Nanocrystalline diamond coating tools for machining high-strength

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International Journal of Refractory Metals & Hard Materials 26 (2008) 135–144

Nanocrystalline diamond coating tools for machininghigh-strength Al alloys

J. Hu a, Y.K. Chou a,*, R.G. Thompson b

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

Received 30 March 2007; accepted 22 May 2007

Abstract

Diamond coating tools have been increasingly used for machining advanced materials. Recently, a microwave plasma-assisted chem-ical vapor deposition (CVD) technology was developed to produce diamond coatings which consist of nano-diamond crystals embeddedinto a hard amorphous diamond-like carbon matrix. In this study, the nanocrystalline diamond (NCD) coating tools were evaluated inmachining high-strength aluminum (Al) alloy. The conventional CVD microcrystalline diamond coating (MCD) tools and PCD toolswere also tested for performance comparisons. In addition, stress distributions in diamond coating tools, after deposition and duringmachining, were analyzed using a 2D finite element (FE) thermomechanical model.

The results show that catastrophic failures, reached in all except one machining conditions, limit the NCD tool life, which is primarilyaffected by the cutting speed. In addition, coating delamination in the worn NCD tools is clearly evident from scanning electron micros-copy (SEM) and force monitoring in machining can capture the delamination incident. At a high feed, coating delamination may extendto the rake face. Furthermore, SEM observations of coating failure boundaries show intimate coating-substrate contact. Though theNCD tools are inferior to the PCD tools, they substantially outperform the MCD tools, which failed by premature delamination.The diamond coating tools can have high residual stresses from the deposition and stresses at the cutting edge are highly augmented.Further machining loading causes the stress reversal pattern which seems to correlate with the tool wear severity.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Coating delamination; Diamond coating; Machining forces; Nanocrystalline diamond; Tool wear

1. Introduction

Diamond coatings, owing to their extreme propertiescomparable to or even exceeding synthetic polycrystallinediamond (PCD), processed by high pressures and high tem-peratures, have been frequently explored for cutting toolapplications. Hot-filament chemical vapor deposition(CVD), in which the gas phase activation is controlled byheating the filament, is one of primarily used processesbecause of the relatively low equipment cost and maturefabrication technologies. Thin film CVD diamond over

0263-4368/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmhm.2007.05.012

* Corresponding author.E-mail address: [email protected] (Y.K. Chou).

50 lm thick has been deposited on various materialsincluding tungsten carbides (WC).

Investigations of CVD diamond coating tools have beenfrequently reported. There are, however, mixed results ofthe CVD diamond tool performance. In a few applications,CVD diamond-coated tools show a tool life comparable toPCD tools in machining. Oles et al. tested diamond coatingtools with different pre-deposition treatments in machiningof high-Si Al alloys [1]. The results showed that the CVDdiamond tools can meet or even exceed the PCD tool in toollife. However, the part surface finishes produced by theCVD diamond tools are poor than the finish by the PCDtools. Shen evaluated CVD diamond tools from a varietyof sources in industrial machining settings and reportedlarge variations of machining results and that a few types

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136 J. Hu et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 135–144

of diamond coating tools have the performance comparableto the PCD tools [2,3]. Belmonte et al. showed that thickCVD diamond tools (�0.3 mm film) can top PCD toolsand have greater flank-wear resistance in machining hard-metal, WC [4]. Karner et al. investigated CVD diamondtools in a broad range of production processes [5]. In turn-ing copper armature, the CVD diamond tools doubled thetool life of PCD tools. On the other hand, several studiesreported that the wear resistance of CVD diamond toolsis still distant to the PCD counterparts, though some arguedthat CVD diamond tools have potential economic benefitbecause of multiple cutting tips. Schafer et al. assessedCVD diamond coatings on high-speed steel substrates inmachining AlSi10Mg alloys and concluded that the CVDdiamond tools had much poor performance than the PCDtools [6]. In machining silicon-carbide (SiC)–Al matrix com-posites, Andrewes et al. reported that the PCD tools per-form significantly better than the CVD diamond tools,over three times [7]. D’Errico and Calzavarini indicated thatmajority of diamond coating tools, obtained from five dif-ferent manufacturers, have shorter tool lives than thePCD tools in machining Al based composites [8]. Davimreported that in machining SiC–Al matrix composites,PCD tools have a much longer tool life, over 10 times, thanthe CVD diamond tools and can be used at much highercutting speeds, over five times used for the diamond coatingtools [9]. Polini et al. also tested CVD diamond tools withvaried coating parameters in dry turning of alumina-rein-forced Al matrix composites [10]. The authors noted thatthe majority of the tested CVD coating tools have shortertool lives than PCD tools.

Several diamond coating tool studies further discussedcoating tool wear. Andrewes et al. indicated that bothabrasion and adhesion are the wear mechanism with theformer dominant during the initial cutting but the latterdetrimentally limiting the CVD diamond coating tools[7]. Karner et al. showed that flaking of diamond films atthe flank surface is the major wear mechanisms of CVDdiamond coating tools [5]. D’Errico and Calzavarini exam-ined coating failure boundary and observed the coatingdetachments and gaps between the coating and the sub-strate [8]. Chou and Liu also demonstrated that coatingdelaminations at the tool flank can be of catastrophic nat-ure and is the tool-life limiting factor for CVD diamondcoating tools [11]. High stresses and/or degraded adhesionduring machining may result in coating failures and subse-quent rapid wear of the exposed substrate.

In CVD processing, the coating-substrate system returnsto the room temperature from a deposition temperature.The largely mismatched thermal strains between the dia-mond coating and substrates generate high stresses in thecoated tool system as well as the stress discontinuity atthe interface. Diamond coatings bear a compressive resid-ual stress and the carbide substrates receive a stress in ten-sion [12]. Residual stresses in diamond coatings have beenwidely studied using different techniques [13–16]. The resid-ual stresses in diamond coatings depend upon substrate

materials, surface treatments, and deposition temperatures,etc., and can be as high as 6 GPa in compression [17]. Sucha high level of residual stresses has a compound impact tothe coating performance [18].

Recently, a microwave plasma-assisted CVD technologywas developed to increase the diamond growth rate, and byusing nitrogen gas, this process can produce ultrafine dia-mond grains in the order of 10 nm [19]. The produced dia-mond coatings consist of nano-diamond crystals embeddedinto a hard amorphous diamond-like carbon matrix andhave high hardness and low surface roughness [20]. Thistechnology has been applied to fabricate nanocrystallinediamond (NCD) coating tools. The objective of thisresearch was to evaluate the newly developed NCD toolsin machining high-Si Al alloys. Machining performancesuch as tool wear and process variables such as cuttingforces were evaluated in a range of machining conditions.Tool wear conditions were examined in details by scanningelectron microscopy (SEM). In addition, the conventionalCVD diamond coating tools and PCD tools were also eval-uated for comparisons. Moreover, to investigate the stressfield in diamond coating tools, finite element (FE) model-ing was applied for thermal and mechanical simulations,both after the deposition and during machining.

2. Experimental set-up

2.1. NCD coating processing

For NCD coating tool fabrications, the substrates usedwere 6 wt.% cobalt fine-grain tungsten carbides of square-shape inserts (SPG422), 12.7 mm wide and 3.2 mm thick.The NCD film was produced by a high-power microwaveplasma-assisted CVD process using an in-house fabricatedreactor. A gas mixture of methane in hydrogen was used asthe feedstock gas. Nitrogen, maintained at a certain ratioto methane, was inserted to the gas mixture to obtain nano-crystalline microstructures by preventing cellular growth.The pressure was about 90 Torr the substrate temperaturewas 800 �C, and the deposition rate was roughly 1 lm/h.All conditions were fixed to produce NCD tools with consis-tent quality. The coating thickness at the rake surface wasabout 30 lm, comparable to commercial CVD diamondcoating tools. The coating was uniform over the rake faceand the thickness at the flank surface linearly diminishedto nil at about 0.9 mm from the substrate bottom. Commer-cial CVD diamond coating with 30 lm film (named MCDfor its micro-crystal sizes) and PCD tools of the same shapeand size (SPG422) were evaluated as well. A separate studyshowed that the grain sizes of the MCD and PCD tools are3–5 lm and 10–20 lm in average, respectively [21]. On theother hand, the NCD tool has ultrafine grains [21].

2.2. Machining experiment

In machining, a 25.4 mm thick steel tool-holder wasused together with the NCD tools to form 0�, 11�, and

Table 1Composition of A390 alloy

A390 Si Fe Cu Mn Mg Zn Ti Other Al

wt.% 16–18 0.5 4–5 0.1 0.45–0.65 0.1 0.2 0.2 Balanced

0

0.2

0.4

0.6

0.8

0 5 10 15 20Cutting time (min)

VB

(mm

)

3 m/s, 0.2 mm/rev3 m/s, 0.8 mm/rev10 m/s, 0.2 mm/rev10 m/s, 0.8 mm/rev

Fig. 1. Flank wear of NCD tools at different machining conditions.

J. Hu et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 135–144 137

15� of the rake, relief, and lead angles, respectively. Theworkpieces were solid round bars made of A 390 alloy(18 wt.% Si), composition in Table 1. Outside diameterturning using the NCD tools was carried out in a precisioncomputer-numerical-control (CNC) lathe (Hardinge Cobra42) in dry. Machining conditions included four combina-tions of two cutting speeds: 3 m/s and 10 m/s, two feeds:0.2 mm/rev and 0.8 mm/rev, and fixed 1 mm depth ofcut. The commercial CVD diamond coating and PCD toolswere also tested at the most aggressive condition, i.e., 10m/s and 0.8 mm/rev. A triaxial piezoelectric force sensor(Kistler 9257B) with a data acquisition system was usedto continuously monitor three components of cuttingforces during machining, i.e., tangential, radial and axial,respectively. Tool wear, in flank wear-land width (VB),was periodically measured off-line by optical microscopy.Note that, during machining, the work materials depositedand covered the tool flank wear-land, partially or entirely.In order not to alter the tool conditions in the continuedmachining experiment, the wear-land size was estimatedwithout the material removed, if any. In the end of themachining test, the tools were chemically cleaned using a10 vol.% hydrofluric acid solution to remove the adheredwork materials. The exposed wear-land was also measuredand compared to the with-deposit measurements. After thecompletion of machining testing, the worn tools werealso studied by SEM, before and after the deposit cleaning,to examine the wear mechanisms at different machiningconditions.

3. Tool stress study

To evaluate the stress field in a diamond-coated tool,after the deposition and during machining, a simplified2D FE model using ANSYS software was developed forthermal and mechanical analyses with the plane strain con-dition assumed, detailed in [22]. For deposition-stress sim-ulations, static structural analysis with thermal strainsconsidered was conducted. A deposition temperature of800 �C was set as the uniform initial condition and a roomtemperature of 25 �C as the final temperature. Linear-elas-tic material models were applied to both the diamond andWC.

To simulate the stress distributions in the tool duringmachining, the thermal and mechanical contact loads werefirst estimated based on the cutting mechanics analysisusing measured data from the machining test including cut-ting forces, cutting chip thickness, and the chip-tool con-tact length. The measurements were input in the cuttinganalysis [23], orthogonal cutting approximation, to esti-mate the heat flux and heat partitioning as well as the nor-

mal and shear stresses at the tool rake face. Then, transientheat conduction analysis was performed, with the parti-tioned heat flux at the tool-chip contact, to obtain the tem-perature distributions in the diamond coating tool. Thesubstrate bottom was approximate as the room tempera-ture and other surfaces adiabatic. Next, a static structuralanalysis was continued, carrying final temperatures fromthe thermal analysis and initial stresses from the depositionsimulations. The machining contact stresses at the rakeface, also estimated from the cutting mechanics analysis,were applied as the mechanical boundary conditions. Thefinal stress distributions in the diamond coating tool weresimulated and the stresses at the coating-substrate interfacewere analyzed and compared for different machiningconditions.

4. Results and discussion

Fig. 1 compares flank wear vs. cutting time at four dif-ferent machining conditions. For all except the low-speedand low-feed condition, abrupt wear growths occurred dur-ing the last cutting pass and consistently showed cata-strophic coating failures as the tool life limit. It is alsoobserved that the cutting speed is dominant to the tool life.For the most aggressive machining condition (10 m/s and0.8 mm/rev), the tool life was about 2.6 min. with a flankwear-land of 0.6 mm. On the other hand, for the 3 m/sand 0.2 mm/rev condition, coating failures were not notedafter about 16 min. of cutting time with a VB less than0.1 mm.

Fig. 2 shows SEM images of the worn NCD tools afterthe machining test. For the low-speed low-feed condition,the cutting edge has minor traces of wear. The other threemachining conditions resulted in larger worn surfaces andcoating failures were evident with clear wear-land bound-aries. The work material deposited and adhered around

Fig. 2. Worn NCD tools from SEM.


the tip is another wear-related feature, with the leastamount in the mild machining and the most in the aggres-sive condition, covering the most of the wear-land. Fig. 3shows the details of the worn NCD tools. For the low-speed high-feed conditions, it is observed that the exposedsubstrate extended beyond the wear-land contact zone,Fig. 3b. For the high-speed high-feed condition, the workmaterial deposited at the wear-land appears to experiencesevere plastic deformation during machining. The tool usedin the low-speed and low-feed machining seems to retain anundamaged cutting edge.

The worn NCD tools after cleaning the metal depositsare shown in Fig. 4. At the rake surface, the crater wearis not observed, and for the high-feed conditions, the coat-ing delamination extended to the rake face considerably.With the deposit removed, the wear-land contact can beclearly identified and coating delamination extendedbeyond the contact zone, implying that delaminationoccurred first followed by consequent massive WC/Cowear and increased wear-land contact. On the other hand,coating thinning due to the gradual wear process is notobserved, again indicating that the material loss was sub-stantial followed by the severe nose wear. Fig. 5 showsan example of high-magnification views around the coatingfailure boundaries, 10 m/s and 0.8 mm/rev. The coating-substrate interface appears to be intact without visible gaps

or flaking and the interface contact remains completelyintimate, implying good coating interface.

Fig. 6 compares cutting forces at different machiningconditions. As expected, the feed has dominant effects oncutting forces, mainly the tangential and radial compo-nents. The cutting speed effect on cutting forces is marginalwith the tangential component more noticeable and theother two components little affected. Before the coating fail-ure, machining forces were stable, then followed by rapidincreasing during the tool failure pass for the high-speedconditions. For the low-speed high-feed machining, sharpforce increasing was not observed during the tool failurepass, even with large flank wear growing. Fig. 7 is an exam-ple of the force signal changes during the coating failurepass at the high-speed low-feed condition. The large magni-tude variations are clearly evident and associated with thecoating delamination, with the axial component being themost noticeable. The abrupt decreasing of the machiningforces was due to the loss of materials and decreased inti-mate contact, and the rapid development of wear-land con-tact resulted in the subsequent force climbing. Both theworn tool features from SEM and machining force datapoint out that the coating delamination is the major toolwear mechanism and triggers the catastrophic failure.

For different diamond tools, the flank wear developmentduring machining is shown in Fig. 8. The aggressive

Fig. 3. Detailed wear conditions of NCD tools.

Fig. 4. Worn NCD tools after the deposit was removed.


machining conditions, 10 m/s and 0.8 mm/rev, causedrapid tool wear. For the conventional CVD diamond coat-ing tool (MCD), premature coating failures led to a drastictool wear growth, reaching about 1.0 mm VB at just1.2 min. of cutting time. On the other hand, the NCD tools

had better wear resistance and the tool wear was close tothe PCD tool till around 2 min. cutting time. However,rapid wear of the NCD tool then developed and thewear-land size became larger than the PCD tool wear.Fig. 9 compares machining forces of different diamondtools. It is noted that initial forces were in similar rangesfor all three different diamond tools.

Majority of surveyed literature in Section 1 reportedthat CVD diamond-coated tools are still distant to PCD

Fig. 5. SEM images of detailed wear areas of NCD tools after the depositremoval.

0

100

200

300

400

500

0 5 10 15 20

Cutting time (min)

Ft

(N)


(a) Tangential component

0

100

200

300


Fr

(N)


(b) Radial component

0

100

200

300


Fa

(N)


(c) Axial component

Fig. 6. Cutting forces in machining A390 alloy at different cuttingconditions.


tool performance. In this study, the machining results alsoindicate that the conventional CVD diamond tools (MCD)have a much lower tool life than PCD tools in machiningA390 alloy. On the other hand, the NCD tools were able

to main low wear comparable to PCD tools before delam-ination causes catastrophic wear. The NCD tools substan-tially outperformed the MCD tools possibly due to thefollowing reasons. The nanoindentation test shows thatthe NCD tools have a much greater hardness than theMCD and PCD tools, about 81 GPa vs. 57 and 50 GPa,respectively [21]. The high hardness will benefit the abra-sive wear resistance. In addition, the NCD tools have thelowest elastic modulus among the three types of diamondtools, 684 GPa vs. 1027 and 1019 GPa of the MCD andPCD tools, respectively [21]. Since the thermal strain mis-match is proportional to the elasticity, the low elasticityof the NCD tools implies a lower deposition-induced resid-ual stress that may reduce the adhesion problem.

Fig. 10 is a typical stress contour in a diamond coatingtool after the deposition, showing the normal stress parallel

0

0.3

0.6

0.9

1.2

0 2

Cutting time (min)

VB

(m

m)

MCDPCDNCD

1 3

Fig. 8. Flank wear of different diamond tools in machining A390 alloy.

0

100

200

300

400

500

0 1 2 3

Cutting time (min)

Ft

(N)

MCD

PCD

NCD

(a) Tangential component

0

100

200

300

400

500

0 1 2 3Cutting time (min)

Fr

(N)

MCD

PCD

NCD

(b) Radial component

0

100

200

300

0 1 2 3Cutting time (min)

Fa

(N)

MCD

PCD

NCD

(c) Axial component

Fig. 9. Cutting forces in machining A390 alloy using different diamondtools.

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30

Time (s)

For

ce (

N)

FtFaFr

Fig. 7. Cutting force evolution during the tool failure pass in machiningA390 alloy (10 m/s, 0.2 mm/rev, and 1 mm).


to the substrate top surface. The area away from the edgehas a uniform compressive stress around 4.0 GPa in thecoating and 0.7 GPa of tension in the substrate. However,around the edge, the stress concentrations are considerablyhigh. The plot also shows the distortion caused by theresidual stresses, about 1.7 lm deflection in the thicknessdirection at the center of the insert.

Fig. 11 plots three components of stresses around thecutting edge. rr and rh are the normal stresses in the radialand tangential directions (with respect to the round edge),respectively, and srh is the shear stress in the tangentialdirection. The stresses plotted are at the coating–substrateinterface and around the edge, also extended to the flatarea of the top surface (coordinate 0 is where the edgecurve begins and negative values are toward the rake face).Severe stress concentrations around the cutting edge can bequantified. For the radial normal component, high tensilestresses were developed, about 1.8 GPa. The high tensilestresses can be detrimental in brittle fracture by crack prop-agations and require greater adhesion strength. On theother hand, the tangential normal stresses around the edgeare highly compressive, maximum 4.5 GPa. The large com-

pressive tangential stresses have been viewed beneficial toabrasive wear reductions, however, buckling could beanother mechanism risky to the coating failure [24]. Thetangential shear stresses have values in a range from�1.0 GPa to 0.8 GPa.

Fig. 12 shows stress evolutions from the deposition tomachining at 10 m/s and 0.2 mm/rev. The stress changesresulted from the combined thermal and mechanical loadsin machining. The plots also illustrate the decoupled effectsof the thermal and mechanical loads from machining. Thecurve of ‘‘thermal’’ means that only the contact heat fluxwas imposed. While ‘‘thermomechanical’’ indicates that

Fig. 10. An example of deposition stress distributions in a diamond coating tool.

-5

-4

-3

-2

-1

0

1

2

-0.25 -0.15 -0.05 0.05

Distance (mm)

Stre

ss (

GP

a)

_r__rθ

σ_θσ

τ

Fig. 11. Stress profile around the cutting edge along the interface.

-0.5

0

0.5

1

1.5

2

0 0.01 0.02 0.03 0.04 0.05 0.06

Distance (mm)

σr

(GP

a)

DepositionThermalThermomechanical

(a) Radial normal stress

-4.6

-3.6

-2.6

-1.6

-0.6 0 0.01 0.02 0.03 0.04 0.05 0.06

Distance (mm)

σθ

(GP

a)

DepositionThermalThermomechanical

(b) Tangential normal stress

Fig. 12. Stress evolutions from one cutting example (10 m/s, 0.2 mm/rev).


both the heat flux and contact stresses were applied. Theelevated machining temperatures caused stress relief andresult in reduced radial normal stresses, from maximum1.8 GPa to about 0.3 GPa. Mechanical contact-loadingfrom machining was compressive, and thus, furtherreduced rr to about maximum 0.1 GPa, and the locationof the maximum stress shifted toward the flank surface.From the results, it can be inferred that the thermal effectis more dominant to the stress reversal conditions. Forthe tangential normal and shear stresses, a similar trendis also observed. Fig. 13 shows machining parameter effectson the stress modifications, rr and rh, respectively. Asobserved in Fig. 12, machining loading generally lowersthe magnitudes of the radial and tangential normal stresses,considered as a stress reversal pattern. Comparing to thetool wear results, Fig. 1, the stress reversal pattern seemsto be correlated to the tool life; e.g., the high-speed andhigh-feed machining had the largest stress reversal andthe shortest tool life.

5. Conclusions

Nanocrystalline diamond coatings were produced usinga microwave plasma-assisted CVD process and depositedon common tungsten carbide tools. The NCD tools were

-1

-0.6

-0.2

0.2

0.6

0 0.01 0.02 0.03 0.04 0.05 0.06

σr (

GP

a)


(a) Radial normal stress

-3.7

-3.2

-2.7

-2.2

-1.7

-1.2

-0.7

-0.2

0.3

0 0.02 0.04 0.06

Distance (mm)

σθ

(GP

a)


(b) Tangential normal stress

Distance (mm)

Fig. 13. Machining parameter effects on interfacial stress distributions.


investigated in machining high-Si Al alloy at a range ofmachining conditions with tool wear and cutting forcesexamined and analyzed. In addition, commercial CVD dia-mond coating and PCD tools were also tested and com-pared against the NCD tool performance. Moreover, anFE model was developed to study the stress modificationsin diamond coating tools after the depositions and duringmachining at different conditions. The results are summa-rized as follows.

(1) Catastrophic failures, reached in all except onemachining conditions, limit the life of the NCD tools.The cutting speed has a dominant effect on the toollife.

(2) Coating delamination of the NCD tools is clearly evi-dent by SEM. In addition, machining force monitor-ing can capture the delamination event.

(3) At a high feed, coating delamination extends to therake-face contact. Furthermore, SEM observationsof coating failure boundaries indicate intimate coat-ing-substrate contact.

(4) The NCD tools substantially outperform the MCDtools, which also failed by the delamination. TheNCD tool wear was close to the PCD tool beforerapid wear development caused by delamination.

(5) The diamond coating tools can have a depositionstresses of 4 GPa in compression and stress concen-trations at the cutting edge are high.

(6) Further machining loading causes the stress reversalpattern which seems to correlate with the tool wear;the larger the stress reversal, the shorter the tool life.

Acknowledgements

This research is supported by NASA through AlabamaSpace Grant Consortium. RGT also acknowledges supportfrom NSF (IIP-0349769). The authors also thank sp3

and Dr. Raja Kountanya of Diamond Innovations forsupplying some diamond tools.

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Nanocrystalline diamond coating tools for machining high-strength