Study on Crystallographic Orientation Effect on Surface Generation of Aluminum in Nano-cutting

Transcription

1 Xu et l. Nnoscle Reserch Letters (2017) 12:289 DOI /s NANO EXPRESS Study on Crystllogrphic Orienttion Effect on Surfce Genertion of Aluminum in Nno-cutting Feifei Xu 1,2, Fengzhou Fng 1*, Yunqing Zhu 2 nd Xiodong Zhng 1 Open Access Astrct The mteril chrcteristics such s size effect re one of the most importnt fctors tht could not e neglected in cutting the mteril t nnoscle. The effects of nisotropic nture of single crystl mterils in nno-cutting re investigted employing the moleculr dynmics simultion. Results show tht the size effect of the plstic deformtion is sed on different plstic crriers, such s the twin, stcking fults, nd disloctions. The minimum uncut chip thickness is dependent on cutting direction, where even negtive vlue is otined when the cutting direction is {110}<001>. It lso determines the mteril deformtion nd removl mechnism (e.g., shering, extruding, nd ruing mechnism) with decrese in uncut chip thickness. When mteril is deformed y shering, the primry shering zone expnds from the stgntion point or the tip of stgntion zone. When mteril is deformed y extruding nd ruing, the primry deformtion zone lmost prllels to the cutting direction nd expnds from the ottom of the cutting edge merging with the tertiry deformtion zone. The generted surfce qulity reltes to the crystllogrphic orienttion nd the minimum uncut chip thickness. The cutting directions of {110}<001>, {110}<1-10>, nd {111}<1-10>, whose minimum uncut chip thickness is reltively smll, hve etter surfce qulities compred to the other cutting direction. Keywords: Nno-cutting, Surfce genertion, Plstic deformtion, Cutting mechnism Bckground Ultr-precision cutting is one of the most efficient nd low-cost methods in relizing the nnometric surfce roughness nd su-micrometric form ccurcy. However, the mchined surfce qulity is ffected y mny fctors, such s mteril properties [1 3], mchine tools [4, 5], nd cutting tools [6, 7]. The mteril property is one of the most importnt fctors tht could not e neglected due to the ever-reduced uncut chip thickness (UCT) mking the mteril removl t nnoscle. It is smller thn the mteril grin size cusing the significnt ppernce of the size effects of mterils [8]. The nisotropic nture of single crystl mterils would exhiit in the cutting processes, even the mchined mterils re polycrystlline, such s the vrition of surfce roughness otined t * Correspondence: fzfng@tju.edu.cn 1 Stte Key Lortory of Precision Mesuring Technology & Instruments, Centre of MicroNno Mnufcturing Technology, Tinjin University, Tinjin , Chin Full list of uthor informtion is ville t the end of the rticle different crystllogrphic orienttion of grins in copper [9]. To etter understnd the influence of nisotropy on surfce genertion of single crystl mterils, much ttention hs een ttrcted. Lee et l. investigted the nisotropy of surfce roughness for three different crystl plnes, {100}, {110}, nd {111} [3]. It ws thought tht the nisotropy could e explined y the dependency of Young s modulus on the grin orienttion which cuses the different mount of recovery fter the tool cutting through. To et l. found tht the est surfce finish is otined in mchining single crystl luminum with {100} plnes [2]. The nisotropy of single crystl 3C-SiC during nno-cutting hs een investigted y Goel et l. using moleculr dynmics (MD) simultions, nd three esy deformtion directions hve een found [1]. MD simultions hve lso een conducted to investigte the orienttion effects in nno-cutting of single crystl mterils, nd three modes of deformtion comined with different disloction genertion forms were oserved in the sher zone [10]. The Author(s) Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution 4.0 Interntionl License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde.

2 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 2 of 13 Besides tht, when the UCT is comprle to the cutting tool edge rdius, the tool could no longer e simplified s shrp edge. The interctions etween the cutting tool edge nd the mterils mke the mteril deform in different wys. For instnce, the shering plne which used to e plne [11 13] extends to shering zone [14, 15]. Woon et l. [15] systemticlly investigte the effect of tool edge rdius on the mteril deformtion ehvior in wide rnge of UCT. In nnocutting process, Fng et l. [16, 17] propose tht the mterils re extruded in front of the cutting tool when the UCT is much less thn tool edge rdius. Woon et l. [18] lso found n extrusion-like mteril deformtion ehvior t criticl comintion of UCT nd tool edge [18]. Simoneu et l. [19] found the chip formtion chnges from the shering to qusi-sher-extrusion mechnism with decrese of UCT. When UCT decrese to criticl vlue, the mteril cnnot remove stly or just no formtion of chip. The threshold vlue is defined s the minimum UCT. It is key prmeter which strongly reltes to the mteril seprtion mechnisms in front of the tool edge nd determines the mchined surfce qulity. Two mjor mechnisms hve een proposed in descriing the mteril seprtion t the cutting tool edge [20, 21]. One is sed on the existence of stgntion point t the tool edge [22], nother one is sed on the formtion of stgntion region in which the mteril flow velocity is lmost zero [23]. The stgntion point or the tip of stgntion region is where the workpiece mteril strts to split into two prts to form the chip or the mchined surfce. The mteril deformtion mechnism influenced y tool edge would further ffect the generted surfce qulity. The cutting edge with lrge edge rdius results in higher verge surfce roughness vlues thn tht with smll edge rdius [24], nd the effect of the cutting edge rdius on the surfce roughness decresed with n increse in workpiece hrdness. The spring ck of the mchined mteril which influences the surfce roughness is lso ffected y the cutting tool edge [25]. In this study, the plstic deformtion nd surfce genertion of single crystl luminum in nno-cutting re investigted employing MD simultions. The effects of the crystllogrphic orienttion nd the tool edge rdius re considered in terms of disloction evolution, stcking fult evolution, shering plne evolution, tom displcement, cutting force, surfce morphology, nd mteril removl mechnism. This study contriutes to etter understnding of the surfce genertion for single crystl mterils nd even polycrystlline mterils in nno-cutting. Methods MD simultion is employed to investigte the plstic deformtion of luminum during nno-cutting. As shown in Fig. 1, the MD simultion model consists of rigid dimond tool nd n luminum workpiece. The edge rdius of the tool is 5 nm. The rke ngle nd clernce ngle is 0 nd Size of the workpiece is 50 nm 20 nm 8 nm contining out 600,000 toms. Atoms of workpiece re ctegorized into three prts: oundry lyer, thermostt lyer, nd Newtonin lyer. Atoms in oundry lyer re fixed t spce to prevent the unexpected movement under the ction of cutting force, nd the thermostt lyer djcent to it is kept t constnt temperture of 293 K to imitte the het dissiption in nno-cutting. The rest toms tht would e under the cutting of tool re in the Newtonin lyer oeying the Newton s lw. Periodic oundry condition pplies long the z direction in the model to reduce the size effect of the nno-cutting process. Seven cutting directions, including {100}<001>, {100}<011>, {100}<012>, {110}<1-10>, {110}<001>, {111}<1-10>, nd {111}<11-2>, re employed to investigte the effect of crystllogrphic orienttion on plstic deformtion mechnism in nno-cutting. UCT is t the rnge from 0.1 to 5 nm nd to reduce the simultion time, the cutting speed is set to 100 m/s t the negtive x direction. The cutting distnce of the model is out 40 nm. Initil temperture of the cutting model is equl to the constnt temperture in thermostt lyer. Fig. 1 Schemtic description of nno-cutting

3 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 3 of 13 The emedded tom method (EAM) potentil [12] is employed to descrie the interction mong the luminum toms. The tool energy E is given s the following function: E ¼ X i 1 X F i ρ i þ ϕ 2 i;j r ij i;j;j i ð1þ where F i (ρ i ) is the emedding energy to emed tom i into the electron density ρ i, nd ϕ i,j (r ij ) is the pir potentil energy etween toms i nd j. The electron density ρ i cn e clculted y the following form: ρ i ¼ X f j r ij ð2þ j;j i where f j (r ij ) is the electron density csing y tom j which hs distnce of r ij to the loction of tom i. The interction etween the cron toms is ignored due to the dimond is much hrder thn luminum nd the dimond tool is thought s rigid. The interction etween the rigid dimond tool nd luminum toms is depicted y the Morse potentil: h i E ¼ D 0 e 2α ð r r 0 Þ 2e α ð r r 0Þ ð3þ where E is the pir potentil energy, D 0 is the cohesion energy, α is constnt determined y mteril properties, r 0 is the distnce t equilirium, nd r is the distnce etween two toms. The MD simultion is sed on the Lrge-scle Atomic/Moleculr Mssively Prllel Simultor, nd the microstructurl evolution of the workpiece under the cutting process is nlyzed sed on common neighor nlysis, n lgorithm to chrcterize the locl structurl environment for pirs of toms nd disloction nlysis using disloction extrction lgorithm [13] with softwre OVITO. The microstructure, such s fce-centered cuic (FCC) nd hexgonl close-pcked (HCP) structures, nd disloction type, such s perfect disloction, Shockley prtil, nd stir-rod disloctions, of the workpiece system could e identified. A single HCP lyer denotes coherent twin oundry (TB). Two HCP lyers with or without FCC lyer etween them indicte intrinsic stcking fult (ISF) nd extrinsic stcking fult (ESF), respectively, [26 28]. Results nd Discussion Cutting-Induced Plstic Deformtion with Lrge UCT The snpshots of the MD simultion with UCT of 5 nm re shown in Figs. 2, 3, 4, 5, 6, 7, nd 8 in which the HCP structures re red nd other type of toms such s disloction cores nd surfce toms re white. The toms in FCC structure re green nd not displyed in the figure of microstructure evolution. Disloction lines re colored ccording to their types: perfect disloctions (lue line), Shockley prtil disloctions (green line), stir-rod disloctions (purple line), nd Frnk prtil disloctions (ple lue line). The red HCP lyers on {111} Fig. 2 Snpshots of the microstructure evolution t cutting direction of {100}<001>, displcement vector sliced t 4 nd c 6 nm in z direction c

4 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 4 of 13 Fig. 3 Snpshots of the microstructure evolution t cutting direction of {100}<011>, displcement vector sliced t 4 nm in z direction crystl plnes indicte the genertion of TB, intrinsic or extrinsic stcking fult. To investigte the mteril removl nd chip formtion mechnism, the displcement vector of the workpiece toms is lso nlyzed, s shown in Figs. 2, 3, 4, 5, 6, 7, nd 8. The red rrows indicte the mteril flow direction. Plstic Deformtion t Different Cutting Directions At the cutting direction of {100}<001>, s shown in Fig. 2, lrge numer of Shockley prtil disloctions nuclete in the plstic deformtion zone in front of the cutting edge ccompnied with ISF nd ESF which is long the {111} plnes. The Shockley disloctions re t the edge of ISF nd ESF. Stir-rod disloctions which re the meet nd rection of Shockley prtil disloctions on different {111} plnes re lso found in the plstic deformtion zone. Besides tht, perfect disloctions re generted in the deformtion zone which nuclete without stcking fult. While the dimond tool cutting through the surfce, point defects, nd severl kinds of disloctions re left on the mchined susurfce. In the nno-cutting process, the evolution of Shockley prtil disloctions domins the plstic deformtion. Figure 2 is the displcement vector of the cutting plne sliced t 4 nm in z direction with cutting direction of {100}<001>. The displcement vectors of workpiece toms hve rupt chnges t the stcking fult oundry on {111} plne. This plne is seen s the shering oundry or the shering plne. The included ngle etween it nd the cutting direction is shering ngle. In this condition, the shering ngle is supposed to e 45. However, due to the workpiece in front of the cutting edge hs 10 pile-up, the shering ngle in this figure is ctully out 35. In front of the cutting edge, there is zone in which the displcement vectors lmost equl to zero. It mens tht the toms re entrpment y the Fig. 4 Snpshots of the microstructure evolution t cutting direction of {100}<012>, displcement vector sliced t 4 nm in z direction

5 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 5 of 13 Fig. 5 Snpshots of the microstructure evolution t cutting direction of {110}<001>, displcement vector sliced t 4 nm in z direction cutting edge. It tkes more time nd cutting distnce for the toms in the zone to determine whether to e prt of chip or mchined surfce. The zone is lso clled s stgntion region. The shering oundry strts t the tip of the stgntion region. The shering oundry on the {111} plnes is not perpendiculr to the cutting plne, while the included ngle etween them is 55. Therefore, in different slice distnce in z direction, the strting point of the shering oundry s well s the position of the stgntion region re chnged, s shown in Fig. 2c which is the cutting plne sliced t 6 nm in z direction. The stgntion region is split into two smll regions influencing the movement of the toms round the tool edge. When the cutting direction is {100}<011>, the minimum included ngle etween {111} plnes nd the cutting direction is 54.7 which is too lrge to initite the disloctions sliding continuously long {111} plnes. Therefore, the shering oundry or the shering plne is not on the {111} plnes, ut disloction slide plne which is perpendiculr to the cutting plne genertes in front of the tool edge, including perfect disloctions, Shockley prtil, Frnk prtil, stir-rod disloctions, nd other disloctions on it s shown in Fig. 3. In the cutting process, disloction density in the shering plne increses resulting in disloction tngle nd refining the grin size of the removed workpiece mteril. It mkes the removed chip to e polycrystlline with the grin size in nnometer. After the dimond tool cutting through, stcking fults nd the disloctions under the mchined surfce re not completely recovered cusing lrge numer of point defects, TB, ISF, ESF, s well s different kinds of disloctions left in the mchined susurfce. This phenomenon would deteriorte the generted surfce roughness. The displcement vector is shown in Fig. 3, the displcement vectors chnge Fig. 6 Snpshots of the microstructure evolution t cutting direction of {110}<1-10>, displcement vector sliced t 4 nm in z direction

6 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 6 of 13 Fig. 7 Snpshots of the microstructure evolution t cutting direction of {111}<1-10>, displcement vector sliced t 4 nm in z direction ruptly t the disloction oundry which is the shering plne with shering ngle of 29 in this figure. The shering plne lso strts t the tip of stgntion region formed in the cutting edge. As shown in Fig. 4, mterils pile up in front of the cutting edge while the cutting direction is {100}<012>. The pile-up zone is ounded y disloction slide plne ABC which strts t the ottom of cutting edge nd expnds long the cutting direction. After severl tens of nnometers, the disloction slide plne expnds towrd the free surfce long line BC with shering ngle of out 45. On the disloction slide plne, different kinds of disloctions, such s perfect disloctions, Shockley prtil, Frnk prtil, stir-rod disloctions, nd other disloctions, nuclete nd move during the cutting process. Disloctions move etween the disloction slide plne nd the top surfce of the pile-up chip nd tend to escpe from the free surfce left micro steps on it. The pile-up mterils would finlly e removed nd ecome the chip. Displcement vector lso displys the motion of toms under the ction of cutting tool, s shown in Fig. 4. The displcement vectors of toms ove line AB pproximtely equl to zero which mens this prt of toms stick to the cutting edge nd move with it. Aove line BC, directions of tom displcement vector ruptly chnge cusing the rottion of the workpiece mteril lttice. Therefore, the disloction slide plne AB could lso e seen s grin oundry ove which the crystl plne is {111} nd elow which the crystl plne is {100}, s shown in Fig. 4. If just tking the shering plne AB into considertion, the shering ngle should e 0. With the cutting direction of {110}<001>, s shown in Fig. 5, the shering plne where displcement vectors of toms chnge ruptly is TB on the {111} plne. It expnds eneth the cutting edge nd segments y Shockley prtil disloctions into two or more prts. During Fig. 8 Snpshots of the microstructure evolution l t cutting direction of {111}<11-2>, displcement vector sliced t 4 nm in z direction

7 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 7 of 13 the cutting process, the Shockley prtil disloctions move long the TB nd mking the segmented TB move forwrd. Beneth the tool edge, tringulr shering zone ABC ounded y the tool edge profile, TB, ISF, nd shering oundry chnges the displcement vector of toms under the tool edge s shown in Fig. 5. Stgntion region forms ove the tringulr zone, nd stcking fult oundry expnds t the tip of the stgntion region cusing the second shering of toms on the oundry. At the cutting direction of {110}<001>, the primry shering plne is the TB nd the shering ngle is out 35 which is the included ngle etween the {111} plne nd cutting direction. Due to the TB tht does not strt from the tip of the stgntion region nd hs severl nnometer distnces to the tool edge, the mteril in front of the cutting edge is pile up y the shering t TB. Then, they seprte t the stgntion region. Therefore, thicker mterils re removed compred to the uncut chip thickness. Similr to simultion results otined with cutting direction of {100}<011>, disloctions moving long {111} plnes re not initited t the cutting direction of {110}<1-10> s shown in Fig. 6. However, shering oundry on which different kinds of disloctions such s perfect disloctions, Shockley prtil, nd stir-rod disloctions initite nd move long it during cutting process. The shering oundry strts t the tip of the stgntion region to the free surfce with shering ngle of 27 s shown in Fig. 6. The displcement vectors of toms on this oundry lso chnge ruptly. Unlike the cutting direction of {100}<011>, the chip does not trnsform to polycrystlline. The {111}<1-10> cutting direction which is crystllogrphic slip direction of single crystl luminum mkes lrge numer of Shockley prtil disloctions initite in front of the cutting edge ccompnied with ISF, s shown in Fig. 7. The disloctions re concentrted on two shering oundries: one expnds from the ottom of the cutting edge long the cutting direction nd nother strts t the tip of the stgntion region to the free surfce, s shown in Fig. 7,. Except for the Shockley prtil disloctions, stir-rod disloctions lso initite on the first shering oundry due to the interction of Shockley prtil disloctions on different crystllogrphic plnes, nd lrge numer of perfect disloctions re lso formed on the second shering oundry cusing the mterils removed in shering mechnism. Besides tht, mss of disloctions including Shockley prtil nd stir-rod disloctions re evolution in the chip in front of the tool rke fce. According to Fig. 7, the shering ngle is out 40 which is the lrgest compred to the cutting process t other cutting directions. This is ecuse lrge stgntion region formed in front of the cutting edge works like uild-up edge shrping the tool edge nd mking the rke ngle of the cutting tool positive. The size of uild-up edge tends to increse nd ttin stle stte with the increse of cutting distnce. Therefore, uild-up edge mking the cutting tool with positive rke ngle nd shrp edge increses the shering ngle in the cutting process. After the cutting edge pss through, lmost no disloctions nd stcking fults re initited nd left in the mchined susurfce. It mkes the mchined surfce hs etter roughness. At the cutting direction of {111}<11-2>, stgntion region is lso formed in front of the cutting edge nd is lrger thn cutting on the {100} nd {110} crystllogrphic plne, s shown in Fig. 8. Unlike the cutting direction of {111}<1-10>, n expnded shering zone is formed strting t the tip of stgntion region. The shering ngle of the shering zone is out 38 which is slight smller thn cutting t {111}<1-10> direction. The lrge nd shrp stgntion region, to certin extent, increses the shering ngle, ut lrge numer of the disloctions nd stcking fult expnd the shering zone, s shown in Fig. 8. The displcement vectors of toms in the zone chnge grdully compred to the cutting process with other cutting directions. After the cutting process, Shockley prtil nd stir-rod disloctions move deep into the mchined susurfce left ISF nd ESF in it, which would influence the generted surfce roughness. Shering Angle nd Cutting Force Sttisticl results of the shering ngles with UCT of 5 nm t different cutting directions re displyed in Fig. 9. The {110}<1-10> cutting direction hs smllest shering ngle compred to the other cutting directions. At {111}<11-2> cutting direction, the shering ngle could ttin vlue lrger thn 45, ut the verge vlue of it is lmost similr s the cutting direction of {111}<1-10> nd Fig. 9 Shering ngle in different cutting directions

8 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 8 of 13 {100}<001>. The verge shering ngle of {110}<001> cutting direction is slightly smller thn the TB-induced shering ngle (35 ) due to the smll shering ngle t the erly stge of the cutting process, ut the upper ound of the shering ngle t the cutting direction is 35. The verge shering ngle of the {100}<011> cutting direction is similr to the {110}<001> cutting direction except the upper ound of it tht is lrger thn tht of {110}<001>. The processing forces normlized y the cutting width nd the rtios of processing force t cutting direction F c to the feed direction F f re illustrted t Fig. 10. The processing force is the verge vlue when the cutting process ttins stle stge. The results show tht the rtio F c = F f hs little reltionship to the shering ngle since the mteril removl mechnism strongly reltes to the plstic deformtion mechnism of single crystl luminum. In nno-cutting process, the size effects of mterils pper mking the genertion of shering plne sed on different plstic crriers, such s the twin disloctions in different crystl plnes. This would cuse the discrepncy etween the cutting force nd shering ngle. The feed force of {110}<001> cutting direction is the smllest compred to the other processing forces. It is ecuse the mteril in front of the cutting edge is pile up y the shering t TB. Then, they seprte t the stgntion region which is closes to the rke fce of the cutting tool edge. Therefore, lrge prt of the pile-up mterils re compressed to form the mchined surfce. The whole process mkes the feed force fluctute over greter rnge, even ttin zero nd negtive vlue t the cutting process. Therefore, the verge feed force for of {110} < 001 > cutting direction is reltively smll. Therefore, the phenomenon tht the cutting force is fr greter thn the feed force could lso e seen in the nno-cutting process, due to the size effect nd nisotropy of mterils. Seprtion Height nd Recovery Height Figure 11 shows snpshots of the MD simultions t different cutting distnces. Atoms tht tend to e removed s the chip or to e the mchined surfce re colored with yellow nd green, respectively. Except for the two kinds of toms, the rest of the workpiece toms re colored with red. Therefore, red lyer etween the chip lyer nd the mchined surfce lyer could e oviously seen in Fig. 11. It is the seprtion lyer nd its verge height relted to the ottom of cutting tool edge is the seprtion height h s. In the cutting process, toms in the seprtion lyer re trpped y the cutting tool edge forming smll red tringulr region in front of the cutting tool edge, s shown in Fig. 11. The tringulr region is recognized s the stgntion region in which the displcement vector or the velocity of the toms pproximte zero, s shown in Figs. 2, 3, 4, 5, 6, 7, nd 8. It mens more time nd cutting distnce the toms in the stgntion region should tke to determine whether to e the removed chip or the mchined surfce. After the tool edge cutting through, recovery hppens t the mchined surfce. The recovery height is the distnce from the mchined surfce to the ottom of the cutting tool edge. At different cutting direction, the seprtion height nd recovery height re displyed in Fig. 12. The seprtion height reltes the minimum uncut chip thickness of the mteril to e removed with tool edge rdius of 5 nm [22]. In ddition, the recovery height determines the mchined surfce qulity, such s the polycrystlline mteril in which the different recovery heights of different grins deteriorte the mchined surfce roughness. The seprtion height of the {110}<001> cutting direction is negtive which mens more mterils, even the mteril elow the cutting tool edge, would e removed in the cutting process. It is ecuse the 35 pile-up in front of the tool edge mking the mteril elow the tool edge moves to the stgntion region nd seprtes t the stgntion tip. Then, the mterils under the stgntion tip re pressed down to form the mchined surfce with negtive recovery height. Besides tht, the seprtion heights of other cutting direction re positive. The seprtion heights of {100}<001> nd {111}<11-2> cutting Fig. 10 Averge cutting force nd feed force, rtio of cutting force to feed force t different cutting direction

9 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 9 of 13 c Fig. 11 Snpshots of the MD simultions with the cutting direction of {100}<001> nd the cutting distnce of 0, 12, nd c 35 nm directions re the lrgest in ll six cutting directions. The seprtion height of {111}<100> cutting direction is smller thn the {100}<011> cutting direction, nd the seprtion height of {110}<1-10> cutting direction is in the middle of them. The recovery height of six cutting directions is lso different nd is smller or equl to the seprtion height. The difference of the recovery height would finlly influences the mchined surfce qulity of polycrystlline luminum. Fig. 12 Seprtion height nd recovery height t different cutting direction with 5 nm UCT Cutting-Induced Plstic Deformtion with Smll UCT With lrge UCT, shering plnes re formed in front of the cutting tool edge mking the mteril removed in shering mechnism. However, when the UCT is smller thn or similr to the minimum UCT, the plstic deformtion nd mteril removl mechnism re different from the former shering mechnism. In this study, MD simultions hve een employed in investigting the plstic deformtion of mteril with UCT round the seprtion height otined ove. The displcement vector plots of different cutting directions nd UCTs in Fig. 13 illustrte the chip formtion nd the plstic deformtion with smll UCT. The toms tht tend to form the chip re colored in lue. When the cutting direction is {100}<001>, no shering plne is formed in front of the tool edge. For the UCT of 0.45 nd 0.7 nm which is less thn or equl to seprtion height, workpiece surfce is firstly pressed down y the tool edge in elstic deformtion. Then, the upper lyer, usully the first lyer, of the workpiece mteril is removed y the cutting tool in n extrusion wy. The rest prt of the mterils flow to the flnk fce of the cutting tool experience the elstic-plstic deformtion nd form the mchined surfce. For the UCT of 0.95 nm which is lrger thn seprtion height, more mterils re removed to form the chip. However, no shering plne forms in front of the tool edge nd the mterils re still removed y extruding. Similr results otin t cutting direction of {100}<011>{110}<1-10> nd {111} <11-2>. Minor difference in the cutting processes is tht the cutting tool edge rus on the workpiece mteril surfce nd lmost no mterils re removed, when the UCT is less thn the seprtion height t these three cutting directions. In ruing mechnism, the workpiece mteril experiences elstic-plstic deformtion due to the interction with the tool edge nd flnk fce, which, to some extent, ffects the generted surfce. For the cutting direction of {110}<001> whose seprtion height is negtive, the upper lyer mterils could e removed even the UCT is 0.1 nm. This is ecuse the mterils in front of the tool edge firstly pile up y shering mechnism nd then re extruded or secondly shered y the cutting tool edge. The mteril could lso e removed when the UCT is 0.25 nm for the cutting direction of {111}<1-10>, s shown in Fig. 13. Mterils re extruded up t distnce of severl nnometers wy from the cutting tool edge. The distnce of the mterils strting to e extruded increses with the cutting distnce. Therefore, when the UCT is smller thn seprtion height, the mteril undergoes the ruing nd extruding mechnism. In this condition, no mteril or just the first lyer of the mteril is removed. When the UCT is lrger thn the seprtion height, more mteril would e removed in extruding mechnism.

10 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 10 of 13 c d e f Fig. 13 Displcement vectors t different UCT nd cutting directions: {100}<001>, {100}<011>, c {110}<001>, d {110}<1-10>, e {111}<1-10>, f {111}<11-2>, mking the mteril remove in ruing or extrusion mechnism

11 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 11 of 13 Thus, the seprtion height with the UCT of 5 nm which is equl to the tool edge rdius could e simply seen s the minimum UCT. It determines the mteril deformtion nd removl mechnism with decrese of UCT. Sher Strin t Ruing, Extruding, nd Shering Mechnisms Figure 14 is the distriution of the sher strin of different surfce genertion mechnism, including shering, extruding, nd ruing mechnisms which re determined y mteril properties nd minimum UCT. At the shering mechnism, the primry deformtion zone (PDZ), secondry deformtion zone (SDZ), nd tertiry deformtion zone (TDZ) could e oviously seen in Fig. 14 c. The PDZ which is ctully shering plne mentioned ove expnds from the stgntion point or the tip of stgntion zone. However, when the UCT is less thn or similr to the minimum UCT, the PDZ expnds from the ottom of the cutting edge or just merges with the TDZ, s shown in Fig. 14d f. It is different from the results otined y Woon et l. tht the PDZ merges with the SDZ with decrese of the rtio of UCT to tool edge rdius [29]. In the ruing mechnism, the strin hppens t the surfce of the workpiece mteril which is under the tool edge, nd the sher strin zone lso merges with the TDZ during the ction of tool flnk fce. In ruing nd extruding mechnism, the strin zone lmost prllels to the cutting direction. The differences etween the extruding nd ruing mechnisms re tht the strin hppens t surfce or susurfce of the workpiece mteril. Surfce Genertion Figure 15 shows the effect of cutting direction nd UCT on the surfce genertion in nno-cutting. The cutting directions of {110}<001>, {110}<1-10>, nd {111}<1-10>, whose minimum UCT is reltively smll, hve etter surfce qulities compred to the other cutting directions with UCT chnging from 0.1 to 5 nm. At cutting direction of {100}<001>, the surfce qulity gets etter with the UCT of 5 nm nd gets worse when the UCT is 0.7 nd 0.95 nm which is similr s the seprtion height (0.7 nm) otined t 5 nm UCT. Similr results could lso e seen t the cutting direction of {100}<011>, whose seprtion height is 0.5 nm, the surfce qulity gets worse with the UCT of 0.5 nd 0.75 nm. However, smll vrition of the surfce qulity is found in the cutting Fig. 14 Sher strin of different surfce genertion mechnisms. {100}<001>, UCT: 5 nm. {110}<001>, UCT: 5 nm. c {110}<1-10>, UCT: 5 nm. d {100}<001>, UCT: 0.75 nm. e {110}<1-10>, UCT: 0.5 nm. f {111}<1-10>, UCT: 0.75 nm. g {100}<011>, UCT: 0.5 nm. h {110}<1-10>, UCT: 0.25 nm. i {111}<11-2>, UCT: 0.25 nm

12 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 12 of 13 Fig. 15 Surfce genertion t different cutting directions nd UCTs direction of {111}<11-2>. The surfce qulity does not get worse when the UCT is similr s the seprtion height. In terms of the overll results, the surfce qulity of the cutting processes with smll UCT is reltively etter thn tht of the lrge UCT. This is ecuse of the smll mount of the mteril cting with the tool edge. From nother direction, the surfce genertion qulity, to some extent, reltes to the contct length which reflects the interction of the workpiece nd the flnk fce of cutting edge, s shown in Fig. 15. The cutting direction of {110}<001>, {110}<1-10>, nd {111}<1-10> whose surfce qulity is etter hve reltively smller contct length thn tht of the others, nd the improved surfce qulity of {100}<001> cutting direction is ccompnied with the decrese of contct length when the UCT is 5 nm. Conclusions The effects of crystllogrphic orienttion on plstic deformtion nd surfce genertion of single crystl luminum in nno-cutting re investigted employing MD simultions. The conclusions cn e drwn s follows: 1. During the nno-cutting, the size effects of mterils mke the genertion of shering plne sed on different plstic crriers, such s the twin, stcking fults, nd disloctions on different crystl plnes. The shering ngle hs little reltionship to the rtio F c =F f s the mteril removl mechnism strongly reltes to the plstic deformtion mechnism in different cutting directions. 2. The seprtion height with the UCT of 5 nm which is equl to the tool edge rdius could e simply seen s the minimum UCT. Its vlue chnges in different cutting directions, nd even negtive vlue is otined in the cutting direction of {110}<001>. 3. The minimum UCT determines the mteril deformtion nd removl mechnism with decrese of UCT. When the UCT is considerly lrger thn the minimum UCT, the mteril is removed y shering mechnism. When the UCT is smller thn or similr s the minimum UCT, the mteril is removed y extruding. For further decresing the UCT, ruing hppens nd no mteril is removed. 4. At the shering mechnism, the PDZ, SDZ, nd TDZ exist t nno-cutting process. The PDZ expnds from the stgntion point or the tip of stgntion zone. However, in ruing nd extruding mechnism, the PDZ is lmost prllel to the cutting direction nd expnds from the ottom of the cutting edge or just merges with the TDZ. The

13 Xu et l. Nnoscle Reserch Letters (2017) 12:289 Pge 13 of 13 differences of the sher strin etween the extruding nd ruing mechnisms re tht the strin hppens t surfce or susurfce of the workpiece mteril. 5. The generted surfce reltes to the crystllogrphic orienttion nd the UCT. The cutting directions of {110}<001>, {110}<1-10>, nd {111}<1-10>, whose minimum UCT is reltively smll, hve etter surfce qulities compred to the other cutting directions. The surfce qulity gets worse when the UCT is similr s the minimum UCT for cutting directions of {100}<001> nd {100}<011>. The surfce genertion qulity lso reltes to the contct length which reflects the interction of the workpiece nd the flnk fce of cutting edge. Arevitions CD: Cutting distnce; ESF: Extrinsic stcking fult; FCC: Fce-centered cuic structure; HCP: Hexgonl close-pcked structure; ISF: Intrinsic stcking fult; MD: Moleculr dynmics; PDZ: Primry deformtion zone; SDZ: Secondry deformtion zone; TB: Twin oundry; TDZ: Tertiry deformtion zone; UCT: Uncut chip thickness Acknowledgements The uthors thnk the supports of the Ntionl Nturl Science Foundtion (grnt no nd ), the Ntionl Key Reserch nd Development Progrm (grnt no. 2016YFB ), nd the 111 Project y the Stte Administrtion of Foreign Experts Affirs nd the Ministry of Eduction of Chin (grnt no. B07014). Authors Contriutions FFZ nd ZXD designed nd supervised this work. XFF nd ZYQ performed the simultion nd wrote the mnuscript. FFZ nd ZXD revised the mnuscript. All uthors red nd pproved the finl mnuscript. Competing Interests The uthors declre tht they hve no competing interests. Pulisher s Note Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions. Author detils 1 Stte Key Lortory of Precision Mesuring Technology & Instruments, Centre of MicroNno Mnufcturing Technology, Tinjin University, Tinjin , Chin. 2 Institute of Mechnicl Mnufcturing Technology, Chin Acdemy of Engineering Physics, Sichun , Chin. Received: 17 Jnury 2017 Accepted: 9 Mrch 2017 References 1. Surv G, Alexnder S, Xichun L, Anupm A, Roert LR (2013) Anisotropy of single-crystl 3C-SiC during nnometric cutting. Model Simul Mter Sc 21: To S, Lee WB, Chn CY (1997) Ultrprecision dimond turning of luminium single crystls. J Mter Process Technol 63: Lee WB, To S, Cheung CF (2000) Effect of crystllogrphic orienttion in dimond turning of copper single crystls. Script Mter 42: Chen W, Ling Y, Sun Y, Huo D, Lu L, Liu H (2014) Design philosophy of n ultr-precision fly cutting mchine tool for KDP crystl mchining nd its implementtion on the structure design. 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