Friction and wear are greatly affected by factors such as material properties, normal load, contact area, adhesion, sliding velocity, temperature, etc. In this thesis, the effects of material hardening on the friction response at the microscopic and macroscopic scales are studied. At the micro-scale, the role of hardening in the response of a single asperity is determined using finite element analysis (FEA). Then a statistical homogenization approach is used to propagate the hardening effects to the macro-scale.

The sample materials for the asperity are Al 2024-T3 and Ti6Al4V, which are modeled as bilinear plastic and include Johnson-Cook damage. The friction results for two different damage models (Johnson-Cook and Bao-Wierzbicki) are also compared for the perfectly plastic case.

In the simulations the asperity is initially compressed in order to induce a normal preload and then sheared to study the friction response. The maximum shear load supported by an asperity is determined by studying the damage in the asperity-rigid surface contact region. These simulations help determine the friction resistance provided by a single asperity until failure occurs while being subjected to different material properties, normal loads, and sliding velocities. The micro-asperity friction coefficients are then used in a statistical homogenization model in order to extrapolate them to the macroscale, i.e., a rough surface.

Results of this study show that the static coefficient of friction is inversely proportional to the normal preload on the micro-asperity. On the other hand this coefficient increases during shearing until the contact region fails. Strain hardening causes the coefficient to increase as well, although rapid loading rates (upto 10 m/s), show negligible increase. These macroscopic results show that the macroscopic friction coefficient is dependent on both material properties as well as surface properties.