含硼單晶鑽石之火花熔蝕加工特性探討

Abstract

本研究旨在對單晶鑽石(Monocrystalline diamond, MCD)，以脈衝寬度調變電阻電容(PWM-RC)電源，進行火花熔蝕加工的特性探討。鑽石具有極高阻抗、極高硬度及極高崩潰電壓的特性，屬難加工材料，目前業界大多採用研磨或雷射方式加工，前者加工效率差，後者設備昂貴，且鑽石受雷射高溫影響，易由sp3的結構降至sp2的結構，對鑽石的硬度影響很大。本研究嘗試以PWM-RC放電電源對單晶鑽石進行火花熔蝕加工研究。PWM-RC電源可對電容波形的放電時間作調變，形成具高頻且可變脈衝寬度的放電波列，因此放電能量容易達到鑽石的崩潰電壓。實驗以具有微弱導電性的含硼單晶鑽石(Boronous monocrystalline diamond, BMD)為材料，進行放電加工。結果顯示，依放電電壓、放電頻率及放電波列的各脈衝調變時間等條件，歸納出含硼單晶鑽石的四種材料移除機制，分別為: 熔化、汽化、汽化伴隨熔化和片狀剝蝕。含硼單晶鑽石以汽化及片狀剝蝕的機制移除時，拉曼光譜圖上並未發現石墨變質層，且汽化機制下，鑽石的表面粗糙度可達Ra 0.1 μm。而在0.006 mm/min的電極進給率下，含硼單晶鑽石不但沒有發生石墨變質層，且有最好的火花熔蝕率(0.0176 mm/min)。此外，本研究以輔助電極法，利用含硼單晶鑽石為輔助電極，對完全不導電的單晶鑽石進行放電誘導加工，發現在汽化及片狀剝蝕的機制下，能有效地移除不導電的單晶鑽石材料，而無法以汽化方式移除。本基礎研究針對鑽石的放電進行深入探討，對於鑽石的加工研究有實質幫助。The purpose of this study is to explore the characteristics of spark erosion processing of monocrystalline diamond (MCD) with the Pulse Width Modulated Resistance-Capacitance (PWM-RC) power supply. Diamond has an extremely high-impedance, high-hardness and high-breakdown voltage, which is a difficult-to-machine material. At present, most of the industry uses grinding or laser processing. The former has poor processing efficiency, the latter requires expensive equipment. Also, it is easy to change the sp3 into the sp2 structures when machining by the laser due to high-temperature, which has a great influence on the hardness of diamond. In this study, the spark erosion processing of MCD is attempted by using the PWM-RC power supply. The PWM-RC power supply can modulate the discharge time of the capacitance waveform to form a discharge pulse train with high-frequency and variable pulse width so that the discharge energy can easily reach the breakdown voltage of the diamond. In the experiment, the boronous monocrystalline diamond (BMD), which has weak electrical conductivity, was used as the material for the discharge process. Experimental results show that four material removal mechanisms of BMD are summarized according to the discharge voltage, discharge frequency, and the modulation time of each discharge of the pulse train: melting, evaporation, evaporation accompanied by melting, and flake ablation. No graphite metamorphic layer is found in the Raman spectra when the BMD is removed by evaporation and flake ablation mechanisms. Under the evaporation mechanism, the surface roughness of the diamond can reach Ra 0.1 μm. At an electrode feed rate of 0.006 mm/min, the BMD not only has no graphite metamorphic layer but also has the best spark erosion rate (SER) (0.0176 mm/min). In addition, by using BMD as auxiliary electrode and conducting discharge-induced machining on completely non-conducting MCD, it is found experimentally that the non-conductive MCD can be effectively removed by evaporation and flake ablation mechanisms, but not by evaporation. This fundamental study will be of substantial help to the diamond machining research.