Advanced Scratch Testing for Evaluation of Coatings Suresh Kuiry, - - PowerPoint PPT Presentation
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. May 8, 2012 1 Introduction Scratch Tests
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Bruker Nano Surfaces Division Tribology and Mechanical Testing, 1717 Dell Ave, Campbell, CA 95008, U.S.A.
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Coatings are used for optical, microelectronic, packaging, biomedical, and decorative applications to improve:
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.
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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]: πΌπ‘ = πΌπ ππ
ππ‘ ππ ππ ππ ππ π
π‘
2
β¦(1) where, subscripts βsβ and βrefβ 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.
progressive (~linearly increasing) or constant load [2-4].
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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
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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.
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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.
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Through Thickness Cracking
cracks; open to the direction of scratch; straight and semi-circular; formed behind the stylus.
cracks within the scratch groove
while coating try to conform to the groove;
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
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tip; irregularly wide arc-shaped patches missing; opening away from scratch direction.
wedging ahead to separate the coating; regularly spaced annular-circular that extends beyond the edge of the groove.
elastic recovery behind the stylus and plastic deformation in the substrate.
common in coating with low adhesion strength.
Gross Spallation Buckling Wedging Recovery
Scratch Direction
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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 Hardness High Low
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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: πΊ
π¦ = π3 12π πΌπ + π 4 ππ2 + ππ’πΌπ
β¦(2) where, d is scratch width, R is the tip radius, Hs and Hc are the hardness
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)
Al-coated glass specimen.
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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: ππ‘ =
πΌπ‘π π2βπ2
β¦(3) where, a is the contact radius between the tip and the coating (a β d/2). The model gives a measure of adhesion in terms of critical shear stress by substituting βaβ 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: ππ =
ππ πππ π2βππ2
β¦(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.
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Laugier: Total compressive stress (sx) under the leading edge of the indenter is expressed as [8,9]: ππ¦ =
πΊ
π¨
2ππ2 4 + Ξ½π‘ 3ππ 8 β 1 β 2Ξ½π‘
β¦(5) where, Ξ½π‘ is the Poissonβs 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
3 4 πΊ π¨π 1βΞ½π‘2 πΉπ‘
+
1βΞ½π2 πΉπ
β¦(6) Ξ½π is the Poissonβs ratio of the coating, Es and Ec are the Youngβs moduli of the substrate and the coating, respectively.
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For a << R, the shear stress (t) acting on the coating-substrate interface at the lip of the indentation was approximated as : Ο β ππ¦π
π
β¦(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=
ππ2 2πΉπ π’
β¦(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 βa >> tβ. The model predicted results on carbide and nitride coatings reasonably.
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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: ππ =
πππ2 8 2πΉππ π’
1 2
β¦(9) where, W is the work of adhesion, dc is the scratch width at the critical load.
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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: ππ =
π΅π Ξ½πππ 2πΉππ π’
1 2
β¦(10) where Ac is the cross-sectional area of the scratch track at the critical load: π΅π = π2π‘ππβ1
ππ 2π β ππ 2 π2 β ππ 2 2 1/2
β¦(11) Equation (10) could be used to calculate work of adhesion (W).
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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
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.
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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.
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CETR-Universal Materials Tester (CETR-UMT) is a unique test and measurement system that can be used for Scratch Testing:
displacement (Z-encoder)
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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
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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
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Specimen: 1-mm thick DLC coating on Titanium substrate. Tool: Rockwell Diamond indenter 200-mm tip radius Adhesion: 25 N
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Specimen: 3.5-mm thick DLC coating on Steel substrate. Tool: Rockwell Diamond indenter 200-mm tip radius Adhesion: 25 N
X [ um ]
50 100 Z [um]
0.5 1 1.5
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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 (οY) and depth (D)
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Specimen: 10-mm thick Zn-coating on steel. Tool: Rockwell Diamond indenter 200-mm tip radius Adhesion: 9.7 N
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Scratch depth profile is measured using either Cap sensor or Z-encoder
scratch (black), scratch (blue), and post-scan (red) steps took care of sample-tilt. Image tag displays scratch width (οY) and depth (D)
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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 (οY) and depth (D)
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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 ]
100 200 300 Z [um]
5 10 15 20 25
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Specimen: 3.5-mm thick DLC coating on steel substrate. Tool: Diamond stylus 12.5-mm tip radius 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; Coating failed at 245 mN
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Specimen: 3.5-mm thick DLC coating on steel substrate. Tool: Diamond stylus 12.5-mm tip radius 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
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Specimen: 3.5-mm thick DLC coating on steel substrate. Tool: Diamond stylus 12.5-mm tip radius 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
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Specimen: 3.5-mm thick DLC coating on steel substrate. Tool: Diamond stylus 12.5-mm tip radius 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.
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1. ASTM Standard G171 (03) β Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus. 2. ASTM Standard C1624 (05) βStandard Test Method for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Single Point Scratch testing 3.
4.
5.
6.
7. S.J. Bull, D.S. Rickerby, A. Matthews, A. Leyland, A.R. Pace, J. Valli, Surf. Coat.
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.
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