Simulation Of Flow Field And Experimental Study On Friction-wear Characterastics Of Silicon Carbide Ceramic Polishing

Silicon carbide has become a new material for spacial light reflecting mirrors with large size owing to its excellent physical and chemical properties. However, silicon carbide has high hardness, high brittleness, low impact resistance and sensitive to defects,so it is difficult to obtain high surface quality and achieve efficient processing. Chemical mechanical polishing(CMP) and traditional mechanical processing methods damage and obtain poor flatness with surface/subsurface damage due to the presence of abrasive particles. Tribological chemical polishing technology(TCP) do not use abrasiv and can obtain higher surface quality, but its efficiency is low.Tribo-electrochemical polishing(TECP) is a new technology for machining materials with high efficiency and high surface quality, but the characteristics of Si C-TECP and its mechanism is not clear. Through simulation analysis of flow fields and experimental study on friction and wear characteristics of Si C polishing, the effects of various processing conditions on polishing characteristics are investigated, which has important theoretical and practical significance for obtaining high surface quality Si C with high efficiency.Two-dimensional finite element model for perforated polishing pad was set up to simulate flow field with the software Fluent. Through the simulation analysis, the influence of polishing speed, polishing pad thickness, pore size and fluid film thickness on flow characteristics was studied. The 3D electric field model was set up and simulated by Ansoft Maxwell to study the influence of polishing pad conductivity and thickness. The original gear-train type planetary-moving polishing machine was improved, and the data acquisition system based on Lab VIEW was developed to monitor pressure, friction force and friction coefficient online. By the improved machine to polish Si C, the influence of different applied voltages, polishing liquids and different pads on the friction and wear characteristics was investigated.The simulation analysis shows that pore diameter and the number of pores on the perforated polishing pad have an important impact on the load-carrying capacity and friction. For constant number of pores on the perforated polishing pad, with the increasing pore-size, the load-carrying capacity and friction force decrease. For constant ratio of pore-pore distance to the pore diameter, with increasing pore number on the pad, the load-carrying capacity increases at first, then decreases, while the friction force decrease. The applied voltage on Si C shows a small influence range in polishing liquid and the polishing pad in which the electric field generated will not overlap with each other. The electric current density on the polishing surface could be improved by increasing the conductivity of polishing pad and reducing the pad thickness.On the experimental aspect, the effects of various processing conditions on friction and wear characteristics are investigated. For Si C grinding with cast iron disc, the grinding efficiency of 3% Na OH is a little higher than that of deionized water; the material removal efficiency of applying positive voltage or no voltage is about 3 times of that under negatively charged. For Si C pieces grinding with different sand-papers, the larger the partical-size, the stronger the mechanical removal action, so the material removal efficiency and friction coefficient for large particle-size papers are greater than those for the small one; For Si C pieces grinding with fine sand-papers, the mechanical removal action is low while the chemical action, especially the electrochemical action dominates for 3% Na OH. In this case, the removal efficiency of applying positive voltage is about 1.5 times of those under no voltage or negatively charged. For Si C polished by leathers, the friction coefficients are almost the same under all electrical states, but the material removal efficiency of applying positive voltage is 8~10 times of those of under no voltage or negatively charged cases. This shows that in the case of applying positive voltage between the Si C and the polishing platen, the electrochemical action for Si C removal is greately enhanced. The friction coefficient for perforated pad is larger than that of the unperforated pad due to unevenness of the perforated pad. Polishing efficiency of W5 diamond suspension slurry is greater than that of 3% Na OH solution, while the polishing efficiency of the mixture of the diamond suspension slurry with 3% Na OH is inbetween.