Study On Ni Based Composite Coating Reinforced By Carbon Nanotubes By Laser Cladding

During the braking process of high-speed and heavy-hauled trains,great friction force and heat will bring much losses and cracks to the brake disc,resulting in the decrease of service life.Therefore,the surface modification of the brake disc is particularly significant.In this paper,CNTs/SiC/Ni composite coating is prepared on the surface of 45 steel by the laser cladding technology using synchronous powder feeding.Effect of the purification of carbon nanotubes,powder milling,laser processing parameters and the content of SiC and carbon nanotubes on coating is researched.Based on the analysis of microstructure,friction coefficient,wear rate and hardness of the samples,the optimum process parameters and the strengthening mechanism of CNTs on coating were obtained.The conclusions are made as follows.?1?The purification and dispersion of carbon nanotubes are improved obviously with the increase of reflux time.When the reflux time was 9 hours,the dispersion is the most uniform,the weight loss ratio of CNTs is about 18.4%.The loss of mass is mostly the impurity atoms and the carbon atoms of the carbon nanotube wall defects.The appearance of many hydroxyl and carboxyl functional groups would strengthen the activity.With the increasing of time,the carbon nanotubes are more dispersive,but the oxidation of carbon nanotubes on make CNTs fracture and thin easily.?2?With the increase of the rotating speed of the ball mill,the dispersion of powders is good at first and then became bad.When the ball milling speed is 300r/min,the powder distributes most uniformly,the damage of carbon nanotubes is the lowest,and there is no agglomeration of carbon nanotubes.When the speed is low,the powder is seriously agglomerate.When the rotational speed is too high,and the carbon nanotubes are cut off,which lead to serious destroy of the structural integrity.?3?When the content of SiCP was 15%?volume fraction?,the microstructure of the coating reach the optimum value,and its performance is the best.The hardness is 394.6HV_0.2?1.6 times as the substrate?,and the wear loss is only 0.0248 g.When the carbon nanotube is 16%?volume fraction?,the grain is the most uniform,and the carbon nanotubes are distributes in the grain boundaries.The mechanical properties are also the best,with the hardness of 446.5HV_0.2?2.2 times as that of the substrate?,the wear loss of 0.0069 g and the friction coefficient of 0.149.When the content of carbon nanotubes continue to increase,the phenomenon of agglomeration would appear.Meanwhile,the burning loss,porosity and pores of the coating appear,the microstructure is coarser and the grain boundaries become wider,resulting in the poor mechanical properties.?4?With the increase of laser power,scanning speed,powder feeding rate and air flow rate,the microstructure and properties of the coatings show peak value.When the 1400 W laser power,the 250mm/min scanning speed,the 30g/min powder feeding rate and the 4L/min gas flow rate are obtained,the cladding layer is smooth and uniform.The coating bond well with the substrate.