Tribological Performance Of The Surface Of Magnetic Rheological Polishing Wheel Treated By Micro-arc Oxidation

Magnetorheological finishing is an advanced and ultraprecise optical technology, which is hailed as revolutionary technology in the optical manufacturing industry. However, in the process of magnetic rheological polishing, the polishing wheel has poor anti-wear performance. It is frayed more easily by the magnetorheological fluid recycling equipment. Thus, the precision on the magnetic rheological polishing is seriously affected. Micro-arc oxidation(MAO) is a novel and emerging surface treatment technique to in-situ produce ceramic oxidation coatings on surface of aluminium and other metals or their alloys. MAO coatings have high hardness and excellent adhesion to substrate, thus the wear resistance of substrate is improved dramatically.The LY12 aluminium polishing wheel is used as substrate in this paper. During MAO, the two power supply modes are utilized, which are constant voltage and constant current density. Orthogonal experiment is used respectively in the two power supply modes to optimize the best experimental parameters. Finally, the wear resistance is analyzed in the magnetorheological fluid(MRF) for aluminium polishing wheel and the MAO ceramic films fabricated with two power supply modes. The conditions of friction test are 560 r/min for rotate speed of disk, 2 mm for frictional radius and GCr15 steel bobble for bearing steel.The orthogonal experiment of three factors and three levels is designed with constant voltage. The factors to consider are sodium silicate, sodium hydroxide and sodium fluoride. It is found that the breakdown voltage for each experiment in orthogonal test is too different. A constant voltage is not suitable for each experiment. In order to perform each experiment successfully, the final voltage is to add a same value to the breakdown voltage of each experiment. The microscopic structure and wear resistance in the dry condition are analyzed for the MAO ceramic films prepared in each experiment. The results show that the MAO coatings fabricated at constant voltage has low surface roughness, high porosity and narrow apertures. The performance of anti-wear is better for MAO coating with thicker layer and including more α-Al2O3 strengthening phase. Compared with the aluminium substrate, MAO coating has lower mass loss under the load of 2.5 N in the dry condition. However, the MAO coating is abrasion failure under the load of 5 N.The orthogonal experiment of three factors and three levels is designed as well with constant current density. The factors to consider are sodium silicate, potassium hydroxide and positive current density. MAO experimental program is optimized based on the thickness and mass loss of coatings as the indexes. The best experimental parameter is sodium silicate 5 g/L, potassium hydroxide 0.5 g/L, positive current density 10 A/dm2. According to the best experimental parameter in the mode of the constant current density, the influence of single factor in the orthogonal experiment on the microscopic structure and wear resistance under the load of 5 N in the dry condition for the ceramic coatings are investigated.The results show that with the thickness of ceramic coatings increases with added values of three factors, yet surface roughness and crack raise, friction coefficient and mass loss enhance as well. Compared with the aluminium substrate, the mass loss reduces obviously for oxide film fabricated at constant current density which is abrasion failure.The tribological properties on aluminium polishing wheel and MAO ceramic coatings prepared at the best experimental parameters of two power supply modes are study under the load of 5 N in the MRF. The results show that MRF has lubrication and result in the decrease of friction coefficient. However, MRF can accelerate abrasion because of including numerous magnetic particles. After 12 min of friction, the coating fabricated at constant voltage is abrasion failure, and the coating fabricated at constant current density shows more better wear resistance. The mass loss reduces one order of magnitude compared with the aluminium substrate. With the increasing time of friction, the mass loss for aluminium substrate enhances linearly, and the mass loss for MAO ceramic coating prepared at constant current density increases more slowly.