Scratch testing and instrumented indentation are widely used for mechanical characterization of surfaces. Advanced instruments like the Ultra Nanoindentation Tester (UNHT) and the Nano Scratch Tester (NST) use high-tech measurement techniques to offer precise information.
Oliver and Pharr Method
“Oliver & Pharr Method” is generally used to examine indentation data. This method uses the unloading-part of the load-displacement curve to determine the mechanical properties of materials such as hardness and elastic modulus.
However, this method presumes the sample to be a monolithic halfspace and does not consider the impact of the substrate in the collective response of the coated surface to the load-induced deformations and stresses, and hence the results are effective values of the entire sample.
Thus, it is rather difficult to get the precise mechanical characteristics of the coating through this "Oliver & Pharr Method". Conversely, the “Oliver & Pharr extended for coatings” model enables a physical examination of indentation measurements as it determines the exact values of generic material properties of the coating’s layer like the elastic modulus.
It also helps in analyzing the yield strength as it determines the total elastic contact field at the point of initial unloading.
This model can also be applied to scratch test data, since it considers the extra load component and measurement effects. Hence, one can now obtain generic and more complete material properties, for instance the critical stresses of each fracture mode (I, II, and III).
Figure 1. Von Mises stress distribution of an assumed monolithic halfspace without considering additional loads and measurement effects
Figure 2. Von Mises stress distribution
The above figure shows the effect on the Von Mises stress distribution of those variations between the standard and extended model, although the standard "Oliver & Pharr Method" does not allow one to determine the entire elastic field and its stress-strain components.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.