| Resin based friction materials are widely used in the field of transportation and machinery industry.With the rapid development of transportation and machinery industry,the requirements of resin based friction materials are more and more strict.The friction and wear properties of friction materials are strongly dependent on the reinforcement and friction modifier.It has become one of the focuses of current researchto develop new type of high performance friction materials by studying the new components and adjusting the friction and wear properties.As the main component of the resin based friction material,the reinforcement component has an important influence on the performance of the friction material.In recent years,the potassium titanate whisker with the character of green environmental protection,low hardness,high temperature resistance,abrasion resistance,excellent physical and mechanical properties,stable chemical properties and excellent infrared reflection ability,have become important research topic.Although this topic has been widely studied,most of the work wasfocused on the effect of content,size and morphology on tribologicalproperties.To the best of our knowledge,there are limitedstudies regarding the effect of crystallinity of potassium titanatewhiskers on tribological properties.Nevertheless,the compositionand microstructure of the reinforcements play a key role in the overallperformance of braking systems.It is known that the crystallinityof the material is the main reason that may alter the mechanicalproperties.As the friction modifier of the resin based friction material,the friction increasing component has an important influence on the friction and wear properties.Mainly to improve and stabilize the friction coefficient,improve the fade resistance and recovery performance.As a kind of special structure composed of hard zirconia particles,granulated zirconia is considered to be an ideal material for friction increasing.The composition of the friction material has an important influence on the structure and quantity of the friction layer.Friction layer is a bridge to understand the relationship between component and tribological behavior.The friction model is usually used to study the temperature field and stress field.Here the effect of friction layer on temperature field and stress field during friction process is studied.Based on this,in this paper,the effect of content,morphology and crystallinity of potassium titanate whisker on the friction and wear properties of friction material were investigated,the effect of content and structure of granulation of zirconia on the friction and wear properties of friction material were investigated.Using PSI method,the comprehensive performance of friction materialsevaluated and optimized.A micro friction model of friction layer,the effect of size and number of friction layer on temperature field and stress field were discussed.Specifically,the following conclusions are obtained:1.A series of potassium titanate whiskers was synthesized via the hydrothermal method.The effect of reactant concentration,hydrothermal temperature,hydrothermal time and calcination process on morphology and properties of potassium titanate whiskers were investigated.The results show that potassium titanate whisker radially grow and aggregation of productwas fasciculate when the concentration of reactantwas low(10 mol/L).When the concentration was high(15 mol/L),the growth of potassium titanate whiskerswas inhibited and the aggregation type of potassium titanate whiskers was changed.Finally,the potassium titanate whiskers stacked densely.The increase of the hydrothermal reaction temperature and hydrothermal reaction time were in favor of potassium titanate whisker crystallization andgrowth.The main influence factor of crystalinity was calcination process.The high quality potassium titanate whiskers with high crystallinity were obtained via calcination process.The optimum technological conditions were obtained: KOH concentration 10mol/L,the reaction temperature 240 oC,the reaction time 24 h.Under the conditions,the potassium titanate whiskers with regular morphology,high aspect ratio and good dispersion can be obtained.After calcination process with 900 oC and 5 h,the crystallinity of potassium titanate whisker was greatly improved.The product is K2Ti6O13 and the whisker grow along the [110] direction.2.The potassium titanate whisker reinforced resin based friction materials were prepared.The effects of potassium titanate whisker on the friction coefficient and wear rate offriction material were investigated.The results show that the 10 wt% potassium titanate whisker showed best overall performance for friction material.Becausethe wear resistance of the friction material was poor when the potassium titanate whisker content is too low and the friction coefficient was not stable when the potassium titanate whisker content is too high.The appropriate content of potassium titanate whisker improve wear resistance of friction material and effectively inhibit the occurrence of adhesive wear of the friction surface.The potassium titanate whisker with certain whisker aspect ratio and good dispersioncan stabilize the friction coefficient and lower wear rate.The higher crystallinity whiskers showed enhanced ability in improving the friction coefficientthan the lower crystallinity whiskers.During the fade testing process,low crystallinity whiskers were moreeffective to stabilize the friction coefficient.During the recovery testing process,high crystallinity whiskers weremore effective to stabilize the friction coefficient.High crystallinity whiskers were more effective in theimprovement of wear resistance.In order to understand the friction and wear behaviors,the worn surfaces andwear debris were analyzed by means of SEM.The results revealed that the higher crystallinity whiskers wassignificant to form stable contact plateaus.By comparing with reference samples,the wear rate of friction material was reduced by 23% by adding KTO-9.With the increase of potassium titanate whiskers crystallinity,the wear resistance is improved with the increase of friction materials reinforced by.This is mainly because of the high crystallinity of potassium titanate whiskers can form more stable contact area on the friction surface.3.The granulated ZrO2 reinforced resin based friction materials were prepared.The effects of granulated ZrO2 on the friction coefficient of friction material were investigated.The results show thatthe friction coefficient of composites increases first and then decreases with the increase of the content of granulated ZrO2.The friction coefficient is increased because of hard particles increase scratch and shear stress,the maximum friction was converted to output;the decrease offriction coefficient is mainly due to the "rolling effect" of granulated ZrO2 in the interface and it reduces the shear stress.When the content of granulated ZrO2 was 10%,the friction coefficient of the composites was the highest,the range of values was 0.46~0.53,and the wear rate was almost the same as that of the reference sample.The friction coefficient of the composites was greatly increased by the addition of granulated ZrO2.The performance matrix of resin based friction material was established,and the PSI method was used to evaluate the performance of the friction material with different content of granulated ZrO2.The GZ10 sample was the best friction material.Granulated ZrO2 can control the friction and wear properties of composites with different resin content,which can adjust the friction coefficient,stabilize the friction coefficient and improve the wear resistance.10% granulated ZrO2 was best to improve the friction coefficient and wear resistanceof the friction material with the resin content of 5%,10% and 15%.4.Based on the experimental results,the finite element model of the friction layer was constructed to study the influence of the number and size of the contact plateau on the temperature field and stress field.The results show that under the same load and speed conditions,the more the number of contact area,the lower the temperature of the contact interface between the friction material and the disk during the friction process.Under the same load and speed conditions,the larger the size of the contact area,the lower the temperature of the contact interface between the friction material and the disk.In the friction process,the temperature field of the contact plateau in different positions is different,and the temperature of the contact plateau in the front of the friction direction is the highest.At each contact plateau,the highest temperature appears in the most forward direction along the friction direction,and the back radiation is lower and lower.With the increase of the number and size of the contact plateau,the actual contact area between friction material and disk increases,which is conducive to the dispersion of contact zone stress and temperature will be low.It can avoid damage and wearfriction material due to thermal decomposition of resin.In addition,the larger the actual contact area is,the smaller the change of the friction stress field after the friction process. |
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