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 standards12, 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.
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.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
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