Analytical modelling utilising the effective indenter concept for stress calculations is a powerful tool for getting the most out of nanomechanical and nano-tribological tests.
Detailed simulated stress distributions enable data to be interpreted more effectively. They can provide mechanistic information which can be the key to unlocking exactly where and why coatings systems fail in scratch and fretting tests, and then to designing coatings with improved performance.
The effective indenter concept has also been applied to determine the coating elastic modulus free of substrate effects in a more reliable method than the standard unloading approach. At high temperatures indentation creep can make elastic modulus measurements less reliable, as shown at room temperature in the ISO standard, ISO14577-4, and to improve the accuracy of the high temperature data analysis a time-dependent effective indenter approach can be used.
Advantages of the NanoTest:
Micro Materials work with the Saxonian Institute of Surface Mechanics (SIO), an innovative modelling company who have developed these models and the accompanying intuitive software interface, to fully integrate this advanced modelling capability to the NanoTest system.
Key strengths of the NanoTest design (such as high lateral rigidity for straight scratch tracks, ultra-low thermal drift at elevated temperature etc) enable production of high-quality artefact-free data to be used as direct input to the physical-based models.
Example – Nanoindentation on very thin coatings on low yield stress substrates
Nanoindentation on very thin coatings (100 nm or less) is more challenging when they are deposited on softer substrates. When coatings are very thin plastic flow can occur in the substrate before the true coating hardness is reached. Sensitivity to film properties is improved by using a sharper indenter so that coating-only properties could be measured.
In tests of 40 nm and 80 nm ta-C films deposited on glass a sharp cube corner diamond indenter was used to ensure the measured hardness is due to the coating hardness and not a lower value due to plastic flow in the substrate.
Determining coating-only elastic properties is also challenging. When the relative indentation depth is 10% of the coating thickness there is some contribution from the less stiff glass substrate and the measured elastic modulus is higher than the real coating value.
Two methods were used to avoid this:- (i) an ISO 14577–4 type approach – multi-cycle load-partial unload tests and extrapolate the results to zero depth (ii) analytically account for the substrate contribution to the measured result. It can be seen below that both methods provide improved accuracy, particularly for the 80 nm film, with the modelling being experimentally more convenient.
Illustrative stress distributions on the point of unloading for (a) 40 nm (b) 80 nm film. (c) NanoTest load-partial unload data and ISO extrapolation method
High temperature micro-scratch
Experimentally it has been shown that on AlCrN and AlTiN coatings deposited on WC-Co the critical load for coating failure can be higher at elevated temperature than at 25 °C . The modelling shows that this is not a result of improved adhesion at 500 °C.
For AlTiN in particular the stress distributions were strongly temperature dependent. At 25 °C the coating failure occurred by interface weakening from substrate yield in combination with high tensile stress at the surface. At 500 °C the yield occurred in the coating suggesting the deformation proceeds by a different mechanism. Analytically determined maximum shear stresses at Lc2 were lower at 500 °C suggesting the coating-substrate bonding strength is actually slightly reduced.
Temperature-dependence of the von Mises stress distributions at Lc2 when scratching AlTiN coating with 25 µm diamond. Over-stressed regions are hashed
For more application examples, download our analytical modelling application note using the link at the top of the page.