| A crucial component of the non-asbestos organic brake friction materials is copper,impacting the brake noise,high-temperature friction performance,wear resistance,and the thermal conductivity of the material,directly.Nonetheless,copper induces potential risks to health and the environment,making the use of eco-friendly materials in the international brake industry imperative,to mitigate the problems of high wear,high noise,and severe thermal fade of the copper-free,non-asbestos organic brake friction materials.Inspired by the multi-level structure of the bovine teeth,this study developed biomimetic brake friction materials by adding NaOH-pretreated rice husk charcoal,and explored the anti-fading mechanism of expanded graphite,generating rice husk charcoal /expanded graphite synergistically reinforced biomimetic brake friction materials.These developments significantly improved the wear,fade rate,and the brake noise.The study provides data support and theoretical guidance for the development of a new environmentally friendly and high-performance brake friction material,expanding the avenues for the high-value utilization of agricultural biomass waste.It holds significant implications for sustainable agricultural development and the resourceful utilization of agricultural waste.The major research contents and achievements of this study could be delineated as under:(1)Bionic structural design and development of braking friction materials.Through the analysis of the composition,microscopic morphology,microhardness,and indentation of bovine molars,the energy absorption and crack-arrest mechanism of bovine molars were elucidated.The results indicated that the porous structure in tooth dentin could absorb stress and impede the extension of crack tips.Building upon this understanding,a biomimetic structure for brake friction materials was designed.By incorporating ordered and porous rice husk charcoal,materials with an internally structured biomimetic brake friction material resembling teeth were successfully synthesized.(2)Study on the influence of pyrolysis temperature of rice husk charcoal on the performance of biomimetic brake friction materials.The research investigated the impact of pyrolysis temperature on the composition of rice husk charcoal.Five types of biomimetic brake friction materials were prepared with different pyrolysis temperatures,and the influence of pyrolysis temperature on the physical,mechanical,and tribological properties of the materials was analyzed.Finally,in conjunction with the microscopic morphology of the worn surface,the friction and wear mechanism was explored.The results indicated that at pyrolysis temperatures of 1200℃ and 1400℃,the biomimetic brake friction materials exhibited increased hardness,impact strength,density,friction coefficient,wear rate,and recovery rate.In contrast,at 800℃,the materials demonstrated good resistance to fade and wear.Analysis of the rice husk charcoal component and the worn surface morphology revealed that at 1200℃ and 1400℃,the generation of SiC from rice husk charcoal resulted in severe abrasive wear and surface fatigue wear on the material’s worn surface.This led to increased wear and surface roughness,enhancing mechanical interlock at the friction interface,and consequently,an increase in the friction coefficient.However,SiC was effective in rapidly removing the denatured surface material at high temperatures,improving recovery performance.(3)Study on the influence of rice husk charcoal content on the performance of biomimetic brake friction materials.The research investigated the effect of the content of rice husk charcoal pyrolyzed at 800℃ on the physical,mechanical,tribological properties,as well as the brake noise and vibration of biomimetic brake friction materials.The results revealed that adding 2-4 wt% of rice husk charcoal stabilized the friction coefficient between 100-300℃ and reduced wear.Furthermore,adding 2-6 wt% of rice husk charcoal had a damping effect on noise reduction.Analysis of the microscopic morphology of the worn surface and the emission of wear debris particles unveiled the mechanism influencing friction and wear,as well as brake noise.The porous structure of rice husk charcoal impeded stress transfer,alleviated delamination wear,and influenced the size of abrasive particles.This led to a reduction in the area of detachment pits and the width and depth of plow grooves on the worn surface,effectively avoiding unstable friction and significant wear.Additionally,the appropriate amount of rice husk charcoal improved the uniformity of stress distribution at the friction interface,increased system damping,mitigated brake system vibration,and consequently reduced brake noise.(4)Study on the influence of NaOH pretreatment concentration of rice husk charcoal on the performance of biomimetic brake friction materials.The research utilized NaOH solution to pretreat rice husk charcoal,investigating the impact of different concentrations of NaOH solution pretreatment on the physical,mechanical,tribological properties,as well as the brake noise and vibration of biomimetic brake friction materials.The results indicated that with the increase in NaOH solution concentration,the impact strength increased,while the friction coefficient and wear rate decreased.The resistance to fade and recovery performance improved,with lower fade rates at 9 wt% concentration and higher recovery rates at 3 wt% concentration.Brake noise decreased,effectively suppressing high-frequency squeal noises.Comparative analysis of the components and microscopic morphology of rice husk charcoal pretreated with different concentrations of NaOH solution,along with the microscopic morphology of the material’s worn surface,revealed the mechanism influencing friction,wear,and brake noise.As the NaOH solution concentration increased,the SiO2 content in rice husk charcoal decreased,and the surface fibrous degree increased.Consequently,this strengthened the material’s bonding strength,enhanced the energy absorption and crack-arrest capability of the biomimetic multilevel structure,and weakened the cutting action of hard abrasive particles.This effectively lowered the friction coefficient and wear,reduced the vibration of the brake system,and diminished brake noise.(5)Study on the performance and anti-fading mechanism of expanded graphite reinforced brake friction materials.Different amounts of expanded graphite were incorporated into brake friction materials,and their physical,mechanical,and tribological properties,as well as braking noise and vibration,were tested.The results showed that adding 2 wt% expanded graphite increased the friction coefficient of the brake friction material at 350℃ by 24.1%,reduced the fade rate by 19%,and increased the recovery rate by 11.2%,demonstrating a noticeable resistance to thermal fade.Adding 4 wt% expanded graphite led to a 37.3% reduction in the total wear rate,with a 40% decrease in the wear rate at 350℃.Through analysis of the microscopic morphology of the worn surface and high-temperature friction performance,the anti-fading mechanism was revealed.Expanded graphite inhibited the expansion of subsurface cracks at high temperatures,reducing delamination wear and enhancing the stability of the friction coefficient at elevated temperatures.This improvement contributed to the anti-fading performance of the brake friction material.Additionally,the increased friction coefficient due to expanded graphite intensified the vibration of the braking system,leading to an elevation in braking noise.(6)The performance of rice husk charcoal/ expanded graphite collaboratively reinforced biomimetic brake friction materials was studied.Based on the aforementioned research findings,biomimetic brake friction materials were prepared by combining 4 wt%of rice husk charcoal pre-treated with a 9 wt% NaOH solution and 3 wt% of expanded graphite.The materials were systematically examined for their performance,and the synergistic effects of the composite were analyzed.Compared with the initial brake friction materials,the fade rate decreased from 10.87% to 7.16%,and the recovery rate increased from 95.51% to 97.54%.The total wear rate decreased by 43.61%,signifying enhanced wear resistance.The equivalent sound pressure level of the braking noises decreased by14.46% with a decrease in the maximum frequency of braking noises from 17983 Hz to3118 Hz.The generated materials inhibited the high-frequency squeal noises.The rice husk charcoal/expanded graphite synergistically reinforced biomimetic brake friction materials demonstrated a significant synergistic enhancement in performance. |
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