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Surface & Coatings Technology

Characterizations of nano-crystalline diamond coating cutting tools

J. Hu a, Y.K. Chou a,⁎, R.G. Thompson b, J. Burgess c, S. Street c

a Mechanical Engineering Department, The University of Alabama, Tuscaloosa, Alabama, United Statesb Vista Engineering, Inc., Birmingham, Alabama, United States

c Chemistry Department, The University of Alabama, Tuscaloosa, Alabama, United States

Available online 2 August 2007

Abstract

Diamond coatings made by processes such as chemical vapor deposition (CVD) have been increasingly explored for cutting tools applications,in particular, for machining lightweight high-strength materials. However, most literature to date reported that the wear resistance of CVDdiamond tools is still distant to the polycrystalline diamond (PCD) counterparts. Recently, a microwave plasma-assisted CVD technology wasdeveloped to increase the diamond growth rate, and by adding nitrogen gas, this process can produce nano-structured coatings which consist ofnano-diamond crystals embedded into a hard amorphous diamond-like carbon matrix and have high hardness and low surface roughness.

In this study, the nano-crystalline diamond (NCD) coating tools were characterized, compared to the conventional microcrystalline diamondcoating (MCD) and PCD tools, in surface topography, grain sizes, carbon bonds, and mechanical properties. Moreover, the diamond tools wereevaluated in machining Al-matrix composite and high-strength Al alloy. The results show that the NCD tools have smoother surfaces than theMCD tools, but rougher than the polished PCD tools. As to the diamond crystals, the NCD tools have ultrafine grains and the PCD tools have thelargest grains. For the cutting edges, the PCD tools have the sharpest edge resulted from polishing and the NCD tools have larger edge roundingthan the MCD counterparts. In addition to the nature-diamond peak identified in the Raman spectra, the NCD and MCD tools have additionalRaman shifts associated with graphite and other disordered carbon bonds. The NCD tools have a much higher hardness, but a lower elasticity, thanboth the MCD and PCD tools. In machining testing, the NCD tools substantially outperform the MCD tools and are comparable to the PCD tools.For both the NCD and MCD tools, coating delamination is the major tool wear mechanism that leads to catastrophic tool failures.© 2007 Elsevier B.V. All rights reserved.

Keywords: Diamond coating; Nano-crystalline diamond; Machining; Material characterization; Tool wear

1. Introduction

Diamond coatings produced by different CVD processes canhave a range of properties due to the diamond purity, micro/nano-structures, and the interface with the substrate, etc.Diamond purity and bond characteristics are generally evalu-ated using Raman spectroscopy, which relates molecularvibration frequencies to the inter-atomic bonds [1,2]. Diamondcrystalline sizes and qualities have been evaluated by X-raydiffractometry (XRD) [3]. Nano-indentation has been particu-larly used for elasticity and hardness measurements [4].

Despite of their exceptional tribological properties for wearresistance, coating delamination is themajor failuremode ofCVD

⁎ Corresponding author. The University of Alabama, Mechanical EngineeringDepartment, 290 Hardaway Hall, 7th Ave., Tuscaloosa, Al 45487-0076, UnitedStates. Tel.: +1 205 348 0044; fax: +1 205 348 6419.

E-mail address: kchou@eng.ua.edu (Y.K. Chou).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.07.050

diamond coatings [2]. The adhesion problem at the interfacecombined with interfacial stresses lead to premature failuresduring machining by debonding. Various techniques have beenproposed to enhance the adhesion strength [5–8]. Diamondcoating adhesion has been qualitatively evaluated using aRockwell hardness tester [5,9–11] or sliding/scratch test [12].

Diamond coatings, again owing to their extreme properties,have been investigated for cutting tool applications. Studies ofCVD diamond coating tools, mostly tungsten carbide (WC)substrates, have been frequently reported. There are, however,mixed results of the CVD diamond tool performance. In a fewapplications, CVD diamond shows tool life comparable to PCDtools in machining high-Si Al alloys [6,13,14]. However, Shencited large variations of coating performance [14]. On the otherhand, several studies reported that wear resistance of CVDdiamond tools is still distant to PCD counterparts [15–21],though some argued that CVD diamond tools have potentialeconomical benefit because of multiple cutting tips per insert

1114 J. Hu et al. / Surface & Coatings Technology 202 (2007) 1113–1117

[18]. Several studies further observed coating flaking and gapsbetween the coating and the substrate after machining, e.g.,[18]. Chou and Liu also demonstrated that coating delaminationat the tool flank can be of catastrophic nature and is the tool-lifelimiting factor for CVD diamond tools [22].

Various new technologies have been renovated to improvediamond coatings. Nano-crystalline diamond coating, for exam-ple, has been developed and evaluated. Deuerler et al. examinedthe wear behavior of micro- and nano-crystalline diamond filmswith cavitation test [23]. The authors indicated that the nano-structured diamond with its smooth surface betters the micro-structured counterpart for complex-shape geometries. Recently, ahigh-power microwave plasma-assisted CVD technology wasdeveloped to increase the diamond growth rate, and by usingnitrogen gas, this process can produce ultrafine diamond grains inthe order of 10 nm [24]. The produced nano-structured coatingswhich consist of nano-diamond crystals embedded into a hardamorphous diamond-like carbon matrix have high hardness andlow surface roughness [4]. The objective of this research was tocharacterize the newly developed nano-crystalline diamond(NCD) coated tools compared to the conventional CVD diamondcoating tools and PCD tools. The three different diamond toolswere also tested inmachiningwith toolwear conditions evaluated.

2. Experimental details

The substrates used were 6% Co fine grain tungsten carbidesof square-shape inserts (SPG422), 12.7 mm wide and 3.2 mmthick. The inserts were chemically etched for surface cobaltremoval to facilitate diamond coating. The NCD film wasproduced by a high-power microwave plasma-assisted CVDprocess using an in-house built reactor. A schematic example ofmicrowave plasma CVD can be found in literature [25]. Thedeposition conditions were an average H2 flow rate of 1650standard cubic centimeters per minute (sccm), about 50 sccmCH4 flow rate, and 5 sccm N2 flow rate. The chamber pressurewas less than 90 Torr and the substrate temperature was about800 °C. The coating thickness at the bulk area is about 30 μm,comparable to commercial CVD diamond coating tools. Thecoating is uniform over the inset rake surface and the coatingthickness at the flank surface decreases linearly to zero at about0.9 mm from the substrate bottom. The NCD tools werecharacterized and tested in machining compared to commercialCVD diamond coating (named MCD for its micro-crystal sizes)and PCD tools. All tools were of the same size as the NCD toolspecified above. The substrates of coated tools, both NCD andMCD, were commercial carbide inserts that had a nose radius of0.8 mm and a nominal edge radius of 25 μm. The PCD tool alsohad a 0.8 mm nose radius, but a much smaller edge radius.

The diamond tools were first examined in details byscanning electron microscopy (SEM), Philip 30XL, includingthe surface topography, microstructures, grain sizes, and thecutting edges. EDS was also employed to confirm the diamondtool surface composition. A stylus profilometer, which wascalibrated against a standard surface and showed a 0.03 μmdeviation, was used to measure the surface roughness (Ra) of thetool rake faces.

Analysis using Raman spectroscopy was performed on thetools to examine the carbon bonding characteristics. The Ramanscans were conducted using a Jobin-Yvon HR800 UV confocalmicroscopy. The excitation line of 632.81 nm came from anHe–Ne laser with approximately 12 mWof power at the sample.The shifts were detected using a peltier cooled CCD detector.The image used a 10× objective lens and all scans were fixedusing a 100× objective lens. All tools were scanned around thecenter of the rake face in the as-received conditions.

Nano-indentation was performed using a Hysitron TriboIn-denter with a Berkovich pyramidal indenter. Before any test, thetip area function was calibrated by performing a series ofindents on a standard fused quartz specimen at various contactdepths. The contact area at different load was calculated andplotted vs. the corresponding contact depth, and then fitted to asix-order polynomial. The standard procedure for nano-indentation analysis using the region between 20% and 95%of the unloading portion of the load–displacement curve, furtherfitted to a power law relation. The derivative of the power lawrelation with respect to the displacement was evaluated at themaximum load to determine the contact stiffness for the contactdepth estimate, which is subsequently used to compute theindentation hardness. The reduced modulus calculated from thecontact stiffness and the contact area at the calculated contactdepth were then used to estimate the elasticity of the diamondcoating or PCD, assuming an 1140 GPa of Young's modulus forthe diamond indenter and a Poisson's ratio of 0.07 for diamond.

For the machining test, the diamond tools, all indexableinserts, were used with a 25.4-mm thick steel tool-holder thatresulted in 0°, 11°, and 15° of the rake, relief, and lead angles,respectively. Two types of workpieces were used, namely,A359/SiC/20p (20 vol.% SiC particles in the matrix of Al A359alloy) composite and A390 alloy, both in bar stocks. Outsidediameter turning using different diamond tools was carried outon a precision CNC lathe in dry. For composite machining, theprocess parameters were 4 m/s of cutting speed, 0.05 mm/rev offeed, and 1 mm of depth of cut. For A390 machining, 10 m/s ofcutting speed, 0.8 mm/rev of feed, and 1 mm of depth of cutwere used. Part surface finish was periodically measuredthrough the machining test using a profilometer. Tool wear, inflank wear-land width (VB), was measured off-line by opticalmicroscopy. Note that, the work material covered, partially orentirely, the tool flank wear-land in machining and in order notto alter the tool conditions, the wear-land size was estimatedwithout removing the deposit. In the end of the machining test,the deposit was chemically cleaned to expose the actual wear-land and compared to the width-deposit measurements. Worntools were also observed by SEM to examine the wear patternsof different diamond tools.

3. Results and discussion

Fig. 1 shows SEM images comparing the microstructures ofthree different diamond tools. The MCD tool exhibits clearmulti-facet diamond crystals and the grain sizes are estimatedaround 3 to 5 μm in average. The PCD tool shows polishedgrains with the cobalt binder around the grain boundary. The

Fig. 1. Microstructures at the rake face of different diamond tools: (a) MCD, (b)PCD, and (c) NCD.

Fig. 2. Raman spectra of three different diamond tools.

Fig. 3. Mechanical properties of different diamond tools.

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averaged grain size is estimated as 10 to 20 μm. On the otherhand, the NCD tool has ultrafine grains not resolvable even at amagnification of 4000 for the SEM used. In addition, it wasobserved that the MCD tool has the roughest surface because ofdiamond crystals. In contrast, the PCD tool has the smoothestsurface resulted from polishing. Further, the NCD tool also

shows a fairly smooth surface that replicates the texture of thesubstrate surface. Measured surface roughness values, in Ra, ofthe tool rake surfaces were 0.47 μm, 0.08 μm, and 0.33 μm inaverage for the MCD, PCD, and NCD tools, respectively. As tothe cutting edge, coated tools are expected to have a roundededge due to the added materials, while the polished PCD toolhas the sharpest cutting edge from polishing. It is further noticedthat the NCD tool has a more rounded edge than the MCD tool.This may be due to higher local temperatures around the edgesduring the CVD process [26].

The Raman spectra, Fig. 2, show that all three types ofdiamond tools have a clear peak associated with the naturediamond (sp3, 1332.5 cm−1). However, no noticeable peak-location shifting was observed from the coating tools. TheMCD and NCD tools have additional peaks possibly associatedwith amorphous carbon (1540 cm−1), graphite (1585 cm−1),and disorder sp3 (1150 cm−1) and sp2 (1350 cm−1), etc. [2]. TheRaman spectrum of the NCD tool also supports that it has muchsmaller crystal sizes [27].

The nano-indentation results, Fig. 3, indicate that the NCDtool has a much greater hardness than the MCD and PCD tools,about 81 GPa vs. 57 and 50 GPa, respectively. On the otherhand, the NCD tool has the lowest elasticity among the three

Fig. 4. Flank wear of different diamond tools in machining Al-matrix composite:(a) MCD, (b) PCD, and (c) NCD.

1116 J. Hu et al. / Surface & Coatings Technology 202 (2007) 1113–1117

diamond tools, 684 GPa vs. 1027 and 1019 GPa for the MCDand PCD tools, respectively. Since the thermal strain mismatchis proportional to the elasticity, the low elasticity of the NCDtool implies a lower deposition-induced residual stress gener-ated in the NCD tool.

Fig. 4 shows the tool wear history, in flank wear-land width(VB), in machining Al-matrix composite using differentdiamond tools. The MCD tool reached 0.8 mm VB at 2.8 minof cutting time vs. 0.1 mm VB at 12.5 min for the NCD tool.The abrupt wear increase of the MCD tool indicates catastrophiccoating failures. On the other hand, the NCD tool retainedsteady wear growth similar to the PCD tool. Surface roughness,in Ra, of machined parts were in a similar range, 0.60 to1.01 μm, 0.75 to 1.05 μm, and 0.46 to 0.75 μm by the MCD,PCD, and NCD tools, respectively. Interestingly, the larger edgerounding of the NCD tool did not adversely affect the partsurface finish produced.

For machining A390 alloy, the flank wear development ofdifferent diamond tools is shown in Fig. 5. Note that during themachining test, metal deposit sometime occurred around thecutting zone including the wear-land. The aggressive machiningconditions, 10 m/s and 0.8 mm/rev, caused rapid tool wear. Forthe MCD tool, premature coating failure occurred and led to asharp tool wear growth, reaching about 1.0 mm VB at just1.2 min of cutting time. On the other hand, the NCD tool had

Fig. 5. Flank wear of different diamond tools in machining A390 alloy: (a)MCD, (b) PCD, and (c) NCD.

greater adhesion and the tool wear approached to that of thePCD tool. However, coating failures eventually occurred anddictated the tool life, at about 2.6 min with a flank wear-land of0.6 mm. Fig. 6 shows SEM images of the worn diamond toolsafter the machining testing and after the metal deposit waschemically removed for the coating tools. Consistently, coatingdelamination is noted as the major tool failure mode fordiamond coated cutting tools.

Fig. 6. SEM images of worn diamond tools after machining A390 alloy: (a)MCD, (b) PCD, and (c) NCD.

1117J. Hu et al. / Surface & Coatings Technology 202 (2007) 1113–1117

The machining results concluded that the NCD toolssubstantially outperformed the conventional MCD tools andwere comparable to the PCD tools. From the nano-indentationtesting, the much higher hardness of the NCD tools appears tobenefit the wear resistance. In addition, a lower elasticity of theNCD tools leads to lower residual stresses that may alleviate theadhesion problem.

4. Conclusions

Nano-crystalline diamond coatings were produced using amicrowave plasma-assisted CVD process and deposited oncommon tungsten carbide tools. The NCD tools were thencharacterized, together with the MCD and PCD tools, in surfacetopography, carbon bonds, and mechanical properties. More-over, the three types of diamond tools were tested in machiningof an Al-matrix composite and a high-strength Al alloy with toolwear evaluated. The results are summarized as follows.

(1) The NCD tools have smoother surfaces than the MCDtools, but rougher than the polished PCD tools. As to thediamond crystals, the NCD tools have ultrafine grainsfollowed by the MCD and then PCD tools.

(2) From the Raman spectra, the NCD and MCD tools haveadditional Raman shifts associated with graphite and otherdisordered bonds, in addition to the carbon sp3 bond.

(3) The NCD tools have a much higher hardness, but a lowerelasticity, than both the MCD and PCD tools.

(4) In machining of both Al-matrix composite and A390alloy, the NCD tools substantially outperform the MCDtools and are comparable to the PCD tools.

(5) For both the NCD and MCD tools, delamination is themajor coating failure mechanism that leads to catastrophictool wear and limits the tool life. In addition, the metaldeposit accumulated at the tool flank wear-land seems toaffect the onset of the coating delamination.

Acknowledgements

This research is supported by NASA through Alabama SpaceGrant Consortium. RGT also acknowledges support from NSF(IIP-0349769). The authors would also like to thank sp3 and Dr.

Raja Kountanya of Diamond Innovations for supplying somediamond tools. Assistance fromM.L.Weaver of UA on the nano-indentation testing is also acknowledged.

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