| Resin-based friction materials have been widely used in the automotive braking field due to the simple preparation process and low cost.However,the thermal degradation of organic components in composites at high temperatures can result in severe fade,which limits the application prospects.Ceramic ingredients of high mechanical strength and lower temperature sensitivity can be used to improve the fade resistance effectively.But relative research has not yet been systematically and in-depth studied enough.Firstly,ceramic ingredients in traditional ceramic reinforced resin matrix friction materials were used as abrasives,so the size was controlled in nano/micro level(?150 ?m).As a result,ceramic ingredients were only used to assist the resin matrix to carry friction load,which could not take a good advantage of the good wear resistance and high friction stability.Secondly,the research quantifying friction behavior during sliding was rarely performed.Finally,most of the study was focused on the tribology properties but rare on the mechanical performance.For that,the idea that using larger silica particles to reinforce cashew nut shell oil modified phenolic resin matrix was proposed.The skeleton effect of large particles may effectively improve the fade resistance,wear resistance and friction stability of friction materials.This paper systematically studied the influence of silica particles on mechanical properties and tribology properties of materials,as well as the friction mechanisms.The main contents and conclusions were listed as follows:(1)The worn surfaces of cashew nut shell oil modified phenolic resin based friction materials under different temperatures were observed through SEM in a fixed-point observation method.Through that,the dynamic tribological properties were deeply studied.Results showed that the dynamic tribological performance has a good consistency with the evolution of worn surfaces.Under low temperatures(100?),the worn surfaces kept stable as a function of friction distance,leading to the constant friction coefficient and wear rate.At high temperature(250?),the real contact area between the friction couple increased gradually,as well as the Fe in contact plateaus.As a result,the friction coefficient increased gradually,while the wear rate decreased.Under on-changing temperature conditions(100?350?),the fade occurred when the temperature was above the thermal degradation of the binder(250?),which was due to the decreasing surface mechanical properties.(2)The effects of silica particles on the mechanical properties of composites were studied by using three point bending and impact toughness tests,as well as the distribution of elastic stress field in composite.Theoretical model and experimental results showed that the addition of ceramic particles causing a disturbance of stress field had an important effect on the mechanical properties.When the particle size is 3.0 mm and the content is 50vol.%,the three-point bending strength is about 48 MPa,and the impact toughness is 0.31 J/cm2.Compared with the material without ceramic particles,they both reduced.The fracture cross section exhibits a brittle fracture characteristic,and the crack passes through the particles.It was found that the stress value at the edge of the particles is the largest.The maximum stress increases with the ratio of particle elastic modulus to matrix elastic modulusas.The average stress of the ceramic reinforced resin-based friction materials is positively related to the content and shape complexity of particles.When the particles size varies from 700 ?m to 3.0 mm,the three-point bending strength and toughness decrease with the increasing particles size.(3)The influence of silica particles size on friction coefficient materials wear and disc wear was studied by using constant speed friction machine,as well as the tribological mechanisms.It was found that the friction coefficient and materials wear increase first and then decrease with the particles size(10 ?m-3.0 mm),and reach a maximum at about 80 ?m.Below 700 ?m,the disc wear increases with the increasing particles size,but when larger than 700 ?m,it decreases with the particles size.The roles of particles during friction vary with the particles size.Small particles(?10 micron)were indirectly involved in friction in the form of "secondly plateaus".With the increase of size,particles(e.g.,80?m)detached from the matrix participate in the friction in the form of "three-body abrasion".The particles ranging from 700 um to 3.0 mm were firmly nailed to the friction interface to participate in the friction in the forms of "two-body abrasion" and providing "sticky" joints.A physical model quantifying the friction behavior of resin-based friction materials reinforced with larger particles shows that the friction coefficient is proportional to the content of particles and the shear strength of friction film,but inversely proportional to the normal load.(4)Based on the effects of particle size on materials mechanical properties,friction and wear performance,3.0 mm silica particles were selected to reinforce the resin matrix.The wear resistance and fade resistance were evaluated by the constant speed friction tester,while the friction stability under different speed and pressure was tested by MM100-? friction machine,in contrast to materials without particles and with small particles.The resin-based friction materials reinforced with 3.0 mm particles has high fade resistance,good friction stability,and extremely low wear.The fade rate at 350? is only about 6%.Moreover,the wear resistance is doubled compared to materials without ceramic particles.In addition,the stability and reliability under different speed and pressure conditions are better than those of well-known commercial brake materials. |
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