| 摘要: | 硬脆材料（如碳化鎢、碳化矽、矽、氧化鋁、Zerodur 和許多玻璃材料等）因為可以滿足精密零件在光學、電子與機械性質上的特殊要求，所以其應用領域十分廣泛；但由於其具有高硬度和脆性的特質，所以一般多被歸類為難加工材料，因此如何瞭解並改善這些硬脆材料的加工方式，以滿足經濟化生產的需求，是相當重要的議題。在本論文中首先以微壓痕及刮痕實驗，作為探討材料機械性質及瞭解材料加工行為之基礎研究，從而對硬脆材料之延/脆性加工模式轉換進行分析。此外，本論文也分別使用金屬結合與樹酯結合鑽石砂輪來磨削碳化鎢與碳化矽試片，探討砂輪/加工參數（如縱向與橫向進給、切深和主軸速度等）及材料機械性質，對輪磨加工後材料的表面形貌/粗糙度，及次表面顯微結構如非晶層、差排、微裂痕及破裂情形等的相關性。研究結果顯示加工參數之選取對延/脆性加工模式有決定性之影響；而材料機械性質與材料移除機制及所獲致之表面性狀間亦有很強之關聯性；例如碳化鎢中的顆粒尺寸和鈷含量的影響，當碳化鎢含鈷量為0%時，其臨界切深值約為2.5μm，當鈷含量提高時，其臨界切深值會隨之增加。而由於磨削加工造成之溫升，會使試片中的鈷在加工過程中因熱擴散而流失，材料主要之破裂模式會因而從穿晶破壞轉為沿晶破壞，加工表面也因此易產生晶粒被拔除之現象；此效應在高含鈷量時尤為明顯。相較之下，碳化矽試片則因具有相對較高的硬度和較小之臨界切深，因此如果加工條件超過延性加工範圍時，表面和次表面會有較深之中央裂縫分佈，因此較碳化鎢材料更難加工。
Brittle materials such as WC, SiC, Si, Al2O3, Zerodur and various optical glasses are difficult-to-machine materials for their high hardness and extreme brittleness. However, owing to their advanced optical, electronic and/or mechanical properties, demands for precision parts made of these brittle materials are increasing at a very fast rate. Advances in manufacturing processes are therefore important to the economic production of these parts. To ensure the quality of the generated surface and to improve the process efficiency, efforts have been made in this research to correlate the material properties, and machining processes/ conditions to the obtained surface integrity. Indentation and scratching tests were adopted in this study to gain fundamental understanding of the materials behavior, especially ductile to brittle transition, when subjecting to “cutting” operations. Both metal-bonded and resin-bonded diamond wheels were used in experiments of grinding WC and SiC specimens. Grinding parameters such as in-feed/cross-feed, cut depth and spindle speed were also studied. The results showed that, depending on the wheel and machining conditions, amorphous layer, dislocations, micro-cracks and fracture could be observed on the obtained surface. The grain size of WC and cobalt concentration also played important role in the achievable surface integrity. It is found that the critical cut depth of WC-C0%0 is around 2.5μm. The critical cut depth gets bigger as the cobalt concentration gets higher. The diffusion and depletion of cobalt induced by the elevated grinding temperature might result in the fracture mode changed from trans-grains to inter-grains and induce grain pull-out on the machined surface. SiC, having a relatively higher hardness and smaller critical cut depth, is much more difficult to be machined than WC for its surface/subsurface would be covered by scattered deep-penetrating median crack if any of the machining conditions goes beyond the ductile regime. |
