The pre-sliding and static friction force behaviour at asperity level between a smooth ball and a smooth flat surface at different normal loads, as well as friction behaviour during full slip has been studied. The normal load dependence of the friction force and the preliminary displacement is discussed when the mean contact pressure is kept under 100 MPa. The theoretical model to calculate the shear stress and the preliminary displacement in the contact is discussed and the experimental data were used to verify the model. The results show that for low applied normal loads the adhesion force has an influence on the friction force measurements. Furthermore, the results for the friction force and preliminary displacement show good agreement with the theoretical trends. The experiments along with the model can be used to analyse the tangential traction in the contact and the behaviour of the stick-slip area. The measurement results along with the model were used to calculate the maximum shear stress at the point of sliding for different applied normal loads. It is also shown that at low applied normal loads the shear stress is not constant as compared to relatively high applied normal loads due to the presence of adhesion force.

Geometrical and chemical changes in the wear track can cause a drift in friction level. In this paper, chemical and geometrical surface changes in wear tracks are analyzed. For this, a setup with a confocal height sensor was developed to measure the local height changes on the wear track, combined with confocal Raman spectroscopy to determine the chemical changes at the surfaces. Pin-on-disc experiments were performed at room temperature and at elevated temperature (600 1C) to understand the material behavior between mild and severe wear regimes. The wear tracks developing between the two ceramics, alumina (Al 2 O 3 ) and zirconia (Y-TZP), were analyzed using these techniques. The results of confocal height sensor showed significantly more geometrical changes in surface roughness at 600 1C compared to the test conducted at room temperature. The developed roughness in the wear track was approximately 250 times larger at 600 1C due to the higher degradation of the mechanical properties of ceramic. Further, material transfer was observed for the test conducted at 600 1C using Raman Spectroscopy. Material transfer at room temperature is difficult to observe because surface changes are less evident in mild wear regimes. The results show that the changes in the micro-geometry of the surface and the chemical compositions of the surface influence the friction level and wear processes. The confocal height sensor and Raman Spectroscopy were used to measure and understand the geometrical and chemical changes occurring on the surface of a wear track during sliding in a single setup.

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 SUMMARYvii 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. In het ontwerp van oppervlakken ten behoeve van positioneringsnauwkeurigheid is het belangrijk dat ten eerste de drift minimaal is en dat daarnaast het contact zo stabiel mogelijk zal opereren, ook bij eventuele veranderingen. Getextureerde oppervlakken, bestaande uit regelmatige structuren, zijn beter in staat om drift te minimaliseren dan random ruwe oppervlakken. Berekeningen laten zien dat een oppervlakte sam...