frictional surfaces showed that thefillers of nano-SiC or nano-ZrO2were able to enhance the adhesion of the transfer films of the PFW/PU coating to the surface of counterpart ring,so they significantly reduced the wear of the PFW/PU coating.FTIR analyses indicated that the occurrence of the tribochemical changes during friction process mainly involved disordering and partial decomposition of the polyurethane binder.©2006Elsevier B.V.All rights reserved.Keywords:Polyfluo wax;Polyurethane;Nano-SiC or nano-ZrO2;Composite coatings;Tribological properties1.IntroductionThe concomitant decrease of both friction and wear is not systematically required.In some applications,such as coated tools,the challenge is to increase the wear resistance to extend the lifetime of the components.In other applications,a strong reduction of the friction in the0.01ranges is paramount to save energy[1].Composite coatings,made of a matrix and particles with low friction coefficient have been developed to obtain coat-ings for bearing application[2].Recently,nano-particles have been considered for fabrication of the wear-resistant polymer composites since less abrasive action during frictional sliding and greater surface area will contribute to improving the tribo-logical performance[3–9].Many types of composite coatings ∗Corresponding author.Tel.:+869314968098;fax:+869314968098.E-mail address:zzzhang@(Z.-Z.Zhang).are prepared by nanocale particles incorporation to enhance the anti-wear and mechanical properties.Zhang et al.[10]reported that the PEEK composite coating with7wt.%nano-SiC exhib-ited better wear resistance in most test conditions.Jung-Yeob Lee et al.[11]reported that nano-diamond particles were added up to4wt.%to PTFEfilm to improve the wear resistance and thermal stability.Toshifumi Sugama and Keith Gawlik[12]also studied the effect of nano-scale boehmitefiller on corrosion-and wear-resistant behaviors of the PPS coating and found that the filler reduced markedly the rate of blasting wear and increased the PPS’s glass transition temperature and thermal decomposi-tion temperature.Polyurethane(PUR)elastomers are largely applied to indus-try and consumer products,particularly in thefields of heavy pressure,load,impact and wear because they possess excellent comprehensive properties such as high wear-,oil-and corrosion resistance,high elasticity and damping,good adhesion to other materials and so on[13].Polyfluo wax(PFW)is a new-style0921-5093/$–see front matter©2006Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2006.03.10460H.-J.Song,Z.-Z.Zhang/Materials Science and Engineering A426(2006)59–65solid pared with the solid lubricant of PTFE,ithas lower melt point and is easy to disperse in solvent.In orderto meet the demand of engineering and design driven by eco-logical and economical reasons,some researches contributed toprepare PU coatings on lightweight metallic substrates recently[14–16].Ceramics or nano-composites of a matrix containing adispersion of second-phase particles usually have various novelproperties,such as dispersion hardening,self-lubrication,hightemperature inertness,good wear and corrosion resistance,andchemical and biological compatibility[17–21].In this work,nanometer SiC or ZrO2reinforced PFW/PU coatings have beenprepared by spray technique.Tribological performances of thecoatings under dry sliding condition were evaluated using anMHK-500ring-on-block wear tester.The influences of curingtemperature,applied load and sliding speed on friction and wearbehaviors of the coating were investigated.The present workwas expected to broaden the application of PFW/PU coating indry-sliding bearings.2.Experimental details2.1.MaterialsSteel45,a kind of medium carbon steels with0.42–0.50%C,0.17–0.37%Si,0.50–0.80%Mn,≤0.035%P,≤0.035%S,≤0.25%Cr,≤0.25%Ni,≤0.25%Cu and balance Fe,was used as substrate of the composite coatings.Single-component PU wasprovided by xinhua resin company of shanghai,the ash contentand the isocyanate content(NCO)were50and5–8wt.%,respec-tively.Polyfluo150wax(PFW)(density1.17g/cm3)in a diam-eter of approximately3–4m,were provided by MircopowderCompany of USA.The mixed acetone/xylene/cyclohexanone ina volume fraction of4:2:1was employed in the present work as asolvent.Nano-SiC(grit size about30–40nm,determined usinga TEM)and nano-ZrO2(grit size about20–30nm,determinedusing a TEM)were prepared at our lab.2.2.Coating preparationThe AISI1045block(12.7mm×12.7mm×19mm)waspolished with300grade water proof abrasive paper in turn andthen cleaned with acetone in an ultrasonic bath for5min.Thepowder of polyfluo150wax(PFW)were dispersed in the mixed solvent with ultrasonic stirring for5min,the quantity of the solid lubricant of PFW used was30%of PU mass.Prior to the PU, the nano-SiC and nano-ZrO2particles were dried at120◦C in vacuum for24h.For having an even dispersion of nano-SiC or nano-ZrO2particles in PU,both the binder and thefillers with desired proportion were carefully mixed by mechanical stirring and ultrasonic treatment before the lubricant additive(PFW) was added.The solution mixture was continuously stirred for 1h,and then subjected to an ultrasonic bath mixing until the liquid system became afine dispersion of solution.The coatings on blocks were prepared by spraying the coating precursors with 0.2MPa nitrogen gas.After solvent evaporation,a thinfilm was obtained on the substrate.It is then cured at60,120,180and 250◦C for2h,respectively.The thickness of the cured coatings was40–50m.2.3.Characterization of coatingFig.1shows the SEM pictures of the composite coatingfilled with5wt.%nano-ZrO2surfaces and sections.It is noted that the surfaces of the nano-ZrO2filled composite coating was quite smooth.The nanoparticlesfillers were seen to be well dispersed in the coating matrix.However,some cavities were apparent in the coating matrix,which may be caused by the solvent evapo-ration from the matrix and the poor adhesion between thefillers and the PU matrix.2.4.Evaluation of the tribological behaviors of the coatingsAn MHK-500ring-on-block wear tester(made by the Jinan Testing Machine Factory,China)was used to evaluate the fric-tion and wear behaviors of the bonded solid lubricant coatings. An AISI-C-52100steel ring of49.2mm in diameter and12mm thick(Hardness Hv850)was rotated against the PFW/PU coat-ingsfilled with nano-SiC or nano-ZrO2particles under dry friction conditions.Before each test,the steel ring was abraded with900grade water proof abrasive paper.Then the steel ring is cleaned with acetone followed by drying.The sliding distance was calculated from the product of the sliding speed and the sliding time.The wear life of the coatings was calculated after dividing the sliding distance by the corresponding coating thick-ness in micrometers.Thus,the wear life of the coatings took a unit of m/ m.All the friction and wear tests were carriedout Fig.1.The SEM photograph of the composite coatingfilled with5wt.%nano-ZrO2surface and sections.H.-J.Song,Z.-Z.Zhang/Materials Science and Engineering A426(2006)59–6561at20–25◦C and a relative humidity of40–60%.The data pre-sented in the current word are the averages of three replicate measurements.3.Results and discussion3.1.Friction and wear behaviors of the PFW/PU coatingPresented in Fig.2is the effect of content of nano-ZrO2or nano-SiC on the friction and wear behaviors of the PFW/PU coating.It can be seen that the addition of nano-SiC or nano-ZrO2can greatly improve the friction and wear behaviors of the PFW/PU coating.The friction coefficients of the PFW/PU coating sharply decreased when nano-ZrO2is below1.0wt.% then gradually increased with further increasing content of nano-ZrO2with the exception of10.0wt.%.Furthermore,it is found that the friction coefficients of the PFW/PU coating slightly decreased with increasing content of nano-SiC up to 3.0wt.%.The lowest friction coefficient was obtained when con-tent of nano-SiC is5.0wt.%.Then the friction coefficient rapidly increased with further increasing content of nano-ZrO2.For the coatingfilled with nano-SiC,the wear life tended to increase with increasing content of nano-SiC up to5.0wt.%and then decreased as content of nano-SiC went unceasingly -pared with the wear life of the nano-SiCfilled coating,that of the coatingfilled with nano-ZrO2was similarly affected by the filling of nanometer ZrO2.Interestingly,the biggest wear life was observed atfiller of nano-SiC or nano-ZrO2mass fraction of5wt.%for the nano-SiC or nano-ZrO2filled the PFW/PU coating.3.2.Variations of the friction and wear life with curing temperatureFig.3shows the comparison of the friction coefficient and the wear life of the PFW/PU coatingfilled with5.0wt.%nano-SiC or5.0wt.%nano-ZrO2with varying curing temperature.It can be seen that the friction coefficient of the PFW/PU coating filled with nano-SiC sharply increased when curing temperature is below60◦C and then gradually increased with increasing cur-ing temperature.However,below120◦C,the friction coefficient of the coatingfilled with nano-ZrO2was almost not affected by the curing temperature in a monotonic way.The friction coef-ficient sharply increased from120to180◦C and arrived at a steady-state valuefinally.For the coatingfilled with nano-SiC, the wear life slightly increased with increasing curing temper-ature up to120◦C,but decreased from120to180◦C.With further increasing curing temperature,the friction coefficient kept a steady-state value.The wear life of the coatingfilled with nano-ZrO2initially decreased then slightly increased from60 to120◦C.As curing temperature went unceasingly up,the wear life lineally decreased.As a result,the anti-wear andreduction-Fig.2.Effect of content of the nano-SiC and nano-ZrO2on the friction coefficient and the wear life of the PFW/PU coatings(2.56m/s,320N).(a)Friction coefficient vs.filler content.(b)Wear life vs.fillercontent.Fig.3.Effect of curing temperature on the friction coefficients and the wear life of the PFW/PU coatingfilled with5.0wt.%nano-SiC or5.0wt.%nano-ZrO2(320N;2.56m/s).(a)Friction coefficient vs.curing temperature.(b)Wear life vs.curing temperature.62H.-J.Song,Z.-Z.Zhang /Materials Science and Engineering A 426(2006)59–65Fig.4.Effect of sliding speed on the friction coefficient and the wear life of the PFW/PU coating filled with 5.0wt.%nano-SiC or 5.0wt.%nano-ZrO 2under 320N.(a)Friction coefficient vs.sliding speed.(b)Wear life vs.sliding speed.friction properties of the PFW/PU coating filled with nano-ZrO 2were better than those of the coating filled with nano-SiC at low curing temperature.3.3.Effect of sliding speed and applied load on the friction and wear propertiesVariations of the friction coefficient and the wear life of the PFW/PU coating filled with 5.0wt.%nano-SiC or 5.0wt.%nano-ZrO 2with sliding speed under the same load are shown in Fig.4.It can be seen that the coating filled with nano-SiC or nano-ZrO 2exhibited much higher friction coefficient and lower wear life at low sliding speed.With the increase of sliding speed,the friction coefficient of the coating filled with nano-SiC or nano-ZrO 2gradually decreased.This reduc-tion in the friction coefficient attributed to surface softening arising from frictional heating.At the same time,we find that the wear life of the coating filled with nano-ZrO 2significantly increased with increasing sliding speed when the sliding speed is below 2.56m/s.Especially,a sharp drop of the wear life was observed with increasing sliding speed from 2.56to 3.84m/s.For the coating filled with nano-SiC,the wear life increased with increasing sliding speed and arrived at a steady-state value finally.From the above analysis,it can be concluded that the coating filled with nano-SiC or nano-ZrO 2generally exhib-ited lower friction coefficient and higher wear life at moderate speed.The friction coefficient and the wear life of the PFW/PU coat-ing reinforced with 5.0wt.%nano-SiC or 5.0wt.%nano-ZrO 2under different applied load at a speed of 2.56m/s are shown in Fig.5.It can be seen that the friction coefficients of the filled coatings tended to decrease with the increase of applied load.For the coating filled with nano-SiC,the wear life little by little decreased with increasing applied load.Meanwhile,the coating filled with nano-ZrO 2exhibited a highest wear life at intermedi-ate applied load of 420N.With the increase of applied load,the anti-wear property of the coating filled with nano-ZrO 2become poor.The results indicated that applied load must be related to plastic deformation of the coating,which causes the transition to severe wear when applied load goes up.3.4.FTIR analysis of the PFW/PU coatingAs shown in Fig.6by Fourier transform infrared microscope of the PFW/PU coating filled with 5.0wt.%nano-ZrO 2at dif-ferent curing temperature.With increasing curing temperature,NCO was consumed by the OH and NH 2groups in matrix.This would be confirmed by the decrease of NCO peaks at 2272cm −1.NCO was greatly consumed when curing tempera-ture was around 250◦C,but slightly at 120◦C.Fig.5.Effect of applied load on the friction coefficient and the wear life of the PFW/PU coating filled with 5.0wt.%nano-SiC or 5.0wt.%nano-ZrO 2under 2.56m/s.(a)Friction coefficient vs.applied load.(b)Wear life vs.applied load.H.-J.Song,Z.-Z.Zhang/Materials Science and Engineering A426(2006)59–6563Fig.6.FTIR spectra of the PFW/PU coatingfilled with5.0wt.%nano-ZrO2at different curing temperature.The chemical reactions during the curing process of the coat-ings can be described as follows:R–NCO+H2O→[R–NH–COOH](1) [R–NH–COOH]→R–NH2+CO2(2) R–NCO+R –NH2→R–NH–CO–NH–R (3) Because the curing of PU was strongly dependent on the reac-tions between the NCO radical in the binder and the OH and NH2 radical originated from H2O in air and matrix,a certain relative curing temperature was essential for the curing chemical reac-tions[16].If the curing temperature was too high,the reaction would be too quick and accelerate the evaporation of the sol-vent or CO2from the coating,which leads to many holes on the surface of the coating and makes the anti-wear and mechanism properties of the coating poor.In order to investigate the structural changes of the PFW/PU coatingfilled with nano-ZrO2in the friction process,the struc-ture of the wear tracks of the coating was analyzed with a FTIR microscope.The original coating surfaces and the worn sur-faces were analyzed,and the results were shown in Fig.7.It can be seen that the strong absorption bands of OH,NH2,CH,Fig.7.FTIR spectra of the surface of PFW/PU coating with5.0wt.%nano-ZrO2 before and after the wear tests.(320N,2.56m/s).NCO and CO appeared on the original coatingfilled nano-ZrO2, and these bands were faded and disappear after the wear tests. This indicated that the occurrence of the tribochemical changes during friction process mainly involved disordering and partial decomposition of the polyurethane binder[22].In combination with the results of FTIR analysis at different curing temperature,it can be concluded that the structural change of the PFW/PUfilled with nano-ZrO2during the friction process was due to the friction heat,which caused disordering and partial decomposition of the polyurethane binder.3.5.SEM analysis of the frictional surfacesIn order to explore the wear-reducing mechanism of nano-SiC and nano-ZrO2and the effects of sliding speed and applied load on the friction and wear behaviors of the coatingfilled with 5.0wt.%nano-SiC or5.0wt.%nano-ZrO2,SEM(JSM-5600LV) and optical micrograph observations of the worn surfaces and the transferfilms were carried out.Fig.8shows the worn surfaces of the PFW/PU coatingfilled with different content of nano-SiC or nano-ZrO2under320N and at a speed of2.56m/s.It can be seen that obvious signsof Fig.8.SEM micrographs of the worn surfaces of the PFW/PU coatingfilled with different content of nano-SiC or nano-ZrO2(320N,2.56m/s):(a)with1.0wt.% nano-SiC;(b)with5.0wt.%nano-SiC;(c)with10.0wt.%nano-SiC;(d)with1.0wt.%nano-ZrO2;(e)with5.0wt.%nano-ZrO2;(f)with10.0wt.%nano-ZrO2.。