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CRACK MacroMachine 3.1 CRACK !FULL!


Depositing metal through sputtering can avoid thermal stress and cracking but can impart severe film stress, warping the Parylene film and forcing released devices to curve. Figure 15a is an image of a Parylene substrate patterned with a negative profile photoresist for metal lift-off (AZ 5214E-IR) and sputter coated with 2000 Å of platinum. The rippled appearance is typical of Parylene films coated by sputtering and highlights the severity of film stress (Figure 15b).

Epoxy/GNP nanocomposites have higher surface roughness compared with plain epoxy. This may be due to two reasons. One is the toughening mechanism of GNP [9]. During the machining process of epoxy/GNP, a crack deflect is generated on the deformation area [9]. The tilting and twisting of the crack front as it is forced to move out of the initial propagation plane also forces the crack to grow locally under mixed-mode (tensile/in-plane shear and tensile/anti-plane shear) conditions. Crack propagation under mixed-mode conditions is associated with higher driving force than in Mode I (tension) and higher fracture toughness of the material. Increased fracture toughness directly increases surface roughness [40]. Another reason for the higher surface roughness is that the glass transition, Tg, slightly increases with the GNP content increase [41].

Figure 8 shows SEM images for the machined surface at various feed per tooth values (from 0.05 to 6.0 μm). There are many small cracks which appear on the surface of epoxy/GNP 0.1 wt% compared with plain epoxy. There are two reasons for this. First is that the presence of GNP changes the fracture mode. Atif et al. [37] has shown that the fracture mode changed significantly after adding GNP to the polymer. The fracture mode of plain epoxy is brittle fractures mode. The fracture mode of epoxy/GNP is nonlinear and parabolic fracture patterns mode. Second, a change in fracture mode causes the deformation process to change, which directly affects the surface morphology [42,43,44]. Rafiee et al. [10] have been demonstrated that the addition of 0.1 wt% GNP causes a significant increase in fracture toughness, as shown in Fig. 9. Anstis et al. [45] and Chantikul et al. [46] have shown that a change in fracture toughness causes the shape of surface cracks to change.

Epoxy/GNP 1.0 wt% has fewer surface cracks compared with epoxy/GNP 0.1 wt%, as shown in Fig. 8. There are two reasons for these differences in the surface. One is that fracture toughness is reduced when GNP content increases [10], as shown in Fig. 9. The reduction in fracture toughness leads to a change in the shape of surface cracks [45]. The other reason is that the regions of GNP agglomeration also cause changes in the shape of surface cracks. When the GNP content exceeds 0.3 wt%, GNP agglomeration will happen in the matrix material. Arora et al. [29] observed microcracks/fragments which were caused by the tool feed at the GNP agglomeration regions. The microcracks around the GNP agglomeration regions caused changes to the cracks on the machined surface.

Figure 10 shows that the cutting force of epoxy/GNP is higher than that for plain epoxy. These may be the two reasons. One is that the crack deflection of epoxy/GNP absorbs more energy compared with plain epoxy. Crack deflection is the process by which an initial crack tilts and twists when it encounters a rigid inclusion. With the addition of GNP, more crack deflection occurs during the machining process. This generates an increase in the total fracture surface area, resulting in the absorption of more energy and greater cutting force compared with plain epoxy [10]. Secondly, the overall strength and modulus of the epoxy/GNP nanocomposites are greater, because the rough and wrinkled surface texture of graphene enhances the mechanical interlocking/adhesion with the polymer matrix [8, 48,49,50].

Edge chipping is another critical factor in micromachining since it affects the capability to meet desirable tolerance and geometry criteria [40]. Thus, it is important to explore the edge chipping either by developing strategies for its minimization or applying new post-processing technology for edge chipping [29, 46]. Figure 12 shows up and down milling slot edges from the up and down milling of plain epoxy and epoxy/GNP (0.1 wt% and 1.0 wt%) nanocomposites at 4 levels of FPT (0.1, 0.5, 4.0, 6.0 μm). The epoxy/GNP shows significant differences in machined chipping compared with plain epoxy. There are many cracks in the slot edge of epoxy/GNP. This may be due to the addition of GNP nanofillers which significantly increase the fracture toughness [10]. The increase in fracture toughness causes the fracture mode to change [37]. The slot edge of plain epoxy exhibits burrs, and the epoxy/GNP shows the edge chipping. Epoxy/GNP nanocomposites at 0.1 wt% exhibit smaller machined chipping breakage on the edge compared with those at 1.0 wt%. This is mainly due to the increase in GNP content which leads to reduced fracture toughness [10], so that consequently, edge cracks are reduced.

Epoxy/GNP nanocomposites present the different crack trends compared with plain epoxy. There are two reasons for this. Firstly, the presence of GNP changes the fracture mode, and secondly the increase in fracture toughness leads to changes in fracture mode from Mode I to mixed-mode. Due to the addition of GNP, nanofillers make the slot width slightly decreased, and GNP can slightly improve the shape accuracy. 153554b96e


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