The Nano-Hardness Tester (NHT) from CSM Instruments is especially suited to providing quantitative data on the hardness and modulus of sub-micron thin films and coatings, using an indentation method where a tip of known geometry is driven into the sample surface. The instrument itself can be seen in Figure 1 and features automated optical microscopic inspection before and after indentation.
The force on the indentor (all geometries can be used) is applied by an electromagnetic actuator, whilst the displacement is measured via a capacitive system, giving a force resolution of 10μN and displacement resolution <1nm.
Figure 1. The Nano-Hardness Tester (NHT)
For this study, a series of indentations were performed on an MoS2/Pb multilayer that had been built-up by PVD magnetron sputtering, each layer having a thickness of 20nm. The total coating was of thickness 500nm.
The measured Vickers Hardness, HV, was 11.2GPa and the Young’s Modulus, E, was 332GPa for penetration depths in the order of 50nm. A typical force/displacement curve is shown in Figure 2 for a larger indentation with depth 176nm.
Figure 2. Force/displacement curve for MoS2/Pb multilayer
Further investigation was possible with CSM’s AST positioning system which allows a specific sample area to be located with micron precision under a high resolution scanning force microscope. This made the location of a particular imprint very easy and gave valuable additional information as to the effects of the indentation and the subsequent material response.
A typical AFM image of a 10mN indentation is shown below in Figure 3, this corresponding to the curve in Figure 2. Although it was not possible to distinguish between each 20nm layer, the surface roughness around the imprint together with slight pile-up of material could be quantitatively measured.
The NHT has several advantageous features, in particular its differential measurement of the sample surface, made possible by a sapphire reference ring which allows exact relative positioning of the indentor tip. Thus the elasticity of the sample and holder is compensated, as is thermal drift during measurement. Full automation, together with optical observation make this an ideal instrument for better characterisation of thin films and coatings.
Figure 3. AFM image of 10mN indentation on MoS2/Pb multilayer