Study Of Effects Of Abrasives On Properties Of Non-asbestos Organic(NAO)brake Friction Materials

Non-asbestos organic（NAO）brake friction materials usually are polymer matrix multivariate composite materials.Modified phenolic resins and synthetic rubbers are mainly used as binders.Organic fibers,inorganic fibers or few metal fibers are used as reinforcing materials.Abrasives and antifriction fillers are used as friction performance modifiers.Some inorganic and organic powders are used as fillers.This kind of brake friction materials is mainly used for the brake function accessories in automobile,elevators,high-speed rails and a variety of industrial and agricultural machinery and equipment.Though they have no asbestos,they have very stable friction coefficients and low wear rates.But their friction coefficients are a little low leading the emergency braking torque too small,which could not reach the requirements of emergency braking.They also have low hardness,which make abrasive wear more serious.In order to meet the industry requirements better,the purpose of this experiment is to study the effects of abrasives on the properties of NAO type brake friction materials.In this study,the optimal formula of the friction materials was found through the orthogonal experiment formula in which certain mechanical strength and hardness of the friction materials were achived.They also have got high friction coefficients which were in the rage between 0.50 and 0.60,the wear rate was below 1.0×10^-77 cm³/（N.m）.In this experiment,the mass fraction gradients of feldspar powders and brown fused alumina powders were 0%,3%,6%and 9%;the mass fraction gradient of chromite powders was 0%,4%,8%and 12%.So three groups of NAO type brake friction materials with three different abrasives were prepared.When the effects of the contents of feldspar powders,chromite powders and brown fused alumina powders on comprehensive performance of the friction material were studied,the results showed that the rockwell hardness of the materials was increased when the contents of the feldspar powders,chromite powders or brown fused alumina powders were increased.Density was decreased with the increase of mass fraction of feldspar powders or brown fused alumina powders;but the density was increased slightly with the increase of the contents of chromite powders.Feldspar powders could improve the mechanical properties of the friction materials.The compression strength and impact strength were first decreased then increased with the increase of the contents of chromite powders.When the addition of the brown fused alumina powders,the compression strength and impact strength were increased significantly.Feldspar powders could improve the friction coefficients of the friction materials at the low and medium temperatures,and the wear rate was increased with a rise of temperature,peaked at 300℃.Chromite could improve friction coefficients of the friction materials at the low and high temperatures.Brown fused alumina could obviously increase the friction coefficients of the friction materials at the high temperature,but after adding brown fused alumina,the wear rate of the materials was also significantly increased.The friction materials showed typical of abrasive wear at300℃,and the materials which contained feldspar powders showed the adhesive wear at 200℃.Comprehensive consideration of the friction and wear properties of the materials,the suitable mass contents of three abrasives were as followed:feldspar powder 3%⁶%,chromite 4%⁸%,brown fused alumina 3%⁶%.In order to further optimize the formulations of NAO brake friction materials,this experiment adopted the orthogonal test which had three factors and three levels to study the best combination of feldspar powder（A）,chromite（B）and brown fused alumina（C）.Selected the three content levels of feldspar powders or brown fused alumina powders were 2%,4%and 6%,and the three content levels of chromite powders were 4%,6%and 8%.The results of the orthogonal test were analyzed,which showed that the optimal formula for friction materials was A₂B₁C₂,namely,the mass fraction of feldspar powder,chromite and brown fused alumina was 4%respectively.Further studies of the wear surface were carried by scanning electronic microscope（SEM）and energy dispersive X-ray spectrometer（EDS）;through the wear mechanism analysis,the friction material was most likely to be the hot wear.