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Precision Engineering 36 (2012) 424– 434

Contents lists available at SciVerse ScienceDirect

Precision Engineering

j o ur nal homep age: www.elsev ier .com/ locate /prec is ion

xperimental characterization of hydrodynamic nanopolishing of flat steel plates

rinal Joshia, Sudhir Morea, Ramesh K. Singha,∗, Suhas S. Joshia, R. Balasubramaniamb, V.K. Surib

Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai, IndiaPrecision Engineering Section, Bhabha Atomic Research Center, Trombay, India

r t i c l e i n f o

rticle history:eceived 7 January 2011eceived in revised form8 December 2011ccepted 19 January 2012vailable online 28 January 2012

eywords:icro-scale abrasion

lurry erosionther surface engineering processes

a b s t r a c t

Nanoscale polishing finds applications in medical, industrial, telecommunication, optics, electronic andmilitary fields. Typically, rigid tool-based methods such as diamond turning, grinding and honing areemployed for nanoscale polishing. These methods have inherent limitations in creating nanopolishedsurfaces on hard and profiled surfaces. To address the issue, this work is focused on experimental inves-tigation of hydrodynamic polishing (HDP) as a nanopolishing method. The soft rubber tool and theworkpiece are submerged in a slurry during hydrodynamic nanopolishing. An elastohydrodynamic filmis formed between the tool and the workpiece due to the tool rotation which is responsible for nanopol-ishing. A HDP experimental setup was fabricated and experiments were conducted on oil hardened andnon-shrinking steel (OHNS, 58-62 HRC) with colloidal alumina suspensions of different particle sizes. Theexperiments were designed using Taguchi techniques to study the effect of four main factors (contact

load, tool stiffness, spindle speed and abrasive particle size) and three important two-factor interactionsat four different polishing times. Statistical analysis of the results shows that concentration of abrasivein the slurry is a significant factor in the hydrodynamic polishing. The best surface finish of 3.5 nm wasobtained using 1 �m abrasive particle size colloidal suspension at 7.5 N load, 2400 rpm spindle speed, 90shore A tool stiffness and 3 min of polishing time. The change in surface morphology and topography dueto polishing also confirm the efficacy of the HDP process.

© 2012 Elsevier Inc. All rights reserved.

. Introduction

Nanofinished surfaces find wide applications in the areasf biomedical devices, telecommunication appliances, precisionptics and electronics. Nanometric surface finish is a functionalequirement for some of the applications in these fields. Theolished area is small and local, and often times the shape is compli-ated which necessitates use of special nanofinishing techniques.se of conventional superfinishing processes, such as diamond

urning, precision grinding, honing, hard lapping and magneticbrasive finishing [1–3] for such applications have inherent lim-tations, especially when surfaces are very hard and bear variousrofiles. The typical requirements of the conventional superfinish-

ng are: the machine resolution should be much smaller than theesired finish, the machining method should be able to conform tohe form of the profiled surface, and finally, the machining process

ust be deterministic. The effect of random disturbances on theachine precision should be at least an order of magnitude lower

han the required precision. The methods such as diamond turning

∗ Corresponding author. Tel.: +91 22 25767507; fax: +91 22 25726875.E-mail addresses: ramesh@me.iitb.ac.in, rsingh@iitb.ac.in,

ameshksingh@gmail.com (R.K. Singh).

141-6359/$ – see front matter © 2012 Elsevier Inc. All rights reserved.oi:10.1016/j.precisioneng.2012.01.004

and precision grinding are deterministic at sub-micron level fin-ishing but have limitations of structural stiffness, closed loop servocontrols, cutting vibrations, machining resolution and other miscel-laneous disturbances [4]. It should also be noted that the machiningresolution in a plastic deformation based process cannot be lessthan the spatial volume of a dislocation [5].

Other important superfinishing method used in semiconductorand photovoltaic industry for silicon wafer polishing is chemicalmechanical planarization (CMP). Although CMP is primarily usedfor silicon but it can also be used for other materials [6]. In anotherstudy by Chandra et al. [7], it was observed that the fractal dimen-sion of the slurry agglomeration process played an important rolein determining the scratch generation propensity in CMP of siliconwafer. The scratch generation significantly influenced the chemicaletching in the CMP process. Although CMP is capable of achievingnanofinished surfaces, it is limited to processing planar surfaces.

For an ideal nanopolishing process, the method should be inher-ently a diminutive volume removing process. In the case of abrasiveslurry-based process, every abrasive particle should in principle beable to create atomic or nanometric surfaces [8]. In addition, the

vibration between tool and work surface should not be transferredonto the work surface. Note that the machining rate of such processat a specific spot is governed by the machining capability of a par-ticle and the number of particles at that spot. For a given particle

ngineering 36 (2012) 424– 434 425

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M. Joshi et al. / Precision E

istribution and work surface combination, the machining capabil-ty is determined by the contact load and the relative speed betweenarticle and work surface. To facilitate deterministic machiningehavior, the load, the particle speed and particle distribution muste controlled accurately during the process. Hydrodynamic pol-

shing (HDP) or elastic emission machining process has all suchesired characteristics [4,5,8]. It can also machine any profiled sur-ace by controlling the total machining time and the tool motion4]. The HDP process has been investigated by a few researchers;hey have worked on analyzing the effect of parameters like initialurface roughness [9], surface profiles [10,11], tool wear [12], mate-ial removal rate [13,14]. It appears that out of many factors thatnfluence the process, the factors like polishing load, stiffness of theolishing tool and time dependence of the polishing process haveot been adequately investigated through the specifically designedxperiments. The objective of this paper is to carry out extensivenvestigation on the HDP process so that the role of the above fac-ors and their contribution to the process of nano-finished surfaceeneration is illustrated. The preliminary experiments to explorehe HDP process feasibility were conducted by Joshi et al. [15],hich showed that the process was feasible and the abrasive parti-

le size affected the process significantly. This information helpedn selecting the process parameters and their levels for the exper-mental design presented in this paper. The experimental designsed in the present work is Taguchi method-based L27 orthogonalrray. The experimental factors affecting the surface roughness areontact load, tool stiffness, spindle speed and abrasive particle size.n addition, the surface roughness values were measured at four dif-erent time intervals to quantify the effect of polishing time. Thisaper is focused on detailed experimental characterization of HDProcess on oil hardened non-shrinking steel (OHNS, 58-62 HRC)late to investigate the effect of input parameters on the processesponse, i.e., average surface roughness, Ra. High resolution scan-ing electron micrographs and surface topography images are usedo characterize the change in the surface morphology due to theydrodynamic nanopolishing process.

. HDP mechanism

Hydrodynamic polishing setup has a rotating tool made of softubber. Both the tool and the workpiece are submerged in a poolf slurry. The slurry should ideally be a suspension of abrasive

Fig. 2. Picture of the fabricated experimental setup for the

Fig. 1. Schematic presentation of the HDP process.

particles in a viscous fluid. During machining, the soft deformabletool applies light load onto the surface of the workpiece and con-forms to the workpiece shape. The tool is typically made of a lowelastic modulus material, such as polyurethane and silicone rubber.A schematic of the setup is shown in Fig. 1.

When the tool rotates, an elastohydrodynamic film is formedbetween the tool and the workpiece. The material removal rateof this process is governed by the shear stress of slurry flow. Thematerial is removed through an adhesive mechanism if the abra-sive particles are much smaller than the elastohydrodynamic film.When a particle comes into contact with the work surface, an inter-facial bonding between particle and surface atoms can occur. If thisbonding is stronger than the bonding between the surface layerand the sub-surface layer atoms, the surface atoms may be removedwhen the particle flows out [4,5]. In principle, this process can yieldsub-nanometer surface roughness.

3. Experimental work

3.1. Nanopolishing setup design and fabrication

Fig. 2 shows a picture of the experimental setup fabricated forhydrodynamic polishing. The set-up used in this work consists of

stacked X–Y stages and a decoupled Z-stage. These stages can pro-vide coordinated 3-axis motion. The positioning resolution of thestage is 0.5 �m. The stage has a travel of 100 mm and can withstand250 N axial force. The maximum translational speed of the stage is

study of HDP process (a) in process (b) closer view.

426 M. Joshi et al. / Precision Engineering 36 (2012) 424– 434

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fore, total dof for this problem is 26, which includes 6 dof for theerror. Consequently, L27 type of OA has been chosen for this set ofexperiments which can accommodate 26 degrees of freedom.

Table 1Factors and their levels.

Factors Levels

1 2 3

Fig. 3. Logic for se

estricted to 100 mm/s. A rotating spindle is mounted on the Z-xis and the work-piece holding tank containing abrasive slurry isounted on the X–Y stacked stages.The workpiece is fixed inside an acrylic tank filled with abrasive

lurry. This tank in turn is mounted on a dynamometer, which isxed to table-top of X–Y stacked stages. The stacked X–Y stages are

ocated away from the Z-axis, hence, a spacer is required to align theolishing tool to the workpiece. A miniature electric motor spindle

s mounted on an aluminum matching plate attached to a spacermade of an aluminum C-section) fastened to the Z-stage. A con-roller has been fabricated to provide variable voltage supply to the

otor for controlling the rotational speed of the spindle. Soft sili-one rubber cylindrical tool with a 15 mm diameter hemisphericalorking end was used in this study. The spindle can be mounted at

ny inclination between 0◦ and 90◦ to the Z-axis. The spindle wasxed at an angle of 45◦ because this orientation yielded the bestesults in the preliminary experiments [15].

.2. Experimental design

Using the process knowledge gained from the preliminaryxperiments, a Taguchi method-based experiment was designednd performed to determine the effect of process parameters onhe process response, i.e., average surface roughness (Ra). Taguchi

ethod is a design of experiments wherein orthogonal arraysre used to study the main and interaction effects. These designsequire fewer runs as compared to a full factorial design of experi-ents. This design uses a priori information about the interaction

f the factors. Consequently, in addition to the main effects, onlyhe relevant interactions can be studied. The major steps in Taguchixperimental design are: selection of response, selection of inde-endent input variables and their interaction effects. Based onhis information, an appropriate orthogonal array (OA) could beelected, following which, the effect of main factors and their inter-ction could be analyzed via analysis of means (ANOM) [16].

.2.1. Selection of independent variables and interactionsThe independent variables considered in this investigation are

f three types: (1) variables related to process conditions, (2) vari-bles related to abrasive slurry and (3) variables related to thetiffness of tool. It is known that the applied load and spindle speedre the major polishing parameters, which influence the surfacenish by affecting the polishing pressure and the hydrodynamiclm thickness [4]. It is believed that the abrasive particle size has a

ignificant influence on the process as it affects the elastohydrody-amic lubrication and consequently, the applied shear stress on theork surface. In addition, the stiffness of the tool was considered

s a parameter of study as it could affect the polishing pressure in

n of interactions.

the hydrodynamic film. Thus, four variables, viz. applied load, spin-dle speed, tool stiffness and abrasive particle size were chosen asindependent variables for this experimentation.

The interaction between the independent variables could influ-ence the contact area, contact pressure and hydrodynamic filmvelocity which in turn could affect the shear stress and its dis-tribution as shown in Fig. 3. Hence, three two-factor interactions,viz.—load and spindle speed, load and stiffness, and load and abra-sive size were finally selected. The higher order interactions aregenerally not significant in engineering applications and hencewere neglected [16].

3.2.2. Selection of levels for the independent variablesTable 1 shows the four experimental factors and the three levels

for each factor. These levels are coded from 1 to 3, 1 being the lowestvalue and 3 being the highest. The tool stiffnesses were kept atvarying degrees of softness from soft (50 shore A) to hard (90 shoreA). Typical spindle speeds used in HDP reported in the literature arearound 2500 rpm. To study the effect of spindle speed, the lowestlevel was chosen as 1000 rpm and the highest as 3600 rpm. Loadwas kept at three levels: 1 N, 4.5 N and 7.5 N. Below 1 N load, thepolished area was not sufficient to study the process. Above the7.5 N load, the process would lead to high vibrations, instabilityin set-up causing early dugout on the work surface. Three differentcolloidal alumina suspensions with different abrasive particle sizesof 0.05 �m, 0.3 �m and 1 �m were used.

3.2.3. Selection of an OAIn this investigation, an L27 OA [16] as shown in Table 2 has been

selected. Here, four columns have been assigned to independentvariables and the remaining six columns to the two-factor interac-tions. A three-level factor and its interactions with other three-levelfactors have 2 and 4 degrees of freedom (dof), respectively. There-

Tool stiffness (shore A) 50 75 90Spindle speed (rpm) 1000 2400 3600Load (N) 1 4.5 7.5Abrasive size (�m) 0.05 0.3 1

M. Joshi et al. / Precision Engineering 36 (2012) 424– 434 427

Table 2L27 orthogonal array with assigned factors and response variable.

Assignment of columns to control factors and their interactions Responses of trials

1 2 3 4 5 6 7 8 9 10 Roughness, Ra �m (Polishing time, min)

Expt. run Load (L) Abrasivesize (A)

L × A Stiffness(S)

L × S Spindlespeed (SS)

L × SS T(3) T(13) T(23) T(35)

1 1 1 1 1 1 1 1 1 1 1 0.0101 0.0077 0.0092 0.0062 1 1 1 1 2 2 2 2 2 2 0.0064 0.0064 0.007 0.013 1 1 1 1 3 3 3 3 3 3 0.0071 0.0097 0.0065 0.0074 1 2 2 2 1 1 1 2 2 2 0.0069 0.01 0.007 0.0085 1 2 2 2 2 2 2 3 3 3 0.0081 0.0086 0.0093 0.0066 1 2 2 2 3 3 3 1 1 1 0.006 0.004 0.007 0.0077 1 3 3 3 1 1 1 3 3 3 0.0061 0.007 0.007 0.0078 1 3 3 3 2 2 2 1 1 1 0.008 0.01 0.0108 0.0119 1 3 3 3 3 3 3 2 2 2 0.005 0.0082 0.007 0.00810 2 1 2 3 1 2 3 1 2 3 0.0082 0.009 0.0074 0.00811 2 1 2 3 2 3 1 2 3 1 0.006 0.0056 0.0101 0.01212 2 1 2 3 3 1 2 3 1 2 0.0107 0.011 0.0115 0.01213 2 2 3 1 1 2 3 2 3 1 0.0059 0.007 0.0066 0.00914 2 2 3 1 2 3 1 3 1 2 0.0052 0.007 0.009 0.0115 2 2 3 1 3 1 2 1 2 3 0.004 0.008 0.0098 0.00816 2 3 1 2 1 2 3 3 1 2 0.012 0.0088 0.008 0.00917 2 3 1 2 2 3 1 1 2 3 0.007 0.009 0.0064 0.00718 2 3 1 2 3 1 2 2 3 1 0.0077 0.0065 0.0077 0.00419 3 1 3 2 1 3 2 1 3 2 0.011 0.0113 0.0116 0.00820 3 1 3 2 2 1 3 2 1 3 0.0078 0.0071 0.0088 0.01721 3 1 3 2 3 2 1 3 2 1 0.014 0.0157 0.0194 0.01922 3 2 1 3 1 3 2 2 1 3 0.0083 0.0044 0.0147 0.00923 3 2 1 3 2 1 3 3 2 1 0.0064 0.0081 0.012 0.01324 3 2 1 3 3 2 1 1 3 2 0.0085 0.0064 0.0094 0.010

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.2.4. Selection of response variablesIn this study, average surface roughness (Ra) is chosen as the

esponse variable. It was observed from the study of preliminaryxperiments that the time of polishing influences the process andence response was measured at four different time instances dur-

ng polishing. The minimum time of polishing was chosen as 3 min,here a significant effect of polishing could be noticed. It was

bserved in the preliminary experiments that for 0.3 �m abra-ive particle size, a dugout is formed in the polishing area withn increase in time of polishing. Hence, the maximum time of pol-shing was chosen as 35 min, where a dugout could be expected inll the runs of the experimental design.

.3. Experimental procedure

The ONHS (60-65 HRC) plate is polished in a colloidal alumina.he initial roughness of the unpolished samples was of 0.11 �m.

soft silicone tool (with varying degree of softness) with a hemi-pherical end was used for polishing. The setup was equipped toeasure forces using a three-component dynamometer (Kistlerini-dyne® 9256C2). In these experiments, the Z-direction forceas applied and monitored by the dynamometer. The Y-direction

s in the plane of workpiece and in the direction of motion of thelurry in which polishing action takes place and the X-direction iserpendicular to this direction in the plane of workpiece as shown

n Fig. 2.The surface roughness values were measured using a Taylor

obson surface profilometer (Form Tallysurf 120 L) having a sub-anometer resolution. The length of the recorded profile was 1 mm.otion to the Z-axis was provided through the controller by steps

f micrometers to apply the desired load and was locked in position

nce the desired load was reached. The spindle speed of the motoras controlled through a variable voltage power supply. The factorsere set to the desired levels according to the L27 orthogonal array

hown in Table 2. At each run, four different specimens were used

3 2 1 0.0048 0.0063 0.0072 0.0081 3 2 0.0128 0.0085 0.0068 0.0052 1 3 0.0035 0.0058 0.004 0.008

to perform experiments for four different polishing times. Polishingwas done on the first specimen for 3 min, on the second specimenfor 13 min, on the third specimen for 23 min and on the fourth spec-imen for 35 min. Hence, experiments were conducted on a total of108 test specimens.

4. Results and discussions

4.1. Results from polishing experiments

The results of these experiments are analyzed using normalprobability plot (NPP) and analysis of means plots. The NPP identi-fies the factors that could significantly affect the response variable,whereas, the ANOM shows the mean effect of the various levels of agiven factor on the response. The measured values of surface rough-ness of the polished surfaces corresponding to all the experimentalruns for different times of polishing are given in Table 2.

The normal probability plot is a plot between the residuals andthe percentage probabilities. The residual is the difference betweenthe values predicted by a regression model obtained by fitting thedata and the experimentally observed values. Note that if the nor-mal probability plots are linear, then it can be construed that thedata is normally distributed. However, the presence of outliers indi-cates that these are these points are influential observations and ifremoved from this would significantly alter the statistical analysis.These points are typically known as leverage points as they havethe potential to influence the results.

The NPP of various process parameters for different polishingtimes are shown in Fig. 4(a)–(d). It is evident from these plots thatduring the first three stages of polishing, i.e., for 3, 13 and 23 min

of polishing, only the abrasive particle size appears to be the out-lier. Consequently, it can be inferred that it is the most influentialfactor on the average surface roughness, Ra. On the other hand, fora polishing time of 35 min as shown in Fig. 4(d), the NPP does not

428 M. Joshi et al. / Precision Engineering 36 (2012) 424– 434

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Fig. 4. NPP plots for major effects (a) 3 min polishing time (b) 13

how any outliers indicating that the process does not have anyignificant single parameter.

.2. Analysis of means for main factor effects

The next few sections present the effects of main factors studiedn the L27 OA at different polishing times.

.2.1. Effect of applied load on average surface roughness, Ra

Fig. 5 shows the plot of effect of load on the surface finish. Ashe load increases from 1 N to 4.5 N, there is no significant differ-nce in the roughness, however as the load increases to 7.5 N, theoughness of the polished surface increases from 7 nm to 8.5 nm.

similar effect is obtained for 13 min of polishing time as seenn Fig. 5. As the load increases from 1 N to 7.5 N, the roughnessf the polished surface increases by 43% from 7.5 nm to 10.7 nmfter 23 min of polishing time. After 35 min of polishing, the sur-ace roughness increases by 55% from 7.7 nm to 11 nm when the

oad is increased from 1 N to 7.5 N.

This shows that with an increase in polishing time, the effect ofoad (i.e., increase in the surface roughness with an increase in theoad) becomes more prominent. This could be attributed to the fact

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Time(23 minutes)

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Residual

lishing time (c) 23 min polishing time (d) 35 min polishing time.

that an increase in the load increases the polishing pressure therebyincreasing the chances of scratches being made by the abrasive par-ticles. A similar effect is also observed with an increase in time ofpolishing and hence, a large increase in roughness is seen whenboth load and time of polishing are at the maximum value.

4.2.2. Effect of abrasive size on average surface roughness, Ra

Fig. 6 shows the plot of effect of abrasive size on surface finish,Ra at different polishing times. The surface roughness decreases by23% from 8 nm to 6.9 nm when the abrasive particle size changesfrom 0.05 �m to 0.3 �m. If the particle size is further increasedto 1 �m, the surface roughness increases by 12% from 6.9 nm to7.7 nm. A similar effect is seen after 13 min of polishing. The trendis different after 23 min and 35 min of polishing. It is observedthat unlike the previous two cases, the average surface roughnessgoes on decreasing with an increase in the abrasive size. It is mostpronounced after 35 min of polishing, where a decrease of 24% is

as shown in Fig. 6. It is evident that with an increase in polishingtime, the optimum value of the abrasive size changes from 0.3 �mto 1 �m. This could be attributed to the fact that with an increase

0

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Fig. 6. Effect of abrasive size on surface roughness, Ra, for different times of polish-ing.

M. Joshi et al. / Precision Engineering 36 (2012) 424– 434 429

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Spindle speed (rpm)

ig. 7. Effect of spindle speed on surface roughness, Ra, for different polishing times.

n polishing time, chances of scratches being made by the abrasivearticles increase.

The aforementioned results showing rougher surface finish withner slurry is counterintuitive. It is expected that this effect isue to the concentration of abrasives in the slurry. The colloidaluspension of abrasives used in these experiments had same con-entration of abrasives by weight but not by number of particlesthe weight/volume of all three suspensions are same). Equal con-entration of abrasives by weight for a given volume implies thathe number of 0.05 �m abrasive particles is much higher than theumber of 0.3 �m abrasive particles per unit volume of the suspen-ion and 1 �m slurry has the least number of abrasive particles pernit volume. Consequently, the smaller abrasive size slurry mayave a tendency to scratch the surface due to increased numberf abrasive particles. This tendency could be aggravated due tohanges in other parameters, such as load, spindle speed, stiffnessnd time of polishing.

Similar trend was observed in the preliminary experimentsherein the surface roughness increased after a certain polishing

ime for 0.3 �m abrasive particle size slurry but similar behav-or was not observed for 1 �m abrasive particle size slurry [13].n the current work, 0.05 �m abrasive particle size slurry hasven more abrasive particles and hence the surface roughnessould deteriorate further due to scratching in even lower polishingimes. Experimental corroboration of this hypothesis is observed inhe analysis of polished surface profiles. The experimental resultsevealed that 0.05 �m abrasive particle size slurry demonstratedcratches on the polished surfaces. Note that this phenomenon ofncrease in the surface roughness and scratches was observed with.3 �m abrasives only under aggravated process conditions, suchs, higher loads, higher spindle speeds and longer polishing time.

.2.3. Effect of spindle speed on average surface roughness, Ra

Fig. 7 shows the plot of effect of spindle speed on surface fin-sh for different polishing times. When the spindle speed increasesrom 1000 rpm to 2400 rpm, the surface roughness decreases by0% from 8.5 nm to 6.8 nm but as the speed is increased to 3600 rpm,he surface roughness increases by 22% from 6.8 nm to 8.3 nm after

min of polishing. A similar effect is seen after 13 min of polish-ng, a 13% decrease in Ra is observed if the speed is increased from000 rpm to 2400 rpm and an increase of 32% in Ra is obtained whenpeed is further increased to 3600 rpm. For 23 min of polishing time,

hen the spindle speed changes from 1000 rpm to 2400 rpm, the

urface roughness remains about the same but shows an increase of6% at 3600 rpm as shown in Fig. 7. Similarly, after 35 min of polish-

ng, the nature is different than the other three cases, Ra increases

Fig. 8. Effect of tool stiffness on surface roughness, Ra, for different polishing times.

when the spindle speed is between 1000 and 2400 rpm and thenremains relatively constant.

In the HDP process, the surface finish and the material removalrate (MRR) are largely dependent on the shear stress developed inthe abrasive slurry, which depends on the cutting velocities pro-vided to the abrasive particles through the spindle speeds. If thespindle speed is less, the shear stress developed is not sufficientfor polishing and when the spindle speed is more, the high shearstress provided to the abrasive particles leads to scratches therebyincreasing the roughness. It is observed that at smaller durationof polishing, the optimum value of spindle speed (correspondingto the smoothest surface) lies in the middle at 2400 rpm. With anincrease in polishing time, the optimum spindle speed shifts from2400 rpm to 1000 rpm.

4.2.4. Effect of stiffness on average surface roughness, Ra

Fig. 8 shows the plot of effect of stiffness on surface finish, Ra

for different polishing times. Note that there is some effect of thetool stiffness for the polishing times of 3 and 35 min whereas noappreciable effect of tool stiffness on surface roughness is observedafter 13 and 23 min of polishing. It can be inferred that tool stiffnessis not as important as other factors on the surface roughness.

After 3 min of polishing, a change in tool stiffness from 50 shoreA to 75 shore A results in a decrease of 11% from 8.7 nm to 7.7 nmin the surface roughness. The roughness is further decreased by6% to 7.2 nm when the stiffness is increased to 90 shore A. Thiscould be attributed to the fact that with lower stiffness, adequatepolishing pressure is not developed to get good surface finish atlower polishing time. For 13 min of polishing time, the sufficientpolishing pressure is developed even with lower stiffness to achievegood surface finish and the surface roughness increases by 10%when stiffness is increased to 90 shore A. For 23 min and 35 minof polishing times, no significant effect of stiffness is observed.

Table 3 summarizes the key effects of the process parameterson the average surface roughness and the plausible physical expla-nations of the observations.

4.3. Analysis of means for interaction effects

4.3.1. Effect of interaction of load and abrasive size on averagesurface roughness, Ra

Fig. 9 shows the plots of interaction effect of load and abrasivesize on the average surface roughness. The lowest effect of interac-

tion on the response variable is observed when the polishing loadis at the lowest level of 1 N. At the same time the highest level ofinteraction effect on the response variable is observed at the maxi-mum level of load at 7.5 N. Also, at the maximum level of load, both

430 M. Joshi et al. / Precision Engineering 36 (2012) 424– 434

Table 3Effect of the factors on average surface roughness in HDP process.

Factor Observed effect Physical explanation

Applied Load • With an increase in the polishing time, the effect of the loadbecomes more prominent. The increase in the surface roughnesswith an increase in the load becomes more pronounced as thepolishing time increases.

• This could be attributed to the fact that an increase in the loadincreases the polishing pressure thereby increasing the chances ofscratches being made by the abrasive particles on the polishedsurface.

Abrasive size • For 3 and 13 min of polishing times, the surface roughnessdecreases when the abrasive particle size changes from 0.05 �m to0.3 �m. If the particle size is further increased to 1 �m, the surfaceroughness increases.

• It is expected that this effect is due to concentration of abrasivesin the slurry. The colloidal suspension of abrasives used in theseexperiments had same concentration of abrasives by weight. Equalconcentration of abrasives by weight for a given volume impliesthat the number of 0.05 �m abrasive particles is much higher thanthe number of 0.3 �m abrasive particles per unit volume of thesuspension and 1 �m slurry has the least number of abrasiveparticles per unit volume. Consequently, the smaller abrasive sizeslurry may have a tendency to scratch the surface due to increasednumber of abrasive particles.

• The trend is different after 23 min and 35 min of polishing andthe average surface roughness goes on decreasing with an increasein abrasive size.• Thus, it can be inferred that with an increase in polishing time,the optimum value of the abrasive size changes from 0.3 �m to1 �m.

Spindle speed • For all the polishing times except at 35 min, the surfaceroughness decreases if the spindle speed increases from 1000 rpmto 2400 rpm. However, the surface roughness again increases if thespeed is increased to 3600 rpm.

• In the HDP process, the surface finish is largely dependent on theshear stress developed in the abrasive slurry, which depends onthe cutting velocities provided to the abrasive particles. If thespindle speed is less, the shear stress developed is not sufficient forpolishing and when the spindle speed is more, the high shearstress provided to the abrasive particles leads to scratches. Thisexplains the optimal response at the intermediate spindle speed.

Tool stiffness • No appreciable change in the surface roughness as a function oftool stiffness is observed after 13 and 23 min of polishing.Although, some variation is seen for the polishing times of 3 and

• The lower stiffness is expected to develop lower polishingpressures in the slurry, but, the levels of tool stiffness studied inthese experiments do not yield any significant difference.

tb

mbd

4s

soilt

35 min.

he best and the worst surface finish are observed. Exception to thisehavior is found for 3 min of polishing time.

The best surface finish occurs at the highest level of load andiddle level of abrasive size for 3 min of polishing time, but this

ehavior changes at 7.5 N load and the abrasive particle size of 1 �muring the later three stages of polishing.

.3.2. Effect of interaction of load and spindle speed on averageurface roughness, Ra

Fig. 10 shows the plots of interaction effect of load and spindlepeed on average surface roughness. Here as well, the lowest effect

f interaction on the response variable is observed when the polish-ng load is at the lowest level of 1 N. At the same time, the highestevel of interaction effect on the response variable is observed athe maximum level of load at 7.5 N.

3 Minute polishing time 13 Minute polishing time

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

n R

a (

m)

Load (N)

0.05 0.31

Abrasive size (um )

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

n R

a (

m)

Load (N)

0.05 0.31

Abrasive size (um)

Mea

n R

a (

m)

Fig. 9. Interaction effect plot of load and abrasi

As mentioned previously, 2400 rpm shows the best surface fin-ish for the initial phases of polishing. After 35 min of polishing, thebest finish is obtained for 3600 rpm spindle speed at 1 N load butfor 4.5 N load, 1000 rpm spindle speed gives best surface finish. Theworst surface finish is found with a combination of the maximumspindle speed of 3600 rpm and maximum load of 7.5 N for a pol-ishing time of 3 min. With an increase in polishing time, the worstsurface finish is observed for lowest spindle speed of 1000 rpm andmaximum load of 7.5 N.

4.3.3. Effect of interaction of load and stiffness on average surface

roughness, Ra

Fig. 11 shows the plot of interaction effect of load andtool stiffness on average surface roughness. It can be observedthat the interaction of tool stiffness and load on the response

23 Minute polishing time 35 Minute polishing time

0.005

0.007

0.009

0.011

0.013

0.015

4.51 7.5

Load (N)

0.05 0.31

Abrasive size (um)

0.006

0.008

0.01

0.012

0.014

0.016

4.51 7.5

Mea

nR

a (

m)

Load (N)

0.05 0.31

Abrasive size (um)

ve size for average surface roughness, Ra.

M. Joshi et al. / Precision Engineering 36 (2012) 424– 434 431

3 Minute polishing time 13 Minute polishing time 23 Minute polishing time 35 Minute polishing time

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

nR

a (

m)

Load (N)

1000 24003600

Spindle speed (rpm)

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

nR

a (

m)

Load (N)

1000 24003600

Spindle speed (rpm)

0.005

0.007

0.009

0.011

0.013

0.015

4.51 7.5

Mea

nR

a (

m)

Load (N)

1000 24003600

Spindle speed (rpm)

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

nR

a (

m)

Load (N)

1000 24003600

Spindle speed (rpm)

spindl

vbtlt

4

nIsiTds

Fig. 10. Interaction effect plot of load and

ariable is quite complex. One important observation, which cane made from the interaction effect plots shown in Fig. 11 ishat better surface finish obtained with 90 shore A tool at 1 Noad as opposed to 7.5 N load for 3, 13 and 23 min of polishingimes.

.4. Main factor effects on surface roughness, Rz

The main factor effect plots for ten point mean surface rough-ess, Rz, for different polishing times are shown in Fig. 12(a)–(d).

t can be observed that the trends of the factor effects for Rz are

imilar to those for Ra. Hence, the HDP process is equally affect-ng the larger and smaller peaks and valleys of the surface profile.herefore, an important conclusion can be made that HDP processoes not cause an increase in the Rz and hence it does not harm theurfaces, like inflicting micro-cracks in it.

3 Minute polishing time 13 Minute polishing time

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

nR

a (

m)

Load (N)

50 7590

Stiffness (shore A)

0.004

0.006

0.008

0.01

0.012

4.51 7.5

Mea

nR

a (

m)

Load (N)

50 7590

Stiffness (shore A)

Mea

nR

a (

m)

Fig. 11. Interaction effect plot of load and stif

e speed for average surface roughness, Ra.

4.5. Identification of optimal and worst-case process responsefrom Taguchi design

Based on the Taguchi design, a set of optimal polishing condi-tions corresponding to the lowest Ra values are given in Table 4. Theworst case condition is also presented in Table 4. The ideal processconditions for obtaining lowest surface roughness should be closerto optimal conditions and away from the worst case condition.

Fig. 13(a) and (b) shows the 3-D surface profile of the polishedarea of the specimens with best (0.0035 �m) and the worst surfacefinish (0.019 �m) respectively.

4.6. Characterization of surface morphology and topography

The surface morphology and topography were character-ized using scanning electron micrographs (obtained from JEOL

23 Minute polishing time 35 Minute polishing time

0.005

0.007

0.009

0.011

0.013

0.015

4.51 7.5

Load (N)

50 7590

Stiffness (shore A)

0.006

0.008

0.01

0.012

0.014

0.016

4.51 7.5

Mea

nR

a (

m)

Load (N)

50 7590

Stiffness (shore A)

fness for average surface roughness, Ra.

432 M. Joshi et al. / Precision Engineering 36 (2012) 424– 434

0.03

0.035

0.04

0.045

0.05

4.51 7.5

surf

ace

rou

gh

nes

s (µ

m)

Load (N)

3 13

23 35

Time (minutes)

0.025

0.03

0.035

0.04

0.045

0.05

0.05 0.3 1

surf

ace

rou

gh

nes

s (µ

m)

Abrasive size (µm)

3 13

23 35

Time (minutes)

0.03

0.035

0.04

0.045

0.05

50 75 90

Su

rface

rou

gh

nes

s (µ

m)

Stiffness (shore A)

3 13

23 35

Time (minutes)

0.025

0.03

0.035

0.04

0.045

0.05

1000 2400 3600

Su

rface

rou

gh

nes

s (µ

m)

Spindle speed (rpm)

3 13

23 35

Time (minutes)a b c d

Fig. 12. Effect of different parameters on surface roughness, Rz, (a) load (b) abrasive size (c) stiffness of tool (d) spindle speed.

Table 4Optimal and worst combination of parameters from the results of the Taguchi method-based designed experiment.

Factors

Load (N) Abrasive size (�m) Spindle speed (rpm) Stiffness (shore A) Polishing time (min) Average roughness, Ra (�m) Remarks

7.5 1 2400 90 3 0.0035 Optimum4.5 0.3 2400 50 3 0.004 Optimum4.5 1 2400 90 35 0.0042 Optimum

Jfmpt

Fs

1 0.3 1000 90

7.5 0.3 2400 50

7.5 0.05 3600 90

SM-840A-40A) and white light interferometery images (obtainedrom WYKO NT 9100), respectively. Fig. 14 shows the surface

orphology and the topography before and after polishing. Theolishing was carried out at a load of 1 N, abrasive size of 0.3 �m,

ool stiffness of 75 shore A and a spindle speed of 1000 rpm.

It is evident from the scanning electron micrographs (seeig. 14(a) and (b)) that the surface prior to polishing exhibits apecific grinding texture and sporadic scratches accompanied by

Fig. 13. 3-D Surface profile of specimen with (a) be

13 0.004 Optimum13 0.0044 Optimum35 0.0193 Worst

microscopic burrs. On the other hand, the hydrodynamic nanopo-lished surface clearly shows that the grinding texture, scratchesand associated micro-burrs have been eliminated. There is evi-dence of occasional pitting but altogether surface finish appears

to have improved significantly. This is further validated by thesurface topography plots obtained from the white light interfer-ometer (see Fig. 14(c) and (d)) which show that the 3-D averageroughness is reduced from 100 nm to 13 nm. These results show

st surface finish and (b) worst surface finish.

M. Joshi et al. / Precision Engineering 36 (2012) 424– 434 433

face;

ts

5

foauff

1

2

3

4

Fig. 14. Scanning electron micrographs: (a) unpolished surface (b) polished sur

hat the HDP is an effective process for creating nanofinishedurfaces.

. Conclusions

The research work was focused on identifying a suitable processor polishing and finding the optimal parameters for nanopolishingf hardened steel plates. Hydrodynamic polishing was identifieds a suitable nanofinishing process. Taguchi method-based designsing L27 orthogonal array was used for conducting experiments atour different polishing times. Based on the experimental analysis,ollowing specific conclusions can be drawn from this study.

. HDP has the capability to produce nanofinished surfaces on hardmaterials and the best Ra value obtained was 3.5 nm.

. The normal probability plots for different polishing timesrevealed that concentration of abrasive is the most significantfactor controlling the HDP process.

. Surface roughness increases with an increase in load. At ini-tial stages of polishing, an increase in the surface roughness ofabout 15% was observed with an increase in the load from 1 Nto 7.5 N. At later stages of polishing, for same increase in load,an increase in the surface roughness of about 55% was observed.A plausible explanation for this behavior is that the increase inthe polishing pressure in the presence of concentrated abrasiveslurry increases shear stresses in the hydrodynamic film whichcould potentialy result in the formation of scratches and dugout

on the surface.

. Optimum spindle speed is found to be 2400 rpm for earlystages of polishing (till 23 min of polishing time) which shiftsto 1000 rpm for the last stage (after 35 min) of polishing.

White light interferometry images: (c) unpolished surface (b) polished surface.

5. Slurry containing abrasive particles of 0.3 �m gave the best sur-face finish for lower polishing times (till 13 min of polishingtime) and slurry containing abrasive particles of 1 �m abra-sive gave the best surface finish for more polishing times (after35 min).

6. Surface roughness was found to be the worst when the parame-ters of polishing were all at their maximum level, i.e., maximumconcentration of abrasive in the slurry (0.05 �m abrasive parti-cle size slurry), maximum load (7.5 N), maximum spindle speed(3600 rpm) and maximum tool stiffness (90 shore A).

7. Surface finish was better when the parameters of polishingwere all at their minimum levels, i.e., minimum concentrationof abrasive in the slurry (0.05 �m abrasive particle size slurry),minimum load (1 N), minimum spindle speed (1000 rpm) andminimum tool stiffness (50 shore A).

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