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Wear 267 (2009) 991–995

Contents lists available at ScienceDirect

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hort communication

elamination wear of nano-diamond coated cutting tools inomposite machining

. Qina, J. Hua, Y.K. Choua,∗, R.G. Thompsonb

The University of Alabama, Mechanical Engineering Department, Tuscaloosa, AL 35487, USAVista Engineering and Consulting, LLC, 2500 1st Ave. N., Suite B117, Birmingham, AL 35203, USA

r t i c l e i n f o

rticle history:eceived 1 September 2008eceived in revised form3 December 2008ccepted 24 December 2008

eywords:coustic emissionelaminationiamond coatingachining

ool wear

a b s t r a c t

Nanostructured diamond (nano-diamond) coated cutting tools have a potential to supplant costly poly-crystalline diamond tools. However, coating delaminations remain the primary wear mode that oftenresults in catastrophic tool failures. Studying tool wear will help to understand machining parame-ter effects on coating delaminations. Moreover, monitoring coating delamination events can preventproduction loss and assist process planning.

In this study, nano-diamond coated cutting tools were investigated in machining aluminium matrixcomposites. Outside-diameter turning with a wide range of cutting conditions was conducted. Tool flankwear-land was periodically measured by optical microscopy and worn tools were examined using scan-ning electron microscopy. A dynamometer and an acoustic emission (AE) sensor were also used to monitortool conditions during machining operations.

The results are summarized as follows. (1) Tool wear evolutions include a low wear rate followed by anabrupt increase of flank wear due to coating delaminations. Such behaviour is consistent in all machiningconditions tested. (2) Once coating is delaminated, cutting forces increase sharply along cutting with ahigh level of dynamic forces, especially for the radial and axial components. (3) AE signals including raw

data, root-mean-square (RMS) values, and frequency responses all show distinct features before and aftercoating delaminations.

Significant conclusions drawn from this study include the following. (1) The feed has a more dominanteffect on cutting durations prior to coating delaminations because of the increased mechanical load.(2) Nano-diamond coating tools have greater coating delamination wear resistance than conventional

-coaed by

microcrystalline diamondoccur, which are recogniz

. Introduction

Synthetic polycrystalline diamond (PCD) is commonly used inhe industry to machine non-ferrous materials because of its excep-ional tribological properties. However, processing and fabricationsf PCD tools are of high cost. On the other hand, diamond-coatedools made by chemical vapour deposition (CVD) processes haveeen developed and evaluated in various machining applications1,2]. Surveys of CVD diamond-coating tool performance [3] showhat there are mixed results of CVD diamond tool performance. The

ajority reported that wear resistance of CVD diamond tools is

till distant to PCD counterparts. Some studies further observedoating flaking and gaps between the coating and the substratefter machining [4]. Several new deposition technologies have beenenovated to improve diamond-coating tools. For example, a high-

∗ Corresponding author. Tel.: +1 205 348 0044; fax: +1 205 348 6419.E-mail address: kchou@eng.ua.edu (Y.K. Chou).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2008.12.065

ted tools. (3) AE signals may be used to identify if coating delaminationssignificant reductions of AE-RMS values.

© 2009 Elsevier B.V. All rights reserved.

power microwave plasma-assisted CVD technology was developedto increase diamond-coating quality with nanostructured diamondfilms (nano-diamond), offering high hardness, smooth surfaces, andimproved adhesion [5,6].

Coating delaminations are the primary wear mechanism andoften result in catastrophic tool failures that impart part quality andinterrupts machining operations. Therefore, tool wear monitoring,with coating delaminations emphasized, can prevent productionloss and assist process planning. Moreover, studying how delami-nation wear is affected by machining parameters will understandfactors triggering coating delaminations.

Acoustic emission (AE) sensors have been utilized as a feasi-ble method for in-process tool wear monitoring due to the greatsensitivity of AE signals to tool wear [7]. Moriwaki and Tobito

reported that there was a transition from the continuous to burst-type AE amplitudes with the progression of tool wear [8]. Choand Komvopoulos performed the frequency analysis of the AE sig-nals collected when machining AISI 4340 steel using coated tools[9]. Their results indicated that plastic deformation generated low-

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requency continuous signals, whereas coating delaminations andarbide removals produced high-frequency signals.

In this study, machining of aluminium (Al) matrix compositesas conducted to investigate nano-diamond-coated cutting tools.wide range of cutting conditions was tested. Flank wear-land

evelopments were analyzed with tool wear patterns character-zed by scanning electron microscopy (SEM). In addition, AE signals

ere monitored during machining. The objective was to character-ze nano-diamond-coating tool wear, investigate cutting parameterffects on delamination wear, and to monitor coating delaminationvents during machining.

. Experimental procedures

.1. Materials and geometry

For nano-diamond-coating tool fabrications, the substrates usedere fine-grain tungsten carbides (WC) with 6 wt.% cobalt (K68

rom Kennametal). The substrates were square-shaped insertsSPG422) that are 12.7 mm wide and 3.2 mm thick with a 0.8 mmorner radius. The diamond film was deposited using a high-powericrowave plasma-assisted CVD process. A gas mixture of methane

n hydrogen was used as the feedstock gas. Nitrogen, maintainedt a certain ratio to methane, was inserted to the gas mixture tobtain nanostructures by preventing cellular growth. The pressureas about 90 Torr, the substrate temperature was about 800 ◦C, and

he deposition rate was roughly 1 �m/h. The coating thickness athe rake surface was estimated as 25–30 �m. Conventional micro-rystalline diamond-coating tools (named MCD) of the same shapend size (SPG422) with comparable coating thickness were evalu-ted as well. Fig. 1 compares the microstructure, around the cuttingdge, of both nano-diamond and MCD coated tools. A separate studyhowed that the grain sizes of the MCD tools are 3–5 �m in average10]. On the other hand, the nano-diamond-coated tool shown hereas ultrafine grains.

.2. Testing device and method

A computer numerical control (CNC) lathe, Hardinge Cobra 42,as used to perform machining experiments, outside-diameter

urning, to evaluate the wear behaviour of nano-diamond-coatedools. The nano-diamond-coated cutting inserts formed a 0◦ rakengle, 11◦ relief angle, and 75◦ lead angle when mounted on a

Fig. 1. Microstructures around the cutting edge of diam

Fig. 2. Machine and instrument setup for the cutting experiment.

tool holder used (CSRNL-164D). The workpieces were round barsmade of A359/SiC-20p composite. Machining parameters used con-sisted of two levels of cutting speeds (4 m/s and 8 m/s) and twolevels of feeds (0.15 mm/rev and 0.3 mm/rev). The depth of cutwas kept constant at 1 mm. Machining was conducted at roomtemperature without coolant. For each machining condition, twotests were repeated. During machining testing, the cutting insertswere periodically inspected by optical microscopy to measure flankwear-land. Worn tools after testing were also examined by SEM. Inaddition, cutting forces were monitored during machining using aKistler dynamometer (9257B). An AE sensor was also employed toacquire both AE-RAW (raw data) and AE-RMS (root-mean-squarevalue) data during the entire machining operation. The sensor usedwas 8152B Piezotron sensor from Kistler. The AE-RAW and AE-RMSwere collected at a 500 kHz sampling rate each. Fig. 2 shows thesetup of machining experiments.

3. Results and discussion

3.1. Tool wear vs. cutting conditions

Fig. 3 shows tool wear growth, flank wear-land width (VB), alongcutting time at different machining conditions. Results of two repli-

ond-coated tools: (a) nano-diamond and (b) MCD.

F. Qin et al. / Wear 267

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ig. 3. Flank wear-land (VB) vs. cutting time at different machining conditions.

ates are shown. In general, the tools showed a slow increase ofool wear followed by an abrupt increase of wear-land in one singleass. It is believed that, during that specific pass, coating delamina-ion occurred and resulted in rapid wear of the exposed substrate

aterial, i.e., carbide. Tool wear and onset of coating delaminationabrupt wear increase) are strongly dependent on machining con-itions. The feed is more dominant, compare to the cutting speed, tooating delaminations, which is due to excessive mechanical loads.or the high feed and low speed condition (0.3 mm/rev, 4 m/s), toolife before the abrupt wear increase was less than 1 min. However,or the low feed and high speed case (0.15 mm/rev, 8 m/s), the toolasted about 2 min before coating delaminations. On the other hand,or MCD tools tested at 4 m/s and 0.3 m/rev, coating delaminationsccurred at just 12 s of cutting time. Further, as a baseline compar-son, a plain carbide tool, same as the substrate of the coated tools,

as tested as well at the gentle machining condition, namely, 4 m/s

nd 0.15 mm/rev. Flank wear-land reached 0.6 mm in just about 15 s,n contrast to 0.3 mm at between 130 s and 150 s cutting time forhe nano-diamond-coated tool. Thus, it is concluded that the devel-

Fig. 4. Worn nano-diamond-coated tools after testing: (a) 4

Fig. 5. Detailed SEM images around the coating failure boundary (highlighted

(2009) 991–995 993

oped nano-diamond-coated tools are far more effective, one orderof magnitude better, than widely used carbide tools.

3.2. Tool wear patterns

Fig. 4 shows examples of worn tool images, from SEM, aftermachining testing. The tools were cleaned, by 10 vol.% hydrochlo-ric acid, to remove the metal deposit covering some cutting area.Flank wear-land is the major wear pattern and crater wear is notidentified. Moreover, both mild and aggressive cutting conditions,i.e., 4 m/s and 0.15 mm/rev, and 8 m/s and 0.3 mm/rev, respectively,show coating delaminations. Fig. 5 shows high-magnification SEMimages of the highlighted areas in Fig. 4, around the coating failureboundary.

3.3. Cutting force analysis

Fig. 6a shows cutting force evolutions during initial cutting (firstpass) at 8 m/s and 0.3 mm/rev. The force values, both tangentialcomponent (Ft) and radial component (Fr), were steady during theentire pass. Fig. 6b, on the other hand, shows cutting force evo-lutions during the tool failure pass when coating delaminationsoccurred. The initial forces in the tool failure pass were slightlyhigher than those in the first pass. But, a sharp increase along thecutting time with a high level of dynamic forces is noticed. In partic-ular, the radial component increased significantly and approachedto the level of the tangential component.

3.4. AE signal analysis

Fig. 7a and b shows typical AE-RAW signals during machin-ing, one for initial cutting pass and the other for tool failure pass,which shows reduction in intensity compared to initial cutting.Fig. 8a compares AE-RMS evolutions during two cutting passes:

m/s and 0.15 mm/rev and (b) 8 m/s and 0.3 mm/rev.

boxes in Fig. 4): (a) 4 m/s and 0.15 mm/rev and (b) 8 m/s and 0.3 mm/rev.

994 F. Qin et al. / Wear 267 (2009) 991–995

Fig. 6. Cutting force evolutions during machining (8 m/s, 0.3 mm/rev): (a) initial cutting pass and (b) tool failure pass.

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Fig. 7. AE-RAW signals: (a) initial cutting pas

nitial cutting and tool failure. Compared to initial cutting, the RMSntensity during the tool failure pass is lower and shows noticeableontinuous decreasing, over 20% intensity reduction in less than0 s cutting. The intensity decrease is due to the change of con-act pairs (diamond-composite vs. WC-composite), which stronglyffect AE generations. The frequency responses of AE signals alsohow distinct features between before and after coating delamina-ions. Fig. 8b compares Fast Fourier Transform (FFT) spectra of AEignals. For the tool failure pass, the amplitude of low-frequencyeaks (∼20–30 kHz), which are correlated to chip formations, were

ubstantially reduced. On the other hand, the intensity of high-requency components (∼120–130 kHz) becomes relatively higherue to intense tribological contacts with the increased wear-landrea; the ratio of high- to low-frequency components is greaterhan 1.0 vs. less than 0.5 at initial cutting. As shown earlier, coating

Fig. 8. AE signal comparisons between initial cutting pass and tool fail

(b) tool failure pass (8 m/s and 0.3 mm/rev).

delamination is the major wear mechanism of diamond-coated cut-ting tools, which is followed by catastrophic tool failure/breakagethat may damage the part or machine tool. Prediction of delamina-tion occurrence is still of great difficulty. Thus, the significance ofAE signals is that it may provide tool users with a means of con-dition monitoring of diamond-coated tools because AE response issensitive to coating delamination. In particular, the AE-RMS valuesdecrease noticeably because the contact of the tool material withthe workpiece changes from diamond to carbide.

Cutting parameters effects on AE signals were also examined.

AE intensity is more dominated by the cutting speed, insteadof the feed. For the repeated tests, AE signals show the sametrend between the duplicated tests; i.e., once coating delamina-tions occurred, the RMS values were reduced and continuouslydecreased, and for the FFT spectra, the amplitude of low-frequency

ure pass: (a) RMS signal and (b) FFT spectra (8 m/s, 0.3 mm/rev).

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eaks was significantly lowered. However, the signals were notuantitatively repeatable, because AE is very sensitive to anyinute changes.

. Conclusions

Nano-diamond-coated cutting tools were evaluated in machin-ng composites at a wide range of cutting conditions. Tool wearas periodically measured and analyzed, with special attention

o abrupt wear-land increases due to coating delaminations. Inddition, cutting forces and acoustic emission were continuouslyonitored to relate to tool wear events during machining. After theachining experiments, worn tools were also examined by SEM.The results can be summarized as follows:

Tool wear evolutions include a low wear rate followed by anabrupt increase of flank wear due to coating delaminations, whichoccurred at all cutting conditions tested. Such behaviour is con-sistent in all machining conditions tested. In addition, the feedhas a more dominant effect on cutting durations prior to coatingdelaminations because of the increased mechanical load.Tool life of nano-diamond-coated tools is substantially longerthan that of plain carbide tools. Moreover, nano-diamond-coatingtools show significantly greater delamination wear resistancethan conventional microcrystalline diamond-coated tools.Cutting force data shows, during the tool failure pass, a sharp

increase along cutting with a high level of dynamic forces, espe-cially for the radial and axial components.For AE signals, the RMS intensity becomes lower after coatingdelaminations and shows continuous decreasing with increasingcutting, which may be used to monitor coating failures. Though

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AE FFT spectra also exhibit some distinct features before and aftercoating failure, it is less ideal for coating delamination monitoringdue to post-processing needed.

Acknowledgement

This research is supported by NSF, Grant No. CMMI 0728228.

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