CeO_(2)nanoparticles are potential anti-wear additives because of their outstanding anti-wear and load-bearing capacity.However,the shear-sintering tribo-film formation mechanism of oxide nanoparticles limits the tribo-film formation rate and thickness greatly.In this study,by compounding with zinc dioctyl dithiophosphate(ZDDP),ultra-fine CeO_(2)nanoparticles modified with oleylamine(OM)can quickly form 2μm ultra-thick tribo-film,which is 10-15 times thicker than that of ZDDP and CeO_(2),respectively.The ultra-thick tribo-film presents a nanocomposite structure with amorphous phosphate as binder and nano-CeO_(2)as filling phase,which leads to the highest loading capacity of composite additives.The results of adsorption experiments tested by dissipative quartz crystal microbalance(QCM-D)showed that the Ps value of additive has nothing to do with its equilibrium adsorption mass,but is directly proportional to its adsorption rate in 10 s.The compound additive of CeO_(2)and ZDDP presented the co-deposition mode of ZDDP monolayer rigid adsorption and CeO_(2)viscoelastic adsorption on the metal surface,which showed the highest adsorption rate in 10 s.It is found that the tribo-film must have high film forming rate and wear resistance at the same time in order to achieve super thickness.Cerium phosphate was formed from ZDDP and CeO,through tribochemistry reaction,which promotes the formation of an ultra-thick tribo-film with nanocomposite structure,which not only maintains the low friction characteristics of CeO,but also realizes high Pg and high load-carrying capacity.
In this work,we evaluated the effect of the counter-body material(the same or dissimilar)and contact configuration(moving or stationary body),at similar contact tribological conditions,on the tribochemical and nanotopography characteristics of adsorbed surface films.Zinc dialkyldithiophosphate(ZDDP),the best performing anti-wear additive,was used in self-mated steel/steel and DLC/DLC contacts,which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests.The macroscale(tribometer)and nanoscale(atomic force microscopy)friction,thickness,topography,and chemical(attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy)properties of the tribofilms were studied.The results revealed unexpectedly large differences in all the studied tribofilm parameters;this is because all the tribofilms are completely different;this includes the chemical composition,which is known to have a crucial effect on the nano-and macro-scale tribological properties.These results clearly demonstrate that the surface material,additives,and common contact operating parameters,that is,pressure,velocity,and temperature,crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact,and the material of the moving/stationary bodies.
To investigate the effect of diesel soot on the distribution,composition and mechanical properties of ZDDP tribofilm,a HFRR tribometer was applied to study the tribological performance.Worn surfaces lubricated with ZDDP and soot were analyzed by laser microscopy,SEM/EDS,Raman spectroscopy,XPS,and a nano-indentation equipment.Results show that soot scrapes off the ZDDP tribofilm and can be embedded into the worn surface,leading to the reduction of film thickness and non-uniform distribution of tribofim.The phosphate structure in ZDDP tribofilm changes from short chain pyrophosphate to long chain metaphosphate due to the increased contact stress caused by the soot abrasive wear,which can promote the cross-linking of ZDDP.The hardness(H)and elastic modulus(E)of the worn surfaces increase,while the ratio of hardness to elastic modulus,H/E,decreases,which indicates that the reduction of wear resistance is caused by the soot.
In biodiesel-fueled compression-ignition(CI)engines,dilution by unburned biodiesel has been found to have adverse effects on the boundary lubrication properties of additives in fully formulated engine lubricants.Such dilution of engine lubricants could be even more pronounced for CI engines running on higher blend concentrations of biodiesel.Given the nanoscopic nature of the interaction,this study seeks to determine the nano-tribological properties of an engine lubricant additive(e.g.,zinc dialkyldithiophosphate(ZDDP))when diluted with a fatty acid methyl ester(e.g.,methyl oleate).Using lateral force microscopy(LFM)together with a fluid imaging technique,the lowest nanoscopic friction forces and coefficient of friction values(0.068-0.085)were measured for ZDDP when diluted with 70 vol%of methyl oleate.These values are also observed to be lower than those measured for neat ZDDP and neat methyl oleate,respectively,under similar conditions.Subsequently,interpreting the data with the Eyring thermal activation energy approach,it could then be elucidated that the lower frictional losses observed for the contact lubricated with this volumetric mixture are a result of the lower potential energy barrier and activation energy required to initiate sliding.These energy values are approximated to be 2.6%and 28.9%(respectively)lower than that of the contact lubricated with neat ZDDP.It was also found that the mixture,at this volumetric concentration,possesses the highest possible pressure activation energy(load-carrying capacity)along with the lowest possible shear activation energy(shearing),potentially indicating optimum tribological conditions for boundary lubrication.Thus,the findings of this study suggest that an optimum concentration threshold exists in which a synergistic nano-tribological interaction between additives and fatty acid methyl esters can be attained,potentially reducing boundary frictional losses of lubricated conjunctions.Such findings could prove to be essential in effectively formulating synergistic ad
Siti Hartini HAMDANChiew Tin LEEMei Bao LEEWilliam Woei Fong CHONGCheng Tung CHONGSuhaila Mohd SANIP
