Four kinds of high-resistivity carbon brushes with MoS2 contents of 0%,2%,4%and 6%(mass fraction)were prepared, respectively.Wear tests of brushes were conducted on an alternate current(AC)motor.Scanning electron microscopy(SEM)and energy dispersive X-ray analysis(EDS)were used to analyze the worn surface of brushes,and a thermocouple was used to measure the bulk temperature of the brush.The results show that wear rate of brush made without MoS2 in 10%relative humidity(RH)is two times larger than that in 50%RH,whereas the wear rates change little for the brushes made with MoS2.The wear of brushes has much to do with the surface film.In low humidity,the surface film can not be formed for the former brush while a sulfur layer can be formed for the latter brushes,which can reduce sparks,frictional heat and wear rate,and play a role like the water film in high humidity.
Cu-coated graphite composites were prepared by an electroless plating method,and the composites were examined by scanning electron microscopy(SEM). The copper content of the composites was measured to reach 73%(mass fraction). Brush samples made with composites contents of 10%,15% and 20% and with copper powders content of 11% were obtained. The results show that the resistivity of the brush samples decreases and hardness becomes higher with increasing composites content. Copper is dispersively distributed in the brush samples made with the composites,whereas copper agglomerates in Cu-graphite mechanical mixture. For the same Cu content,Cu-coated graphite composites possess lower resistivity and higher hardness than Cu-graphite mechanical mixture. A new type of electric brush was obtained by using Cu-coated graphite composites,the wear and durability tests carried out on a HGH5435M120 AC motor indicate that its service life reaches 250 h. The electric brushes possess relatively low resistivity(275 μΩ·m) and good wear resistance,so they are very suitable for high-speed micromotors.
Carbon brushes with a resin binder were prepared according to an industrial process and the effects of the molding pressure, grains size and cure temperature on the properties of brush samples were discussed. The results show that the bulk density, bending strength and Rockwell hardness increase, while resistivity decreases with increasing molding pressure. Cure temperature has much more influence on the properties of brushes than molding pressure and grains size. Isothermal differential scanning calorimetry(DSC) was used to estimate the degree of cure of resin binder and a novel method of using the true density to measure the degree of cure of resin binder was presented and discussed briefly. Based on optimal process parameters carbon brushes were manufactured, durability tests for brushes were carried out on an alternate current motor and scanning electron microscope(SEM) was adopted to observe the morphology of worn surface of brushes. The results show that a luster oxide film can be formed on the surface of brushes and their service life reaches 380 h.
