Volume 4, Issue 8 (September 2007)
Investigation of Friction in the Meso Normal Force Range on DLC and TiN Coatings
In recent years “low load” tribology has received more attention due to the emergence of special devices like MEMS, and new materials such as bioimplants, polymers, and textured surfaces. The tribological characterization of MEMS materials is challenging, because the devices operate at nominal contact pressures of only a few MPa and low wear rates of nm/h. The evaluation of materials with conventional high load tribological tools or with AFM type techniques is not appropriate because the contact pressures or contact geometries are not suitable. Furthermore, even in many conventional applications, the contact pressures and wear rates are more moderate than the ones achieved in conventional tribometers.
In this work, the friction and wear behavior of the commonly used industrial coatings titanium nitride (TiN) and diamond-like carbon (DLC) are investigated with normal forces in between Newton (conventional tribology) and nanoNewton (nanotribology). The investigation of friction at such low contact pressures was achieved using a high precision microtribometer MUST® operating in a linearly reciprocating ball-on-flat configuration; a ball-on-disk configuration is also possible. The equipment works on the principle of deflection measurements of a flexible cantilever beam with light optical sensors bearing a force resolution of 0.1 μN. As the applied normal force reduces from mNs to μNs, the effect of surface roughness becomes strikingly apparent in the recorded tangential force. The friction of TiN coating appears to be less sensitive to surface roughness variations and chemical changes of the countermaterial than DLC. The behavior of DLC at low contact pressures is different from the well known low friction behavior of DLC coatings at high contact pressures. The importance of the appropriate tribological tool for evaluating materials at low contact pressures has been illustrated through this study.