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(This is a sample cover image for this issue. The actual cover is not yet available at this time.)
This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Brittle–ductile transition during diamond turningof single crystal silicon carbide
Saurav Goel a, Xichun Luo a,b,n, Paul Comley c, Robert L Reuben a, Andrew Cox d
a School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH144AS, Scotland, UKb School of Computing and Engineering, University of Huddersfield, Canalside East (CE3/17), Huddersfield HD13DH, UKc School of Applied Sciences, Cranfield University, Cranfield, Bedfordshire MK430AL, UKd Contour Fine Tooling, Wedgwood Court, Stevenage, Hertfordshire SG14QR, UK
a r t i c l e i n f o
Article history:
Received 10 August 2012
Received in revised form
31 August 2012
Accepted 5 September 2012Available online 13 September 2012
Keywords:
Silicon carbide
Diamond turning
Brittle–ductile transition
a b s t r a c t
In this experimental study, diamond turning of single crystal 6H-SiC was performed at a cutting speed of
1 m/s on an ultra-precision diamond turning machine (Moore Nanotech 350 UPL) to elucidate the
microscopic origin of ductile-regime machining. Distilled water (pH value 7) was used as a preferred
coolant during the course of machining in order to improve the tribological performance. A high
magnification scanning electron microscope (SEM FIB- FEI Quanta 3D FEG) was used to examine the
cutting tool before and after the machining. A surface finish of Ra¼9.2 nm, better than any previously
reported value on SiC was obtained. Also, tremendously high cutting resistance was offered by SiC
resulting in the observation of significant wear marks on the cutting tool just after 1 km of cutting length.
It was found out through a DXR Raman microscope that similar to other classical brittle materials (silicon,
germanium, etc.) an occurrence of brittle-ductile transition is responsible for the ductile-regime
machining of 6H-SiC. It has also been demonstrated that the structural phase transformations associated
with the diamond turning of brittle materials which are normally considered as a prerequisite to ductile-
regime machining, may not be observed during ductile-regime machining of polycrystalline materials.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Silicon carbide (SiC) is an ultra-hard ceramic material possessinghighly desirable engineering properties such as chemical inertness,high thermal conductivity, high carrier saturation velocity, highspecific stiffness (E/r) and high-temperature resistance [1]. Forthese reasons, SiC is an appropriate choice for the purpose ofquantum computing applications as a substitute to diamond [2],in space based laser mirrors [3,4] and for moulding dies used forhot-press moulding of aspherical glass lenses. However, the extre-mely high micro-hardness of SiC makes it a difficult to machinematerial even with the hardest known diamond cutting tool [5].
Single point diamond turning (SPDT) is an established ultra-precision manufacturing method used to produce optics on avariety of classical engineering materials in a single machining
pass using a single point diamond cutting tool [6]. SPDT is preferredfor its unique capability to efficiently produce three dimensionalfreeform structures. Moreover, the components produced throughan SPDT operation have a much better metallurgical structure thanthe one obtained through polishing and lapping processes [7]. Thiscouples further with the fact that SPDT offers flexibility of gener-ated figure, better step-definition, deterministic form accuracy andeconomy of fabrication time [8]. Therefore, SPDT of silicon carbide(SiC) is of significant technological interest and economic advantagefor various industrial applications [9,10]. The aim of this work is toprincipally investigate the microscopic origin of ductile-regimemachining of 6H-SiC and demonstrate the attainable surface rough-ness on this ceramic for the purpose of producing optical surface. Ingeneral, the average surface roughness (Ra) on the materials shallbe within 20 nm in order to qualify as a good optical candidate [5].
2. Literature review
Brittle materials including SiC, exhibit low fracture toughness(CVD 3C-SiC being an exception) and are therefore difficult tomachine. It is however possible to machine such brittle materialssimilar to machining a metal at a relatively smaller length scaleusing appropriate machining parameters. Execution of such kind ofmachining process on brittle materials where the chips are
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generated through a mode of plastic deformation rather thanfracture is known as ductile-regime machining. The possibility ofmachining brittle materials in the so called ductile-regime was firstobserved by King and Tabor [11] in the year 1954 during frictionalwear of rock salts. They realized that although there were somecracks and surface fragmentations, there was some plastic deforma-tion involved. Similarly, Bridgman and Simon [12] recognised that abrittle material such as glass exhibited ductility under high hydro-static pressure. Subsequently, Lawn and Wilshaw [13] observed thesame ductile behaviour of glass during nano-indentation testingwhich lead to the identification of elastic–plastic transition. Theyrealized that the ductile behaviour causes the material to expand ina radial core (hydrostatic core) which exerts a uniform hydrostaticpressure on its surroundings. This radial core is encased within anintermediate core of ‘‘plastic region’’ that is surrounded by a regioncalled ‘‘elastic matrix’’. This observation leads to the identificationof elastic–plastic response of brittle materials during their nano-indentation. It was realized at this time that under the influence oflarge hydrostatic stress almost any material, including diamond,can be deformed plastically even at low temperatures [14]. In thesubsequent work, Lawn and Marshall [15] proposed an empiricalrelation for the required lower bound of the critical load P and theresulting critical crack length c in the substrate material which theycorrelated with the fracture toughness and hardness of the sub-strate material:
P¼ l0Kc
4
H3
" #ð1Þ
c¼ m0
Kc2
H2
" #ð2Þ
where l0 and m0 are the geometrical constants dependent on thematerial, P is the critical load, c is the crack length, Kc is the fracturetoughness which is the resistance to fracture and H is the hardnessof the material which is defined as the resistance to the plastic flow.Further development lead to the identification of the critical indentsize (CIS) [16] as follows:
CIS¼ m Kc
H
� �2
ð3Þ
where mpE/H and E being the elastic modulus of the material.Subsequently, Bifano et al. [17] postulated that each material will
be apt to undergo a brittle–ductile transition when subjected to asmall infeed rate. At this small in-feed rate the energy required topropagate a crack is larger than the energy required for plasticyielding, so plastic deformation will become dominant. It was as lateas 1990 when Scattergood and Blake [18] suggested that despite thedynamic and geometric differences in material removal mechanismduring nano-scratching and a nano-indentation process, there areessential similarities in both these processes. They identified that acritical chip thickness dc separates the portion of plastic deformationfrom fracture removal. Accordingly, they proposed a new machiningmodel to explain the ductile regime machining of brittle materials.As per their machining model, a material exhibiting minimum cracklength is preferable in order to avoid the penetration of the crack
underneath the machined surface during the course from machining.Usually, the estimation of the various geometrical parameters fromthis model is about 50% off the actual experimental value [19]. This isdue to the unaccountability of the associated structural transforma-tions and associated volume changes (�20%) of the cutting chips intheir machining model [20]. However, this model is still quiterelevant to relate theoretical understanding with the experimentaloutcome [21]. Evaluations of the critical parameters such as criticalcrack length and critical chip thickness of 6H-SiC are shown inTable 1. These calculations are based on the empirically knownrelations as shown earlier.
With this brief but essential background, initial SPDT trialsperformed to date on SiC were mainly concerned with studyingits technical feasibility. These works are presented in Table 2,highlighting their experimental outcome, type of work materialused and the coolant used.
Table 2 suggests that SPDT, in spite adopting a high feed rate,successfully generated a very fine machined surface of Ra value14 nm on RB-SiC using copper nanoparticles as a coolant. On theother hand, an inferior Ra value of 23 nm was obtained on the sameRB-SiC while dry cutting. Similarly, 3C type polycrystalline CVD-SiCwas machined upto an Ra value of 83 nm against as received Ra valueof 1.158 mm using an alumina and silica based specialized coolant.
Surprisingly, no surface roughness data has been reported onsingle crystal SiC despite the fact that there are significant differ-ences in the nature of bonding, microstructure, extent of plasticdeformation and number of slip systems between single crystal andpolycrystalline SiC. Although, polycrystalline SiC is relatively easierto machine than single crystal SiC [27], the above differences asanticipated should provide a better surface finish on single crystalSiC compared to polycrystalline SiC. Therefore, the SPDT trial in thecurrent work was performed on single crystal SiC (6H-type) in orderto measure the attainable surface roughness on this material in asingle pass. Since abrasion alone was identified as the cause of toolwear during SPDT of single crystal SiC [28], distilled water (pH value7) was used as a preferred coolant as it was the one whichsignificantly improved the tribological performance of the diamondduring its abrasion with another diamond [29]. In the subsequentsections, the experimental results are presented and discussed.
3. Experimental details
The SPDT trial was performed on an ultra-precision diamondturning machine (Moore Nanotech 350 UPL). This machine tool has
Nomenclatures
dc critical chip thicknessE elastic modulus of the materialfmax critical feed rateH hardnessKc fracture toughness
N spindle speedR nose radius of the cutting toolRa average surface roughnesstmax maximum critical chip thicknessV cutting speedYc critical crack length
Table 1Critical properties of 6H-SiC.
Sl. no. Material properties Unit of
measurement
Values
Fracture toughness Kc MPa m1/2 1.9
Hardness (H) GPa 22
elastic modulus E GPa 347.01
1 Critical crack length Yc ¼ 120Kc2=H2 mm 0.895
2 Critical chip thickness dc ¼ 0:15 EH
Kc
H
� �2 mm 0.01764
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a liquid cooled air bearing spindle with a motion error of less than50 nm while its driving system resolution is up to 0.034 nm [30].A snapshot of the total experimental assembly is shown in Fig. 1.
A three-component force dynamometer unit (Kistler 9257-B)was used for the measurement of the cutting forces. A non-contact measurement of surface roughness was done through awhite light interferometer (Zygo NewView 5000) while a formTalysurf surface profilometer was used to measure the surfacefinish via contact measurement. The cutting tool was examined ina high magnification scanning electron microscope (SEM FIB- FEIQuanta 3D FEG). The workpiece specimen used was a N type-6H-SiC wafer of diameter 50 mm and thickness 5 mm with crystalorientation (001). Conventionally, round nose cutting tools andlow feed rates are preferred to obtain a crack free machinedsurface while machining brittle materials [31]. This experimentalstudy also adopted a round nose cutting tool. A single crystaldiamond cutting tool (cubic orientation) having negative rakeangle of 251, 2 mm tool nose radius and 101 clearance angle wasused. The machining parameters used in this study were calcu-lated by combining the experimental variables and empiricallyknown relations shown in Table 3.
4. Experimental observations
4.1. Brittle–ductile transition, chip formation and cutting forces
Fig. 2 shows a cross sectional image of the ductile-regimemachining model. Since than, this model has been used todemonstrate the machining mechanism for all the brittle
materials. During SPDT, the undesirable fracture damage is assumedto originate at the critical chip thickness (dc) which propagates to adepth yc. As long as the fracture damage does not penetrateunderneath the finished machined surface, ductile regime machin-ing could be executed consistently. The fact to be noticed here isthat in the remaining region of uncut shoulder, even if the fracturedamage occurs, the fractured material is carried away by the tool inthe subsequent cuts. This phenomenon highlights the fact that thematerials possessing short critical crack length are more amenableto SPDT.
In the current work, a DXR Raman microscope developed byThermo Scientific Limited was used to obtain an image shown inFig. 3 from an uncut shoulder of 6H-SiC. Fig. 3 clearly shows theoccurrence of brittle–ductile transition and the associated experi-mental measurements in accordance with the ductile-regimemachining model. Similar to other classes of brittle materials suchas silicon and germanium, the occurrence of brittle–ductile transi-tion was thus found to occur in 6H-SiC as well which explains theroot of the ductility offered by 6H-SiC during the SPDT operation.
An interesting fact to be noted is that the critical depth of cutdc for 6H-SiC is only 70 nm [25] in contrast to the critical depth ofcut of another polytype of SiC e.g. 4H-SiC where dc was obtainedas 820 nm [32]. This observation suggests that the materialremoval rate (MRR) under the same ductile-regime machiningconditions would be significantly higher in 4H-SiC in comparisonto 6H-SiC. However, a trade-off among the quality of finishedsurface, sub-surface deformation lattice layer depth, tool wearand machining efficiency would dictate a choice between thesetwo polytypes of SiC [21].
On the other hand, polycrystalline SiC has been found moremachinable compared to a single crystal SiC on account of theease of chip formation as shown in detail in Fig. 4 obtained bymolecular dynamics simulation [27]. Fig. 4 is a comparison of chipmorphology between machining a single crystal SiC and a poly-crystalline SiC. As evident from Fig. 4, the absence of grainboundaries causes tremendous lattice distortion which could beresponsible for the structural transformation of the cutting chipsof single crystal SiC [33].
Such phase transformations are, however, obstructed by thepresence of grain boundaries during machining of polycrystallineworkpiece such as RB-SiC. In an RB-SiC workpiece, the grains ofSiC are oriented in different crystal orientations. Since grainorientation changes from one crystal to another in polycrystallineSiC, the cutting tool experiences work material with differentcrystallographic orientations and directions of cutting. Thus, someof the grain boundaries cause the individual grains to slide alongthe easy cleavage direction. This causes the build-up of stresses atthe grain boundaries. Consequently, the cutting chips in RB-SiCare not deformed by plastic deformation alone rather a combina-tion of the phase transformation at the grain boundaries and slipof the large size grains both precede in tandem. This is the reason
Table 2Experimental trials reported so far on nanometric cutting of SiC.
Sl. no. Work material Reference study Experimental outcome Coolant used
1 RB-SiC Yan et al. [22] Ra: 23 nm (at high feed rate of 72 mm/rev) Dry cutting
2 RB-SiC Yan et al. [23] Ra: 20 nm; Rz: 400 nm Ra: 14 nm; Rz: 300 nm Grease of MoS2 nanoparticles Grease of Cu nanoparticles
3 3C-CVD SiC Ravindra and Patten [10] Ra: 83 nm and Rz: 530 nm
against as received
Ra: 1.158 mm and Rz: 8.486 mm.
Masterpolish 2 final polishing suspension (contains
alumina and colloidal silica with a pH�9)
4 6H-SiC Patten et al. [4,9,24] Ra: Not specified Dry cutting
5 6H-SiC Jacob et al. [25] Only a scratching test was performed to establish DBT depth which was found to be about 70 nm for 6H-SiC
6 4H-SiC Ravindra and Patten [5] Only a scratching test was performed to establish DBT depth which was found to be about 820 nm for 4H-SiC
7 4H-SiC Shayan et al. [26] Laser assisted nano-scratching was done to observe the improvement in the machinability of SiC
Fig. 1. Experimental setup.
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that while silicon bonds underwent amorphization, no phasetransformation of 6H-SiC grains was observed while RB-SiC wasdiamond turned [22]. Since, the cleavage of 6H-SiC grains couldoccur in a random fashion, this mechanism of chip formationexplains the observation of high machined surface roughness onRB–SiC compared to single crystal SiC.
The former part of Fig. 4 also shows schematically theorientation of the components of cutting force acting on thecutting tool during a cutting operation. The ‘‘tangential cuttingforce’’ (Fx) acts in the x direction, the ‘‘thrust force’’ (Fy) acts in they direction and Fz acts in the direction orthogonal to the X and Y
planes. The evolution of these cutting forces over the period of 2 sis presented in Fig. 5.
It is evident from Fig. 5 that the thrust forces were almost4 times higher in magnitude than the tangential cutting forces.This could be attributed to the use of high negative tool rake anglewhich is central to any SPDT operation. A negative rake causes anincrease in the thrust forces [34,35] in contrast to the conven-tional macro-scale machining, where positive rake angle tools arenormally used. It is of interest to note that an MD simulationstudy reported that the dominance of thrust forces over cuttingforces during nanometric cutting is a necessary requirement toexecute ductile-regime machining conditions [36]. While thisappears in accordance with the current experimental trial onsingle crystal 6H-SiC, this is not the case observed in nanoscalefriction based studies where cutting forces were found dominantover thrust forces [21,37]. Therefore, this is an area of investiga-tion yet to be researched. Furthermore, in contrast to machiningsilicon [38], both tangential cutting forces and thrust forcesduring SPDT of 6H-SiC were found almost two and half timeslarger, suggesting that the cutting resistance of 6H-SiC is sig-nificantly higher than that of silicon [21].
4.2. Surface roughness
Fig. 6 shows the experimental measurement of machinedsurface roughness on 6H-SiC during first kilometre of cuttinglength. The Ra value obtained through Form Talysurf was found to
qNote: values of dc and yc are taken from Table 1. (mm/rev) 0.61–say 0.65
5 Maximum critical depth (d) of cut for 6H-SiC nm 70 [25]
6 Spindle speed N¼ ð1000VÞ=ðpDÞ RPM 382
7 Maximum critical chip thickness when f offiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Rd�d2
p, where Rbf, Rbd and R is in mm. tmax ¼ f
ffiffiffiffi2dR
q[22] nm 5.438
8 Coolant pH value 7 Distilled water
Fig. 2. Ductile-regime machining model [18].
Fig. 3. Measured uncut shoulder of diamond turned 6H-SiC using a DXR Raman
microscope.
8.52 nm
Single crystal SiC workpiece
Polycrystalline SiC workpiece
Tool
Tool
14.24 nm
Fig. 4. Difference in chip formation mechanism between single crystal and poly-
crystal SiC [27].
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be 9.2 nm while white light interferometer measurement wasrecorded to be 10 nm.
Comparing the Ra value obtained in this work with the pre-viously reported Ra values (shown in Table 1) confirms the earlierspeculation that a single crystal SiC provides a better measure of Ravalue than polycrystalline SiC [27]. It is also now known thatwavelengths in the IR spectral region are longer than those of thevisible region, hence, surface roughness specifications are not verystringent for visible components [39]. The surface roughnessmeasurement on 6H-SiC obtained in the current work demon-strates that SPDT is capable of generating a surface suitable forvisible optics on 6H-SiC directly in a single pass, albeit, for a smallercutting distance.
4.3. Tool wear
SiC is known to be chemically inert and therefore the influenceof tribochemistry on the wear of diamond tools, unlike machiningsilicon, becomes negligible [40,41]. However, abrasive wear is aptto occur during the tribological contact of diamond and SiC owing
to their ultra-high hardness [28]. Fig. 7 shows a SEM image of thediamond cutting tool before cutting.
It can be seen from Fig. 7 that before cutting, the cutting edgewas extremely sharp and both tool flank face and tool rake facewere prepared extremely fine without any visible wear marks onthe edge or the surface of the tool. Fig. 8 shows the SEM snapshotof the diamond tool on the same magnification after 1 km ofcutting length. It can be seen that the cutting tool has started toshow wear marks on the flank face and the edge radius hasstarted to lose its sharpness. In some areas recession of thecutting edge is also visible.
Besides recession of the cutting edge, significant wear markson the tool flank face can also be seen. An interesting observationhowever was that the cutting chips were observed to cling to thetool rake face despite using the liquid coolant. The clinging ofcutting chips to the rake face of the cutting tool suggests theexistence of very high stress and high flash temperature in thecutting zone during machining of SiC [28,33]. A recent simulationbased study has showed interfacial abrasion to be the dominantmechanism of tool wear during SPDT of SiC which results ingraphitization of the diamond [28,42]. The ability of distilled
122112201219
0
2
4
6
8
10
12
14
Forc
es (N
ewto
n)
Time (seconds)
FzFy (Thrust force)Fx (Tangential cutting force)
Fig. 5. Experimental measurement of cutting forces during SPDT of single crystal
6H-SiC.
Fig. 6. Ra of 9.2 nm measured by form Talysurf surface profiler after cutting length of 1 km.
Fig. 7. SEM image of the diamond cutting tool before cutting.
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water to suppress tool wear will be investigated in a future workalong with a comparison with other coolants especially coppernanoparticles [23], which were found to provide superior perfor-mance while machining RB-SiC.
5. Conclusions
Over the past decade, the proliferation of single point diamondturning (SPDT) investigations has enabled generation of opticalfinished surfaces on various categories of brittle materials. SPDTof 6H-SiC in the current work is yet another benchmark to thissequence. This study in its current format provides an impetus tounderstand the microscopic aspects of brittle–ductile transitionduring SPDT of single crystal silicon carbide. The followingconclusions are made based on the discussions made in theearlier sections:
1. Single crystal 6H-SiC was diamond turned using a specificcoolant of distilled water with pH value of 7. A surfaceroughness of Ra value¼9.2 nm was obtained, making SPDTas a feasible option to generate visible range optics on singlecrystal SiC in a single pass, albeit, for smaller cutting distances.
2. The microscopic mechanism for material removal in singlecrystal SiC involves ductile deformation and brittle fracture, inaccordance with the ductile-regime machining model pro-posed long back in the year 1990. The material removalbehaviour seems to be influenced by the type of coolant usedwhich provided an improved machined surface roughness inthe current investigation.
3. Significant wear marks on the tool cutting edge and clinging ofthe cutting chips/debris despite the usage of liquid coolantwere observed which impeded the successful execution ofductile-regime machining operation on a larger size siliconcarbide workpiece.
4. The occurrence of brittle–ductile transition was capturedthrough a state-of-art DXR Raman microscope. The cuttingforces during SPDT of 6H-SiC were found to be on very higherside. They were almost two and half times the magnitude ofcutting forces while machining single crystal silicon, signifyingtremendous cutting resistance of SiC than that of silicon.
Thrust forces were almost four times the cutting forces whichare attributed to the use of high negative tool rake angle.
5. The chip formation mechanism in the case of single crystal SiC andpolycrystalline SiC (RB-SiC) is significantly different. While it hasbeen realized over the past decade that structural transformationsof brittle materials are responsible for their ductile response orductile regime machining, this is not the case with RB-SiC. It hasbeen shown in this work that RB-SiC, unlike single crystal SiC,involved a different mechanism of chip formation which wasfound responsible for not observing the phase transformation of6H-SiC grains in a previously reported experimental study.
Acknowledgements
The authors would like to thank Mr. Alan Heaume (CranfieldUniversity) and Dr. Jining Sun (Heriot-Watt University) for theirexperimental assistance. Helpful suggestions of Dr. John Patten(Western Michigan University, USA) and Dr. Jiwang Yan (KieoUniversity, Japan) are sincerely appreciated. This work is a part ofPh.D. project which was funded through Scottish Overseas ResearchStudents Award (with additional funding from the Neilson fund)from the School of Engineering and Physical Sciences of Heriot-WattUniversity, UK, to which the first author (SG) will remain deeplyindebted.
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