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 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 2
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 3
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]: 2 𝑋 𝑠𝑓𝑔 𝑀 𝑡 𝐼 𝑡 = 𝐼 𝑠𝑓𝑔 …(1 ) 𝑀 𝑠𝑓𝑔 𝑋 𝑡 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 4
Progressive Load Scratch Test A stylus is moved over a specimen Load surface with a linearly increasing load until failure occurs at critical Stylus loads (Lc i ). Normal force (Fz) and Coating tangential force (Fx) are recorded. The failure events are examined by an optical microscope. Acoustic Emission (AE) is also measured during the test. Scratch Direction 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. June 5, 2012 5
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 6
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 7
Damage Features Scratch Direction Through Thickness Cracking Chevron Cracks • Brittle Tensile Cracking: Nested micro- cracks; open to the direction of scratch; straight and semi-circular; formed behind Arc Tensile the stylus. • Hertz Cracking: Series of nested micro- Hertz cracks within the scratch groove • Conformal Cracking: micro-cracks form while coating try to conform to the groove; Conformal open away from the direction of scratch. Chipping Scratch Direction Rounded regions of coating removal extending laterally from the edges of the scratch groove Chipping June 5, 2012 8
Damage Features Spallation Scratch Direction • Buckling : coating buckles ahead of the stylus Buckling tip; irregularly wide arc-shaped patches missing; opening away from scratch direction. • Wedging : Caused by a delaminated region Wedging wedging ahead to separate the coating; regularly spaced annular-circular that extends beyond the edge of the groove. Recovery • 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 • Gross Spallation : Large detached regions; common in coating with low adhesion strength. June 5, 2012
Failure Mechanisms of Coating Substrate Hardness Low High Plastic deformation of Plastic deformation and coating and substrate conformal cracking of produces tensile and coating, followed by Low Coating Hardness conformal cracking with spallation and buckling buckling failure of failure in coating as coating substrate cracks. Tensile crack followed Tensile and Hertzian High cracks in coating by chipping and progressing to chipping spallation of coating and spallation of coating as substrate is deformed June 5, 2012 10
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 1 st model can be summarized as: 𝑦 = 𝑒 3 4 𝜐𝑒 2 + 𝑒𝑢𝐼 𝑑 12𝑆 𝐼 𝑇 + 𝜌 𝐺 … (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 1 st , 2 nd , and 3 rd 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 ( t c ) of the coating-substrate interface. This model was found to work well with Al-coated glass specimen. June 5, 2012 11
Scratch Models The 2 nd model of Benjamin and Weaver is based on normal force that describes scratching in terms of shear stress ( t s ) at the lip of stylus tip: 𝐼 𝑡 𝑏 𝜐 𝑡 = …(3) 𝑆 2 −𝑏 2 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 H S in equation (3) by F z / p a 2 , resulting in a critical shear stress given by: 𝑀 𝑑 𝜐 𝑑 = …(4) 𝜌𝑏 𝑑 𝑆 2 −𝑏 𝑑2 where, L c = critical load and a c = contact radius at the critical load. This model was able to yield semi-quantitative results for DLC films. June 5, 2012 12
Scratch Models Laugier: Total compressive stress ( s x ) under the leading edge of the indenter is expressed as [8,9]: 𝐺 3𝜌𝜈 𝑨 𝜏 𝑦 = 2𝜌𝑏 2 4 + ν 𝑡 8 − 1 − 2ν 𝑡 …(5) where, ν 𝑡 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: 𝑨 𝑆 1−ν 𝑡2 1−ν 𝑑2 𝑏 3 = 3 4 𝐺 + 𝐹 𝑑 …(6) 𝐹 𝑡 ν 𝑑 is the Poisson’s ratio of the coating, E s and E c are the Young’s moduli of the substrate and the coating, respectively. June 5, 2012 13
Scratch Models For a << R, the shear stress ( t ) acting on the coating-substrate interface at the lip of the indentation was approximated as : τ ≈ 𝜏 𝑦 𝑏 …(7) 𝑆 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: 𝜏 𝑑2 W = 2𝐹 𝑑 𝑢 …(8) The critical stress ( s c ) 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. June 5, 2012 14
Scratch Models 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: 1 2 𝜌𝑒 𝑑2 2𝐹 𝑑 𝑋 𝑀 𝑑 = …(9) 8 𝑢 where, W is the work of adhesion, d c is the scratch width at the critical load. June 5, 2012 15
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