Research On Electrochemical Micromachining Of Micro-dimple Array With A Thick Mask

Micro-dimple array plays an important role in improving the tribological performance.It could decrease the friction coefficient as well as reduce the friction wear,which improves the service life of friction pair.For example,Gear pump is the drive part of water circulation system in manned spacecraft,and the friction and wear are unavoidable between the gear face and seal face during the runtime.With the wear increasing,it could influence the efficiency of the gear pump as well as the stability of the whole water circulation system.It has been proved that the seal face designed with a certain scale and distribution of micro-dimple arrays could improve the lubrication condition as well as reduce the wear between the gear face and seal face,and thus enhance its operating efficiency and service life.Therefore,the preparation of micro-dimple arrays with high-efficiency and high-accuracy becomes a research focus.Several methods have been used for generating micro-dimple arrays,such as conventional machining,laser beam machining,electrical discharge machining,abrasive jet machining and electrochemical machining.Electrochemical Machining(ECM),with the advantage of no heat-affected layer,residual stresses and tool wear as well as owning the capacity of micromachining,has become a promising machining technology.As a kind of ECM,photoetching electrochemical micromachining is a popular method with the advantage of generating micro-dimple arrays with large area at one time.In photoetching electrochemical micromachining,the patterned mask is prepared with photolithography,which shows a complex process including spin coating,prebaking,exposure,development and postbaking for every specimen.When large numbers of micro-dimple arrays are generated at one time,the machining accuracy is poor.In addition,the profile of micro-dimple is irregular,and there would be an ?island? phenomenon in the micro-dimple.In this paper,the method for improving the machining accuracy as well as removing island in the micro-dimple is proposed,and a thick mask is also introduced for generating micro-dimple arrays.The major contents of the dissertation are as follows:(1)A method for improving the machining accuracy as well as removing island in micro-dimple array is presented.An auxiliary electrode with positive potential was used to uniform the electric field distribution on the workpiece,which was useful for improving the machining accuracy of micro-dimple array.Further investigation indicated that a thick patterned mask could also improve the machining accuracy of micro-dimple array as well as avoid the island in the micro-dimple.(2)A new method for fabricating PDMS mask with micro through-holes is proposed.Combining the modified photolithography and vacuum-aided process,the PDMS mask with the micro through-hole of 50 ?m,100 ?m in diameter and 250 ?m in thickness was well fabricated.The PDMS mask could be re-used in machining,which improves the machining efficiency greatly.(3)An optimized electrolyte flow mode is proposed for generating micro-dimple arrays in TMEMM with PDMS thick mask.By comparing the quality of micro-dimple arrays generated with different electrolyte flow modes,a modified forward flow mode with a multi-slit structured cathode was designed for generating micro-dimple arrays.The result showed that the modified mode enhanced the adhesion between the mask and workpiece.Moreover,it avoided the distortion of the mask,which was useful for generating micro-dimple arrays with high quality.(4)The reason for generating micro-dimple with high accuracy is explored.The experimental results showed there was only about 3~5 ?m undercutting of the micro-dimples,and no increase in undercutting was observed with greater voltage and longer machining time,which showed a high machining accuracy.In addition,the dimple depth was changed with machining time,and there was little change in depth with the machining voltage.With current efficiency calculation as well as flow flied simulation and CCD observations,it was found that with the thick PDMS mask,the electrolyte near the machining region was in the state of high-pressure and low-speed,which reduced the volume of oxygen bubbles as well as slowed down the removal of bubbles and product.Hence,lots of the bubbles and product accumulated at the edge of micro-dimple,and they protected the edge of micro-dimple from dissolving,which contributed to the high machining accuracy of micro-dimple.(5)The application of micro-dimple arrays generated with the thick PDMS mask is investigated.Micro-dimple arrays were successfully prepared on the seal face of gear pump,which increased the service life of gear pump over one times.The tribological performance of chrome-coated surface with micro-dimple arrays was investigated.The result showed that micro-dimple array with an appropriate depth could reduce the friction coefficient efficiently,and compared with circular micro-dimple,the square micro-dimple with showed a higher reduction of friction coefficient in the same condition,which reached to 40%.