Study On Key Techniques In Arc-enhanced Plasma Machining For Silicon Carbide Mirrors

It is well known that silicon carbide（SiC）ceramics is one of the best mirror materials for space telescope and synchrotron radiation because of its hardeness,chemical stability,polishability and excellent thermal properties.However,it has been found difficult to obtain a high material removal rate（MRR）due to its hardness and chemical stability by conventional machining methods.Atmosphere plasma etching methods have been applied to the machining of optics such as fused silica or ULE.Etching at atmospheric pressure is always a purely chemical process.As a result,it is also difficult to improve the MRR for the machining of SiC.In this paper,we found that the MRR will be significantly improved while the electric spark appears between the plasma and the SiC surface.Therefore,a new plasma source is designed to generate stable arc at the SiC surface to improve the MRR and this new plasma machining method is called arc-enhanced plasma maching（AEPM）technology.The MRR for AEPM is over 10 times as high as the conventional etching method.The method to generate stable arc is introduced in this paper and the radio frequency（RF）plasma voltage is measured.The mechanism of material removal is discussed by studying the voltage charactieristics.The SiC surface is figured with the AEPM plasma generator,verifying the capacity for high effecient figuring using AEPM method.1.AEPM generator design and study on RF voltage charateristics.The channel model of plasma is introduced,with which the plasma torch is designed.A new equivalent circuit model of plasma considering the capacitve coupling is established and the RF voltage and current charateristics of plasma is analyzed by simulating the moedel.The new AEPM generator is designed based the simulation results.Plasma voltage is measured in states of free burning and arcing,and the voltage characteristics at the plasma-metal interface are also studied.The RF voltage of plasma is extremely high to form arc when plasma is close to the SiC surface.It is also found that the distance bewteen the plasma jet and the workpiece(D_jw)affects the arc formation.Moreover,while arc forms,it is proved that the ions can reach the SiC surface.2.Study on the material romoval mechanism of AEPM.Comparing the surface raction kinetics of conventional ICP process,it is found that the interaction of ions in plasma and the SiC surface plays an important role in the reaction process.The stable Si-C bond is broken by ion bombardments and the high energy released by the three-body effects at the surface.The ion bombardment is proved exitent by the arcing machining experimant.The processing efficiency model is prsented by etching trenches on the SiC surface under diferrent D_jws and different temperatures.The activation energy of the reaction system decreases while arc forms.As a result,the MRR is improved.3.Study on the AEPM process.The processing temperature is studied,measured by an infrared thermal imaging instrument.Due to the high thermal conductivity of SiC,the processing temperature is not very high.The MRR is proved irrelevant to the travelling speed of the plasma jet.As a result,it is possible to determine the dwell time using conventional dwell time method.AEPM shows a Gaussian shaped removal function with good stability.It is found that the MRR and surface roughness are affected intensively by D_jw.As a result,it is important to make D_jw remains the same during the process and an optimal D_jw can be determined.AEPM method has demonstrated the capacity for high effecient figuring of SiC mirrors.