Pre-sliding behaviour of multi asperity ceramic contacts

Abstract

Accurate positioning mechanisms are required for the proper and effective functioning of high-tech mechatronic systems. The tribological behaviour of sliding contacts in such mechanisms plays an important role in the performance of these systems. The geometrical changes of the surfaces, where geometry is defined as a multi asperity contact, will influence the frictional behaviour. This thesis focuses on the pre-sliding and sliding behaviour of contacts with the aim of determining the parameters which influence the errors in positioning accuracy. The single asperity contact is the first step in understanding the pre-sliding behaviour between two elements. It is from this model that the tangential displacement is calculated, based on an applied normal load and the coefficient of friction. The normal load can be constant, either increasing or decreasing; this will depend on the operating conditions within the application. Adhesion plays an important role in a single asperity contact, whilst for a multi asperity contact adhesion can often be ignored. This thesis introduces the test setup, which consists of a confocal height sensor for surface roughness measurement and a confocal Raman spectroscopy setup for chemical changes in the contact. Using this setup, it is possible to analyse the formation of chemical transfer layers as well as any geometrical changes in the wear track. The single asperity contact model shows good agreement with the experimental work performed for silicon and silica against glass. With respect to high and low contact pressures, (< 100 MPa) the Mindlin theory can be used to predict pre-sliding behaviour. However, at very low loads the adhesion starts to significantly affect the results. The pre-sliding behaviour of multi asperity contact, so rough surfaces, can be described on the basis of results obtained from theoretical calculations and experiments. The calculation of the tangential displacement for rough surfaces is presented in this thesis. The main assumption in the model presented is that rough surfaces can be modelled as a set of Hertzian contacts, where each asperity has its own radius and summit height. One asperity with a maximum value of tangential displacement will determine when a multi asperity contact starts sliding. The asperities will have no mutual influence apart from sharing the total tangential and normal load. Calculations for different applied loads, surface roughness and autocorrelation lengths show that roughness plays an important role in the preliminary displacement. In the design of surfaces for positioning accuracy, parameters such as surface roughness, applied normal load and tangential load should be taken into account. Textured surfaces give better results to minimize drift as compared to random rough surfaces. A surface composed of asperities with large radii gives less variation in the tangential displacement and, as a result, less drift.