| The ZrB2-SiC-Graphite(ZrB2-SiC-G)ceramic composite has the excellent high temperature resistance,oxidation and ablation resistance,and good electrical conductivity.It is an ideal new material for MEMS and microfluidic components with the high temperature requirement.The ZrB2-SiC-G ceramic is prepared by hot-pressing sintering using three kinds of materials with different properties,and the melting point and hardness of this ceramic are extremely high,which shows the great specificity in terms of material compositions and physical properties.The severe tool wear and low machining efficiency are produced using the traditional contact mechanical machining methods for this material.Micro-electrical discharge machining(EDM)is a non-contact processing method regardless of the strength and hardness of material.Therefore,exploring micro-EDM technology of ZrB2-SiC-G ceramic,and investigating its machining mechanism and processing methods for improving machining quality and efficiency,are of great significance for expanding the application of ZrB2-SiC-G ceramic material.This dissertation takes the ZrB2-SiC-G ceramic machining as the research object,and studies micro-EDM characteristics of the ceramic,machining methods for micro-structures,and the application of machining technology in micro-nozzle fabrication.Based on micro-EDM drilling,the machining characteristics of ZrB2-SiC-G ceramic are studied.The micro-EDM performances of this material are analyzed in the terms of material removal rate,electrode wear ratio and discharge characteristics.And then,the surface topography and erosion products are analyzed thoroughly.It is found that the machined surface is composed of droplets,micropores and a large number of microcracks.The erosion products consist of regular sphere particles,irregular flake particles,and irregular bulk particles.Based on the above analysis,the material removal mechanism of ZrB2-SiC-G ceramic in EDM is revealed,and it mainly includes melting/evaporation and thermal spalling.The thermal spalling mechanism promotes the material removal,but it produces large-scale erosion products.The appearance of large-scale erosion products leads to more abnormal discharges.To solve this problem,the micro-EDM drilling method using the micro sheet-cylinder tool electrode is proposed.The machined results show that the radial overcut is reduced,and the surface quality and machining efficiency are improved.For the machining of 3D micro-structures,micro-EDM milling of ZrB2-SiC-G ceramics was studied.To improve machining precision of micro-structures,the electrode wear compensation strategy and machining path planning strategy are investigated.An electrode wear compensation method based on the scanning volume is presented,and a special micro-EDM milling CAM system is developed based on this compensation method.The NC code including the compensation information can be generated using this system,which helps to realize the automatic compensation during the machining process and improve machining accuracy in the vertical direction.The effects of electrical and non-electrical parameters on material removal rate,electrode wear ratio and machining gap are investigated,and the processing rules are obtained.According to the machining gap error,the machining path of micro-structure is planned.With the compensation method and machining path planning,the high-precision complex micro-cavity is machined,which lays the foundation for machining the ZrB2-SiC-G ceramic 3D micro-parts.To improve the surface quality of ZrB2-SiC-G ceramic 3D micro-structures,a micro-EDM and micro-ultrasonic machining(micro-USM)combined milling method is proposed,and the relevant combined milling device is set up.The effects of processing parameters on the recast layer removal and surface quality are analyzed,and the suitable combined milling parameters are determined.In order to achieve the complete removal of recast layer and high-precision machining,the micro-tool feeding strategy in combined milling is studied.Finally,various types of micro-structures are machined using this combined milling method.The machining results show that the EDM recast layer containing a large number of microcrack defects is completely removed,and the machining of 3D micro-structures with high surface quality is achieved.Based on the research about micro-EDM characteristics and machining methods for the ZrB2-SiC-G ceramic,its application in micro-nozzle fabrication is explored.A ZrB2-SiC-G ceramic micro-nozzle is designed,fabricated and tested preliminarily.The main structural parameters of micro-nozzle are determined by theoretical calculation.The influence of structural shape on the thrust performance is simulated,and the structural dimensions are optimized.The processing route for micro-nozzle is determined.Firstly,the micro-nozzle is machined by micro-EDM rough milling using electrodes with different diameters.The high machining accuracy and efficiency are obtained,which lays the foundation for subsequent finishing.Secondly,based on the 2D reverse copying plate machined by micro-WEDM,the micro-forming tool is prepared on-machine by micro-EDM grinding.Finally,the micro-nozzle is finished on-machine by micro-USM using the micro-forming tool.The recast layer in inner walls of micro-nozzle is entirely removed,which improves surface quality.A micro-thrust test platform based on the cantilever beam force transducer is set up for the developed micro-nozzle.The preliminary test experiment is carried out to verify the feasibility of ZrB2-SiC-G ceramic micro-nozzle. |
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