Tribological Behaviour And Application Technology Of Microcrushed Rape Straw Fiber Reinforced Friction Materials

Brake friction materials are related to the safety and stability of automotive braking,is the core component materials in the automotive braking system.In recent years,China’s brake friction material industry has developed rapidly,occupying more than half of the global market share.In fact,as people become more aware of environmental protection,environmentally friendly friction materials are highly favoured.To this end,the Chinese government has issued guidance documents such as the"Made in China 2025"strategy and the"14th Five-Year Plan"for green transport to promote pollution and carbon reduction,and to collaboratively promote the sustainable development of green transport.The use of green raw materials is an important way to develop environmentally friendly friction materials.Among them,natural plant-reinforced fibers are becoming a hot spot for research due to their biodegradability,good physical and mechanical properties,wide range of sources and low prices.However,in terms of engineering applications of natural fiber friction composites,there are problems such as lack of basic physicochemical data on natural fibers,poor homogeneity of mixed materials,and the material structure also needs further improvement to enhance its tribological properties.In this paper,friction composites were prepared using rape straw cellulose fibers as reinforcing fibers,and a series of studies were carried out on raw material components,processing processes and material structures.（1）The effect of sodium hydroxide modification on the flowability of rape straw fiber powder was investigated.Four alkali-modified fiber samples were prepared according to the particle size of the straw fibers,and the effects of the differences in surface microscopic morphology,chemical composition and morphological characteristics on the powder flowability of the fiber samples before and after modification were investigated.The results showed that sodium hydroxide treatment decreased the flowability index of fiber powders by 6 to 11.The removal of lubricating substances（waxes,pectin）,rough surface and increased anisotropy of the fiber structure due to alkali treatment were the main reasons for the decrease in the flowability of fiber powders.In addition,the chemical modification had a slimming effect on the straw fibers,reducing the average diameter of the fiber samples by 9.4%-30.8%,and the modified fibers had a larger aspect ratio,implying that the chemical modification increased the anisotropy of the individual fiber structures.FTIR and fraction analysis showed that this slimming effect was due to the removal of lubricating substances,lignin and hemicellulose from the fiber surface by the chemical modification.This is evidenced by SEM images of the microscopic morphology of the fiber samples.In terms of overall distribution,box plot analysis shows that the modification resulted in a more concentrated and regular distribution of straw fiber form factors.（2）Experimental optimization of hybrid feedstock mixing process based on box-behnken design/response surface method.The rape straw fiber powder was mixed with other raw materials.The uniformity and compressibility of the blended raw material powders were used as the optimisation index.The response values of the factors such as fiber dispersion time,particle size and addition amount were obtained by box-behnken design,combined with response surface method regression analysis,and optimised to find the best blending process.The range of tests was determined by single-factor tests,where the uniformity exceeded 95%and the compression rate was in the range of 15%-20%for dispersion times of 60s,120s and 180s,particle sizes of355-250μm,500-355μm and 600-500μm,and addition rates of 3wt.%,6wt.%and 9wt.%.The rape straw fiber powder was mixed with other raw materials.The uniformity and compressibility of the blended raw material powders were used as the optimisation index.The response values of the factors such as fiber dispersion time,particle size and addition amount were obtained by box-behnken design,combined with response surface method regression analysis,and optimised to find the best blending process.Firstly,a single-factor test was conducted to provide a reference test range for the multi-factor test,and then a second-order polynomial regression model was established by Box-Behnken design,and the test showed that the prediction error rate of this model was not more than 5%.An analysis of variance（ANOVA）based on the response surface method showed a significant antagonistic interaction between fiber size and dispersion time on powder compression,and a significant offsetting effect between dispersion time and addition amount.Using Design-expert software,the optimum mixing process was predicted to be 130s dispersal time,355-250μm particle size and 6wt.%addition.Validation tests showed that the blended powder under this condition had a compression rate of 16%and a uniformity of 98.6%,an improvement of 4.6%over the comparison group.On this basis,composite specimens were prepared and tested for friction stability using the hot pressing method.The results showed that the thermal recession rate of the optimised material was reduced by 42.1%and wear morphology analysis showed that the uniform distribution of the reinforced straw fibers was beneficial to the friction stability of the composite.（3）Preparation of low resin friction composites by wet granulation and tribological behaviour analysis.Granulated straw fibers,which have a different shape from other powder materials,were wrapped in pellets,thus improving the flow of the material and reducing the amount of binder.An orthogonal test and extreme difference analysis revealed that the best pelletization rate（96.2%）and pellet density（1.21 g/cm²）were achieved at 10 wt.%,6 wt.%and40 wt.%of straw fiber,seafoam fiber and corn starch water-linked bridge solution,respectively.Based on this formulation,the effect of the secondary binder on the tribological properties of the composite was investigated.The test results show that the composites exhibit good friction coefficient stability and wear resistance when a multi-binder with phenolic resin,sulphur and tin powder all added at 5wt.%is used.The reduction in the amount of resin binder and the vulcanisation reaction of sulphur and tin powder at high temperatures resulted in friction materials with good performance at high temperatures,with friction stability coefficients,recovery coefficients and total wear rates of 0.917,1.070 and 1.644×10^-7cm³（N·m）^-1 respectively.In addition,the wear surfaces of several friction material specimens were observed by SEM.The wear forms of sphere-structured friction materials are mainly ploughing wear,adhesive wear and fatigue wear,based on which the friction model of sphere-structured composites was established to further explain the wear mechanism.（4）Preparation of micro-crushed rape straw fiber pellet composites and experimental analysis of their tribological properties.The rape straw was physically modified by micro-crushing through a ball mill grinder,in the hope of further improving the pelleting quality and the tribological properties of the material using the easy agglomeration of the micro-crushed particles.The results show that pelletizing using 180-150μm micro-crushed straw had the highest overall rating of 2.678,with the pelletizing rate and spherical particle density reaching 0.968 and1.22 g/cm³ respectively,which is better than that of ordinary crushed straw.Based on the previous formulation and process,four friction material specimens were prepared using micronized straw fibers,of which specimen M15,a 180-150μm micronized straw fiber reinforced composite,showed the best friction stability and wear resistance.The friction stability coefficient was 0.934,the recovery coefficient was 1.072 and the total wear rate was 1.587×10^-7cm³（N·m）^-1,an increase of 3.5%over the normal straw fiber.Micro-morphological observation and roughness analysis of the wear surface of the friction material specimens showed that the surface roughness of specimen M15 was low,with S_a and S_z values of 3.71μm and 19.43μm respectively,and that the secondary contact plateau improved the tribological properties.（5）Bench test verification and comparative analysis of rape straw fiber reinforced brake materials.On the basis of the previous paper,three brake linings were selected for inertia bench testing.The test was divided into two parts:the first was a systematic evaluation of the micro-crushed straw fiber pellet brake linings in accordance with the standard;the second was a comparative test to analyse the braking performance of the research results at various stages,including unpelleted,normal crushed straw pelleting and micro-crushed straw pelleting.The first part of the inertia bench test results showed that the friction linings of the micro-crushed straw fiber pellets met the standard requirements in several iterations of the braking efficiency test and the thermal braking test,and had stable braking performance,as well as good resistance to thermal recession and recovery performance.The wear measurement results showed that the inner piece of the micro-crushed fiber material specimen had a wear of 0.39mm and a mass consumption of 47.4g,while the outer piece had these two measurements of 0.31mm and 35.6g respectively,which were better than the same type of product on the market.After the test,there were no obvious defects in the appearance of the friction lining and the brake system operated stably.The results of the second part of the comparison test showed that the 180～150μm micro-crushed rape straw fiber-reinforced friction lining with a spherical pellet structure had a better friction coefficient performance and braking time,and its wear was the smallest of all the specimens at 0.632mm,an improvement of 46%compared to the unpelleted specimen.