Study On The Tribology Characteristic Of Nano/micro Additives

Lubrication is one of the oldest technologies in human history. Historically, lubrication is more an art than science.There also has been ever pressure to develop the automobile that has more fuel efficiency now. From the viewpoint of the tribology, this means to increase load, speed and temperature for the major components and also to use lower viscosity lubricants to lubricate them. This leads to decrease lub-film thickness between the interacting surfaces of these components.It is the bigger challenge and oortunity for china auto industry to save energy according the requirements from the china government.In order to face the energy crisis, china auto industry needs to pay much efforts to introduce some new technical during the products designing, to use some new materials, to establish the new standard for the auto parts, and also requiring immediate action to carry out tribology strategy to all products. Tribology is an interdisciplinary science which involving physics, chemistry, fluid mechanics, materials, circumstances and human factors.In this study, we use SRV as the friction tester to test the lubricants with different nano/micro additives, take SEM, wear scar and friction coefficient as the results to identify if the lubricants with the trobology charaeristics.In order to understand the tribology, one should better understand about artistic Stribeck Curve first.This curve shows relationships between thickness of lub-film, friction coefficient, viscosity, speed and load. According to Stribeck Curve, there are four characteristic regions to be described:(A). hydrodynamic lubrication model (HL); (B). elastohydrodynamic lubrication model (EHL); (C). In mixed lubrication model (ML); (D). In boundary lubrication model (BL)The highlights of this paper is the development of lubricant electric conductibility tester. The tester is developed to determine breakdown voltage of lub-film, in other words, proportional to electric conductibility of lub-film.In this study, based on the Ampere's law, choosing Timken as the main tester, taking the ASTM D 2782《Test Method for Measurement of Extreme Pressure Properties of Lubricating Fluids》(Timken) as the testing procedure, choosing the ring and block of the Timken as the two electrodes, data collection software, data transform software (A/D) and the computer are used to develop this electric conductibility tester.When the external voltage is 5V, according to the ratio of R1 /R2, the partial voltage of R2 is 35mV. Thus the film presents 35mV insulation voltage. In general, base on the thickness and characteristic of lub-film, the breakdown voltage may change from 0 to 35mV.When the ring and block are sliding against each other, the lub-film between the ring and block will produce many micro-colligations due to the friction. Each micro-colligation has its electrical double layer, in order to confront the voltage alied to the ring and block, all the micro-colligation may be rearranged in the same direction under the electric field force. Therefore, the break-down voltage of lub-film is the quantity and the density of micro-colligations. it redefined as the electric-hydrodynamic lubrication model; the electric-elastrohydrodynamic lubrication model; the electric-mixed lubrication model and the electric-boundary lubrication model. The abbreviation is eHL, eEHL, eML and eBL.It is clear at a glance if the lub-film is too thin to withstand the higher load and higher temperature, The micro-colligation can't be found between the ring and block, and eBL model can be reverted into BL (or metal to metal contact) model. With the help of this tester, we examine the different kinds of lubricant, especially the lubricant with powdered MoS2 (molybdenum disulfide), revealed the four phases HL, EHL, ML, BL according to Stribeck Curve.The second highlights of this study is the investigation of electric-elastohydrodynamic lubrication model.When the breakdown voltage is larger than 35 mV, it may be defined as the function of electric-elastohydrodynamic lubrication model, the abbreviation is eHL.It is shown that the eHL model and also the eEHL model. This model is considered as strong electric-elastric physical phenomenon. With such solid particles as MoS2, under the friction of the ring and block, each particles forms strong electric-charged cells which get together and produce charged aggregation, that is the electric-charged micelle particles or micro-colligation;every aggregation possesses the same charge towards the outside; the single aggregation could interconnect to each other, and then become greater aggregations; all the aggregations cover the friction region. The external voltage tries to breakdown this lub-film, so it needs to overcome not only the physics absorbed film of the liquid lubricant, but also the additional reversed arrangement electrical charge. Therefore, the value of breakdown voltage is increased in some way, which exceeds 35 mV.It is shown that MoS2 can boost up the quantity and density of micro-colligation. Some points of breakdown voltage are larger than 35mV, the max breakdown voltage is 40.83mV, increasing 5.83mV, VeHLMoS20N =5.83mV, It is strong and robust eHL model.Even if the influence is not the remarkable factor in Stribeck Curve, temperature has a remarkable influence on the viscosity and the breakdown voltage of the lub-film.Generally speaking, the lub-film with solid nano/micro MoS2 particles, the thickness of the film could increase after self-assembly or self-reparability, therefore enhance the bearing capacity of the lubricant.In this study, even if not showing the influence of different particles size of MoS2 on the breakdown voltage, it could deduced when solid particles sizes are larger than nanometer may disorder the adsorbed layer of the polarity molecule, and then weaken the electrical double layer, the main reason of this is the competitive adsorption of some solids nano/micro particles with polarity molecules.When the solid nano/micro particles penetrating in the frictional area, there exists the adsorption competition between the polarity molecule of lubricant and the solid nano/micro particles.The solid nano/micro particles could produce three different results in the adsorption competition with polarity molecules:(1) Be positive correlation with the polarity molecule and extreme pressure additive.(2) Be negative correlation with the polarity molecule and extreme pressure additive.(3) Without correlation with the polarity molecule and extreme pressure additive.The third highlights of this study is the way to revealing stribeck curve perfectly. The simulated Stribeck Curve could also be divided into four testing regions (A);(B);(C);(D). molybdenum disulfide;5 cycles;80N-60N-40N-20N-0N;each 120 seconds;total 600 seconds)Based on measuring of breakdown voltage of lub-film, the forth highlights is the investigation of cobble-stone lubrication model. Estimating and analyzing in theoretically, the possible primary shapes of the electricity elasticity and micro-aggregations are bigger cobble-stone when testing load is below the OK value, and the shape of the micro-aggregations trends to smaller cobble-stone when testing load is equal to or larger than the OK value.It is indicated that the thickness of lub-film changes along with the load and temperature, following the Stribeck Curve absolutely, whatever the kinds of lubricant be tested. That is to say, when the load increased, the thickness of lub-film trended to be decreased, when the thickness of lub-film trends to be zero, it means metal to metal contact entirely. Homogeneous lub-film may be with weak eHL, eEHL, eML, eBL model.The conclusions of this study is that the eHL, eEHL, eML, eBL models may be one of the common phenomenon with electric-elasto polarity layer plays the role in tribological fields.It is deduced that there would be strong eEHL, eHL, eML, and eBL models for some lub-film with insulation solid nano-particles, and there would be weak or without of eEHL, eHL, eML, eBL models for some lub-film with conductor solid nano-particles.Some solids nano/micro particles with characteristics of electric-elasto could be used in auto lubricant, having the energy-saving capability to meet the needs of future requirements of china auto industry.