Accessing the tribological contact on the nanoscale by means of scanning probe techniques

Tribology represents a research field which has been extensively explored for a long time but a little fundamental understanding of various observations has been achieved. The main reason for this lies in the enormous complexity of the phenomena acting on the forces between two contacting bodies which are moved with respect to each other. However, the understanding of the central role of tribology in the modern society in the context of optimization of the performance as well as the lifetime of many products demands dedicated research in this interdisciplinary field. Today the technological progress in the scanning probe techniques opens up the potential to study contact phenomena on the single asperity level. In this thesis work, the influence of surface roughness, mechanical properties, adhesion force, and external parameters (applying normal load, scratching speed, and loading rate) on friction are identified.
Adhesion force between silica microspheres of different sizes and different rough surfaces (silicon and diamond-like carbon (DLC)) is measured using atomic force microscopy (AFM). The surface roughness, asperity geometry, and size of adhering particles play an important role in determining the adhesion force. Adhesion force between adhering particle and smooth surface linearly increases with size of the adhering particle. On increasing surface roughness, the adhesion force is found to show decreasing trend initially, followed by an increasing trend. The results are compared with existing as well as proposed models.
The influence of applied normal load on the tribological behavior between spherical probe and various rough surfaces such as fused silica, aluminum, DLCs, and Si-C-B-N-O coatings, is experimentally investigated using Nanoindenter and AFM. At a sufficient low level of applied normal load, wherein the contact is elastic, the friction coefficient decreases with load. At higher load, the contact involves the plastic deformation and friction coefficient will be constant. At very high load, friction coefficient increases with applied load. The surface roughness and mechanical properties (hardness and elastic modulus) have significant influence on the friction as they determine the degree of plastic deformation. An additional lateral force due to the intrinsic adhesive force is seen. After eliminating this additional adhesive force term, at a sufficient low level of applied normal load, wherein the contact is elastic, the friction coefficient is constant. By eliminating the adhesion component from friction, at increased normal loads the contact involves plastic deformation and the friction coefficient increases with increasing normal load.

The friction coefficient increases on decreasing the loading rate and increasing the scratching speed. The critical load range for a transition from either predominantly elastic to elastic-plastic contact or elastic-plastic to predominantly plastic contact between the indenter and sample increases with increasing the size of tip and the scratching speed, and it decreases with surface roughness and loading rate. The results are compared with existing models.