| 摘要: | 微放電加工為製造具有高深寬比3D微元件的關鍵技術，然而利用微放電加工法製造的微元件會因微裂縫而發生缺口效應，進而導致應力集中降低疲勞強度，因此，本文將微放電加工法製造的碳化鎢微軸進行微拉伸與微彎曲破壞試驗，探討粗糙度效應與尺寸效應對破壞強度的影響，與試片初始破壞的原因，除此，並研究以微電鍍鉻技術改善碳化鎢放電試片的破壞強度，以及試片尺寸與表面粗糙度對鉻披覆鍍層表面形態之影響。 微破壞測試實驗結果顯示放電電流、軸向表面積、體積與長度都會影響試片的張力與彎曲破壞強度；試片在具有相同尺寸的條件下，平均張力與彎曲強度將隨著放電電流的增加而減少，而試片的破壞機率也會隨之增加；具有相同直徑與放電能量的試片，平均張力破壞強度隨著試片的長度、體積以及軸向表面積增加而降低，而張力破壞機率則隨之增加；此外，試片在具有相同放電能量的條件下，減少試片長度以及軸向表面積將會提高平均彎曲破壞強度，並降低試片的彎曲破壞機率。此外，微電鍍鉻實驗結果顯示，在相同的電鍍條件下，放電表面較粗糙的碳化鎢試片，鉻披覆鍍層的表面亦會較為粗糙，而隨著電鍍的時間增加，披覆鍍層的表面完善性會有明顯的改善，且不同粗糙披覆鍍層間的表面完善性也會隨之接近；此外，在相同電鍍電流作用下，直徑較小的放電試片可鍍出較緻密的鉻鍍層，但會產生較大的結晶塊而形成較粗糙的電鍍層；而經電鍍後的小尺度放電試片，將可提升張力破壞強度，降低張力破壞機率與減少張力破壞強度的散佈範圍。 Micro electrical discharge machining (Micro-EDM) is a critical technology to fabricate high aspect ratio 3D micro-components. However, the surfaces of micro- components manufactured by micro-EDM will exhibit micro-cracks that will produce notch effects, and lead to stress concentration and reduction of fatigue strength. This paper performs micro-tensile and micro-bending tests to investigate the influence of various roughness and sizes on the fracture strength of micro WC-shafts manufactured by micro-EDM, and determines the origin of the fracture in the specimen. Moreover, the further study on improvement of fracture strength of WC EDMed specimen through micro chromium electroplating is conducted. Besides, we also examine the influence of roughness and sizes on the surface topography of chromium coating. The experimental results indicate that the discharge current, axial surface area, volume, and length of the specimen will affect its tensile and bending fracture strength. For specimens with the same size, the mean tensile and bending fracture strength decrease as discharge current increases, and the fracture probability of the specimens also increase. For specimens with the same diameter and discharge capacity, increasing the length, volume or axial surface area reduces the mean fracture strength, and increase the fracture probability. For specimens with the same discharge capacity, reducing the length or axial surface area will increase the mean bending fracture strength, and reduce the bending fracture probability. Besides, the experimental results of micro chromium electroplating indicate that, under the same electroplating parameters, the specimen with rougher discharge surface will exhibit a higher roughness on the chromium coating. Furthermore, the surface integrity effect of coating and the surface integrity within different roughness coatings will also improve with the increase of the electroplating time. Moreover, under the same electroplating current, the EDMed specimen with a smaller diameter can produce a chromium coating with compact build, but will also generate larger crystalline solids that lead to a rough surface. On the tiny scale, an EDMed specimen coated with electroplating will reduce scattered distribution of tensile fracture strength and the tensile fracture probability, and enhance tensile strength. |
