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Coating Thickness and Interlayer Effects on CVD-diamond Film Adhesion to Cobalt-cemented Tungsten Carbides

Ping Lua, Humberto Gomezb, d, Xingcheng Xiaoc, Michael Lukitschc, Delcie Durhamb, Anil Sachdevec, Ashok Kumarb, Kevin Choua a Mechanical Engineering Department, University of Alabama, Tuscaloosa, AL 35487, USA b Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620, USA c Chemical Sciences & Materials Systems Laboratory, General Motors R&D Center, 30500 Mound Road, Warren, MI 48090, USA d Departamento de Ingeniería Mecánica, Universidad del Norte, Barranquilla, Colombia

Abstract

In this study, diamond coating adhesion on cobalt-cemented tungsten-carbide (WC-Co)

substrates was investigated using scratch testing. In particular, the methodology was applied to

evaluate the effects of the coating thickness and interlayer on coating delaminations. In the

coating thickness effect study, substrate surface preparations, to remove the surface cobalt, prior

to diamond depositions was common chemical etching using Murakami solutions. On the other

hand, to study the interlayer effect, by halting the catalytic effect of the cobalt binder, two

different interlayers, Cr/CrN/Cr and Ti/TiN/Ti, were deposited to WC-Co substrate surfaces (no

chemical etching) by using a commercial physical vapor deposition (PVD) system in a thickness

architecture of 200nm/1.5µm/1.5µm, respectively. Diamond films were synthesized by using a

hot-filament chemical vapor deposition (HFCVD) reactor at a gas mixture of 6 sccm of CH4 and

60 sccm of H2, with varied deposition times.

Scratch testing was conducted on the fabricated specimens using a commercial machine,

at a maximum normal load of 20 N and a speed of 2 mm/min. It is noted that the onset of coating

delamination can be clearly identified by high-intensity acoustic emission (AE) signals when

such events occur, which can be used to determine the critical load. Scratched track geometry

was also characterized by scanning electron microscopy.

The results show that the adhesion of the diamond coating increases with the increased

coating thickness, with a nearly linear relation, in the range tested. For the two types of interlayer

materials tested, either of them seems to be effective and the diamond coating with Ti-interlayer

shows poorer adhesion comparing to the Cr-interlayer coating.

 

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1. Introduction

1.1. CVD diamond coated tools

The Chemical Vapor Deposition (CVD) synthesis of diamond on cemented carbides has

been an ideal approach in enhancing cutting tools life and improving their machining

performance due to the exceptional diamond properties such as superior hardness, low

coefficient of friction, and chemical stability. CVD diamond-coated tools provide significant

advantages in terms of cost and flexibility when compared to synthetic polycrystalline diamond

(PCD) tools [1], which are also commonly used in the manufacturing industry. The ability to

form a conformal coating on the tool surface, the relative simplicity of the synthesis as a result of

the new advances in industrial CVD reactors, and the possibility to produce different film

structural characteristics (micro or nano-crystalline), represent a significant advantage of CVD

diamond coatings [2]. However, under cutting operations represented by harsh machining

conditions or high-strength workpiece materials, the diamond coating delamination remains to be

the primary wear mechanism that results in catastrophic tool failures [3]. In cemented carbide

substrates like WC-Co, diamond delamination is due to the insufficient adhesion between the

coating and the substrate, partially as the result of the formation of non-diamond compounds at

the substrate-diamond film interface due to the Cobalt-carbon interdifussion at CVD deposition

temperatures.

1.2. Interface engineering

Several interface engineering approaches have been reported in the last 15 years with the

aim to reduce the undesired catalytic effect of cobalt on diamond adhesion [4-6]. In order to

maximize the practical adhesion of diamond coatings on cemented carbides, any approach must

halt the interdifussion effect of cobalt. The most widely successful techniques discussed in the

literature are related to the cobalt removal in depths ranging in about 3 to 10 µm from the

substrate surface by using chemical etching methods [7], or by halting the cobalt effect on the

surface by depositing interdifussion barrier layers [8], that also diminish the thermal stresses

caused during the diamond growth.

Interface engineering techniques are specifically targeted to improve the diamond coating

adhesion. Since an increased surface roughness has been correlated in enhancing the diamond

 

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nucleation density and promoting a film interlocking behavior, surface pretreatment efforts can

be also tailored accordingly besides suppressing the cobalt catalytic effects. In addition to the

improvements in the diamond growth conditions, the substrate surface plays an important role in

the final adhesion behavior of diamond coatings. Surface textures and surface/subsurface damage

characteristics on the substrate have a direct impact to the subsequent diamond adhesive quality

and wear failure modes; hence the final diamond coating adhesion behavior depends on the

surface pretreatments used and their resulting effects on the substrate surface, which are

ultimately the interface characteristics in the substrate-coating composite system. This interface

requires the formation of strong interfacial chemical bonds between the diamonds crystallites

nucleated at the surface and the atoms at the substrate surface. Moreover, a mechanical

interlocking effect is also desired in order to enhance the coating addition.

The effects of chemical etchings on the surface characteristics of WC-Co substrates have

been studied by far [9, 10] and represent the pretreatment method used in most of the

commercial diamond coated cemented carbides in the industry. This method has the purpose to

produce a selective etching of the cobalt binder by using a two step process composed by an

initial wet treatment in a Murakami solution with the aim of reconstruct and rough the surface by

attacking the WC grains and exposing the Co binder [9]. Then, a second wet etching in an acid

solution (H2SO4 or HNO3 with H2O2) is used to reduce the exposed cobalt in a depth determined

by the etching time [10].

Another approach to avoid the catalytic effect of cobalt is the deposition of carbide and

nitride intermediate layers (CrN, TiN, TiC, SiC, AlN, etc) on the substrate before the final

diamond deposition. These interlayers normally deposited by physical vapor deposition (PVD)

methods must remain stable during the diamond deposition, have a low thermal expansion

coefficient to minimize internal stresses, and provide a carbide formation layer to improve

diamond nucleation [11]. These conditions may also be improved by using nanometer sized

metal thin layers like Cr and Ti at the top or bottom of the interlayer architecture. Additional

diamond particles may be peened in the top interlayer surface to provide additional diamond

nucleation sites and serve as anchors to the final diamond coating [12].

1.3. Coating adhesion and scratch testing

 

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There are several methods commonly used to examine the adhesion of coatings in general

[13]. Scratch testing is one of the most practical approaches in evaluating the adhesion of a hard-

thin coating on a particular substrate [14, 15], since it is reliable, simple to perform, and with no

special specimen geometry or preparation requirements. Coating adhesion is measured as a

correlation between the occurrences of critical load at the coating failure instant. In the event of

an adhesive failure, this critical normal load is taken as a measure of the coating–substrate

adhesion or used to calculate the work of adhesion [16, 17]. During a scratch test, a spherical

indenter tip slides over the surface of the coating to generate a groove under incremental or

constant normal load modes. In addition, the tangential force is measured during the test and the

morphology of the scratches can be observed simultaneously or afterwards, an acoustic emission

sensor is used to capture the coating delamination during scratch tests. When the resolved

compressive mean stress exceeds a critical value, the coating detaches from the substrate

decreasing the elastic energy stored in the coating [18]. Then, the work of adhesion at the

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

instant of detachment as a function of the compressive mean stress of the coating stress over the

delaminating area. Thus, the critical compressive coating mean 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. It is known that coating has a

higher critical compressive stress than tensile and shear stress, but less than the critical

compressive stress may result in coating delamination during scratch test.

1.4. Adhesion characterization

As discussed, the adhesion of coating is measured by the critical load under coating

failures, and there are different ways to determine the critical load determination [19].

Microscopic observation is the most reliable method to detect the coating delamination. This

technique can distinguish cohesive failure within the coating and adhesive failure at the interface

of the coating-substrate composite system. The use of acoustic emission (AE) sensors, which is

insensitive to mechanical vibration frequencies of the instrument, represents another option to

detect the elastic waves generated as a result of the formation and propagation of micro-cracks in

 

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diamond coating along to the tangential force (Ft) values recorded from force fluctuations along

the scratch. The adhesion of CVD diamond coating on molybdenum substrates has been

investigated by scratch testing [11], and results displayed critical normal load values in the range

of 16 to 40 N for CVD diamond films grown after 4 h at a CH4/H2 ratio of 0.5%. However,

diamond films grown after 24 h at a methane concentration of 0.5% do not exhibit any failure

when the force increased to75 N. Moreover, adhesion scratch tests were able to provide a direct

qualitative comparison of the adhesion of diamond coatings on steel and copper substrates [12],

with the aim to investigate the effect of metal substrates (copper and steel) and film thickness on

the adhesion, and these results showed that the diamond coatings on steel exhibits a higher

critical load than on the copper, but thicker films displays a higher critical load than thinner films

for the same kind of substrates.

1.5. Objectives and Approaches

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

micro-scratch testing, and the critical load for coating delamination were used to evaluate the

adhesion of diamond-coated carbide tools, where corresponding process singles would help to

identify the coating delamination. It is essential to investigate that the effect of coating thickness

and interlayer on the adhesion of the diamond coating to better understand the adhesion of

diamond-coated carbide tools.  

2. Experimental details

2.1. Substrate preparation

Experimental samples correspond to WC-Co (6%) square cemented carbide substrates.

The surfaces of the tools display surface characteristics represented by feed marks resulting from

their manufacturing process. These preferential marks are depicted in Figure 1 and constitute the

as-receive state of samples before any surface pretreatment.

The use of chemical etching pretreatment and the pre-deposition of interdiffussion barrier

layers were applied to the samples with the aim to modify the as ground surface before the final

diamond deposition, improving the coating adhesion by halting the effect of the cobalt binder in

 

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the cemented carbide substrate. The conditions of the pretreatments are summarized in Table 1

and were selected from previous work of the authors [10].

2.2. Interlayer preparation

Two different interlayers, Cr/CrN/Cr and Ti/TiN/Ti were deposited to the WC- Co (6%)

substrate surface by using a commercial PVD coating system in thickness architecture of

200nm/1.5um/1.5um, respectively. This physical barrier prevents the diffusion of carbon into the

underlying cobalt phase and the subsequent graphite formation that is so deleterious to diamond

film adhesion. The barrier also provides a stress relaxation barrier layer [20]. Additional

treatments after the interlayer were applied to the top of the surface in order to improve the

surface roughness and diamond nucleation. This surface treatment corresponds to an additional

shoot peening to the final Cr and Ti using diamond powder particles (1μm).

2.3. Coating deposition

Pretreated samples were subjected to a seeding process prior diamond deposition. The

seeding method was performed using a slurry solution, consisting of 1.2 grams titanium

nanopowder, 1.2 grams nanocrystalline diamond powder, and 100 milliliters of methanol.

Diamond films were synthesized by using a HFCVD reactor at a pressure of 20 Torr, two

filaments located at the top of the sample operating at 90 V, and a gas mixture of 6 sccm of CH4

and 60 sccm of H2. In order to investigate the effect of coating thickness on the adhesion of

diamond coating, three different coating samples (coded T-1.5, T-2.5 and T-4.5) were prepared

under the same working parameters except the deposition time for the coating, which will result

in the thin coating thickness ranged from 1.5µm to 4.5 µm. Table 2 shows the sample details for

the diamond coated inserts used in the scratch tests detailed below. For the specimens with either

the Ti or Cr interlayer, the deposition thickness estimated was about 3 μm.

2.4. Scratch test setup, procedure and data acquisition

A Micro-scratch tester from CSM Instruments, model Micro-Combi, was used for the

experiments at room temperature, by using an indenter with tip radius of 50 µm, and a scratch

speed of 2 mm/min with a progressive loading method in order to determine the critical load for

the diamond-coated tools. The scratch length for each test was set to 5 mm. During the scratch

 

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test, tangential forces values, acoustic emission (AE) signals, and the resulting depth of the

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

scratch marks and coating delamination after the test. In addition, a white-light interferometer

(WLI) was used to characterize the morphology of the scratch grooves, and a scanning electron

micrograph instrument (Philips XL30) was used to show how the coating delamination appear

and propagate.

3. Results and Discussion

3.1. Characterization information of different interfaces and diamond coatings

Raman spectroscopy was performed to all CVD diamond samples, corresponding to a

microcrystalline diamond structure represented by the 1332 cm-1 broad peak observed in the

Raman spectra and shown in Figure 2. The crystal structure of the diamond film correspond to

faceted (100){111} polycrystals was shown in Figure 3.

3.2. Scratch test results for diamond coated WC with different thicknesses

Scratch tests conducted on sample T-1.5 included four repeated tests of maximum load of

10 N, three repeated tests of maximum load of 30 N and one test of maximum load of 20 N.

Figure 4 shows the overall images of 8 scratch grooves at the corresponding load (1~4: 10 N,

5~7: 30 N, and 8: 20 N).

Figure 5 shows the AE signal and tangential force (Ft) vs. the applied normal load (Fn)

during the 4th scratch test with a progressive load of maximum 10 N. It is observed that the

tangential force increases smoothly, though varies slightly until reaching 10 N. It is also found

that only a few isolated high peaks (i.e. Spot 1) of AE signals exist before the normal force reach

10 N. This implies coating delamination may not initiate under such a load.

To understand the isolated high peaks, spot 1, the scratch groove was observed in the

digital microscope at 1000X after testing. From the correlation between the load and the distance,

the location corresponding to Spot 1 can be examined to verify whether the coating delamination

has initiated at that point. Figure 6a) shows the digital microscopic image at Spot 1 and the

corresponding normal load is around 8.0 N. It can be seen that the cracks on the coating have

been formed at the spot without coating delamination. Figure 6b) shows the digital microscopic

 

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image at the end of scratch test. It is shown that intensive cracks have been formed at the end of

scratch trace; however, no coating delamination is associated with them.

Figure 7 shows the AE signal and tangential force (Ft) vs. the applied normal load (Fn)

during the test with maximum load of 20N. Similar results are found that the tangential force

increases smoothly with the normal force less than 9 N, but follows with considerable variation.

An abrupt amplitude increase of AE signals (Spot 1) exists at the load around 9 N, followed by a

series of continuous high-amplitude AE peaks. In addition, the highest amplitude of AE peak

occurred at Spot 2, and drastic variation of tangential force was observed at Spot 2 with

corresponding normal force of 13 N.

Figure 8a) and 8b) show the digital microscopic image at Spot 1 and Spot 2, with the

corresponding normal load for Spot 1 around 9 N, and cracks initiation but without coating

delamination is found at such a force. The corresponding normal load for Spot 2 is around 13 N.

Coating delamination has initiated at such a force, with clearly exposing the substrate layer of

WC, near Spot 2. Figure 10c) 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. Figure 9 displays the SEM image for the coating delamination

(Spot 1) at 200X and 800X, it could be clearly seen that multiple micro-cracks existed on the

coating surface before Spot 1, followed with coating material removal from the substrate, where

coating delamination formed.

Sample results of testing on the sample T-4.5 with a maximum load of 20N, repeated

three times, are discussed below. Figure 10 shows the AE signal and tangential force (Ft) vs. the

applied normal load (Fn) during the 1st scratch test with maximum 20 N, similar to previous

observations, the transition with Spot 1 is found for the normal load of 14.6 N. and an abrupt

amplitude increase of AE signals (Spot 1) exists at the spot, followed by a series of continuous

high-amplitude AE peaks, implying the critical load for coating delamination.

Figure 11a) shows the digital microscopic image at Spot 1 with corresponding normal

load around 14.6 N. Coating delamination has initiated at such a force with clear exposing

substrate layer of WC, near Spot 1. Figure 11b) shows the digital microscopic image at the end

 

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of scratch test. It is shown that coating delamination continued, once initiated, to the end of the

final load.

Figure 12 shows the SEM images at the final load locations (20 N) on the three samples;

all the results confirmed that coating spallation along the scratch formed, and severe coating

detachment were found at the end of the scratch. It was also found that T-4.5 had the slightest

coating detachment comparing to the other two samples.

Figure 13 shows the result of the critical load of coating delamination vs. the coating

thickness. It can be noted that the critical load increases with the coating thickness, increasing

from 11.2 N to 14.5 N for the coating thickness of 1.5 μm vs. 4.5 μm. It demonstrates that the

adhesion of the diamond coating will increase with the coating thickness, for the range of this

study. Such an effect has been confirmed by other researchers [21].

3.3 Scratch test results on diamond coating with different interlayers

Figure 14a) and b) show the AE signal and tangential force (Ft) vs. the applied normal

load (Fn), with maximum 5 N load, during the test on the I-Ti and I-Cr samples, respectively. It

is concluded, based on force and AE signal and optical images, that coating delamination

initiated around 1.0 N for I-Ti and 3.5 N for I-Cr.

Figure 15 displays the SEM images at two load locations on the two samples, one is

around the coating delamination, another is at the end of scratch test with 5 N, and same results

with coating spallation along the scratch formed after coating delamination during scratch tests,

and more severe coating detachment were observed on the sample of I-Ti than I-Cr.

The results show that the critical load is only 1 N for the I-Ti, and 3.5N for I-Cr. Thus,

both are not effective with the current approach. The sample of I-Ti has the poorest adhesion

comparing to the samples discussed in this research. Cr-interlayer provides a slightly better

adhesion than Ti-interlayer. The possible reason is that the carbon diffusion in Cr is relatively

low compared to Ti, which should improve the adhesion of diamond to the seeded substrate.

However, the adhesion is still poorer than the diamond coating samples without interface in our

research. This is possible due to a defective chromium carbide layer formed during deposition. It

is known that the multi-phase system could provide an easy path for the micro-cracks due to

 

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transformation among these phases [21]. In addition, the mismatch of the coefficients of thermal

expansion in the multilayer structure aggregates the propagation of micro-cracks in the scratch

test.

Conclusions

Scratch testing of a diamond thin film deposited on a WC substrate has been carried out

using a micro-scratch tester. The objective is to evaluate the coating thickness and interlayer

effects on the coating delamination critical load. During the scratch tests, the normal force, the

tangential force, the acoustic emission signals and the penetration depth were acquired to identify

the delamination initiation event. After scratch tests, the scratch marks were also observed in a

digital microscope and a scanning electron microscope. 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.

(3) The adhesion of the diamond coating increases with the increased coating thickness in the

range discussed. On the other hand,, the Ti-interlayer and Cr-interlayer do not seem to be

effective in interface adhesion enhancement compared to other conventional samples.

Acknowledgements This research is supported by NSF, 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 samples were provided by

the University of South Florida.

 

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MSEC2009-84372.

[3] F. Qin, J. Hu, Y.K. Chou, R.G. Thompson. Wear 267 (2009) 991.

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335.

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Casadei, Diamond and Related Materials, 2006, 15(9),1284-1291.

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[7] J. Oakes, X.X. Pan, R. Haubner and B. Lux. Surf. Coat. Technol., 47 (1991), p. 600.

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Related Materials, Volume 5, Issue 12, December 1996, Pages 1478-1489.

[10] Humberto Gomez, Delcie Durham, Xingcheng Xiao, Michael Lukitsch, Ping Lu, Kevin

Chou, Anil Sachdev, Ashok Kumar, Journal of Materials Processing Technology, Volume 212,

Issue 2, February 2012, Pages 523-533.

[11] J.G. Buijnsters, P. Shankar, W. Fleischer, W.J.P. van Enckevort, J.J. Schermer, J.J. ter

Meulen, Diamond and Related Materials, Volume 11, Issues 3-6, March-June 2002, Pages 536-

544.

[12] F.J.G Silva, A.P.M Baptista, E Pereira, V Teixeira, Q.H Fan, A.J.S Fernandes, F.M Cost,

Diamond and Related Materials, Volume 11, Issue 9, September 2002, Pages 1617-1622.

[13] A.J. Perry. Thin Solid Films 107 (1983) 167.

[14] S.J. Bull in: W. Gissler, H.A. Jehn (Eds.), ECSC, EEC, EAEC, (1992) 31.

[15] H. Ollendorf, D. Schneider. Surf. Coat. Technol. 113 (1999) 86.

[16] R. Jaworski, L. Pawlowski, F. Roudet, S. Kozerski, F. Petit, Surf. Coat. Technol. 202 (2008)

2644.

 

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[17] S. Nakao, J. Kim, J. Choi, S. Miyagawa, Y. Miyagawa, M. Ikeyama, Surf. Coat. Technol.

201 (2007) 8334.

[18] P. Lu, X. Xiao, M. Lukitsch, A. Sachdev, Y.K. Chou, Surface and Coatings Technology,

2011,206(7):1860-1866.

[19] A.J. Perry. Scratch adhesion testing of hard coatings, Thin Solid Films, Volume 107, Issue 2,

16 September 1983, Pages 167-180.

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Table 1 Conditions of the pretreatments of the substrate Denomination Pretreatment Conditions T Chemical Etching 1. Acetone cleaning

2. K3(Fe(CN)6) + KOH + H2O 3. HNO3 + H2O2 4. DIW Rinse

I-Cr PVD Interlayer Cr/CrN/Cr [200nm/1.5um/1.5um] I-Ti PVD Interlayer Ti/TiN/Ti [200nm/1.5um/1.5um]

Table 2 Sample details for diamond coated inserts in scratch tests Denomination Interlayer Coating thickness/µm Surface roughness Ra/µm

T-1.5 N/A 1.5 4.45 T-2.5 N/A 2.5 3.82 T-4.5 N/A 4.5 2.76 I-Ti Ti 2 3.82 I-Cr Cr 4 2.23

 

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List of figures

Figure 1. Surface characteristics of the WC-Co (6%) surface as received from supplier, a) SEM image of surface morphology representing the finishing marks on the tool and b) optical interferometry image of the surface representing the roughness value and pattern.

Figure 2. Raman spectra corresponding to the structure of the CVD films depicting the microcrystalline diamond structure represented by the 1332 cm-1 broad peak.

Figure 3. SEM image corresponding to the diamond film deposited in the samples depicting the faceted diamond polycrystals.

Figure 4. Digital microscopic images of scratch grooves on sample T-1.5.

Figure 5. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 10 N.

Figure 6. Spot for a) load around 8N and b) end of scratch on scratch trace for max load of 10N under microscope(X1000).

Figure 7. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 20 N.

Figure 8. The digital microscopic images of a) Spot 1 around 9N, b) spot 2 around 13 N, and c) the end of the scratch for scratch test under maximum load of 20 N.

Figure 9. The SEM image for the coating delamination on sample T-1.5 (20 N load) at a) 200X b) 800X.

Figure 10. Figure 12. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 20 N, sample T-4.5.

Figure 11. The digital microscopic images of a) Spot 1 around 14.6 N, and b) the end of the scratch for scratch test on sample T-4.5 under maximum load of 20 N.

Figure 12. SEM images at the scratch end location (20 N): a) T-1.5, b) T-2.5, and c) T-4.5.

Figure 13. Critical load for coating delamination vs. coating thickness.

Figure 14. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 5 N on a) I-Ti and b) I-Cr.

Figure. 15. SEM images at the scratch end location (5 N): a) I-Cr, and b) I-Ti.

 

 

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16  

Figure 4. Digital microscopic images of scratch grooves on sample T-1.5.

Figure 5. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a

maximumload of 10 N.

a)Spot around 8N b) End of scratch

Figure 6. Spot for a) load around 8N and b) end of scratch on scratch trace for max load of 10N under microscope(X1000).

10N

30N

20N

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.

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0

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2000

3000

4000

5000

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Figure 7. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum

load of 20 N.

a)Spot 1 b) Spot 2 c) End of scratch

Figure 8. The digital microscopic images of a) Spot 1 around 9N, b) spot 2 around 13 N, and c) the end of the scratch for scratch test under maximum load of 20 N.

a) 200X b) 800X

Figure 9. The SEM image for the coating delamination on sample T-1.5 (20 N load) at a) 200X b) 800X.

Spot 2 Spot 1

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Figure 10. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum

load of 20 N, sample T-4.5.

(a)Spot 1 around 14.6N (b) End of scratch

Figure 11. The digital microscopic images of (a) Spot 1 around 14.6 N, and (b) the end of the scratch for scratch test on sample T-4.5 under maximum load of 20 N.

Spot 1

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N)

Aco

ustic

Em

issi

on(%

)

Normal Force(mN)

AE Ft

 

19  

    

  Figure. 12. SEM images at the scratch end location (20 N): a) T-1.5, b) T-2.5, and c) T-4.5.

Figure 13. Critical load for coating delamination vs. coating thickness.

a) T-1.5 b) T-2.5

c) T-4.5

8

10

12

14

16

0 1 2 3 4 5

Crit

ical

load

(N)

Coating thickness(μm)

 

20  

a)I-Ti

b) I-Cr

Figure 14. Acoustic emission (AE) and tangential force (Ft) vs. normal load (Fn) for a maximum load of 5 N on a) I-Ti and b) I-Cr.

    Figure. 15. SEM images at the scratch end location (5 N): a) I-Cr, and b) I-Ti.

Spot 1

Spot 1

a) I-Cr b) I-Ti

030060090012001500

020406080

100

0 1000 2000 3000 4000 5000

Tang

entia

l For

ce(m

N)

Aco

ustic

Em

issi

on(%

)

Normal Force(mN)

AE Ft

05001000150020002500

0

10

20

30

0 1000 2000 3000 4000 5000

Tang

entia

l For

ce(m

N)

Aco

ustic

Em

issi

on(%

)

Normal Force(mN)

AE Ft