Experimental investigation of surface/subsurface damage formation and material removal mechanisms in SiC grinding

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International Journal of Machine Tools & Manufacture 48 (2008) 698–710

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Experimental investigation of surface/subsurface damage formation andmaterial removal mechanisms in SiC grinding

Sanjay Agarwal, P. Venkateswara Rao�

Department of Mechanical Engineering, Indian Institute of Technology, New Delhi 110016, India

Received 10 May 2007; received in revised form 18 October 2007; accepted 23 October 2007

Available online 18 December 2007

Abstract

The difficulty and cost involved in the abrasive machining of hard and brittle ceramics are among the major impediments to the

widespread use of advanced ceramics in industries these days. It is often desired to increase the machining rate while maintaining

the desired surface integrity. The success of this approach, however, relies in the understanding of mechanism of material removal on the

microstructural scale and the relationship between the grinding characteristics and formation of surface/subsurface machining-induced

damage. In this paper, grinding characteristics, surface integrity and material removal mechanisms of SiC ground with diamond wheel on

surface grinding machine have been investigated. The surface and subsurface damages have been studied with scanning electron

microscope (SEM). The effects of grinding conditions on surface/subsurface damage have been discussed. This research links the surface

roughness, surface and subsurface damages to grinding parameters and provides valuable insights into the material removal mechanism

and the dependence of grinding-induced damage on grinding conditions.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Ceramics; Grinding; Surface integrity; SiC

1. Introduction

There has been increased interest in the use of advancedceramic materials such as alumina, silicon nitride, siliconcarbide and zirconia, in the recent past, due to their uniquephysical and mechanical properties. The advantages ofceramics over other materials include high hardness andstrength at elevated temperature, chemical stability, attrac-tive high temperature wear resistance and low density.Because of these properties of advanced ceramics, it foundapplications in precision bearings for the use in the nuclearindustry; automotive components (sensors, insulators,catalyzers, pistons, jackets, inserts, valves, linings);biocompatible implants (dental prostheses, bone replace-ments, cardiac valves); wear parts (valve seats, mec-hanical seals, guides); refractories (insulators, rocket liningplates, military linings, furnace components); substrates,bases and insulators in electrical components.

e front matter r 2007 Elsevier Ltd. All rights reserved.

achtools.2007.10.013

ing author. Tel.: +91 11 2659 1443; fax: +91 11 2658 2053.

ess: [email protected] (P.V. Rao).

High dimensional accuracy and good surface integrityare frequently required in these structural ceramic compo-nents. Although advances have been made in the near-net-shape technology, grinding with diamond wheels is still themethod of choice for the machining of these structuralceramics. Unfortunately, the ground ceramic componentsare most likely to contain a deformed layer, surface/subsurface microcracks, phase transformation, residualstresses and other types of damage. The major form ofmachining damage usually occurs as surface and subsur-face damage. The first type of damage is due to radialcracks formed on the ground surface which are visible, andthe later damage is due to median and lateral cracks thatare formed below the affected grinding zone which are notvisible [1]. The nature of the grinding damage depends onthe mechanism of material removal. The material removalmechanisms are usually classified into two categories:brittle fracture and plastic deformation [2]. Brittle fracture,analogous to indentation of a brittle material by a hardindenter, involves two principal crack systems; lateralcrack, which are responsible for material removal, and the

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Nomenclature

ae depth of cut (mm)deq equivalent wheel diameter (mm)Esp specific grinding energy (J/mm3)

Ft tangential grinding force (N/mm)Ra surface roughness (mm)tmax maximum undeformed chip thickness (mm)Vw table feed rate (m/min)Vs wheel speed (m/s)

S. Agarwal, P.V. Rao / International Journal of Machine Tools & Manufacture 48 (2008) 698–710 699

median cracks, for strength degradation. In the brittlefracture, material removal is accomplished through voidand crack nucleation and propagation, chipping orcrushing [3]. Plastic deformation is similar to the chipformation in the grinding of metals, which involvesscratching, plowing, and chip formation. The material isremoved in the form of severely sheared chips as obtainedin machining of metals [4].

Low thermal coefficient of expansion, low density andrelatively high thermal conductivity are the special featuresof silicon carbide ceramics. In view of these properties, SiCis expected to be used increasingly for heat-resistant parts[5]. Grinding is often the method of choice for machiningceramics in large-scale production and automation. De-spite various research efforts in ceramic grinding over pasttwo decades, much needs to be established, to understandthe grinding characteristics, to characterize the machiningdamages and the mechanism of material removal. Theperformance and reliability of ceramic are influencedstrongly by the damages introduced grinding, in mostcases, ground ceramics suffering strength degradationcaused by machining-induced damage [6]. So, it would beessential to understand the mechanism of material removaland also to evaluate the significance of the processparameters on the quality of surface produced. This paperpresents an investigation on the machining characteristicsand the material removal mechanism in the grinding ofsilicon carbide by diamond-grinding wheel. Grinding-induced surface and subsurface damage have also beenassessed and characterized using scanning electron micro-scope (SEM).

2. Literature review

A major impediment to engineering applications ofceramics is their hardness and brittleness, which oftenrender them difficult and costly to machine. Abrasivemachining of ceramics by means of grinding with diamondwheels is the primary process used in achieving the desiredtolerances and surface integrity. Grinding of silicon carbideis difficult because of its low fracture toughness, making itvery susceptible to cracking. The grinding process is mostlyconducted under moderate conditions requiring extensivemachining. Efficient grinding of high-performance cera-mics requires judicious selection of operating parameters tomaximize removal rate while controlling surface integrity[7]. Lowering grinding costs by using faster removal rates isconstrained mainly by damage to the ceramic workpiecebecause of the median/lateral cracks that emanate during

grinding. So, it would be essential to understand themechanism of material removal and also to evaluate thesignificance of the process parameters on the requiredresponses.Grinding is most efficient and cost-effective technique to

finish ceramic components. Unfortunately, because of thehard and brittle nature of ceramic materials, groundceramic components often contain damage such as cracks[2,3,8], pulverization layer [9,10], and a little amount ofplastic deformation [11]. The performance and reliability ofceramic components are influenced strongly by thedamages introduced during grinding. Malkin and Hwang[12] have studied and analyzed the mechanism of materialremoval in ceramic grinding with the help of indentationfracture mechanics approach and the machining approach.It was shown with the first approach that median/radialcracks are usually associated with strength degradation,and lateral cracks with material removal. Xu et al. [13] havedemonstrated that the mechanism of material removal andthe effect of machining-induced damage on strength ofadvanced ceramics can be controlled by approximatelytailoring the microstructure. This not only promotes easyand well-controlled material removal by grain dislodge-ment during machining, but also suppresses the formationof strength degrading cracks. Zarudi and Zhang [14]investigated both experimentally and theoretically thesubsurface damage in alumina by ductile-mode grinding.It was found that the distribution of the fractured area on aground mirror surface, with RMS roughness in the rangefrom 30 to 90 nm, was dependent on not only the grindingconditions but also the pores in the bulk material. Surfacepit formation was the result of interaction of abrasivegrains of the grinding wheel with pores. Thus, surfacequality achievable by ductile-mode grinding was limited bythe initial microstructure of a material. The investigationshowed that median and radial cracks did not appear andhence were not the cause of fracture as usually thought.Chen et al. [15] investigated and analyzed theoretically, thefactors influencing the surface quality of brittle materials,during ultra-precision grinding. Grinding experiments werealso carried out to confirm the outcome of theoreticalanalysis, for the brittle materials. The results showed thatthe average abrasive grain size of the diamond wheel had amain influence on the surface quality, and the influence ofthe wheel speed and feed rate were secondary. In order toinvestigate the surface and subsurface integrity of dia-mond-ground optical glasses, Zhao et al. [16] used amachine tool featuring high close-loop stiffness, to conductthe ultra-precision machining of fused silica and fused

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Fig. 1. Microstructure of silicon carbide (SiC).

S. Agarwal, P.V. Rao / International Journal of Machine Tools & Manufacture 48 (2008) 698–710700

quartz assisted with electrolytic in-process dressing(ELID). An acoustic emission sensor and a piezoelectricdynamometer were used to monitor the grinding process tocorrelate the processing characteristics with the generatedsurface and subsurface integrities, which were character-ized by atomic force microscope, scanning electronicmicroscope, and nano-indentation technique. Experimen-tal results showed that for optical glasses the fracturetoughness value could be used to predict the machinability,while its bigger value always means a better surface andsubsurface integrity. During the grinding process of opticalglasses, the smaller amplitude and RMS values of acousticemission signal, as well as the smaller grinding forces andthe ratio of normal force to tangential force, correspond toa better surface and subsurface integrity. With selectedmachining parameters and a very fine grain-sized diamond-grinding wheel, nanometric quality surfaces with minimalsubsurface damage depth can be generated for fused quartzon this machine.

Detailed knowledge on the effect of the grinding processon surface integrity gives the opportunity for a betterexploitation of ceramic materials by improved processconditions. Pfeiffer and Hollstein [17] have used the X-raydiffraction technique for determining the damage inducedin ground silicon nitride and alumina and therebyestablished correlations between microplastic deformationand amount of damage. These investigations show that inthe case of lapped and ground alumina and of groundsilicon nitride, bending strength is dominated by machin-ing-induced damage. In the case of lapped silicon nitride,the effect of damage is compensated by machining-inducedcompressive residual stresses. Unfortunately, the X-raydiffraction technique neither differentiates subsurfacedamage from the bulk structures, nor differentiates theeffects of the damage on the residual strength and surfacetribological properties of a ground component. The scopeis still left for further improvement to provide more preciseand reliable prediction. Daniels [18] has investigated theinfluence of surface grinding parameters such as diamondabrasive type, wheel speed and down feed on the rupturestrength of silicon carbide. It was found that more severegrinding conditions, such as higher normal forces andpower consumption, did not significantly reduce the meanrupture strength of the material. The most encouragingaspect inferred from these results was that grindingconditions could be changed in order to optimize theprocess without significant structural damage to the workmaterial. There were several techniques, viz. flexuralstrength testing, fractography, and non-destructive inspec-tion, which were considered to be useful for assessingsubsurface damage. Ahn et al. [19] were able to detect thesubsurface lateral cracks (at larger scale) associated withindentations in glass and silicon nitride using the ultrasonictechnique and the thermal wave measurement technique.However, since the service life of a ceramic component, in awear and corrosive environment, could be affected by thesecracks produced during the grinding process, Ahn et al.

suggested that these techniques should be applied toinvestigate the small cracks developed during the grindingprocess. Shen et al. [20] conducted a study of the force andenergy characteristics in the grinding of advanced ceramics.In this study, two typical ceramics were ground with a resinbonded diamond wheel on a precision surface grinderunder different grinding conditions. According to themeasured power used by the spindle, the normal andtangential force components operating on the grindingwheel, the force ratio, the specific grinding energy ingrinding of ceramics was analyzed. The result showed thatthe ceramic materials were mainly removed in the fracturemode, while most of the energy was expended by ductile.The friction between the diamond wheel and ceramics weretaken into account in the material removal mechanisms forceramic grinding.Unfortunately, the industrial applications of advanced

ceramics have been restricted by the machining difficultiesand associated high cost with the use. This is mainly due tothe poor machinability of these ceramics, as a result, greateffort were made towards the development of grindingtechnology in an efficient mode [21–23]. High-speedgrinding has been studied in order to achieve a highmaterial removal rate in the grinding of ceramics [23]. Inthe high-speed grinding process, an increase in the wheelspeed would reduce the maximum chip thickness, and thusthe grinding force [24]. This would cause the ductile flow byreducing the tendency for brittle fracture [25]. On the otherhand, the increased speed will cause the increase in thedepth of cut or the feed rate to obtain the higher materialrate, without deteriorating the ground surface integrity.Kovach et al. [26] carried out a feasibility study on theapplication of high speed in the low-damage grinding ofthe advanced ceramics. Hwang et al. [27,28] investigatedthe machining characteristics of silicon nitride under high-speed grinding conditions. This research is focused on thewheel wear mechanism, at low material removal rate.Klocke et al. [23] studied various process strategies for the

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as-castsur facesubjected to grinding

B

A

y

A’

B’

x’D’

D

C

x

C’ y’Step2

Step3

Step4

ground surface

polished side

damagedepth

Step1

polishedsides

as cast surface

groundsurface

damagedepth

polishedsurface

Fig. 2. Schematic illustration of the procedure used in preparation of

SEM samples to study subsurface damage. An as-cast sample was

recombined, as shown in step 1. Grinding was performed (step 2),

followed by cross-sectional and plane-view examinations of the ground

subsurface and surface using SEM (step 3). Step 4 shows the measurement

of subsurface damage depth (an average value is taken as depth is not

uniform).


high-speed grinding of aluminum oxide and silicon-infiltrated silicon carbide at high removal rates. The resultsindicated that the high-speed grinding at high removalrates does not decrease the fracture strength of themachined ceramic components. High removal rate (up to16.6mm3/s per mm) grinding of alumina and alumina–ti-tania was investigated by Yin et al. [29] with respect tomaterial removal and basic grinding parameters using aresin-bond 160mm grit diamond wheel at the speeds of 40and 160m/s, respectively. The results show that thematerial removal for the single-phase polycrystallinealumina and the two-phase alumina–titania compositerevealed identical mechanisms of microfracture and graindislodgement under the grinding conditioned selected.There were no distinct differences in surface roughnessand morphology for both materials ground at eitherconventional or high speed. An increase in materialremoval rate did not necessarily worsen the surfaceroughness for the two materials at both speeds. Also thegrinding forces for the two ceramics demonstrated similarcharacteristics at any grinding speeds and specific removalrates. Both normal and tangential grinding forces and theirforce ratios at the high speed were lower than those at theconventional speed, regardless of removal rates. An in-crease in specific removal rate caused more rapid increasesin normal and tangential forces obtained at the conven-tional grinding speed than those at the high speed.Furthermore, it is found that the high-speed grinding atall the removal rates exerted a great amount of coolant-induced normal forces in grinding zone, which were four tosix times higher than the pure normal grinding forces.However, in these works [23,26–31], no detailed investiga-tions of the effects of high-speed grinding conditions on thematerial removal mechanisms and the surface integrity ofadvance ceramics have been reported. Huang and Liu [32]studied the machining characteristics and surface integrityof advanced ceramics, viz. alumina, alumina–titania, andyttria partially stabilized tetragonal zirconia, under high-speed deep grinding conditions. Material removal mechan-isms involved in the grinding processes were explored. Thematerial removal in the grinding of alumina and alumi-na–titania was dominated by grain dislodgement or lateralcracking along grain boundaries. The removal for zirconiawas via both local microfracture and ductile cutting. Zhanget al. [33] conducted the study on diamond grinding ofadvanced ceramics, including hot-pressed silicon nitride,hot-pressed alumina, slip-cast zirconia, and pressurelesssintered silicon carbide. Grinding-induced damage in theseceramics was assessed and characterized using threedestructive inspection techniques and progressive lappingtechnique combined with scanning electron microscopy(SEM), and transmission electron microscopy (TEM). As aresult, two types of grinding damage were identified,pulverization and microcracking. Damage depth was foundto be related to the properties of ceramic materials,especially their brittleness. For a given grinding condition,damage penetrated deeper in less brittle materials than in

more brittle materials. In addition, two types of grinding-induced microcracks were identified, scattered and clus-tered. The former was observed on all four types ofmaterials tested under various grinding conditions, whilethe latter was only associated with less brittle materialssubjected to relatively aggressive grinding conditions. The

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Table 1

Mechanical properties of SiC used in experimentation

Material Material properties

Density (gm/cm3) Bending strength RT

(MPa)

Hardness (HV10)

(GPa)

Fracture toughness

(MPam1/2)

Modulus of elasticity

RT (GPa)

SiC 3.1 400 25 3.0 410

Table 2

Measured values of surface roughness and tangential grinding force at

different grinding parameters

ae (mm) vw (m/min) Ft (N/mm) tmax (mm) Ra (mm)

5 5 8.1 0.781 0.180

15 5 10.6 1.23 0.287

25 5 11.8 1.89 0.359

35 5 12.2 2.13 0.400

45 5 12.3 2.89 0.353

0

100

200

300

400

500

600

700

0.5 1 1.5 2 2.5 3

max. undeformed chip thickness (�m)

sp

. g

rin

din

g e

ne

rgy (

J/m

m3)

Fig. 3. Effect of maximum chip thickness on specific grinding energy.


mechanisms of damage nucleation and propagation werediscussed. The results provided valuable insights into thedependence of grinding-induced damage on the propertiesof workpiece materials, and on the grit size of grindingwheels.

The literature review above indicates that silicon nitrideand alumina are the most common materials whosebehavior in grinding has been investigated. Alumina beingan oxide ceramic is not preferred for high temperature/strength applications. Most of the studies have beenconcentrated on using silicon nitride as the work material.In recent years along with silicon nitride, silicon carbidehas also emerged, as the work material, whose application,as a structural material, is very limited.

This paper investigates the grinding characteristics andmechanism of material removal during surface grinding ofsilicon carbide by diamond-grinding wheel. The effects ofgrinding conditions on the surface finish and surface andsubsurface damage of the ground specimens were investi-gated and the associated material removal mechanism wasdiscussed. The direct observations of surface and subsur-face damages were used in this investigation as it couldprovide key information on the mechanism of materialremoval mechanism, grinding damage prediction andgrinding characteristics.

3. Experimentation

Grinding experiments were performed on an ‘ELLIOTT8-18’ Hydraulic surface-grinding machine. A resin bonddiamond-grinding wheel, with an average grit size of121 mm was used (with a diamond concentration of 100%).The wheel has a diameter of 250mm and a width of 19mm.The work material used for this investigation was SiC. Theproperties of SiC workpiece material, used for experimen-tation in this work, are as given in Table 1. The specimenshave the dimensions of 20� 20� 5mm3. Grinding wascarried on the 20� 5mm2 surface. Fig. 1 shows themicrostructure of the silicon carbide. The SiC has thegrains ranging from 1 to 10 mm. The wheel speed of 37m/swas used. The depths of cut, for study, were ranging from 5to 45 mm. The feed rate was taken as 5m/min for the study.

The ground surfaces were examined using a SEM. Priorto examination, the ground specimens were cleaned withacetone in an ultrasonic bath for at least 10min and thengold coated for examination. The roughness of the groundsurfaces was measured using a Taylor Hobson Profilometer

(Talysurf-6 with cutoff value 0.8mm). The roughness wasthe average value obtained by scanning rectangularsurfaces of the workpiece. While performing the experi-ments, the grinding forces were measured using adynamometer (Kistler 9257 B).A bonded interface sectioning technique [34] was used to

examine the grinding-induced subsurface damage. In thismethod, the two specimens were ground at same dimen-sions and one surface of each specimen was polished. Thepolished surfaces were subsequently bonded face-to-facewith suitable adhesive. Clamping pressure was then appliedto push the two specimens tightly together, leaving athin layer of adhesive approximately 1 mm thick. Fig. 2shows the schematic illustration of the procedure used inpreparation of SEM samples to study subsurface damage.

ARTICLE IN PRESSS. Agarwal, P.V. Rao / International Journal of Machine Tools & Manufacture 48 (2008) 698–710 703

As shown in this, the two specimens bonded together at thexyx0y0 interface of the two polished surfaces. It is essentialto clamp the two specimens tightly together during

Fig. 4. (A) Ground surface characteristics of SiC under a wheel speed of 3

cut ¼ 15 mm, and (c) depth of cut ¼ 25mm (arrow indicates the fractured area

and a feed rate of 5m/min: (d) depth of cut ¼ 35mm, (e) depth of cut ¼ 45mm (s

arrow indicates the fractured area and pits resulting from grains dislodgemen

bonding, to make a bonded interface narrow. A wideinterface between the two specimens could cause anartificial damage close to the interface during grinding.

7m/s and a feed rate of 5m/min: (a) depth of cut ¼ 5 mm, (b) depth of

). (B) Ground surface characteristics of SiC under a wheel speed of 37m/s

mall arrows indicates the brittle fractures) and (f) an enlarged photo of (e),

t.

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Table 3

Measured values of surface roughness at different values of depth of cut

Depth of cut (mm) Surface roughness (mm) Average roughness (mm)

1 2 3 4 5 6 7

5 0.2 0.19 0.2 0.18 0.17 0.16 0.16 0.180

15 0.3 0.297 0.298 0.297 0.291 0.30 0.226 0.287

25 0.425 0.349 0.345 0.348 0.350 0.342 0.350 0.357

35 0.448 0.401 0.398 0.705 0.376 0.356 0.366 0.400

45 0.391 0.371 0.322 0.341 0.359 0.374 0.313 0.353

0

0.1

0.2

0.3

0.4

0.5

5 15 25 35 45

depth of cut (�m)

su

rfa

ce

ro

ug

hn

ess (�m

)

Fig. 5. Effect of depth of cut on the surface roughness. The lower and

upper error bars show the minimum and maximum surface roughness

measured.


When the interface is narrow, no extraneous damage willbe observed along the interface xyx0y0 compared to theground area away from the interface (Fig. 2, step 2).Grinding was performed on the plane ABCD passingsymmetrically across the line xy. The specimens wereseparated after grinding by melting the glue and werecleaned with acetone in an ultrasonic bath. The polishedsurfaces were gold coated for SEM examination.

4. Results and discussion

The force plays an important role in grinding processsince it is an important quantitative indicator to character-ize the mode of material removal (the specific grindingenergy and the surface damage are strongly dependent onthe grinding forces) in ceramic grinding. So the measure-ment of grinding force is essential. Tangential grindingforce has been measured at each set of grinding conditions,given in Table 2. It has been observed that the tangentialgrinding forces increases with the increase in the depth ofcut. This increase in grinding force is expected because ofincreased chip thickness at higher depth of cut.

In the grinding of advanced ceramics, the materialremoval mechanism depends upon the chip formationcharacterized by plastic flow or brittle fracture as thecutting depth increases. If the cutting depth is large enoughto cause cracks, a chip removal will be due to the fractureof material. When fracture occurs, the specific energyrequirement is lower than that in normal chip formation,but the surface damage occurs leading to strengthreduction. Quantitatively, the parameter tmax (maximumundeformed chip thickness) characterizes the depth ofpenetration of the abrasive grain into the workpiece whenit is engaged in cutting. The value of tmax thus representsthe effect of grinding conditions on the grinding force welland depends on both machine and wheel parameters. Thetmax can be expressed as [35]:

tmax ¼E1

E2

� �0:5484

Cr

Vw

V s

� �ae

deq

� �1=2" #1=2

, (1)

where r is the chip width-to-thickness ratio, C is thenumber of active grits per unit area of the wheel periphery(grit surface density), E1 is the modulus of elasticity of thewheel, E2 is the modulus of elasticity of the workpiece, Vw

is the workpiece feed rate and Vs is the wheel velocity, ae isthe depth of cut and deq is the equivalent wheel diameter.The value of r as in Eq. (1) is difficult to determine and isreported in the range of 10–20 [36]; r was assumed to beequal to 10 in this work. The equivalent wheel diameter insurface grinding is the wheel diameter itself. The modulusof elasticity of the diamond-grinding wheel (E1) is assumedto be the modulus of elasticity of the core material of thewheel itself. This is because the amount of the abrasivelayer on the core is only 4mm thick (6%, v/v) and the corematerial is of 242mm diameter (94%, v/v) in a grindingwheel of 250mm diameter. Aluminum is the core materialused in the diamond-grinding wheels and thus the modulusof elasticity of the wheel is taken as the modulus ofelasticity of aluminum, which is 70GPa. Hence E1 is takenas 70GPa in the present study. The value of modulus ofelasticity of the workpiece (E2) is taken as 410GPa, whichis provided by the manufacturer of the silicon carbideworkpiece. The value of C, in Eq. (1), can be obtained by asimple geometric relationship, derived by Xu et al. [37] asfollows:

C ¼4f

d2gð4p=3vÞ2=3

,


where dg is the equivalent spherical diameter of diamondparticle, v is the volume fraction of diamond in the grindingwheel, and f is the fraction of diamond particles thatactively cut in grinding. The grinding wheel used in thepresent study has a density of 100, or in other words,volume fraction v is 0.25. To obtain the value of C, it isassumed that only one-half of the diamond particles on thewheel surface are actively engaged in cutting [37], or thevalue of f is equal to 0.5. The equivalent spherical diameterof diamond grit (dg) is given [38] as:

dg ¼ 15:2M�1,

where M is the mesh size used in the grading sieve. In thepresent study, a mesh size of 120 is used. The values ofundeformed chip thickness tmax were calculated, aftersubstituting all these parameters in Eq. (1), as shown inTable 2. Eq. (1) shows that not only the machiningparameters (ae, Vw, Vs) modify the tmax but also the wheelparameters, such as C and r. The C and r, in turn, dependon the wheel topography, which includes the distancebetween the consecutive grains. This distance is related tothe grit size, the diamond type and the concentration. Ingeneral, a smaller tmax is desirable to generate smoothersurfaces, as it reduces the machining damage. Forachieving the higher material removal rate, a higher valueof the product (ae, Vw) is desirable. This in turn wouldmake the undeformed chip thickness tmax larger.

In the case of surface grinding process, the specificgrinding energy (Esp) is another important parameter whilegrinding brittle materials since it can be used to know themechanism associated with interaction between the abra-sive grit and the workpiece. The specific grinding energy,Esp, is defined as the ratio of the net grinding power (FtVs)to the volumetric material removal rate. Specific energywas calculated using the formula (FtVs)/(aeVwb), where Ft

is the tangential grinding force in Newtons, Vs is thegrinding speed in m/min, Vw is the feed rate in m/min, ae isthe depth of cut in mm, and b is the width of cut in mm. Thevariation of specific grinding energy with tmax was as shownin Fig. 3. It can be seen from this figure that a reduction intmax resulted in an increase in the specific energy. When thevalue of tmax was above a critical value (which isapproximately 1.75 mm, as shown in Fig. 3), the decreasein the specific energy was minor and gradual. However,when the value of tmax was below this critical value, thespecific energy had a rapid rise with the further decrease intmax. In the grinding of ceramics, the material removalmode, brittle or ductile, was substantially influenced by the

Table 4

ANOVA for surface roughness data

Source of variation Sum of squares Degree of freedom Mean

Depth of cut 0.216 6 0.036

Error 0.171 28 0.006

Total 0.387 34 –

chip thickness or the grain load [39]. When tmax was belowthe critical value, there would be a reduction in the degreeof brittleness of the material removal mechanism. Thiscould be due to the smaller impacts between the abrasivegrain and the workpiece. The prevalent brittle fracturemode would be changed to a ductile flow mode. However,most of the chip removal of silicon carbide was still due tobrittle fracture. This estimation was confirmed through theobservation of ground surface micrographs as shown inFig. 4 (shown by arrows). This was further confirmed byhaving an enlarge view of one of the ground workpiecesurface (Fig. 4(B), subpart f)), where arrow indicated thefractured area and pits resulting from grain dislodgementand was consistent with the observations on machining ofother materials including alumina [40] and glass-ceramics[41]. However, it is difficult to quantitatively identify theeffect of the grinding conditions on the surface character-istics through SEM micrographs.In addition to the tangential grinding force and specific

grinding energy, surface roughness is also an indicator tocharacterize the material removal mode associated with thegrinding process. The quality of surface generated duringceramic grinding process depends upon grinding condi-tions. The surface roughness data obtained from theground surfaces (along with the dispersions in the observedvalues of surface roughness) with respect to the depth ofcut have been shown in Table 3. The variation of surfaceroughness with depth of cut is as shown in Fig. 5. Eachdata point is the average of seven measurements and theerror bars indicated the maximum and minimum values ofsurface roughness measured. Since there are significantdispersions in the measured values of surface roughness foreach set of data points, it would be essential to perform thestatistical analysis to establish the statistical differences intheir mean values. For this, analysis-of-variance (ANOVA)technique is used. The results of ANOVA for surfaceroughness are summarized in Table 4. It could be seen fromthe table that the calculated value of F-ratio was more thanthe tabulated value of F-ratio for a desired level ofconfidence (say 99%). So it could be concluded that theaverage values of surface roughness were statisticallydifferent; that is, the depth of cut significantly affectedthe surface roughness. It could be seen from Fig. 5 thatsurface roughness increased initially and then decreasedwith increase in depth of cut. The initial increase was, dueto the increase in the maximum chip thickness with theincrease in depth of cut, which resulted in an increase insurface roughness. The decrease in surface roughness

square Fcal F0.01,6.28 Remark

0 5.90 3.53 Significant at 99% confidence level

1

ARTICLE IN PRESSS. Agarwal, P.V. Rao / International Journal of Machine Tools & Manufacture 48 (2008) 698–710706

beyond certain value of depth of cut could be due to thelesser requirement of specific energy at high depth of cut(material removal due to brittle fracture) (Fig. 3, which

Fig. 6. (A) Subsurface damage layer of SiC for different depths of cut. The arro

cut ¼ 15 mm. (B) Subsurface damage layer of SiC for different depths of cut. Th

of cut ¼ 35mm and (e) depth of cut ¼ 45 mm.

shows the variation of specific energy with undeformedchip thickness), causing reduction in friction between thewheel and the work, resulting in the improvement in the

w indicates the machined surface: (a) depth of cut ¼ 5mm and (b) depth of

e arrow indicates the machined surface: (c) depth of cut ¼ 25 mm, (d) depth


surface finish. An important consequence of this result isthat there is some scope of improving productivity withoutcompromising the surface finish.

4.1. Grinding-induced subsurface damage

Subsurface damage of the ground specimens, which wereprepared using the bonded interface sectioning technique,was examined using an SEM. Two types of subsurfacedamage, chipping and cracking, induced by grinding wereclearly observed. The damage layer right underneath themachined surface exhibits to be generated via chipping,and the chipping layer exhibits to be induced mainly by

Table 5

Measured values of chipping layer thickness at different values of depth of

cut

Depth of

cut (mm)

Chipping layer thickness (mm) Average

thickness (mm)1 2 3 4 5 6 7

5 7.8 6.8 5.9 5.0 4.2 4.2 3.6 5.0

15 8.4 7.6 6.4 5.1 5.2 5.0 4.7 6.2

25 10.1 8.7 7.7 7.5 6.0 5.8 5.3 7.3

35 10.2 8.8 8.4 6.4 5.9 5.7 5.2 7.2

45 8.8 7.2 7.0 6.4 5.9 5.6 5.3 6.6

0

2

4

6

8

10

12

5 45352515

depth of cut (�m)

Chip

pin

g layer

thic

kness (�m

)

Fig. 7. Effect of depth of cut on the chipping layer thickness. The lower

and upper error bars show the minimum and maximum thickness

measured.

Table 6

ANOVA for chipping layer thickness data

Source of variation Sum of squares Degree of freedom Mean

Depth of cut 29.3 6 4.88

Error 63.8 28 2.27

Total 93.1 34 –

grain dislodgement as shown in Fig. 6. During grinding,the contact of an individual diamond particle with theceramic workpiece produced a damage zone containingdistributed grain-boundary microcracks. This damagedlayer contributes to the dislodgement of individual grainsresulting from the grain-boundary microfractures. Thisfact led to the conclusion that the chipping layers wereinduced by grain dislodgements. The chipping effect forhigher depth of cut was more overwhelming and probablycovered the damage layer. This could be due to fact that asdepth of cut in grinding was increased, the local contactforce and the number of contacting diamond particleswould increase, leading to a possible removal of a segmentof material that contains a number of individual grains,besides the dislodgement of individual grains. Apart fromthis, microcracks were also observed (as indicated byarrows in Fig. 9) in the subsurface layer of the groundsilicon carbide, usually under the chipping layer. Most ofthe microcracks were observed along the grain boundaries.These results are in agreement with the surface character-istics shown in Fig. 4.The chipping layer thickness was measured during the

SEM examination. Since the chipping layer was uneven,measurements at seven locations uniformly distributedalong the grinding width for each specimen, were taken(Fig. 2, step 4), as shown in Table 5. The chipping layerthicknesses, obtained from the ground surfaces (along withthe dispersions in the observed values of chipping layerthickness), with respect to the depth of cut, have beenshown in Table 5. Each data point (Fig. 7) was the averageof seven; there was a significant dispersion in the measuredvalues of chipping layer thickness, it would be essential toperform the statistical analysis to establish the statisticaldifferences in their mean values. For this, ANOVAtechnique was used. The results of ANOVA for chippinglayer thickness, was summarized in Table 6. It could beseen from the table that the calculated value of F-ratio wasmore than the tabulated value of F-ratio for a desired levelof confidence (say 90%). So it could be concluded that theaverage values of chipping layer thickness were statisticallydifferent; that is, the depth of cut significantly affected thechipping layer thickness. The average thickness of chippinglayer was then plotted as a function of depth of cut asshown in Fig. 7. It could be seen from this Fig. 6 that theaverage thickness of the chipping layer was increased withthe increase in depth of cut initially and then decreasedwith further increase in the depth of cut. This confirmedour surface roughness results as shown in Fig. 5. In

square Fcal F0.10,6.28 Remark

2.149 2.00 Significant at 90% confidence level

ARTICLE IN PRESS

Fig. 9. SEM micrographs of the specimen ground at 35mm depth of cut,

with specimen tilted so that both surface and section are visible. The

arrows indicate the grain boundary microcracks and pits resulting from


addition to the chipping layer, microcracks induced bygrinding were observed in the subsurface layer of theground SiC, underneath the chipping layer. Most micro-cracks were observed along the grain boundaries as shownin Figs. 8(a) and 9. Evidently, the material removal was dueto the dislodgement of individual grains, resulting frommicrocracks along the grain boundaries. This fact wassupported by the surface characteristics observed shown inFig. 4 and near-surface structure as shown in Fig. 8(b) andwas consistent with the observations during conventionalgrinding of other materials including silicon nitrideceramics [7], alumina [40] and glass-ceramics [41].

In order to assess the total damage, it would be advisableto club both the modes of damage by defining the totaldamage layer as the layer, which contains both the edgechipping damage and microcracks. The thickness ofdamage layers, measured from SEM micrographs, wasplotted as a function of depth of cut as shown in Fig. 10. Itcould be seen from this graph that the increased depth of

Fig. 8. (a) SEM micrograph of the near-surface structure, showing near-

surface damage, in the form of grain segments, as indicated by arrow and

(b) completely shattered grains below the surface (arrow indicates the pits

resulting from grain dislodgement).

grain dislodgement.

0

3

6

9

12

15

18

dam

age layer

thic

kness (�m

)

5 45352515

depth of cut (�m)

Fig. 10. Effect of depth of cut on the total damage layer thickness. The

lower and upper error bars show the minimum and maximum thickness

measured.

cut resulted in a deeper damage layer; however, the edgechipping damage was decreased (Fig. 7). Also the increaseddepth of cut led to a higher grinding force thus generatinglarger sized cracks. Thus, it could be concluded that thegrinding-induced microcracks had influence on the damagelayer as well as the chipping edge damage. The resultsobtained indicated that material removal mode has animportant bearing on the chipping layer thickness, andthus on the values of surface roughness (Fig. 5). It could beseen from Fig. 11 that there exists a direct relationshipbetween the thickness of chipping layer and surfaceroughness for silicon carbide.

ARTICLE IN PRESS

3

4

5

6

7

8

0.14 0.19 0.24 0.29 0.34 0.39 0.44

surface roughness (�m)

chip

pin

g layer

thic

kness (�m

)

Fig. 11. Relationship between the chipping layer thickness and the

surface roughness.


5. Conclusion

In the present study, the machining characteristics andsurface integrity of ground silicon carbide were studied forexploring the material removal mechanism involved in thegrinding process. The direct observations of surface andsubsurface damage were used in this investigation as theycould provide key information on the mechanism ofmaterial removal and grinding damage prediction. Thesurface and subsurface characteristics of the ground siliconcarbide showed that the material removal associated withthis material was primarily due to the dislodgement ofindividual grains, resulting from microcracks along thegrain boundaries. The grinding force and specific grindingenergy could be, therefore, considerably reduced by takingadvantage of this phenomenon. This provided an impor-tant insight into the ceramic manufacturing that the siliconcarbide could be efficiently ground without causing asignificant loss to the surface integrity. Thus, this under-standing of the material removal process could be used notonly for optimization of ceramic grinding process but alsofor the microstructural design of future silicon carbideceramics. This would lead to increased productivity makingthe silicon carbide grinding process more economical andsilicon carbide a more viable material for industrialapplications.

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Documents

Experimental investigation of surface/subsurface damage formation and material removal mechanisms in SiC grinding