Study Of Film Formation Mechanism And Process Of Micro-Arc Oxidation Based On Loading Characteristics

Micro-arc oxidation technology is simple and easy to operate,almost pollution-free,and the generated film layer has good corrosion and wear resistance,so micro-arc oxidation technology is widely used in various fields such as aviation,aerospace,medical and transportation.The micro-arc oxidation process can be divided into three stages with the increase of input voltage: anodic oxidation,micro-arc oxidation and large-arc discharge,and the large-arc stage is easy to lead to film ablation failure and should be avoided as much as possible.However,the micro-arc oxidation technology is too complex due to its reaction,the film formation mechanism and the large arc forming mechanism still do not have a clear conclusion.In this paper,the main objective is to avoid the tendency of large arc in the micro-arc oxidation process,and to study the correlation between the process state,film characteristics and electrical signal characteristics with the research object of micro-arc oxidation film formation.Based on theoretical analysis plus experimental verification,a micro-area arc discharge mechanism and model are proposed,and load adaptation requirements are proposed through load characteristics study,and a comparative study of power supply modes and parameters is launched to form a load characteristics-based micro-arc oxidation electric parameter regulation method to avoid the occurrence of large arc phenomenon.The main work of the thesis includes the following:The relationship between arc characteristics,film quality and load characteristics of microarc oxidation under unipolar and bipolar power supply output modes is investigated.The arc characteristics and waveform acquisition results show that the micro-arc oxidation load presents an extremely capacitive nature,charging in positive pulse cycles and discharging in negative pulse cycles,and the load itself requires cooling time.The film formation in bipolar power mode is more stable and the film formation rate is faster than that in unipolar power mode.The study on the effect of negative pulse and negative voltage in bipolar power mode found that the negative pulse significantly reduces the tendency of large arc by increasing the cooling time,dissolves the film tip and improves the surface flatness,but the number of negative pulses in a single cycle is greater than 2,which is not conducive to the film growth;the negative voltage changes the tendency of large arc by changing the ionization degree of gas in the discharge channel,and the tendency of large arc increases significantly when the negative voltage increases,which leads to the film denseness When the negative voltage is set to 10～30 V,the film layer is dense and grows rapidly.Thus,a load-based electrical parameter adjustment method was proposed to characterize the large-arc tendency,film morphology and film growth rate during the experimental process by adjusting the frequency and duty cycle,and to derive the influence of cooling time on the film formation process and film structure of micro-arc oxidation.The results show that with the increase of cooling time,the micro-area thermal cycle can be changed to effectively control the large arc tendency,and the film thickness and growth rate both show a trend of first increasing and then decreasing.Considering the effect of cooling time on arc characteristics,film morphology and film formation rate,the stability of the micro-arc oxidation process is high,the growth is rapid and the film is uniform and dense when the cooling time is 0.67～1.2 ms,i.e.,the frequency is 750～1000 Hz and the duty cycle is 30%～50%.