Jasminka Starcevic

Topographic properties of the contact zones of wear surfaces in disc brakes

Disc brakes are widely used in cars, trains, and other vehicles. For understanding the basic processes occurring during the friction process, information on the surface topography of brake pads and discs is needed, in addition to information about chemistry and thermodynamics. In this paper, samples of pads and discs were analysed using white light interferometry. The topography of the surfaces was measured quantitatively. Using software tools described in a previous paper [Wear 248 (2001) 121], the distributions of area, height, and slope of the contact patches were computed. Characteristic properties of the contact patches were found to describe the influence of the test parameters such as friction power and testing time.

Quantitative description of wear surfaces of disc brakes using interference microscopy

Abstract Based on interference microscopy using visible light, methods are described to access and analyze topographic data from wear surfaces. As an example, measurements on discs and pads of disc brakes are presented. It is shown that interference microscopy combined with additional software is an accurate method for the quantitative description of wear surfaces.

Influence of the normal force and contact geometry on the static force of friction of an oscillating sample

The paper is devoted to an experimental and theoretical investigation of the static friction force between a rapidly oscillating sample and a steel plate. The static frictional force is studied experimentally as function of the oscillating amplitude, the normal force and the contact geometry. A simplest model of tangent contact with a constant friction coefficient is proposed and shows a good agreement with experiment. The static friction force is proved to be a universal function of the ratio of the oscillation amplitude, the indentation depth and to the friction coefficient.

Simulation of the influence of ultrasonic in-plane oscillations on dry friction accounting for stick and creep

We consider a pair of bodies contacting on an elastic substrate; the distance between the bodies oscillates harmonically at a high frequency. If a horizontal force is applied to the bodies, macroscopic movement starts only after achieving some critical value, which we identify with the static friction force of the oscillating system. The dependence of the static friction force on the oscillation amplitude is simulated numerically using the method of reduction of dimensionality. Results of simulation are compared with experimental data.

Reduction of friction by normal oscillations. II. In-plane system dynamics

The influence of out-of-plane oscillations on friction is a well-known phenomenon that has been studied extensively with various experimental methods, e.g., pin-on-disk tribometers. However, existing theoretical models have yet achieved only qualitative correspondence with experiment. Here we argue that this may be due to the system dynamics (mass and tangential stiffness) of the pin or other system components being neglected. This paper builds on the results of a previous study [19] by taking the stiffness and resulting dynamics of the system into account. The main governing parameters determining macroscopic friction, including a dimensionless oscillation amplitude, a dimensionless sliding velocity and the relation between three characteristic frequencies (that of externally excited oscillation and two natural oscillation frequencies associated with the contact stiffness and the system stiffness) are identified. In the limiting cases of a very soft system and a very stiff system, our results reproduce the results of previous studies. In between these two limiting cases there is also a resonant case, which is studied here for the first time. The resonant case is notable in that it lacks a critical sliding velocity, above which oscillations no longer reduce friction. Results obtained for the resonant case are qualitatively supported by experiments.

Influence of In-Plane and Out-of-Plane Ultrasonic Oscillations on Sliding Friction

in [m/s]Fig.: Experimental data of the coefficient of friction of asteel-steel-couple,oscillationfrequency of 45 kHz and the oscillationamplitudes: (1) 0.023µm , (2) 0.056µm , (3) 0.095µm , (4)0.131µm , (5) 0.211µm , (6) 0.319µmInfluence of Ultrasonic Oscillations on Friction Elena Teidelt 09.19.2011 www.friction-physics.com 9

Heterogeneity of material structure determines the stationary surface topography and friction

The character of surface roughness and the force of friction in the stationary state after a sufficiently long run-in process are of key importance for numerous applications, e.g. for friction between road and tire. In the present paper, we study theoretically and experimentally the asymptotic worn state of a bi-phasic material that is arbitrarily heterogeneous in the contact plane, but homogeneous in the direction of the surface normal. Under the assumption of Archard’s wear law in its local formulation, the asymptotic shape is found in the closed integral form. Given the surface profile, the coefficient of friction can be estimated, since the coefficient of friction is known to be strongly correlated with the mean square root value of the surface slope. The limiting surface profiles and the corresponding coefficient of friction are determined as functions of size, relative concentration and wear ratio of the phases. The results of numerical calculations are compared to and validated by experiments carried out on simplified model systems. The main conclusion is that the rms value of the surface slope is not influenced by the characteristic linear size of inclusions and depends solely on the relative concentration of phases, as well as the ratio of their wear coefficients.