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MANUSCRIPT REPORT FORM

PAPER NUMBER: ICMCTF-E2-3-12 TITLE OF PAPER: Interface Characterizations of Diamond-coated Tools by Scratch Testing and Simulations NAME OF THE CORRESPONDING AUTHOR: Y.K. Chou FULL MAILING ADDRESS: The University of Alabama Mechanical Engineering Department 290 Hardaway Hall, 7th Ave. Tuscaloosa, Al 35487-0076 TELEPHONE NUMBER: 205-348-0044 FAX NUMBER: 205-348-6419 E-MAIL: kchou@eng.ua.edu

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AUTHOR’S CERTIFICATION

This is to certify that I have obtained the necessary authorization for publication of the enclosed paper

# E2-3-12 in the Proceedings of the ICMCTF 2011 Conference and in Surface and Coatings

Technology/Thin Solid Films and that the paper is original and unpublished and is not being considered

for publication elsewhere.

(Signature) Corresponding Author Y. Kevin Chou

Printed Name The University of Alabama

Organization

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Interface Characterizations of Diamond-coated Tools by Scratch Testing and Simulations

P. Lua, X. Xiaob and M. Lukitschb, A. Sachdevb and Y.K. Choua

aMechanical Engineering Department

The University of Alabama Tuscaloosa, Alabama

bResearch & Development Center General Motors Corporation

Warren, Michigan, USA Abstract

In this study, micro-scratch tests were conducted on a diamond-coated tungsten-carbide

substrate to investigate the coating adhesion. High intensity Acoustic Emission (AE) signals were

detected once the coating delamination initiated during the scratch test. It has also been found

that the tangential force increased gradually with the normal force, but varied significantly when

the critical load of coating delamination was reached. A finite element (FE) model with a

cohesive-zone interface was developed to simulate the scratch process and the coating

delamination phenomena. The preliminary results indicate that it is feasible to use the FE

combined with scratch tests to evaluate the coating interface characteristics.

Introduction

Applying hard coatings such as diamond on cutting tools is an ideal approach for

enhancing tool lifetime and improving the machining quality due to diamond’s high

hardness/strength, low friction coefficient, and chemical stability, etc. Diamond-coated tools

excel in cost and flexibility to various tool geometry and size comparing to synthetic

polycrystalline diamond (PCD) tools [1], which are also commonly used in the manufacturing

industry. Thus, diamond-coated cutting tools have a potential to replace costly polycrystalline

diamond tools. However, in tooling applications, diamond coating delamination remains the

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primary wear mode that often result in catastrophic tool failures [2,3]. Coating delamination is

due to insufficient adhesion between the coating and substrate. Many approaches have been

developed to enhance the coating adhesion such as etching cobalt (Co) phase off from the tool

surface before diamond depositions, or adding an interlayer to prevent Co diffusing to the

interface. In addition, reliable experimental methods to quantitatively characterize the coating

adhesion are critical to compare and further optimize different interface approaches.

There are several methods used to examine the adhesion of coatings [4]. Scratch testing is

one of the most practical means of evaluating the adhesion of a hard, thin coating on a substrate

[5,6], since it is reliable, simple to perform and no special specimen shape or preparation are

required. Adhesion is measured when a critical load is reached at which coating failure occurs.

Provided that the failure is adhesive, this critical normal load is taken as a measure of the

coating–substrate adhesion, or the work of adhesion is derived from the critical normal load [7,8].

During scratch testing, a spherical indenter tip slides over the surface of the coating to generate a

groove under incremental or constant normal loads. The tangential force is measured during the

test and the morphology of the scratches can be observed simultaneously or afterwards. When the

mean compressive stress over an area in the coating exceeds a critical value, the coating detaches

from the substrate to lower the elastic energy stored in the coating [9]. The work of adhesion at

the interface between the coating and substrate is equal to the energy release rate from coating

detachment and this rate is a function of the mean compressive coating stress over the detaching

area at the instant of detachment. Thus, the critical mean compressive coating stress responsible

for the detachment could be a measure of coating–substrate adhesion. On the other hand,

diamond coatings are very brittle. While a coating can withstand compressive stresses induced by

the indenter to a certain extent, it may fracture if a high tensile or shear stress field is induced

simultaneously, in particular, at the interface such as delamination [10].

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This study aims at better understanding the adhesion of diamond-coated carbide tools by

micro-scratch testing; critical load for coating delamination and the corresponding process signals

were focused. In addition, a finite element (FE) model was developed to simulate the scratch

process, with emphasis on the interface behavior, and to evaluate the adhesion between the

coating and substrate using interface characteristics, which can be further applied in studies at

different loading conditions such as cutting.

Experimental Details

The specimen was a diamond-coated tool. The substrate is co-cemented tungsten carbide

(WC-Co), submicron grains, 6% Co, with coating thickness around 4 µm, provided by University

of South Florida. The surface roughness of the samples was about 0.38 µm Ra. A Micro-scratch

tester from CSM Instruments, model Micro-Combi, was used for the experiments at the room

temperature. The diamond indenter with a tip radius of 50 µm was utilized. The scratch speed

was 2 mm/min with the progressive loading method. The scratch length for each test was 5 mm.

During the scratch test, tangential forces, acoustic emission (AE) signals, and the depth of the

scratch were acquired. A KEYENCE digital microscope (VHX-600X) was used to observe the

scratch marks and delamination after the test. In addition, a white-light interferometer (WLI) was

used to acquire the morphology of scratch grooves. To determine the critical load for the tested

diamond-coated tool, scratch tests were carried out with a progressive load, maximum normal

force of 10 N, 15 N, 20 N, 25 N and 30 N. Figure 1 shows the overall images of 5 scratch

grooves at the corresponding load (1: 10 N, 2: 15 N, and 3: 20 N, etc.)

Results and Discussion

Figure 2 shows the AE signal and tangential force (Ft) vs. the applied normal load (Fn)

during the scratch test with a progressive load of maximum 20 N. From the figure, it is observed

that the tangential force increases smoothly when the normal force is less than 4.3 N, but varies

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considerably once the load exceeds 4.3 N. It is also observed that an abrupt amplitude increase of

AE signals (Spot 1) exists at the load around 4.3 N, followed by a series of continuous high-

amplitude AE peaks. This implies coating delamination initiation occurred around 4.3 N.

After testing, the scratched groove was observed in the digital microscope at 500X. From

the relation between the load and the distance, the location corresponding to the high force

variation and sudden intensity increasing (Spot 1) can be examined and verified whether coating

delamination has initiated at that point. Figure 3a shows the digital microscopic image at Spot 1

and the corresponding normal load is around 4.3 N. It can be clearly confirmed that coating

delamination has initiated at such a force, clearly exposing the substrate layer of WC, near Spot 1.

Figure 3b shows the digital microscopic image at the end of scratch test. It is shown that coating

delamination continued, once initiated, to the end of the final load, with a comparable

delamination width.

Using the approach described earlier, the corresponding critical load of delamination for

every scratch condition can be estimated. It is found that the critical load of delamination

initiation is around 4 to 6 N for all tested loads, indicating that the delamination critical load is

not sensitive to the load rates currently applied. Thus, it is concluded, the evaluated diamond-

coated tool with a coating thickness of 4 µm has a critical load for delamination in the range of 4

to 6 N.

The white-light interferometer has also been used to obtain quantitative information of the

scratched geometry. Figure 4a below shows a 3D surface contour image of part of the scratched

track, at ~3 mm distance from beginning (corresponding to 12 N for the 20 N maximum load). It

shows coating delamination propagated outward. Figure 4b is the 2D profile analysis of the

scratched mark at the same location. It is estimated that the delamination width is about 88 µm

and the depth of the track is about 7 µm.

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Scratch Process Simulations

A 3D FE model has been developed to study the micro-scratch test by using ABAQUS

6.8. A half model is used because of the symmetry of the problem. The geometric model of the

specimen shown in Figure 5a is 3000 µm in length and 200 µm in width. The thickness of the

diamond coating and the carbide substrate is 4 µm and 60 µm, respectively. The diamond

indenter is assumed to be rigid with a tip radius of 50 µm. Figure 5b shows the details of the

model around the indenter. The smallest mesh size underneath the indenter is 0.6 µm for the

coating and 5.0 µm for the substrate. Further, a cohesive layer with zero thickness was included

to model the mechanical behavior of the interface between the coating and the substrate.

The materials of the specimen, both the coating and the substrate, were modeled as an

elastoplastic solid with isotropic hardening. According to the Ramberg-Osgood law, the stress-

strain relation can be described as

,

where , and the strain-hardening exponent is generally in the range of 0 to 0.5 [11]. The

yield strength of WC-6wt%Co is in the range of 1.45 to 5.76 GP [12], and the yield strength of

the WC-Co is assumed as 3.605 GPa in this study. Other properties of WC are identical with

those in a previous study [13]. The yield strength and hardening exponent of the diamond can be

obtained by the method proposed by Suresh [14-16] based on the experimental data of

nanonindentation on diamond [17]. In this study, the Young’s modulus of the coating is 1200

GPa, other properties of the coating and substrate are listed in Table 1. The frictional coefficient

was set as 0.1. The cohesive zone properties such as the strength and characteristic length are

derived from a previous study [18] with adjusted property values: 543 MPa maximum strength,

0.43 µm characteristic length and 117 J/m2 fracture energy density, to approximate the critical

load observed from the experiments. During simulations, the spherical indenter first moves

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downward in contact with the coating, then slides along the surface of the coating, at a speed of

500 µm/s, with a linearly increased depth, and the sliding distance is 500 µm. The final load was

about 15 N. At the end, the slider moves upward until fully unloaded.

Figure 6 shows (a) the maximum principal stress and (b) the equivalent plastic strain in

the specimen at the onset of coating delamination. It can be observed that high compressive

stresses, around 20 GPa, occurred in the coating near the indenter and the maximum equivalent

strain is 0.082. Moreover, high tensile stresses occur in the wake of the slider. The corresponding

load at this location is about 4 N, and the delamination has just initiated, to be shown later.

Figure 7 illustrates the normal stress distribution in the cohesive-zone layer at different

times: (a) initial scratch, (b) onset of delamination and (c) end of scratch. At the initial period,

though the normal stress of the cohesive zone has reached the cohesive zone strength (543 MPa),

the damage evolution has not resulted in the cohesive zone failure. Thus, the interface is still

intact. On the other hand, at around 4 N of normal load, the cohesive-zone layer has shown some

damage initiated (Figure 7b); the cohesive zone normal separation has reached its characteristic

length (0.43 µm). Figure 7c shows a large delamination area at the end of scratch simulation. The

coating delamination crack has spread in both the sliding and transverse directions.

Figure 8 further shows the cohesive-zone normal stress vs. normal separation traced along

the scratch time at two different locations. Figure 8a is for a location where the interface remains

attached. The normal stress is first compressive when the slider is approached to that location and

quickly changes to tensile and reaches the interface strength. Then, the stress is reduced in a short

time, but with the normal separation increased. Finally, as the slider moves away from this

location, the stress is reduced, also the separation, but does not return to zero at the end. On the

other hand, Figure 8b is for a location where the interface has been delaminated. The normal

stress vs. separation curve follows the typical traction-separation behavior and the normal

separation has reached the maximum separation and results in the interface failure. In a previous

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study of diamond-coated carbides [18], the ideal cohesive zone properties (as an upper bound)

were estimated from the substrate (WC) toughness. For the current study, note that the cohesive-

zone properties have been modified (1/3 of the upper-bound fracture energy) to approximate the

critical load measured from the experiments. Therefore, the estimated cohesive-zone

characteristics may be used to indicate the interface capacity related to the delamination

behaviors.

Conclusions

Scratch tests of diamond coatings deposited on a WC substrate have been carried out

using a micro-scratch tester. During the scratch tests, the normal force, the tangential force, the

acoustic emission signals and the penetration depth were acquired. After scratch tests, the scratch

marks were also observed in a digital microscope and analyzed by a white-light interferometer.

Moreover, to characterize the interface mechanical behavior, an FE model including the interface

cohesive zone was developed to simulate the scratch process. The results are summarized as the

following:

(1) Coating delamination can be clearly detected by AE signals. It was observed that the abrupt

AE peak jumps followed by several continuous AE high-amplitude peaks are associated with

coating delamination. The tangential force increases smoothly with the normal force before

the initiation of coating delamination, but, varies considerably once coating delamination

initiated. Therefore, tangential force may also be used to monitor the coating delamination

during scratch tests.

(2) The width of coating delamination would increase with the increased loads after delamination

initiated, this is confirmed by the scratch images under digital microscope, which show that

the coating delamination becomes more severe with the increased load.

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(3) The critical load for the tested diamond-coated WC tools was in the range of 4 to 6 N, which

has been confirmed by repeated tests.

(4) The simulation results indicate that it is feasible to use the FE model combined with scratch

tests to evaluate the coating interface characteristics.

Acknowledgements

This study was conducted under an NSF-funded joint project (CMMI 0928627) -

GOALI/Collaborative Research: Interface Engineered Diamond Coatings for Dry Machining,

between The University of Alabama, General Motors and University of South Florida. The

diamond-coated sample was provided by the University of South Florida.

References

1. J. Hu, Y.K. Chou, R.G. Thompson, Trans. NAMRI/SME 35 (2007) 177. 2. P. Lu, Y.K. Chou, R.G. Thompson. Proc. 2009 Int. Manuf. Sci. and Eng. Conf., (2009)

MSEC2009-84372. 3. F. Qin, J. Hu, Y.K. Chou, R.G. Thompson. Wear 267 (2009) 991. 4. A.J. Perry. Thin Solid Films 107 (1983) 167. 5. S.J. Bull in: W. Gissler, H.A. Jehn (Eds.), ECSC, EEC, EAEC, (1992) 31. 6. H. Ollendorf, D. Schneider. Surf. Coat. Technol. 113 (1999) 86. 7. R. Jaworski, L. Pawlowski, F. Roudet, S. Kozerski, F. Petit, Surf. Coat. Technol. 202 (2008)

2644. 8. S. Nakao, J. Kim, J. Choi, S. Miyagawa, Y. Miyagawa, M. Ikeyama, Surf. Coat. Technol.

201 (2007) 8334. 9. F.X. Liu, F.Q. Yang, Y.F. Gao, W.H. Jiang, Y.F. Guan, P.D. Rack, O. Sergic, P.K. Liaw,

Surf. Coat. Technol. 203 (2009) 3480. 10. Y. Xie, H.M. Hawthorne, Surf. Coat. Technol. 155 (2002) 121. 11. Z. Xu, X. Li, Acta Materialia 53 (2005) 1913. 12. K.L. Mittal, V.S.P. Intl Sci. Netheland. (2001). 13. A. Renaud, J. Hu, F. Qin, Y.K. Chou, Int. J. Mechatron. Manuf. Syst. 2 (2009) 490. 14. A.E. Giannakopoulos, S. Suresh, Scr. Mater. 40 (1999) 1191. 15. U. Ramamurty, S. Sridhar, A.E. Giannakopoulos, S. Suresh, Acta. Mater. 47 (1999) 2417. 16. T.A. Venkatesh, K.J. Van Vliet, A.E. Giannakopoulos, S. Suresh, Scr. Mater. 42 (2000) 833. 17. S. Chowdhury, E. de Barra, M.T. Laugier, Diamond Relat. Mater. 13 (2004) 1625. 18. J. Hu, Y.K. Chou, R.G. Thompson, Surf. Coat. Technol. 203 (2008) 730.

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Table 1. Properties of the substrate and coating for the diamond-coated cutting tools

Part Material Young’s modulus/GPa

Poisson’s ratio

Yielding strength/GPa

Hardening exponent

Substrate WC-6 wt. % Co 619.5 0.24 3.605 0.244

Coating Diamond 1200 0.07 26.7 0.23

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List of Figures with captions: Figure 1. Digital microscopic images of scratch grooves on the sample..

Figure 2. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 20 N.

Figure 3. The digital microscopic images of (a) Spot 1 around 4.3 N, and (b) the end of the scratch for scratch test under maximum load of 20 N.

Figure 4. WLI analysis of scratch track at 3 mm scratch distance for 20 N maximum load, showing coating delamination:.(a) 3D morphology, and (b) 2D profile analysis.

Figure 5. 3D FEA model for the scratch test: (a) overall model, and (b) details around the indenter.

Figure 6. (a) Maximum principal stress (unit: 1000 GPa) and (b) Equivalent plastic strain at the onset of delamination.

Figure 7. Cohesive zone stress distribution (unit: 1000 GPa) at the interface: (a) initial scratch, (b) onset of delamination, and (c) end of scratch.

Figure 8. Normal stress vs. separation behavior along the scratch time: (a) location without delamination, and (b) location with delamination.

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Figur

1

re 1

12

Figure 2.

Spot 1

0

2000

4000

6000

8000

10000

12000

0

2

4

6

8

10

12

14

0 5000 10000 15000 20000

Tangen

tial Force(m

N)

Acoustic

 Emission

(%)

Normal Force(mN)

AE

Ft

(a

(

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a) Spot 1 ar

(b) End of t

Figur

3

round 4.3 N

the scratch

re 3.

N

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(a) 3D morphology

(b) 2D profile analysis

Figure 4.

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(a) Overall model

(b) Details around the indenter

Figure 5.

IndenterCoating

Substrate Cohesive Zone

(a)

60 μ

m

4μm

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(a) Maximum principal stress (unit: 1000 GPa)

(b) Equivalent plastic strain

Figure 6.

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(a) Initial scratch

(b) Onset of delamination

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(c) End of scratch.

Figure 7.

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(a) Location A (see Fig 8(c))without delamination

(b) Location B (see Fig 8(c))with delamination

(c) The normal displacement of the interface when fully unloading

Figure 8.

A B

‐0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

‐0.1 0 0.1 0.2 0.3 0.4 0.5Normal stress(Ga)

Normal separation(μm)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5

Normal stress(GPa

)

Normal separation(μm)

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?Figure 4. The digital microscopic images of (a) Spot 1 around 5.4 N, and (b) the end of the scratch for scratch test under maximum load of 10 N.

(b)

(a)

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?Figure 5. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum scratch load of 30 N.

Spot1

0

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14000

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0 5000 10000 15000 20000 25000 30000

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l Force(m

N)

Acoustic

 Emission

(%)

Normal Force(mN)

AE Ft

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?Figure 6. Digital microscopic images of (a) Spot 1 around 3.9 N, and (b) the end of the scratch

(30 N load).

(a)

(b)

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