Physical Characterization of Coatings by Scratch Testing

Known mechanical values are needed to perfect a scratch test for particular surface structures. To this end, the scratch test is a sensitive technique that is widely utilized to determine the coatings’ mechanical stability on various substrates. It is a standard method used to control the consistency of the production process.

The scratch test is built on a number of standards. In this application note, physical characterization of coatings using a CSM Instruments scratch tester is discussed in detail.

Dimensioning of scratch tests

The generic mechanical properties of the materials were determined by physically studying nanoindentation measurements. The following scratch test can be dimensioned as per the flow chart shown below.

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 indicates the stylus and the light gray lower shape indicates the asperity of the sample surface. The white arrows indicate the applied forces, while the red arrow denotes the tilting of the stylus owing to the asperity.

Figure 1. 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 indicates the stylus and the light gray lower shape indicates the asperity of the sample surface. The white arrows indicate the applied forces, while the red arrow denotes the tilting of the stylus owing to the asperity.

In this study, only the TR sample was examined. The objective of dimensioning a scratch test is to get optimum measurement data from the coating, with the intention that a physical analysis of such experiments can explain the failure mechanisms of future applications.

To achieve this aspect, appropriate degrees of freedom of a scratch test need to be calculated. The software, FilmDoctor Studio, enables the modeling and simulation of lateral forces and resulting tilting, and can also be employed to dimension a scratch test where these contact conditions are pertinent.

With spherical indenters of 20 pm, 50 pm, and 200 pm radii and standard loads of 1 N, 20 N and 80 N, respectively, three varied scratch situations are modeled and the ensuing Von Mises stress distribution is measured based on previously calculated elastic modulus of the layers (EC1, EC2).

Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 pm radius with 1 N normal load (a), 50 pm radius with 20 N normal load (b), and 200 pm radius with 80 N normal load (c). The interfaces are denoted by the white dashed lines.

(a)

Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 pm radius with 1 N normal load (a), 50 pm radius with 20 N normal load (b), and 200 pm radius with 80 N normal load (c). The interfaces are denoted by the white dashed lines.

(b)

Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 pm radius with 1 N normal load (a), 50 pm radius with 20 N normal load (b), and 200 pm radius with 80 N normal load (c). The interfaces are denoted by the white dashed lines.

(c)

Figure 2. Simulation of distribution of Von Mises stress for three different kinds of scratches with spherical tips: 20 pm radius with 1 N normal load (a), 50 pm radius with 20 N normal load (b), and 200 pm radius with 80 N normal load (c). The interfaces are denoted by the white dashed lines.

One can select these scratch test parameters based on measured stress distribution of the indentation measurement and according to the available measurement equipment. For initial dimensioning, one can select the tangential force in accordance to coefficient of friction values from literature.

It is evident that these scratch parameters result in different locations of maxima, values of maxima, and stress distribution.

The maxima of Von Mises stress are concentrated in the first and second layer of the coating as shown in figures 1 and 2, respectively, while the von Mises stress is concentrated in the substrate, displayed in figure 3. The von-Mises stress maxima should agree with the depth of interest, since the sensitivity of following scratch tests is expected to be in these depth ranges.

Physical Analysis of Scratch Tests

For physical analysis of a scratch test, a scratch tester must determine certain data. The surface profile before the scratch has to be achieved, apart from the progressively loaded scratch itself, during which time the lateral force, the normal load and the penetration depth under this load are calculated.

In addition, the profile of the surface after the scratch has to be calculated in order to differentiate the plastic deformation from the elastic deformation since it leads to a different contact situation, like location and dimension of contact area, and is hence important for the simulation.

Similar to the pre-scan scratch surface, a three-dimensional topography can be obtained by an AFM measurement or a two-dimensional profile of the residual surface can be achieved by a post-scan.

These measurement data are taken into account for measuring stress-strain field, which occurs during the scratch test and allows a physical analysis of the scratch test. Moreover, the critical load of failure (Lc) is calculated and this will be linked to the simulated contact field so as to identify why the layer failed at that very instant.

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

Figure 5. Profiles of measured information along the scratch track along with an aligned panorama image of the scratch track on top.

The evolution of normal stress in scratch direction indicated 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 6. The evolution of normal stress in scratch direction indicated 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.

Conclusion

Therefore, a physical analysis of mechanical contact measurements such as scratch tests and instrumented indentations allows one to identify why a layer or surface structure fails at a certain moment. These results demonstrate how the coating structure can be enhanced.

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.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit