Loading rate effect on lateral force measurements on nanostructured Ti and TiN thin films

Abstract

Tribological properties of titanium and titanium nitride nanostructured thin films as well as of silicon substrate have been studied. We report on the effect of lateral tip displacement direction and normal force loading mode on lateral force measurements. On hard coatings such as titanium nitride thin films, we show that the tip's scratch direction has almost no effect on lateral force measurements. A completely different behaviour has been observed on titanium films. In this last case, the measured lateral force is very dependent on the scratch direction. Concerning the effect of normal force loading mode, we will show how the loading rate, rather than the sliding speed, is a very important parameter in the case of a ramping normal load. For this purpose we have performed two sets of six scratches, for each sample. Scratch length was fixed to 10 μm in all cases. In the first set, the scratch length and duration were kept constant. In the second one, the scratch duration was tuned in order to achieve a constant value of the normal loading rate ⅆ F z ⅆ t . In this last case, the experimental data leads to a unique curve. This means that during nanoscratch measurements, normal tip displacement is controlled by the normal force loading rate. Consequently, lateral displacement speed ⅆ x ⅆ t must be tuned in order to obtain the required value of ⅆ F z ⅆ t . This behaviour seems to be universal and not related to material hardness and Young modulus.