Evaluating mechanical properties of complex material structures from a new combination of calotte grinding and indentation tests

Depending on the material structure (e.g. layer thicknesses, Young’s moduli and yield strength ratios …) and the indentation device capabilities (e.g. force range), it’s often not possible to measure the true coating parameters without any substrate (or underlying material) effect. A few years ago, SIO developed a model and a dedicated software package called Oliver & Pharr for Coatings (OPfC® [1]) which allows the determination of true generic material parameters (like Young’s modulus and yield strength) for a coating by knowing the parameters of the substrate and all underlying layers.

To apply this approach to a complex multilayer stack of different materials, one needs to stop the production process after every added layer. Then the indentation measurements are performed to determine the material parameters for the top coating. This analysis is repeated for every layer in a possibly very complex coating structure. This approach has severe drawbacks: Firstly it’s not always possible to coat the complete structure layer by layer and perform the measurements after a layer was added. Secondly it is possible that during the coating process the parameters of the underlying layer are changed, because of some interface effects. One of these effects could be the creation of a mixed zone of both materials with different mechanical parameters.

Because the calotte grinding tests are widely used to determine the layer thicknesses, SIO thought about using the possibility to directly access the deeper parts of a complex layer stack by applying a combination of calotte grinding and subsequent indentation testing. We created a new module which analyzes a series of indentation measurements which were performed from the inside (focal point) to the outside of the calotte test crater. So it’s possible to perform tests on the substrate and all layers. This module was built into the software package FilmDoctor® [2], which subsequently analyzes such a measurement series starting with the substrate measurements. All evaluated values are used for the next iteration step of the new analyzis method. At the end the material parameters for all layers are determined without the need of stopping or changing the production process.

The poster will use an actual example from the automotive industry illustrating the new approach. Because high temperatures are present in the later application, the new method was applied at this elevated temperatures using the newest indentation device developments.

References:

[1] www.siomec.de/OPfC

[2] www.siomec.de/FilmDoctor