magnetic field assisted abrasive based micro-/nano-finishing

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Journal of Materials Processing Technology 209 (2009) 6022–6038

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Journal of Materials Processing Technology

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agnetic field assisted abrasive based micro-/nano-finishing

.K. Jain ∗

ndian Institute of Technology, Mechanical Engineering, Kanpur 208016, Uttar Pradesh, India

r t i c l e i n f o

eywords:icro-/nano-machiningagnetic Abrasive Finishing

brasive Flow Machiningagnetorheological Abrasive Flow

inishingagnetorheological Finishing

a b s t r a c t

Micro-/nano-machining (abbreviated as MNM) processes are classified mainly in two classes: traditionaland advanced. Majority of the traditional MNM processes are embedded abrasive or fixed geometrycutting tool type processes. Conversely, majority of the advanced MNM processes are loose flowingabrasive based processes in which abrasive orientation and its geometry at the time of interaction withthe workpiece is not fixed. There are some MNM processes which do not come under the abrasive basedMNM category, for example, laser beam machining, electron beam machining, ion beam machining,and proton beam machining. This paper gives a comprehensive overview of various flowing abrasivebased MNM processes only. It also proposes a generalized mechanism of material removal for theseprocesses. The MNM processes discussed in this paper include: Abrasive Flow Finishing (AFF), MagneticAbrasive Finishing (MAF), Magnetorheological Finishing, Magnetorheological Abrasive Flow Finishing,Elastic Emission Machining (EEM) and Magnetic Float Polishing. EEM results in surface finish of the orderof sub-nanometer level by using the nanometer size abrasive particles with the precisely controlledforces. Except two (AFF and EEM), all other processes mentioned above use a medium whose propertiescan be controlled externally with the help of magnetic field. This permits to control the forces acting onan abrasive particle hence the amount of material removed is also controlled. This class of processes iscapable to produce surface roughness value of 8 nm or lower. Using better force control and still finerabrasive particles, some of these processes may result in the sub-nanometer surface roughness value
on the finished part. Understanding the mechanism of material removal and rotation of the abrasives inthese processes will help in rationalization of some of the experimental observations which otherwiseseem to be contradicting with the established theories. It also explains why a magnet used in MAF shouldhave a slot in it even though the area under the slot has “non-machining” zone. It elaborates based onthe experimental observations why to use pulse D.C. power supply in MAF in place of smooth D.C. power supply.
. Introduction

Fabrication of products deals with the building of machines,tructures or process equipment by cutting, shaping, weldingnd assembling of components made of the same or differentaterials. Fabrication can be classified into two main categories:acro-fabrication and micro-fabrication. The first one considers

he process of fabrication of structures/parts/products/featureshat are measurable and observable by naked eye (≥1 mm inize) while the second category deals/considers the miniaturetructures/parts/products/features which are not easily visible
ith naked eye, and have dimensions smaller than 1 mm
1 �m ≤ dimension ≤ 999 �m). There are various methods/ways byhich micro-fabrication of products can be achieved (Fig. 1). How-

ver, two of them are the most commonly used: (1) material

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deposition, and (2) material removal. This article deals with thematerial removal processes only. Some of the traditional mate-rial removal processes can be used for micro-fabrication. However,they have some constraints hence advanced material removal pro-cesses are more commonly used for this purpose. Fig. 2(a) showsa classification of advanced micro-/nano-machining (MNM) andmicro-/nano-finishing (MNF) processes.

Majority of the advanced material removal processes canbe employed for both, macro-machining as well as for micro-machining. While scaling down the applications of a processfrom macro- to micro (�)-machining, the �-machining processparameters have to be appropriately changed. Further, advancedmicro-machining processes (abbreviated as AMMPs) are used fortwo main purposes: (i) shaping and sizing a part, (ii) finishing a part.
For differentiating between these two classes, the first one is calledas AMMPs and the second one as advanced micro-/nano-finishingprocesses (AMNFPs). The AMMPs can be further sub-categories as(i) those processes which use abrasive particles as tools for remov-ing material from the work piece in the form of micro-/nano-chips,
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V.K. Jain / Journal of Materials Processing

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Fig. 1. Methods of micro-fabrication.

nd (ii) those processes which use direct energy for removal ofaterial by melting and/or vaporization, or electrochemical or

hemical reaction (Fig. 2(a)). The AMMPs deal with fabrication oficrostructures, generation of micro-features (say, micro-grooves,icro-cavity, micro-channels, etc. [Jain, 2009]).This paper deals only with those processes which fall in the

ategory of abrasive based micro-/nano-finishing processes. Somef the abrasive based advanced (micro-/nano-finishing) and tradi-ional finishing processes are given in Fig. 2(b).

. Abrasive based advanced micro-/nano-finishingrocesses

In today’s advanced engineering industries, the designers’equirements on the components are stringent, for example,xtraordinary properties of materials, complex shaped 3D com-onents (Fig. 3(a, i–iv)), miniature features, nano-level surfacenish on complex geometries which are not feasible to achievey any traditional methods (say, thousands of turbulated cool-

ng holes in a turbine blade, Fig. 3(a, iv), making and finishing oficro-fluidic channels in the electrically non-conducting materi-

ls (say, glass), etc. Such objectives can be achieved only throughhe advanced manufacturing processes in general and advanced

achining processes in particular. In this section, the working prin-iples of abrasive based advanced micro-/nano-finishing processesFig. 4) are discussed.

As shown in Fig. 2(a), AMPs (Jain, 2002, 2009) are capable oferforming micro-machining operations. To limit the length of therticle, only some of these advanced abrasive based (micro-/nano)-nishing processes are critically reviewed in the following sections.

.1. Advanced Abrasive Finishing Processes

The Advanced Abrasive Finishing Processes can be dividednto two groups to understand their working principles. First onencludes Abrasive Flow Finishing (AFF), Elastic Emission Machin-ng (EMM) and Chemo-Mechanical Polishing (CMP) where forcesn the work piece acting during the finishing process are notossible to control externally. The second one includes Magneticbrasive Finishing (MAF), Magnetorheological Finishing (MRF),agnetorheological Abrasive Flow Finishing (MRAFF), and Mag-

etic Float Polishing (MFP). In these processes, it is possible toxternally control the forces acting on the workpiece by varyinglectric current flowing in the electromagnet coil or by changinghe working gap while using a permanent magnet. A change in thelectric current changes magnetic flux density in the working zone

Technology 209 (2009) 6022–6038 6023

due to which the normal force exerted by an abrasive particle onthe work piece changes. This change in normal force changes fin-ishing rate and critical surface finish that can be achieved by theprocess under the given finishing conditions.

2.1.1. Nano-finishing processes without external control of forcesIn this category following types of abrasive based nano-finishing

processes are briefly discussed: Abrasive Flow Finishing (AFF),Chemo-Mechanical Polishing (CMP) and Elastic Emission Machin-ing (EEM).

2.1.1.1. Abrasive Flow Finishing. AFF process was originally identi-fied for deburring and finishing critical hydraulic and fuel systemcomponents of aircraft in aerospace industries. It can polish any-where that air, liquid, or fuel flows. Rough, power robbing cast,machined, or EDM’d surfaces are improved substantially regardlessof their surface complexities Fig. 3(a) shows that the AFF mediumacts as a ‘self deformable stone’ adapting itself according to theshape and size of the work piece (concave, convex, hexagonal orturbulated holes). It has been used for finishing micro-fluidic chan-nels made on glass and ceramics. It uses two vertically opposedcylinders (Fig. 4(a)), and extrudes abrasive medium back and forththrough a passage formed by the workpiece and tooling. To for-mulate the AFM medium, the abrasive particles are blended intothe special viscoelastic polymer, which shows a change in viscositywhen forced to flow through a restrictive passage Fig. 4(a) showsradial force (Fn) responsible for indentation of an abrasive particlein to the work piece, and axial force (Ft) responsible for removal ofmaterial in the form of a micro-/nano-chip. Abrasive action is accel-erated by a change in rheological properties of the medium when itenters and passes through the restrictive passage (Rhoades, 1988,1991). The viscosity of polymeric medium plays an important rolein finishing operation (Jha, 2006).

AFF can be applied to a wide range of finishing operationsthat require uniform and repeatable results (Kohut, 1989). Forces(radial and axial) acting during AFF have been evaluated (Goranaet al., 2004) and correlated to a change in surface roughness valueachieved after AFF. Under certain finishing conditions, it has beenfound that the material removal during AFF can take place byploughing mode (material piled up in the sides) also (Fig. 3(b))(Gorana et al., 2004), other than chipping mode (Jain and Jain, 1999).Fig. 3(b) shows a ploughing mode of material removal during AFFprocess. Active grain density (Gorana et al., 2004; Jain and Jain,2004) has been found to influence finishing rate and depends onthe finishing parameters such as abrasive concentration, extrusionpressure, abrasive mesh size and medium viscosity.

A stochastic methodology has been proposed (Jain and Jain,2004) to evaluate the interaction between spherical (assumedshape) abrasive grains and workpiece surface. This simulationenables the prediction of active grain density at different concen-trations and mesh size of abrasive particles which finally control thequality and rate of change in surface finish improvement. A goodcorrelation is obtained between the predicted and microscopicallyobserved active abrasive grain density. Empirical models have beenproposed for material removal and change in surface roughness(�Ra). Authors (Jain et al., 2001a) have investigated the effects ofvarious process parameters on the viscosity of the medium. It isfound that abrasive concentration, medium temperature and abra-sive mesh size have significant effect on medium viscosity. It isalso found that an increase in viscosity of the medium results in anincrease in material removal rate, but decrease in surface rough-
ness (�Ra) value until the critical surface roughness is achieved.Further, an increase in finishing rate becomes zero at critical surfaceroughness value.
Jain et al. (1999a) have analyzed material removal and surfaceroughness produced during AFF using finite element method. In this

6024 V.K. Jain / Journal of Materials Processing Technology 209 (2009) 6022–6038

Fig. 2. (a) Classification of micro-machining processes (USM: ultrasonic machining; AJM: abrasive jet machining; AWJM: abrasive water jet machining; WJM: water jetm lectroP ng; ECE

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achining; EBM: electron beam machining; LBM: laser beam machining; EDM: eCMM: photo chemical micro-machining; ECMM: electro chemical micro-machiniLID: electrolytic in-process dressing). (b) Micro-/nano-finishing processes.

ttempt, a classical abrasion theory has been applied. The model isased on accumulated plastic flow, by repeated indentation of mov-

ng abrasive particles. The dependence of surface roughness valuen various process parameters has been analyzed and the theoreti-al results are found to be in good agreement with the experimentalesults obtained from AFF process. The AFF results have shown thathe axial force, radial force, active grain density and grain depth ofndentation, have a significant influence on the scale of materialemoval. The minimum depth of indentation and minimum loadequired for chip formation, are found to correlate well with theode of material deformation. The theoretical and experimental

esults show that the rubbing mode of material deformation dom-
nates in the present study; however, some evidences of ploughinguring AFF are also reported (Gorana et al., 2004, 2006).
To improve the performance efficiency of AFF process, drill bituided (DBG) AFF process has been proposed (Sankar et al., 2009a).he major difference between AFF and DBG-AFF machines is in its

discharge machining; IBM: ion beam machining; PBM: photon beam machining;SMM: electro chemical spark micro-machining; EDG: electro discharge grinding;

tooling. In AFF machine, circular fixture plate allows the mediumto flow as a cylindrical slug. The abrasive intermixing (or reshuf-fling) purely depends on medium self-deformability and for mostof the time the same active abrasive grains keep taking part infinishing. The abrasive particles follow the shortest contact length(straight line) in AFF. In DBG-AFF process, the cylindrical slug getsdivided in two halves while entering in the finishing zone; at theexit side these two halves recombine resulting in better intermix-ing of the medium. The abrasive intermixing depends not only onthe medium self-deformability but also on the pressure from thedrill bit being exerted on the medium (reciprocating axial flow, flowalong the flute, and scooping flow—all the three flows take place at
the same time). Due to the combination of different modes of flow,the workpiece (AISI 4340)–abrasive contact length is no longer astraight line, rather it becomes inclined (Fig. 5(a)). Hence, the num-ber of peaks that can be sheared off in a single cycle increases,leading to higher material removal rate hence finishing rate also

V.K. Jain / Journal of Materials Processing Technology 209 (2009) 6022–6038 6025

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ig. 3. (a) Abrasive Flow Finishing medium adapts the shape of the workpiece (i)aterial removal by ploughing.

mproves as compared to AFF process. The medium in contact withhe drill bit (inner region of abrasive mixed medium slug) trieso follow the path of the flute at an angle �. However, it is foundhat the abrasion is taking place at an angle �1 which is differentrom the flute angle (�) and the difference has been expressed as �2Fig. 5(b)). The finishing zone becomes more restricted because ofhe presence of the drill bit; hence, the abrasive particles in DBG-FF exert higher pressure on the workpiece surface as compared to

he case of AFF.Nano-finishing of heterogeneous materials such as metal matrix

omposites (MMC) is a challenge for manufacturing engineers.ere, material removal also depends upon the position where anbrasive particle strikes the workpiece (i.e., matrix material, rein-orcement, or interface of these two). Let an abrasive (particle in the

edium) hit the MMC workpiece. If it hits matrix material (Al alloy),t removes the material in the form of a micro-chip. If abrasive (SiC)ncounters reinforcement (SiC) then the abrasive tries to detach (orull out) the reinforcement from the base material. The reinforce-ents pullout takes place only if Fa > Freq where Fa is applied axial

orce and Freq is the force required to pullout the reinforcement (i.e.,esistance offered by the MMC for pull out). Fig. 6(a) shows a pho-ograph of reinforcement pullout during AFF of MMC and Fig. 6(b)hows a model proposed (Sankar et al., 2009a,b) to represent rein-orcement pullout. If Fa < Freq then the abrasive may rotate or crossver the workpiece surface peak without any effective materialemoval, or remain embedded in the medium (Fig. 6(c)) (because itannot machine the material due to the condition Fa < Freq (may beue to higher depth of penetration, or unable to pull out the abra-ive in MMC)). The average surface roughness achieved in finishingl alloy/SiC MMC is 0.2 �m and this can be further improved byptimizing the process parameters.

For achieving the best out of any manufacturing process, para-
etric optimization is essential which has been attempted in case
f AFF (Jain and Jain, 2000; Jain et al., 1999a,b) using artificial neuraletworks (ANN). The optimization has been performed using back-ropagation neural networks. The optimization results of ANNave been compared with the optimization results obtained by

nal, (ii) concave, (iii) convex, (iv) turbulated hole, and (b) a schematic diagram of

employing Genetic Algorithm (GA) to establish the validity of neu-ral networks approach. The important advantage in case of neuralnetworks is that the process optimization can be performed in theabsence of mathematical model of the process, and its accuracydepends on the accuracy of the experimental observations used fortraining of ANN. This feature of ANN is important because mathe-matical modeling of the process is a pre-requisite for all the classicaloptimization methods, and in some cases, it is very difficult todevelop a mathematical model.

2.1.1.2. Chemical Mechanical Polishing. Chemical Mechanical Pol-ishing (CMP) is mainly used in the semiconductor manufacturingindustries and it is a planarization process which involves a com-bination of chemical and mechanical actions. In this process, thechemical reaction takes place between the silica slurry and thework material. The reaction products so formed are removed bymechanical action (abrasion) (Nanz and Camilletti, 1995; Hayashiet al., 2001). A schematic of CMP planarization process is shownin Fig. 4(b). The wafer is pressed downward by carrier and rotatedagainst the polishing pad covered with a layer of silica slurry. A sim-ilar variant is Chemo-Mechanical Polishing in which driving forcefor material removal is chemical action between abrasive particlesand work material followed by mechanical action for the removalof reaction products (Vora et al., 1982; Komanduri et al., 1997). Thisprocess is expected to overcome many problems of surface dam-age associated with hard abrasives, including pitting due to brittlefracture, dislodgement of grains, scratching due to abrasion etc.,resulting in smooth, damage free surfaces (Komanduri et al., 1997).

To explain material removal in CMP, abrasion mechanism insolid–solid contact mode has been proposed (Luo and Dornfeld,2001). Liu et al. (2003) have presented polishing kinetics and themechanism of material removal from the silicon substrate. They
found that the smaller size (15–20 nm) silica sols (abrasives) per-form better than the larger size (50–70 nm). It gives higher averagepolishing rate (200 nm/min), and it keeps changing with polishingtime. It gives lower damage and lower value of surface roughnessaftr CMP. Ahn et al. (2004) compared CMP using colloidal silica


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ig. 4. Nano-finishing processes (a) Abrasive Flow Finishing (AFF), (b) Chemo-Meinishing (MAF), (e) Magnetorheological Finishing (MRF), (f) Magnetorheological Ab

ased slurry and alumina (Al2O3) based slurry. The colloidal-basedilica slurry produced a desirable fine finish on aluminum surfaceith a few micro-scratches. Saka et al. (2008) found that during

abrication of advanced semiconductor devices, undesirable nano-cale scratches are produced.

Polishing both sides concurrently gives defect free surfaces hav-ng better parallelism compared to single side polishing and lessdherence of particles (Jhansson et al., 1989; Wenski et al., 2002).nother use of CMP substrate is in thin film transistor (TFT) tech-
ology and polishing of IC wafers (Venkatesh et al., 1995; Chang etl., 1996).
.1.1.3. Elastic Emission Machining. This process attracts the atten-ion because of its ability to remove material at the atomic level

cal Polishing (CMP), (c) Elastic Emission Machining (EEM), (d) Magnetic Abrasivee Flow Finishing (MRAFF), and (g) Magnetic Float Polishing (MFP).

by mechanical means and to give completely mirrored, crystal-lographically and physically undisturbed finished surface (Tsuwaet al., 1979). The ultra fine abrasive particles strike the individualatoms/group of atoms and separate them out from the parent sur-face (Fig. 4(c)). It has been found that the material removal processis a surface energy phenomenon in which each abrasive parti-cle removes a number of atoms after coming in contact with theworkpiece surface (Mori and Yamauchi, 1987). It has been estab-lished theoretically and experimentally that atomic scale fracture
can be induced elastically producing ultrafine surface finish with-out plastic deformation at atomic scale (Mori et al., 2002). In EEM,the material removal occurs at the atomic level, hence the sur-face finish obtained is close to the order of atomic dimensions(2–4 Å). The type of abrasive and size of abrasive grains used (in the


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ano-range) have been found to be critical to the material removalfficiency.

.1.2. Nano-finishing processes with external control of forcesThis class of processes includes MAF, MRF, MRAFF, and MFP. In

hese processes, the force acting on the work piece surface throughhe abrasive particles is controlled externally by changing the mag-etic flux density as discussed in the following paragraphs.

.1.2.1. Magnetic Abrasive Finishing. In case of a flat large size work-iece made of hard-to-machine material, the processes discussedo far do not qualify to give nano-level surface finish. Magneticbrasive Finishing (MAF) is the process which is capable of preci-ion finishing of such workpieces (Shinmura, 1987). In this process,sually ferromagnetic particles are sintered with fine abrasive par-icles (Al2O3, SiC, CBN, or diamond), and such particles are callederromagnetic abrasive particles (or magnetic abrasive particles-

AP). Fig. 4(d) shows a schematic diagram of a plane MAF processn which finishing action is controlled by the application of mag-etic field across the machining gap between the workpiece topurface and bottom face of the rotating electromagnet pole. Theagnetic field acts as a binder and retains ferromagnetic abrasive

articles in the machining gap. Normal component (Fmn) of theagnetic force due to magnetic field is responsible for abrasive

enetration inside the workpiece surface while rotation of the fer-omagnetic abrasive brush intact to north pole results in tangentialorce (Ft) (not shown in the figure). The sum of the forces Ft and the
angential component of the magnetic force (Fmt), (Fc = Ft + Fmt) isesponsible to remove material in the form of tiny chips (Singh etl., 2005a,b; Kremen, 1994). The MAP join each other magneticallyue to dipole–dipole interaction between the magnetic poles alonghe lines of magnetic force, forming a flexible magnetic abrasive
ig. 6. Mechanisms of material removal during AFF of MMC. (a) Reinforcement pulloutsatrix due to reinforcement pullout in Al alloy/SiC MMC, and (c) abrasive embedded in t

in DBG-AFF (a) macro-view, and (b) micro-view.

brush (FMAB) (usually, 1–3 mm thick). In case of unbounded (un-sintered) magnetic and abrasive particles (homogeneously mixedpowder), the abrasive particles get entangled in between the chainsand within the chains formed by ferromagnetic particles (Singh etal., 2005a). MAF uses this FMAB for surface and edge finishing. TheFMAB has multiple cutting edges and it behaves like a multi pointcutting tool to remove material from the workpiece in the form oftiny chips. Since the magnitude of machining force caused by themagnetic field is very low but controllable, a mirror like surface fin-ish (Ra value in the range of nanometer) is obtained. MAF can alsobe used to perform operations such as polishing and removal ofthin oxide film from high-speed rotating shafts. Researchers (Jainet al., 2001b; Kim, 1997; Komanduri, 1996) have applied MAF tofinish external and internal surfaces of cylindrical workpieces.

Apart from rotary motion of the cylindrical workpiece, axialvibratory motion is also introduced in the magnetic field by theoscillating motion of the magnetic poles or the workpiece toaccomplish surface and edge finishing at a faster rate and witha better quality (Komanduri, 1996; Fox et al., 1994; Yamaguchiand Shinmura, 1999, 2004). The process is highly efficient, and thematerial removal rate and finishing rate depend on the workpiececircumferential speed, magnetic flux density, working gap, work-piece material properties, and size, type and volume fraction ofabrasives. The process performance is also affected the presenceor absence of a slot in the magnet (Jayswal et al., 2004). Finish-ing of stainless steel rollers using MAF process to obtain final Ra of7.6 nm at an average finishing rate of 7.08 nm/s has been reported
(Komanduri, 1996; Fox et al., 1994). MAF can produce mechanicaland electronic components with high accuracy and very low surfaceroughness value having hardly any surface defects. This processhas also been applied for micro-deburring (Fig. 7) using permanentmagnet in place of electromagnet (Madarkar and Jain, 2007).
in MMC in the agglomerated area, (b) corresponding model of indentation in thehe surface.


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.1.2.2. Magnetorheological Finishing. The high precision lenses aresually made of brittle material such as glass, which tends to crackuring machining/finishing. To overcome the difficulties beingaced in finishing the lenses, a technology has been developed toutomate the lens finishing process known as Magnetorheologicalinishing (MRF) (Fig. 4(e)) (Kordonski, 1996). This process relies onunique “smart fluid”, known as Magnetorheological (MR) fluidhich is a suspension of micron sized magnetizable particles such

s carbonyl iron particles (CIPs), dispersed in a non-magnetic car-ier medium like silicone oil, mineral oil, or water. In the absence
f magnetic field, an ideal MR-fluid exhibits Newtonian behav-or. Magnetorheological effect is observed on the application ofxternal magnetic field to the MR-fluid. In the presence of exter-al magnetic field it behaves as non-Newtonian fluid. Fig. 8(a)hows the random distribution of CIPs and abrasive particles in
Fig. 8. (a–c) Material removal mechanism in Magnetorheological Finishing, and

d (b) after deburring by MAF.

the absence of magnetic field. Fig. 8(b) shows that the CIPs magne-tize when the magnetic field is on, and move towards the rotatingwheel where magnetic field strength is higher. Fig. 8(c) shows thatthe abrasive particles are in contact with the workpiece (Lens) sur-face but intact with the fluid (ribbon), and the CIPs are closer to therotating wheel. The normal magnetic force (or penetrating force) istransferred to the work surface through the abrasive particles, andit results in abrasive penetration in the work surface. Due to therelative motion between abrasive particles and work surface, mate-rial removal takes place in the form of micro-/nano-chips resulting
in nano-finishing. Because energy is required to deform and rup-ture the chains, this micro-structural transition is responsible forthe onset of a large controllable finite yield stress (Furst and Gast,2000). There is an increasing resistance to an applied shear strain,� due to this yield stress. When the field is removed, the parti-
(d) MRF experimental setup developed at IIT Kanpur (Mathur et al., 2003).


graph

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Fig. 9. Experimental results of MRF (a) initial surface topo

les return to their random state and the fluid again exhibits itsriginal Newtonian behavior. Fig. 8(c) shows work piece and fluidnteraction region.

The fluid strength (static yield shear stress) increases non-inearly as the applied magnetic field increases because thearticles are ferromagnetic in nature and magnetization in differ-nt parts of the particles occurs non-uniformly (Ginder and Davis,994). The upper limit of the strength of MR-fluid is decided byagnetic saturation. The ability of electrically manipulating rhe-

logical properties of MR-fluid attracts attention of a wide rangef industries, and numerous applications are explored. The MRFrocess is used for finishing optical glasses, glass ceramics, plasticsnd some non-magnetic metals (Lambropoulo et al., 1996; Carlsont al., 1996; Klingenberg, 2001). This finishing process is capableo produce surface finish of the order of 10–100 nm peak to valleyeight, and 0.8 nm RMS value in finishing optical lenses (Kordonski,996). A setup shown in Fig. 8(d) was used to nano-finish glass

enses for 50 min. Fig. 9(b) shows a change in surface texture andurface finish compared to original surface roughness and textureFig. 9(a)). The surface roughness value changed from 53 nm to1 nm in 50 min (finishing rate = 0.84 nm/min) (Mathur et al., 2003).

.1.2.3. Magnetorheological Abrasive Flow Finishing. The abradingorces in AFM process are least controllable by external means,ence lack of determinism. To preserve the versatility of AFF pro-ess and at the same time to have determinism and controllabilityf rheological properties of abrasive laden medium, a new hybrid

Fig. 10. Formation of CIPs chain structure (a) in absence of magneti

y, and (b) after finishing for 50 min (Mathur et al., 2003).

process termed as Magnetorheological Abrasive Flow Finishing(MRAFF) has been developed (Jha and Jain, 2004) by combiningAFF and MRF (Fig. 4(f)). Abrasive mixed viscous base mediumacts as a “self deformable stone” and overthrows shape limitationinherent in almost all traditional finishing processes. This pro-cess has the capability of finishing complex internal and externalgeometries up to nano-level surface roughness value. It impartsbetter control on the process performance as compared to AFFprocess due to in-process control over abrading medium’s rheo-logical behavior through magnetic field (Rabinow, 1948). MRAFFprocess comprises MR-polishing fluid having fine abrasive parti-cles dispersed in it. On the application of magnetic field, the CIPsform a chain like columnar structure with abrasives embedded inbetween and within the chains. Fig. 10(a) and (b) show the actualphotographs taken by an optical microscope for the case when nomagnetic field is applied and the structure formed in the presenceof magnetic field, respectively (Jha, 2006). The abrasive particlesheld by CIPs chains abrade the workpiece surface and shear thepeaks from it. The amount of material sheared from the peaks ofthe workpiece surface by an abrasive grain depends on the bondingstrength provided by the magnetic field-induced structure of MR-polishing fluid and the extrusion pressure applied through piston.
The best finish obtained by the present setup and specified machin-ing conditions (Jha and Jain, 2008) is 30 nm on stainless steel workpiece. However, experimentation with optimum process parame-ters would give still lower Ra value of the surface finished by MRAFFprocess.
c field and (b) on the application of magnetic field (Jha, 2006).

6 ssing Technology 209 (2009) 6022–6038

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The finishing action in MRAFF process relies mainly on bondingtrength around abrasive particles, in magnetorheological polish-ng (MRP) fluid due to cross-linked columnar structure of CIPs. Theuid flow behavior of MRP-fluid exhibits a transition from weakingham liquid like structure to a strong gel like structure on thepplication of magnetic field. The rheological properties of MRP-uid play an important role in MRAFF action which mainly dependsn CIPs and silicon carbide particles size, their volume concentra-ion, magnetic properties and magnetic field strength. Experimentsonducted on silicon nitride using silicon carbide (SiC), boron car-ide (B4C) and diamond abrasives proved MRAFF capability inano-finishing hard ceramics (Jha and Jain, 2006). This process has

mmense possibilities of applications especially in case of finishingf complex shaped 3D components.

.1.2.4. Magnetic Float Polishing. The finishing processes discussedn the preceding sections have been developed for flat surfaces,ylindrical surfaces or their combinations leading to complex 3Durfaces. However, finishing of spherical surfaces is equally impor-ant for which the above discussed processes do not qualify.he Magnetic Float Polishing (MFP) process has been developedFig. 4(g)) to meet this requirement. The best surface finish obtainedsing this process on the ceramic balls is 4 nm Ra and 40 nm Rmax,nd the best sphericity obtained on the Si3N4 balls is 150–200 nm.his process is assisted by magnetic field to support abrasivelurry in finishing ceramic balls and bearing rollers without havingcratches and pits (Umehara et al., 2005).

This technique is based on the ferro-hydrodynamic behaviorf magnetic fluid that levitates a non-magnetic float and abra-ive particles suspended in it by the application of magnetic field.he levitation force applied by the abrasives is proportional to theagnetic field gradient which is extremely small and highly con-

rollable. MFP can be a very cost-effective and viable method foruper finishing of brittle materials with flat and spherical shapes.

bank of strong electromagnets is arranged (alternately northnd south poles) below the finishing chamber. The ferro-fluid isttracted downward towards the area of higher magnetic field andn upward buoyant force is exerted on non-magnetic materials toush them to the area of lower magnetic field (Rosenweig, 1966)Fig. 4(g)). The buoyant force acts on a non-magnetic body in mag-etic fluid in the presence of magnetic field. The abrasive grains,eramic balls, and acrylic float inside the chamber are of non-agnetic materials, and all are levitated by the magnetic buoyant

orce. The drive shaft is fed downward to contact the balls and presshem downward to reach the desired force level. The balls are pol-shed by the relative motion between the balls and the abrasivearticles under the influence of levitation force. Si3N4 balls haveeen finished by MFP for high-speed hybrid bearing in ultra high-peed precision spindles of machine tools and jet turbines of aircraftTani and Kawata, 1984).

. Analysis of selected abrasive based nano-finishingrocesses

Modeling and theoretical analysis of the selected abrasiveased nano-finishing processes have been carried out to explainome of the results obtained during experimentation. Theseesults have not been satisfactorily explained in the existingiterature.

.1. Critical surface roughness

During MAF, AFM and MRAFF processes, it has been observedhat with the increase in finishing time, the surface roughness valueRa value) keeps on decreasing. Beyond a certain finishing time,he relationship between the finishing time and surface roughness

Fig. 11. Relation between surface roughness and finishing time.

value (Ra value) becomes asymptotic (Fig. 11) except the minorfluctuations within a small band of Ra value. This behavior wasobserved while comparing experimental and computational results(Jayswal, 2005) obtained for MAF process. To understand thisbehavior exhibited in Fig. 11, let us analyze the models proposedto explain material removal in MRAFF process which is applicableto MAF as well as AFM with minor modifications. Fig. 12(i) showsan abrasive particle held by chains of iron particles (Jha, 2006). Thedownward or indenting force is a normal component of magneticforce in MAF and MRAFF processes, and radial force (Fr) in AFMprocess. Because of the cutting force (axial in AFM and MRAFF,and tangential in MAF), this bunch of iron and abrasive particlesmoves in the forward direction and shears/removes a very smallamount of material in the form of micro-chip (Fig. 12(ii)). The sizeof the chip removed depends on the magnitude of radial force, axialforce, and the ratio of axial force and the force required to shear offthe roughness peaks. When this bunch of iron and abrasive parti-cles moves further, it separates the micro-/nano-chip (MNC) fromthe workpiece (Fig. 12(iii)). Similar mechanism of material removalwith minor modifications works in case of MAF process. In the sameway, material is removed in the form of micro-chip in case of AFM(Fig. 12(iv)).

This phenomenon of removal of material in the form of MNCis repeated by each bunch of iron/abrasive particles. As a result,the height of the surface irregularities keeps decreasing as shownin Fig. 13(i)–(iii). It also suggests that the iron and abrasive par-ticles size should be larger than the top width of a valley. Ata stage between Fig. 13(i) and (iv), one can get the minimumattainable surface roughness value but that stage is not knownin priori. At this point, the pre-finished marks/scratches are com-pletely removed and the abrasives creat their own finishing marks(peaks and valleys). At the stage (iv) (Fig. 13), the abrasive particlesfurther penetrate into the workpiece and their depth of penetra-tion depends upon the radial force, and the ratio of axial forceand the force required to shear off the surface peaks. This indenta-tion depth decides the critical surface roughness value attainable inthese processes. Unless finishing conditions are changed, the valueof (critical) surface roughness remains unchanged with time. How-ever, real life situation is slightly different because the size of ironand abrasive particles varies in a range hence the depth of pen-etration also as shown in Fig. 13(v). It shows different depth of
penetration for different size of abrasive particles depending uponthe penetrating force acting on a particular abrasive particle. As aresult of this, the critical surface roughness value obtained afterfinishing is not an unique number rather it fluctuates over a smallrange or band. This fluctuation in critical surface roughness value


Fig. 12. (i–iii) Three stages of material removal in case of MRAFF process, and (iv) micro-chip formation in AFM process.

F surfac

cicmi

rifvd

Fcr

ig. 13. Stages ((i)–(iii)) material removal from the workpiece surface, (iv) Critical

an also be caused due to the non-homogeneous mixture of theron and abrasive particles (MAF and MRAFF processes), or vis-oelastic medium and abrasive particles (in AFM). This variationay also result due to the inclusions of MNC in the medium before

t is replaced by the fresh medium.The experimental results of MAF clearly indicate that the surface

oughness approaches to the ‘critical surface roughness’ value dur-ng the experiments (Fig. 11). The critical surface roughness valueor the given finishing conditions is higher for the case of higheriscosity in case of AFM. However, this also has an upper limitepending upon the finishing conditions. It requires further theo-

ig. 14. (a) Cutting just to start, (b) material removal in the form of micro-/nano-chip, (c)utting force (Fc) and force required (Freq) for removal of material from the workpiece. Fc

esultant forces.

e finish obtained for the given finishing condition, (v) Cross-section along AA.

retical and experimental investigations to set these upper limits ofthe ultimate surface finish achievable from AFM and other abrasivebased nano-finishing processes.

3.2. Rotation of abrasive particles

This phenomenon is common for MAF, MRAFF and AFM pro-cesses where abrasive particle is held either by ferromagnetic (iron)particles or viscoelastic polymer. During finishing by any of theseprocesses, one of the following conditions may prevail in the fin-ishing zone (Fig. 14).

rotation of the particle, (d) forces and depth of penetration, (e) three conditions of= available cutting force, Freq = Force required for cutting to take place. R and R’ are

6 ssing Technology 209 (2009) 6022–6038

pcpanttadsraMttdmsl

dpott(AfMcipt

Fig. 15. Depth of penetration during rotation of an abrasive particle in thebrush/medium.


In the equilibrium condition, machining/finishing is just at theoint of starting (Freq = Fc, Fig. 14(a)) while in the material removalondition (Freq < Fc, Fig. 14(b)) the material removal keeps takinglace (a desirable finishing condition). In case of MAF (also in MRFnd MRAFF) process when Fmn (normal component of the mag-etic force) is more than the desirable magnetic field strength inhe finishing zone (or higher radial force in case of AFM process),he abrasive penetration depth in the workpiece also proportion-tely increases (say, hs in Figs. 14(d) and 15). With the increasedepth of penetration of an abrasive particle inside the workpieceurface, the projected area (=Ap) accordingly increases hence theequired force (Freq = Ap × �s) also increases to the extent that thevailable cutting force (Fc) to remove the material in the form ofNC becomes smaller than the required cutting force (i.e. no cut-

ing condition). Now, one of the two events can take place. Eitherhe abrasive particle remains stagnant in this condition (withoutoing any cutting) unless favorable conditions prevail, or it slightlyoves upward (or rotates opposite to the penetration direction)

uch that the reduced penetration depth (h′s in Figs. 14(c) and 15)

eads to the condition of cutting (Freq = A′p × �s < Fc).

With the reduced projected area (A′p), the required cutting force

ecreases. Under certain conditions of finishing, the event that thearticle remains embedded in the surface also has equal probabilityf happening Fig. 6(c). As a result of this, although the penetra-ion depth is increasing due to higher extrusion pressure (or dueo higher magnetic field intensity) but Ra value is not changingFig. 16) substantially because Fc becomes smaller than the Freq.non-uniformity existing in the strength of a FMAB, or that of dif-

erent bunches of iron particles holding abrasive particles (MRF and
RAFF), is also practicable leading to the situation of Fig. 14(c). It
an also explain the nature of Fig. 16 as follows. A slight decreasen the strength of a part of the FMAB (MAF), or a bunch of the ironarticles holding abrasive particle in MRAFF or the medium holdinghe abrasive particle in case of (AFM), (or stiffened in case of MRF (or

Fig. 16. Effect of radial/indentation force in AFM, on Ra (all other conditions remainunchanged including cutting force, Fc).

Fig. 17. (a) Schematic view of plane MAF with a slotted pole and (b) solution domain for plane MAF with slotted pole.


F e fromw of MAw al., 20

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this weakness of S-FMAB, the experiments were conducted usingpulsed power supply to the electromagnet. Fig. 21(b) and (c) showsthe texture obtained by the use of P-FMAB. The surface is moreuniform and has lower Ra value. However, when using 0.4 duty

ig. 18. (a) Variation of normal force on a single abrasive particle with distancith finishing time (working gap = 4 mm, current = 1.12 A, voltage = 15 V, diameterorkpiece = 85 mm × 85 mm × 8 mm, initial Ra = 1.5 �m) (Jayswal, 2005; Jayswal et

RAFF)), force R′ will slightly move the particle upward resultingn the reduction in depth of penetration from hs to h′

s (Fig. 14(c)).ig. 14(c) also shows that if the force Fmn decreases to F ′

mn, then theepth of penetration decreases by an amount equal to hs − h′

s = hnd cutting restarts.

.3. Slot in the electromagnet used in MAF

As shown in Fig. 17(a), electromagnet used in MAF process has alot in it. An attempt has been made to analyze merits and demer-ts of the presence of the slot, in term of variation in forces. Thisnalysis has been carried out using finite element method (FEM).ig. 17(b) shows the solution domain for the plane MAF with a slotn the magnetic pole (Jayswal, 2005; Jayswal et al., 2005a,b).

As shown in Fig. 18(a), the normal magnetic force (Fmn) becomesegative under the slot while under the edge of the slot, the forceecomes larger than that under the flat area of the magnet. Whenhe force becomes negative, there will be ‘no penetration’ by thebrasive particle in the workpiece under the slot but the abrasivearticles near the slot edge will have larger depth of penetrationhan the flat area because at the boundary of the magnet, the mag-etic force (Fmn) value is very large for all the three cases of theagnets (with no slot, with one slot, with two slots) (Fig. 18(a)).

his increase in Fmn at discontinuities of the magnet leads to anncreased finishing rate as well as lowered Ra value until the crit-cal surface roughness is achieved. This is possible only when the

orkpiece is given feed in X and Y directions to finish the wholeorkpiece surface. This characteristic can be seen in the Fig. 18(b)here a comparison is made between the performance of a magnetith a slot and without a slot. The magnet with a slot gives better

urface finish than the magnet without a slot.

.4. Methods of energizing FMAB in MAF

The electromagnet in MAF is usually energized by the DC powerupply (the finishing process is called as DC-MAF) during experi-entation. However, keeping in view the limitations of the DC-MAF

rocess, the attempt was made to investigate this process withulsed power supply to the electromagnet (the process is calleds PC-MAF). DC-MAF results in the static flexible magnetic abra-ive brush (S-FMAB) while PC-MAF results in a pulsating flexibleagnetic abrasive brush (P-FMAB) (Fig. 19).

the centre of the tool (with one and two slots), and (b) variation of Ra valueP = 100 �m, rotation speed of N-pole = 200 RPM, power input = 2.2 kW, size of the04).

The MAF experiments were conducted (Singh et al., 2005a,b)using initial ground surface (Fig. 20(a)), with DC power supply with-out feed to the work piece (Fig. 20(b)) and with feed (in X and Ydirections) to the work piece (Fig. 20(c)). These atomic force micro-graphs clearly indicate that the ground surface has high peaks andvalleys resulting in surface roughness value of 500 nm or so. Whenthis surface is finished by S-FMAB without feed to the workpiece,the average surface roughness value is reduced to 100–200 nm ascan be seen in Fig. 20(b). At some places it has high peaks while atother places it has low peaks or good surface finish. The only rea-son which seems feasible to explain this characteristic is that theS-FMAB may be having non-uniform strength because of inhomo-geneous mixing of iron and abrasive particles. When the workpieceis given feed (X and Y) relative to the magnet, such high peaks getsheared off when they interact with the strong part of the S-FMAB.It is quite obvious while comparing Fig. 20(b) and (c) (Singh, 2005).

The performance of S-FMAB (DC-MAF) was further investigatedin terms of the texture of the finished surface. Fig. 21(a) shows thesurface texture obtained by the use of S-FMAB. Some randomly dis-tributed peaks are clearly visible which are not desirable. Thesehigh peaks show mainly high noise level and to a small extentnon-uniformity in the finished surface also. To find the solution to

Fig. 19. Methods of energizing FMAB.


Fig. 20. Surface produced by surface grinding and S-FMAB. (a) initial surface topography obtained by grinding, (b) surface texture obtained by S-FMAB without feed to thework (current = 0.88 A, working gap = 1.25 mm, RPM = 90, lubricant = 2%, time = 30 min), (c) surface texture obtained by S-FMAB with feed to the workpiece (current = 0.75 A,working gap = 1.75 mm, grain mesh size = 800, number of cycles = 9).

Fig. 21. Comparison of surface produced by S-FMAB and P-FMAB (a) lays obtained by S-FMAB (duty cycle, � = 1.0, t = 45 min), (b) lays obtained by P-FMAB (� = 0.4, ton = 2.0 ms)(Some grinding marks are left on the finished surface), and (c) lays obtained by P-FMAB (� = 0.08, ton = 2.0 ms) (no grinding marks).


00, fin

caa

F0sHT(tFF1ttc

(flftTsHaopaofl

Fg

Fig. 22. Problems at very low duty cycle in PC-MAF (gap = 1.5 mm, RPM = 2

ycle, some grinding marks are left on the finished surface whichre removed when still lower duty cycle (=0.08) is used (Singh etl., 2005b).

Atomic force microscopy of the work pieces finished with P-MAB are further examined. It is found that some pieces with.08 duty cycle have deep pits (Fig. 22(a)), and deep scratches ashown in (Fig. 22(b)) which are not found at higher duty cycles.ence, an attempt is made to further investigate for its reason.wo components force dynamometer is designed and fabricatedSingh, 2005; Singh et al., 2006). The normal magnetic force andhe tangential cutting force are measured in case of DC-MAF (S-MAB) and PC-MAF (P-FMAB). In case of S-FMAB, the variation inc is within approximately 4 N and in Fmn it is approximately within0 N (Fig. 23(a)). In the case of P-FMAB, Fc does not change substan-ially, i.e. minimal fluctuation is seen (because it is originated fromhe motor power used for rotating the magnet whose power is nothanged during the process as can be seen in Fig. 23(b)).

On the other hand, Fmn fluctuates between 1000 N and 3000 NFig. 23(b)) in case of PC-MAF. The high magnitude of fluctuation inorce Fmn leads to such kind of deep scratches and pits. Hence, veryow duty cycle is also not recommended to get the defect free sur-aces. However, the question arises why such a high fluctuation inhe magnetic normal force (Fmn) occurs? It is explained as follows.he magnetic force is directly dependent on the magnetic flux den-ity which depends on the current supplied to the electromagnet.ence, current versus time variation during PC-MAF is recorded,nd it is found that during a voltage pulse current varies by a factor
f approximately three (Fig. 24(a)). Secondly, even during the offeriod the current never attains zero value due to the induced volt-ge as can be seen in Fig. 24(a). During PC-MAF, at the beginningf the voltage pulse the FMAB is quite strong and a large magneticux density is produced (Fig. 24(b, i)). But, during the off period, the
ig. 23. (a) Force variation with time in S-FMAB (current = 0.75 A, working gap = 1.25 mm,ap = 1.50 mm, on-time = 2.0 ms, RPM = 200).

ishing time = 15 min). (a) � = 0.08, ton = 2 ms, and (b) � = 0.16, ton = 1.5 ms.

FMAB becomes weak and due to lower strength it starts breaking(Fig. 24(b, ii)) till the on-time of the next voltage pulse restarts.

This making and breaking of the P-FMAB results in intermixingof the used and unused abrasive particles leading to a more uniformfinished surface and higher finishing rate as compared to S-FMAB.That is why in some cases finishing rate during P-FMAB is as highas three times that of S-FMAB.

3.5. Texture and Measurement of Finished Surface

During the MNF process, one may get the same surface rough-ness value but different surface texture which is important fromwear point of view when the finished part is put in assembly.Fig. 25(a) shows an Atomic Force Micrograph of a piece fin-ished by MRAFF process using SiC abrasive particles which gaveRa = 0.10 �m. It gave quite distorted surface texture. The same piecewas further finished using the medium having diamond particlesas abrasives (Jha, 2006). Then the finished surface was analyzedunder the microscope and the surface texture obtained is shownin Fig. 25(b). It had the same surface roughness value (Ra = 0.10) asin the previous case when it was finished by SiC. To some extentRq and Ry values are changed. This texture in Fig. 25(b) would def-initely increase the product life against wear and tear as comparedto the surface texture shown in Fig. 25(a). Hence, surface textureshould also be examined from wear and tear of the part point ofview, rather than surface roughness values alone.

Fig. 26(a) shows SEM photograph after 30 cycles of MRAFF.
Fig. 26(b) schematically shows that the measured surface rough-ness Ra value also depends upon where it is being measured. If,say, it is measured with 4 mm measurement length, and the valleycomes under this measurement length then it gives the Ra value as0.07 �m (Fig. 26(c)). On the same workpiece, if it is measured away
RPM = 180), and (b) force variation with time in P-FMAB (duty cycle = 0.08, working


Fig. 24. (a) Variation of pulse current and pulse voltage with time during PC-MAF. (b) (i) Formation of FMAB during on-time, and (b) (ii)partial breaking/falling of FMABduring off-time of a voltage pulse (Singh, 2005).

Fig. 25. Initial surface finish (Ra: 0.28 �m). (a) Finished surface after MRAFF for 2000 cycles with SiC abrasive (Ra: 0.10 �m), and (b) finished surface after MRAFF withdiamond abrasive (Ra: 0.10 �m).

Fig. 26. Preliminary experimentation. (a) SEM micrograph of a finished work piece surface, (b) effect of cut-off length during the measurement of roughness and location ofvalleys, (c) intermediate roughness profile, (d) final roughness profile. (Jha, 2006).

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rom this point where the valley does not appear in this measure-ent it gives a better Ra value (=0.04). Hence, the measurement of

a value at one or two points does not give a clear picture about theurface texture of the finished part (Fig. 26(d)).

. Conclusions

From the presented experimental results and analysis, followingonclusions can be drawn:

Each of the nano-finishing process is characterized by its ultimate(or critical) surface finish (normally varies in a small band) whichcan be produced by that process, and it depends on the finishingconditions used during the process.During PC-MAF, use of low duty cycle is recommended but thesurface integrity should be carefully examined for the defectssuch as deep scratches, pits, etc. Increase in magnetic flux densityalone does not give higher finishing rate unless cutting force ishigh enough to remove material.A single slot in the magnet during MAF gives higher finishing rateas compared to the magnet without any slot.Knowledge of only surface roughness is not enough. Surfaceroughness plots should be examined carefully. Surface textureis also important from the wear and tear of the product point ofview.

cknowledgements

Author acknowledges the invitation by Prof. Ian Hutching, Chair-an of 1st International Conference on Abrasive Based Processes,

o give an invited talk on “Abrasive Based Micro-/Nano-Finishingechniques—An overview”. The author sincerely thanks Mr. Ajayidpara, Ph.D. Scholar of Mechanical Engineering Department, IITanpur for his help in the preparation of this manuscript. Thanksre also due to Dr. D.K. Singh of M.M.M. Engg. College, Gorakhpur;r. Sunil Jha of I.I.T. Delhi for sharing their results for the presentanuscript. This paper’s contents were presented on 22 September

008 at Churchill College, Cambridge (UK).

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