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Molecular Dynamics (MD) Simulation of Nanoscale Cutting of Silicon Wafers:

• MD Simulation Model

• Crack Initiation

• "Dynamic Hard Particles"

1.   MD Simulation Model

To understand the underlying science of nanoscale ductile mode cutting better, a molecular dynamics simulation system has been developed, to enable one to understand the chip formation and cutting mechanisms in nanoscale ductile mode cutting of brittle materials at the atomic level (Fig. 1). 

Fig. 1. Molecular dynamics simulation of nanoscale ductile mode cutting of brittle materials.

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2.    Crack Initiation

Two significant findings have been made in our molecular dynamics simulation. Firstly, it was found that cracks could form readily in the chip formation zone when cutting brittle materials (Fig. 2(c)).  This has been related to the tensile stress state in the chip formation zone (Fig. 2(d)).  Our results also show that under the condition such that the tool edge radius is smaller than a critical value and that the undeformed chip thickness is smaller than the tool edge radius, only compressive stresses are present in the chip formation zone (Fig. 2(b)).  The condition of ductile mode cutting of brittle materials is thus realized and cutting would proceed without crack formation (Fig. 2(a)).

                 

                                       (a)                                                                      (c)

                  

                                         (b)                                                                  (d)

Fig. 2.   Results of molecular dynamics simulation: (a) No crack initiation when the undeformed chip thickness is smaller than the tool edge radius. In this case, only compressive stresses are present in the chip formation zone (b). (c) Formation of a crack in the chip formation form when the undeformed chip thickness is larger than the tool edge radius, being a result of the tensile stress state in the chip formation zone (d).

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3.    "Dynamic Hard Particels"

Another interesting finding is that in nanoscale ductile mode cutting of brittle single crystals, dynamic hard particles, i.e., atom groups with significantly shorter bond lengths (Fig.3), would be formed, removed and reformed in the chip formation zone.  These hard particles, in turn, may cause the formation of micro/nano groove wear at the tool flank.  Experimental confirmation of the wear grooves at the tool frank has been obtained in nanoscale ductile mode ductile cutting of single crystalline silicon wafer with single crystalline diamond tools. An example of such grooves is provided in Fig. 4.

                                            

Fig. 3. Presence of “dynamic hard particles” in the chip formation zone in nanoscle ductile mode cutting.

Fig. 4. Micro/nano grooves on the diamond tool flank face in cutting of single crystalline silicon.

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