本研究探索線放電加工(wire electrical discharge machining, WEDM)於多晶鑽石，控制參數對表面形貌產生之破壞形式。多晶鑽石具有高硬度(8000–10000 HV)、高彈性模數(1000 GPa)、高熱傳導率(550–920 W/m K)與低熱膨脹係數(1.2–3.7 µm/K)等特性，因此多晶鑽石之加工皆以高能量製程進行。多晶鑽石於線放電加工時，鑽石晶粒因反覆之熱效應，導致邊脆裂(chipping-off)、裂縫(crack)、晶粒拔除(dislodgement)、凹坑(crater)與石墨化(graphitization)等缺陷，因此探究放電能量之控制參數有其必要性。本研究修正焦耳熱模型、材料移除模型與表面粗糙度模型，並於實驗中操作開路電壓(85–115 V)、脈衝放電時間(0.1–0.2 μs)以及脈衝休止時間(40–60 μs)，建立晶粒破壞形式與特徵化方法。實驗結果顯示，開路電壓增加時，提升放電能量，增加熱能吸收比例與電離通道半徑，造成材料移除率與表面粗糙度提高。此外，多晶鑽石裡之鈷元素，於高溫中誘發晶粒間之內應力，形成邊脆裂與晶粒間裂縫之破壞形貌。當增加之放電能量會使邊脆裂與裂縫之情形加劇。多指標之最佳化結果，可同時求得參數區間內之材料移除率、表面粗糙度、凹坑、邊脆裂與裂縫之最佳化結果。所修正之材料移除率模型精度為81.62% (R2)，表面粗糙度之修正模型精度為94.69% (R2)。多晶鑽石加工表面之三維形貌、顯微組織、破壞形式特徵化結果與控制參數之關聯，皆已分析並詳盡討論。

This study explores the machined surface integrity and fracture morphology when wire electrical discharge machining (WEDM) of polycrystalline diamonds. Polycrystalline diamonds having the characteristics of high hardness (8000–10000 HV), high elastic modulus (1000 GPa), high thermal conductivity (550–920 W/m K) and low coefficient of thermal expansion (1.2–3.7 µm/K) are to be machined with high energy events. During energy being applied, the diamond grains will be damaged by mechanical force and thermal effect, such as chipping-off, crack, delamination, grain dislodgement, crater and graphitization. Therefore, investigations of the control parameters on the surface damages and fracture morphology necessitate understandings of the mechanisms underlied. In this work, the modified Joule heat, material removal and surface roughness model were presented, and validated with experiments. When employing open voltage (85–115 V), pulse-on time (0.1–0.2 μs) and pulse-off time (40–60 μs), the recorded results are characterized, filtered and processed in order to be tested via statistical examinations. The experimental results show that when the open voltage increases, the discharge energy is increased, the thermal energy absorption ratio and the ionization channel radius are increased, resulting in an increase in material removal rate and surface roughness. In addition, the cobalt element in the polycrystalline diamond induces internal stress between the grains at high temperature, causing effects of chipping-off and cracking in the matrix. When the discharge energy is increased, the amounts of chipping-off and cracking increase. In the validation experiment, the model accuracy of the corrected material removal rate is 81.62% (R2), whereas the surface roughness model present an accuracy of 94.69% (R2). In the multi-objective optimisation, preferable results of material removal rate, surface roughness, crater, chipping-off and cracks can be obtained simultaneously. The correlation between the three-dimensional morphology, microstructure, characterization results and control parameters of the processed surface of polycrystalline diamonds has been analyzed and discussed in detail.

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