阴极等离子体辅助微细电解加工方法与机理研究

微细电解加工技术在难加工材料高品质表面微结构加工中具有重要应用，具有无接触、工具电极无损耗和无亚表面损伤等优点。但是，受限于流场、电场的不均匀性和工件材料的化学性质，微细电解加工存在侧向杂散腐蚀、材料去除率低、极间传质困难，以及化学惰性材料难加工等问题。

针对微细电解加工中存在的问题，本研究提出一种阴极等离子体辅助微细电解加工方法，该方法在常规微细电解加工中引入阴极等离子体，实现等离子体-电解能场耦合。以深入揭示阴极等离子体辅助微细电解加工技术原理及特性为目的，本文对阴极等离子体稳定产生机制、气泡-等离子体行为、放电等离子体理化特性及调控方法、阴极等离子体-电解能场耦合方法及加工特性等展开深入研究。

本研究使用高速摄影仪和红外摄像机对加工区域进行可视化观测，分析后得出工具电极之外分别是气膜、离散气泡和电解液。气膜在高电场强度下被击穿形成放电等离子体，放电发射光照射在工具电极周围的气泡上，形成等离子体照亮区域。放电等离子体区域及气膜厚度较薄约0.1 mm，但等离子体照亮区域可以达到1 mm以上。放电等离子体行为受电源脉冲信号调控，脉冲宽度< 3 μs时，等离子体无法被诱导。脉冲幅值影响放电等离子体能量，幅值越大放电能量越高，但是当电压幅值过大时加工机制会转变为火花放电，导致极间间隙被击穿。脉冲宽度影响等离子体能量和稳定性，随着脉冲宽度增大，等离子体强度及直径均增大。调节脉冲占空比可调控阴极放电的热累积和冷却效果，脉冲占空比越高等离子体电离越充分。提高脉冲频率能将放电过程离散化，得到安定、温和的等离子体，有利于提高加工精度和稳定性。

微细电解加工中工具电极表面诱导产生的气膜/等离子体膜具有侧壁绝缘效应，能保证在高加工电压下不会恶化电解加工精度。并且，诱导的气泡流冲刷及放电等离子体热膨胀能促进电解产物从加工间隙内排出，从而实现在无冲液条件下获到高加工表面质量。另一方面，放电等离子体产生的能量能提高电解液温度，从而提高电解液电导率并活化工件表面，实现原位热促进电解加工。

通过脉冲优化可控制阴极等离子体行为机制，更好的实现等离子体-电解的协同耦合。本文提出了双极性脉冲诱导阴极等离子体辅助电解加工技术，在负极性时诱导产生等离子体，进而促进正极性时的阳极溶解效率。基于此方法成功在5 s内将初始直径为200 μm的微棒加工至18 μm，加工效率达到36.4 μm/s，并且微棒长径比达到55:1。进一步地，提出了高低压复合脉冲波形诱导阴极等离子体辅助微细电解加工技术。高脉冲电压时诱导的等离子体和气泡能形成一种激发式流场环境，从而快速排出加工产物，保证已加工表面光滑。等离子体膜和气膜作为高阻态介质在工具电极表面形成侧壁绝缘，从而屏蔽杂散电场，最终实现了异形轴和工件表面微结构加工。

另一方面，针对半导体单晶碳化硅(SiC)材料绿色加工问题，验证了阴极等离子体-电解耦合可在碱性溶液下刻蚀SiC。等离子体具有强氧化性，包含大量自由基离子，同时结合等离子体热辐射，能对SiC表面进行快速氧化，而生成的氧化物在高温碱性溶液中能被快速刻蚀。基于该方法，实现了直径220 μm，深度350 μm的SiC微通孔加工。并且，针对SiC表面纳米多孔结构刻蚀，提出并验证了阴极等离子体平扫刻蚀技术。通过调控等离子体作用到工件表面的强度以及电解液的循环速度，实现了纳米多孔孔径、密度可调加工。其中，在参数优化后，最大刻蚀速率和最大刻蚀深度分别达到540 nm/min和10 μm。

为解决阴极等离子体诱导过程中剧烈气体析出导致加工区缺液问题，本文提出基于管电极内冲液的阴极等离子体辅助微细电解加工技术，并对内冲液环境下的等离子体诱导原理、调控方法及其对微细电解加工过程的影响进行了研究。通过中空工具电极向加工区提供电解液，从而避免加工区缺液。并且，电极端面剧烈析出的气体能隔离其与电解液接触，从而在水下环境营造出气体-液体界面，使电流密度在射流内呈高斯分布，即在水下环境达到近似于空气中射流电解加工的效果。利用内冲液阴极等离子体辅助微细电解加工方法，实现了表面微结构的高品质加工。进一步地，在工具电极进给条件下，实现入口直径980 μm，出口直径750 μm，深径比5.1:1的深小孔加工，并且孔侧壁无重铸层。

Electrochemical micromachining (micro-ECM) has essential applications in fabricating high-surface quality microstructures in difficult-to-process materials. Its unique advantages include no tool wear, no surface damage, and no contact, etc. Meanwhile, further development of micro-ECM technology meets challenges in avoiding lateral stray corrosion, improving material removal rate, enhancing mass transfer in the small inter-electrode gap, and achieving processing of chemically inert materials.

To solve the problems presented in micro-ECM, a cathodic plasma-assisted micro-ECM method was proposed in this research, which introduced cathodic plasma into the conventional micro-ECM to realize the coupling of plasma and electrochemical reaction. Investigations were conducted on the plasma generation mechanism, bubble and plasma behavior, and the interaction between electrical parameters and plasma physicochemical properties. Further, the optimal coupling method and machining characteristics under the application of cathodic plasma are studied in-depth to reveal the principle and mechanisms of cathodic plasma-assisted micro-ECM.

Direct observation of the machining area by a high-speed camera and infrared camera shows that the gas film, discrete air bubbles, and electrolytes are arranged in sequence around the electrode. The gas film is broken down under high electric field strength, forming a discharge plasma. The light emitted by the discharge irradiates the bubbles around the tool electrode, creating a plasma-illuminated area. The thickness of the discharge plasma layer is about 0.1 mm, but the plasma-illuminated area reaches > 1 mm. The power pulse signal regulates the discharge plasma behavior, and the plasma cannot be ignited when the pulse width is < 3 μs. The pulse amplitude affects the discharge plasma energy. The larger the amplitude, the higher the discharge energy. However, when the applied voltage amplitude is too large, the discharge mechanism transforms into spark discharge by directly breaking the gap between electrodes. The pulse width affects the plasma energy and stability. As the pulse width increases, the plasma intensity and diameter increase. The cooling effect of cathodic discharge can be adjusted by pulse duty ratio, and a high pulse duty ratio leads to sufficient ionization. Meanwhile, increasing the pulse frequency can interrupt the discharge process and obtain a stable and mild discharge plasma, which improves the machining accuracy and stability.

The gas/plasma film induced on the tool electrode surface has the characteristics of a sidewall insulation effect, which can ensure the machining accuracy of micro-ECM at high machining voltages. Moreover, the induced dynamic bubble flow and plasma expansion can promote the removal of electrolytic products in the small machining gap, achieving high machined surface quality even without flushing. In addition, the energy generated by the discharge plasma can increase the electrolyte temperature, thus increasing the conductivity of the electrolyte and activating the workpiece surface to realize in-situ thermal promoted ECM.

The cathodic plasma can be controlled by applied pulse waveform to optimize the synergistic coupling effect of plasma-electrolysis. In this research, a bipolar pulse waveform is applied to cathodic plasma-assisted ECM, which generates plasma in negative pulse to promote the anodic dissolution in the subsequent positive pulse. Based on this method, a micro-rod with a diameter of 18 μm and an aspect ratio of 55:1 was successfully fabricated within 5 s from the initial diameter of 200 μm, achieving a maximum machining efficiency of 36.4 μm/s. Further, a high- and low-voltage hybrid waveform is designed to realize cathodic plasma-assisted micro-ECM. Plasma and bubbles induced by the high voltage pulse form a self-excited flow field, efficiently removing the electrolytic products and ensuring the smoothness of the machined surface. In addition, the gas/plasma film of high resistance forms a sidewall insulation effect on the tool electrode surface, shielding stray currents. Finally, the structured micro-rod and the microstructure can be fabricated.

On the other hand, it is verified that the cathodic electrolytic plasma can achieve the machining of SiC single crystal in an alkaline solution, implying the plasma's strong oxidation capacity due to its high temperature and many free radical ions. The generated oxides are etched in the high-temperature alkaline solution caused by plasma. A micro-hole with a diameter of 220 μm and depth of 350 μm is fabricated on the SiC surface. Further, the cathodic plasma assisted-etching method effectively fabricates nanoporous structured SiC by scanning the cathode tool. Adjustment of nanoporous structure is realized by controlling the plasma intensity and the tool scanning speed. The achievable maximum etching rate and etching depth are 540 nm/min and 10 μm, respectively.

The cathodic gas and plasma formation can occupy the entire machining gap, causing a severe lack of electrolytes for ECM. To solve this problem, a tube electrode enabling internal supplying, i.e., flushing, of electrolytes to the machining area is applied. The plasma ignition principle and control method under tube electrode conditions is analyzed. The violently generated plasma gas on the end surface of the electrode can isolate the electrode from contacting the electrolyte, confining the current distribution inside the electrolyte jet and realizing a process environment similar to electrochemical jet machining in air. Based on this method, microstructures with high surface quality can be fabricated. With tool electrode feeding, a deep micro-hole can be machined with an inlet diameter of 980 μm, outlet diameter of 750 μm, and a depth-to-diameter ratio of 5.1:1. Meanwhile, no recast layer presents on the hole sidewall.

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