Advanced Scratch Testing for Evaluation of Coatings Slides

May 8, 2012 1

www.bruker-axs.com

Advanced Scratch Testing for Evaluation of Coatings

Suresh Kuiry, PhD

Bruker Nano Surfaces Division

Tribology and Mechanical Testing, 1717 Dell Ave, Campbell, CA 95008, U.S.A.

June 5, 2012 2

Introduction

• Scratch Tests Fundamentals

• Scratch Failure Regimes and Their Characteristics

• Existing Scratch Models

• CETR-UMT Scratch Tester

• Advanced Scratch Testing with CETR-UMT

• Some Scratch Test Results Obtained Using CETR-UMT

• Q & A

June 5, 2012 3

Why Scratch Test ?

Coatings are used for optical, microelectronic, packaging, biomedical, and

decorative applications to improve:

• tribological (lower friction),

• mechanical (wear/abrasion resistance),

• chemical (barrier to aggressive gases),

• optical, magnetic, and electrical properties of any substrate.

Functional behaviour of a coating is critical to its adhesion to the substrate.

Scratch test is one of widely used, fast, and effective methods to obtain the

critical loads that are related to adhesion properties of coating.

June 5, 2012 4

Scratch Tests

1. Scratch Hardness Test: Scratch with constant normal load on a

specimen and on a reference specimen using a stylus. Scratch width

data are utilized to obtain the scratch hardness of the specimen as

follows [1]:

𝐻𝑠 = 𝐻𝑟𝑒𝑓𝐿𝑠

𝐿𝑟𝑒𝑓

𝑊𝑟𝑒𝑓

𝑊𝑠

2 …(1)

where, subscripts ‘s’ and ‘ref’ stand for the test specimen and the

reference specimen, respectively. The terms H, L, and W denote

hardness, normal load, and scratch width, respectively. The test is

used for bulk and coating materials.

2. Scratch Adhesion Test: This test is performed by applying either a

progressive (~linearly increasing) or constant load [2-4].

June 5, 2012 5

Progressive Load Scratch Test

A stylus is moved over a specimen

surface with a linearly increasing

load until failure occurs at critical

loads (Lci). Normal force (Fz) and

tangential force (Fx) are recorded.

The failure events are examined by

an optical microscope. Acoustic

Emission (AE) is also measured

during the test.

Lc is a function of coating-substrate

adhesion, stylus-tip radius, loading

rate, mechanical properties of

substrate and coating, coating

thickness, internal stress in coating,

flaw size distribution at substrate-

coating interface, and friction

between stylus-tip and coating.

Scratch Direction

Load

Coating

Stylus

June 5, 2012 6

Constant Load Scratch Test

Series of scratch tests are performed with constant normal loads on a

coating to obtain a load where the coating exhibits failure. Each scratch is

examined with an optical microscope for failure. The load at which such

failure of the coating occurs is termed as the critical load (Lc). Acoustic

Emission (AE) and Electrical Surface Resistance (ESR) are also measured

simultaneously during the constant load scratch test to supplement/confirm

the failure.

Constant load test requires more time but it provides greater statistical

confidence.

Progressive load test is suitable for rapid assessment and quality

assurance (QA) of coating. Hence, it is more popular for research and

development work on coating processes.

June 5, 2012 7

Coating Failure during Scratch Test

At sufficient stress, cracks initiate preferentially at defect sites in

the coating and/or coating-substrate interface. Propagation of

such cracks lead to coating failure.

Cohesive Failure: occurs by tensile stress behind the stylus

(Through-Thickness Cracking)

Adhesive Failure: Due to compressive stress, the coating

separates from the substrate either by cracking and lifting

(Buckling) or by full separation (Spallation; Chipping).

Practical scratch adhesion value of coating is defined as the

lowest critical load at which a coating fails. It is an important

parameter related to coating-substrate adhesion that could be

used for comparative evaluation of coatings.

June 5, 2012 8

Damage Features

Through Thickness Cracking

• Brittle Tensile Cracking: Nested micro-

cracks; open to the direction of scratch;

straight and semi-circular; formed behind

the stylus.

• Hertz Cracking: Series of nested micro-

cracks within the scratch groove

• Conformal Cracking: micro-cracks form

while coating try to conform to the groove;

open away from the direction of scratch.

Chevron Cracks

Arc Tensile

Conformal

Chipping

Rounded regions of coating removal extending

laterally from the edges of the scratch groove Chipping

Hertz

Scratch Direction

Scratch Direction

June 5, 2012

Damage Features

Spallation

• Buckling : coating buckles ahead of the stylus

tip; irregularly wide arc-shaped patches

missing; opening away from scratch direction.

• Wedging : Caused by a delaminated region

wedging ahead to separate the coating;

regularly spaced annular-circular that extends

beyond the edge of the groove.

• Recovery: regions of detached coating along

one or both sides of the grove; produced by

elastic recovery behind the stylus and plastic

deformation in the substrate.

• Gross Spallation : Large detached regions;

common in coating with low adhesion strength.

Gross Spallation

Buckling

Wedging

Recovery

Scratch Direction

June 5, 2012 10

Failure Mechanisms of Coating

Tensile crack followed

by chipping and

spallation of coating

Plastic deformation and

conformal cracking of

coating, followed by

spallation and buckling

failure in coating as

substrate cracks.

Plastic deformation of

coating and substrate

produces tensile and

conformal cracking with

buckling failure of

coating

Tensile and Hertzian

cracks in coating

progressing to chipping

and spallation of

coating as substrate is

deformed

Substrate Hardness

Low High

Coating H

ard

ness

H

igh

Low

June 5, 2012 11

Scratch Models

Benjamin and Weaver: They proposed two scratch models [5] based on

(a) tangential force (Fx) at the tip and (b) normal force. The 1st model can

be summarized as:

𝐹𝑥 =𝑑3

12𝑅𝐻𝑆 +

𝜋

4𝜏𝑑2 + 𝑑𝑡𝐻𝑐 …(2)

where, d is scratch width, R is the tip radius, Hs and Hc are the hardness

of substrate and coating, respectively; t is the shear stress at the coating-

substrate interface and t is the thickness of the coating.

The 1st, 2nd, and 3rd terms in the RHS of the equation (2) are the ploughing

force required to deform the substrate, the force to remove coating from

the surface, and the ploughing force required to push aside the sheared

film, respectively. This model can be used to obtain critical shear stress (tc)

of the coating-substrate interface. This model was found to work well with

Al-coated glass specimen.

June 5, 2012 12

The 2nd model of Benjamin and Weaver is based on normal force that

describes scratching in terms of shear stress (ts) at the lip of stylus tip:

𝜏𝑠 =𝐻𝑠𝑎

𝑅2−𝑎2 …(3)

where, a is the contact radius between the tip and the coating (a ≈ d/2).

The model gives a measure of adhesion in terms of critical shear stress by

substituting ‘a’ measured at the critical load.

Ollivier and Matthews: They [6] replaced HS in equation (3) by Fz/pa2,

resulting in a critical shear stress given by:

𝜏𝑐 =𝐿𝑐

𝜋𝑎𝑐 𝑅2−𝑎𝑐2 …(4)

where, Lc = critical load and ac = contact radius at the critical load. This

model was able to yield semi-quantitative results for DLC films.

Scratch Models

June 5, 2012 13

Laugier: Total compressive stress (sx) under the leading edge of the

indenter is expressed as [8,9]:

𝜎𝑥 =𝐹𝑧

2𝜋𝑎2 4 + ν𝑠3𝜋𝜇

8− 1 − 2ν𝑠 …(5)

where, ν𝑠 is the Poisson’s ratio of the substrate and m is friction coefficient

(Fx/Fz) between the indenter and the coating. The first terms originates

from the compressive stress at the leading edge of the indenter induced by

the friction during sliding. The second term describes the radial surface

stress at the edge of the contact circle induced by the force normal to the

surface. Assuming elastic Hertzian contact, the contact radius (a) is

expressed as:

𝑎3 =3

4𝐹𝑧𝑅

1−ν𝑠2

𝐸𝑠+

1−ν𝑐2

𝐸𝑐 …(6)

ν𝑐 is the Poisson’s ratio of the coating, Es and Ec are the Young’s moduli of

the substrate and the coating, respectively.

Scratch Models

June 5, 2012 14

For a << R, the shear stress (t) acting on the coating-substrate interface at

the lip of the indentation was approximated as :

τ ≈𝜎𝑥𝑎

𝑅 …(7)

The value of the t at the critical load is considered a measure of coating

adhesion.

Laugier later [9] introduced practical work on adhesion (W) as:

W=𝜎𝑐

2

2𝐸𝑐𝑡 …(8)

The critical stress (sc) is the sum of external stress and internal stress at

the critical load. This model was purely elastic, and it was assumed that ‘a

>> t’. The model predicted results on carbide and nitride coatings

reasonably.

Scratch Models

June 5, 2012 15

Burnett and Rickerby [10]: The driving forces for removal of coating

consists of components of (i) an elastic-plastic indentation stress, (ii) an

internal stress, and (iii) tangential force. The following relation was derived

for critical scratching load:

𝐿𝑐 =𝜋𝑑𝑐

2

8

2𝐸𝑐𝑊

𝑡

12 …(9)

where, W is the work of adhesion, dc is the scratch width at the critical

load.

Scratch Models

June 5, 2012 16

Bull et al [7] modified Burnett-Rickerby model assuming that the coating

detachment occurs when the tangential compressive stresses in the

coating in front of the stylus induce critical tensile stresses normal to the

coating-substrate interface: The critical load is given by:

𝐿𝑐 =𝐴𝑐

ν𝑐𝜇𝑐

2𝐸𝑐𝑊

𝑡

12 …(10)

where Ac is the cross-sectional area of the scratch track at the critical load:

𝐴𝑐 = 𝑅2𝑠𝑖𝑛−1 𝑑𝑐

2𝑅−

𝑑𝑐

2𝑅2 −

𝑑𝑐

2

2 1/2

…(11)

Equation (10) could be used to calculate work of adhesion (W).

Scratch Models

June 5, 2012 17

The existing models invariably use assumptions and simplifications to deal

with the inherent complexity of any scratching process, which involves

large number of variables. Hence, the existing scratch models experience

great difficulties to give a complete analytical description of the mechanics

of scratch testing. Predicted scratch adhesion values from such models

differ widely from the actual scratch test results.

Evaluation and fine-tuning of such scratch models certainly requires

continuous developmental effort with an objective to obtain a standard

model that would give comprehensive description of any scratching

process.

Limitation of Scratch Models

June 5, 2012 18

For all practical application, we perform scratch test to obtain the critical load

as a useful adhesion parameter for evaluation of coatings to ensure their

fitness for use.

Practical Approach

June 5, 2012 19

Scratch Tester (CETR-UMT)

CETR-Universal Materials Tester (CETR-UMT) is a unique test and measurement system that can be used for Scratch Testing: • Constant load scratch Test for hardness and adhesion

• Progressive load scratch Test for practical adhesion

• Electrical Contact Resistance (ECR)

• Electrical Surface Resistance (ESR)

• Acoustic Emission (AE)

• in-situ scratch depth profiling using capacitance sensor, tip-

displacement (Z-encoder)

• Optical microcopy and AFM for imaging

• 3D-Optical Microscopy (interferometry) for imaging and metrology

June 5, 2012 20

Scratch Tester (CETR-UMT)

June 5, 2012 21

Linear Scratch Test

Specimen: 3.5-mm thick DLC coating on steel substrate.

Tool: Diamond stylus 12.5-mm tip radius

Scratch Parameters: Linear; 1 mm at 0.02 mm/s; Load 20 to 500 mN;

Coating failed at 298 mN

June 5, 2012 22

Linear Scratch Profile with K0



Scratch Parameters: Linear; 1 mm at 0.02 mm/s; Load 20 to 500 mN;


June 5, 2012 23

Linear Scratch

Specimen: 1-mm thick

DLC coating on

Titanium substrate.

Tool: Rockwell

Diamond indenter

200-mm tip radius

Adhesion: 25 N

June 5, 2012 24

Specimen: 3.5-mm

thick DLC coating on

Steel substrate.

Tool: Rockwell

Diamond indenter

200-mm tip radius

Adhesion: 25 N

X [ um ]

-150 -100 -50 0 50 100

Z [

um

]

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

Linear Scratch

June 5, 2012 25

Linear Scratch with Depth Profile

40-mm thick polymer coating on steel; WC-

micro-blade (400-mm tip radius)

Scratch depth profile is measured using Z-

encoder of CETR-UMT. Pre-scan, scratch

(green), post-scan (black) steps are used for

removing the sample-tilt. Image tag displays

scratch width (Y) and depth (D)

June 5, 2012 26

Linear Scratch

Specimen: 10-mm thick Zn-coating on steel.

Tool: Rockwell Diamond indenter 200-mm tip radius

Adhesion: 9.7 N

June 5, 2012 27


Scratch depth profile is

measured using either

Cap sensor or Z-encoder

of CETR-UMT. Pre-scan,

scratch (black), scratch

(blue), and post-scan

(red) steps took care of

sample-tilt. Image tag

displays scratch width

(Y) and depth (D)

June 5, 2012 28


Scratch depth profile of

coated-wire measured with Z-

encoder of CETR-UMT. Pre-

scan, scratch (black), scratch

(blue), and post-scan (red)

steps take care of sample-tilt.

Image tag displays scratch

width (Y) and depth (D)

June 5, 2012 29

Linear Scratch for Adhesion Energy

Specimen: Polymer dots (1.2 mm dia x 25 mm) on ceramic substrate.

Tool: Special Tool-steel micro-blade; Fx increased during delamination of

a polymer dot. Adhesion Energy calculated from area of the triangle under

Fx plot. Adhesion Energy: 490 erg

X [ um ]

-300 -200 -100 0 100 200 300

Z [

um

]

-5

0

5

10

15

20

25

June 5, 2012 30

3D-Scratch



Scratch Parameters: 3D; 1 mm by slider at 0.004 mm/s; 0.15mm stroke

by linear at 0.15 mm/s; Load 20 to 250 mN;


June 5, 2012 31

X-Y Scratch with Reduced Spacing



Scratch Parameters: X-Y; 120-mm long; spacing 100, 80, 60, 50, 40, 30,

20, 10 mm; at 0.01 mm/s; Load 250 mN

Coating did not Fail

June 5, 2012 32

X-Y Scratch with Reduced Spacing



Scratch Parameters: X-Y; 120-mm long; spacing 60,40,30, 20,15,10,5 mm;

at 0.01 mm/s; Load 250 mN

Coating Failed

June 5, 2012 33

Angular Scratch



Scratch Parameters: Angular; X = 48 mm at 2 mm/s; Y = 120 mm at 10

mm/s; Load 250 mN;

Last cycle was not moved and the coating failed.

June 5, 2012 34

Concluding Remarks

• Scratch test is a widely used test procedure for evaluation of coatings.

• CETR-UMT can perform advanced scratch tests (Constant load, Progressive, 3D, X-Y, Angular etc) to evaluate the adhesion properties of coating.

• CETR-UMT Test system can perform comprehensive evaluation of coatings by automated imaging and profiling of scratch.

June 5, 2012 35

References

1. ASTM Standard G171 (03) – Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus.

2. ASTM Standard C1624 (05) –Standard Test Method for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Single Point Scratch testing

3. S. J. Bull, Surf. Coat. Technol. 50 (1991) 25. 4. S. J. Bull, Trib. Inter. 30 (1997) 491. 5. P. Benjamin, C. Weaver, Proc. R. Soc. London, A 254 (1960) 163. 6. B. Ollivier, J. Matthews, J. Adhesion Technol., 8 (1994) 651. 7. S.J. Bull, D.S. Rickerby, A. Matthews, A. Leyland, A.R. Pace, J. Valli, Surf. Coat.

Technol. 36 (1988) 503. 8. M.T. Laugier, Thin Solid Films 76 (1981) 289; 117 (1984) 243. 9. M.T. Laugier, J. Mater. Sci. 21 (1986) 2269. 10. P.J. Burnett, D.S. Rickerby, Thin Solid Films 154 (1987) 403; 157 (1988) 233.

May 8, 2012 36

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Advanced Scratch Testing for Evaluation of Coatings Slides

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