Scratch Test: Physical Characterization of Coated Surfaces

The standard scratch test is routinely used to test the coatings’ mechanical stability on different types of substrates and has become a sensitive method to manage the reliability of the manufacturing process. It is based on different standards [12, 13].

To apply a normal load FN on the sample surface, a diamond stylus (normally spherical diamond tip geometry) is utilized. The sample is simultaneously displaced at a constant speed as the normal load is increased. The resulting stresses in the coating structure can cause chipping or flaking of the coating at some point. The critical load (Lc) at which a particular failure event happens can be measured from the acoustic emission signal, from the fluctuation that occurs in the tangential force, or can be observed as particular surface deformation in the optical microscope. In addition, Lc can be observed as a discontinuity (step) in the post-scan surface. However, it can be impossible or difficult to calculate generic material properties, for example, critical stresses of each failure mode, from these standardized tests, because they are not customized to the surface structure under investigation and as a result, do not produce critical close to the coating structure, but deep down in the substrate. Therefore, the traditional scratch test must be properly dimensioned initially.

Instrumented Indentation on Thermal Spray Coatings

Using the generic mechanical material properties at hand, determined in the previous article (n37) through physically analyzed nanoindentation measurements, an subsequent scratch test can be well-dimensioned, as per the flow chart shown in Figure 2. In this study, only the TR sample was examined. The purpose of dimensioning a scratch test is to get optimum measurement data from the coating of interest, so that a physical analysis of these tests can explain the failure mechanisms of the future applications, like mode-I fracture or mode-II fracture, which are much closer to what occurs in a contact situation from practice compared to a single load-component indentation. To accomplish this aspect, the most appropriate degrees of freedom of a scratch test, which are the applied normal force and indenter geometry, need to be established. The indenter geometry is described by the indenter radius with regard to a spherical indenter (Rockwell), a common scratch test stylus.

The FilmDoctor® Studio software enables the modeling and simulation of lateral forces and resulting tilting, and hence it can also be employed to dimension a scratch test where these contact situations are relevant. As a result, three varied scratch conditions with spherical indenters of 20 µm, 50 µm and 200 µm radii and standard loads of 1 N, 20 N and 80 N, respectively, are modeled and the ensuing Von Mises stress distribution is measured (Figure 1) based on previously measured elastic modulus of the substrate (ES) and the layers (EC1, EC2). These scratch test parameters can be selected based upon experience or measured stress distribution of the indentation measurement and according to the available measurement equipment. For the first dimensioning, the tangential force may be chosen according to coefficient of friction values from literature and the surface is assumed to be plane. It is clear that these different scratch parameters result in completely different stress distribution, locations or maxima and values of maxima.

Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 µm radius with 1 N normal load (a), 50 µm radius with 20 N normal load (b), and 200 µm radius with 80 N normal load (c). The interfaces are indicated by the white dashed lines. The block cross hairs mark the von-Mises stress maxima.

Figure 1. Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 µm radius with 1 N normal load (a), 50 µm radius with 20 N normal load (b), and 200 µm radius with 80 N normal load (c). The interfaces are indicated by the white dashed lines. The block cross hairs mark the von-Mises stress maxima.

The von-Mises stress maxima are concentrated in the first and second layer of the coating as shown in Figures 1a and 1b, respectively, while the von Mises stress is concentrated in the substrate, shown in Figure 1c. The von-Mises stress maxima must agree with the depth of interest, since the sensitivity of following scratch tests is expected to be in these depth ranges. Furthermore, the maximum should exceed the yield strength of the constituent of interest within the limits of the pertinent application situations in order to ensure that a failure will occur.

Physical Analysis of Scratch Tests

For a physical analysis of a scratch test, a scratch tester should determine certain information. Besides the progressively loaded scratch, during which time the lateral force, the standard load and the penetration depth under this load are calculated, the surface profile has be to obtained before the scratch. As shown in Figure 2, the contact situation and the stress-strain fields are considerably different dependent on where and how the stylus is in contact with the surface of the sample. For example, a three-dimensional (3D) topography of the sample surface from an AFM would be ideal, because it can make a difference whether the stylus hits an asperity accurately in the center and will scratch over, which is a very rare case, or if the stylus hits the asperity at a flank side and, therefore, will deflect and cause a multi-axial inclined contact situation. For simple reasons, only a 2D surface profile as achieved by a simple pre-scan will be used here.

Schematic of two different inclined contact situations: when the stylus moves upwards the flank of an asperity (a) and downwards (b), the dark gray upper shape denotes the stylus and the light gray lower shape the asperity of the sample surface. The white arrows denote the applied forces and the red arrow the tilting of the stylus due to the asperity.

Figure 2. Schematic of two different inclined contact situations: when the stylus moves upwards the flank of an asperity (a) and downwards (b), the dark gray upper shape denotes the stylus and the light gray lower shape the asperity of the sample surface. The white arrows denote the applied forces and the red arrow the tilting of the stylus due to the asperity.

In addition, the profile of the surface after the scratch has to be calculated to differentiate the plastic deformation from the elastic deformation as it leads to a different contact situation (e.g. location and size of contact area) and is hence vital for the simulation. Similar to the pre-scan scratch surface, a 3D topography can be obtained by an AFM measurement or a 2D profile of the residual surface can be achieved by a simple post-scan. All of the measurement information is considered for measuring stress-strain field that develops during the scratch test and enables a physical analysis of the scratch test, because particular critical mechanical properties of the constituent of interest (layers/ substrate) can be measured. Furthermore, the critical load of failure (Lc) is calculated and this will be linked to the simulated contact field to determine why the layer failed at that very instant (Figure 3).

Profiles of measured information along the scratch track together with an aligned panorama image of the scratch track on top.

Figure 3. Profiles of measured information along the scratch track together with an aligned panorama image of the scratch track on top.

The software FilmDoctor®, SSA®, or ISA can analyze over 28 field components on the basis of the measurement data. The Von Mises stress (Figure 4) and the normal stress in scratch direction (Figure 5) are only two of them, but the most relevant ones for the typical failure mechanisms mode I fracture and mode II fracture. At the start of the pre-dimensioned scratch test, the stresses are definitely subcritical (2.2 GPa maximum tensile stress and 6.9 GPa maximum Von Mises stress) and as shown in Figures 5a and 5b, the stresses are concentrated in the top layer. Due to the increasing normal load, the maximum of the von-Mises stress moves down to the substrate during the scratch test. As shown in Figure 4b, the maximum von-Mises stress (25.8 GPa) is concentrated in the interlayer in between the start of scratch test and the Lc moment. At this point, plastic deformation takes place (Figure 2), as the residual depth is below the pre-scan surface. However, it should be noted that the yield strength of the interlayer is 30 GPa, so there is no yielding taking place in the interlayer. Rather, the von Mises stress in the substrate (22.1 GPa) has surpassed the yield strength of the substrate (22 GPa) at the interface and, therefore, plastic deformation in terms of yielding occurs in the substrate. In this case, this is the first failure of the system which will ultimately result in complete failure of the coating later. As a result, the coating gradually loses its support from the substrate and is continually stretched at the surface. According to Figure 6b, the tensile stress at the surface behind the stylus is already 8.2 GPa. Eventually, the substrate has produced so much in the failure moment (Lc = 26.3N), that the top layer cracks (mode I fracture) from the surface to the coating-substrate interface behind the stylus. This failure mode is illustrated in a schematic manner in Figure 6a along with an optical picture of the related failure location on the scratch track in Figure 6b. Therefore, the measured tensile stress of the top layer as 10.2 GPa is its critical tensile.

The evolution of von-Mises stress during the scratch test shown at three measurement points: (a) at the beginning of the scratch test, (c) in the moment of LC failure, and (b) in between. The black cross hairs indicate the location of the maximum.

Figure 4. The evolution of von-Mises stress during the scratch test shown at three measurement points: (a) at the beginning of the scratch test, (c) in the moment of LC failure, and (b) in between. The black cross hairs indicate the location of the maximum.

The evolution of normal stress in scratch direction illustrated at three measurement points: (a) at the beginning of the scratch test, (c) in the moment of LC failure, and (b) in between. The black cross hairs indicate the location of maximum tensile stress.

Figure 5. The evolution of normal stress in scratch direction illustrated at three measurement points: (a) at the beginning of the scratch test, (c) in the moment of LC failure, and (b) in between. The black cross hairs indicate the location of maximum tensile stress.

Illustrative scheme of the failure mechanism (a) and an optical image of the post-scratch surface (b) in which the corresponding LC position is marked in red.

Figure 6. Illustrative scheme of the failure mechanism (a) and an optical image of the post-scratch surface (b) in which the corresponding LC position is marked in red.

Therefore, a physical analysis of mechanical contact measurements like scratch tests and instrumented indentations allows one to discover why a surface structure fails in a particular moment. These results offer indications on how the coating structure can be enhanced. However, the improvement of coated surfaces would be beyond the scope of this study.

Example of Application: For Cutting Tools

Advanced design of wear resistant and decorative coatings hinges on the improvement of the mechanical properties (yield strength, elastic modulus, intrinsic stresses, fracture, fretting, wear resistance and adhesion) of the coating-substrate system.

The objective is to find material and structural solutions to maintain the resulting stress-strain field under typical application conditions well below the stability limits of the system.

A scratch test is dimensioned as function of the indenter geometry and the load range, based on nanoindentation measurements achieved from the coating-substrate system. The measured data from the “Physical Scratch Test” is used to calculate spatial or 3D stress profiles of, for instance, the von Mises stress and the normal stress (tensile/compressive) in scratch direction.

In order to improve and understand the coating-substrate interface better, the below flow chart is followed.

A flow chart of the procedure of mechanical characterization and optimization of arbitrary structured surfaces.

Figure 7. A flow chart of the procedure of mechanical characterization and optimization of arbitrary structured surfaces.

Practical Example for Cutting Tools

Step 1: Nanoindentation tests are carried out to accurately determine the materials properties of the coatings and substrate.

Step 2: With the mechanical properties earlier measured by nanoindentation, a calculation of the stress-strain fields of a simulated scratch enables an ideal dimensioning of the scratch test.

Step 3: Scratch testing with the defined conditions is carried out.

Step 4: An advanced analysis together with integration of the pre-scan surface profile, friction coefficient and residual penetration depth is made with the simulation software so as to perfectly recognize the stress-strain field.

Step 5: An animation of the scratch is created for better understanding.

Step 6: Your coating-substrate system.

Bibliography

[7] N. Schwarzer, Q.-H. Duong, N. Bierwisch, G. Favaro, M. Fuchs, P. Kempe, B. Widrig und J. Ramm, „Optimization of the Scratch Test for Specific Coating Designs,“ Surface and Coatings Technology, 2011.

[8] Saxonian Institute of Surface Mechanics, „Material DataBase,“ [Online]. Available: www.siomec.de/database.

[9] Saxonian Institute of Surface Mechanics, „FilmDoctor,“ [Online]. Available: www.siomec.de/FilmDoctor.

[10] Saxonian Institute of Surface Mechanics, „Oliver&Pharr for Coatings (O&PfC),“ [Online]. Available: www.siomec.de/OPfC.

[11] Saxonian Institute of Surface Mechanics, „Indentation and Scratch Analyzer (ISA),“ [Online]. Available: www.siomec.de/ISA.

[12] ISO 20502 Fine ceramics (advanced ceramics -advanced technical ceramics) - Determination of adhesion of ceramic coatings by scratch testing.

[13] ASTM C1624 Standard Test Method for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Single Point SCRATCH Testing.

Acknowledgement

The Authors would like to thank the Saxonian Institute of Surface Mechanics for strong collaboration (Dr Norbert Schwarzer and Nick Bierwisch).

This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.

For more information on this source, please visit Anton Paar TriTec SA.

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