טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLior Kogut
SubjectAn Elastic-Plastic Model for the Contact and Friction of
Rough Surfaces
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Etsion Izhak


Abstract

Friction is a very important phenomenon in science and engineering. It can be either desirable or destructive. In both cases it is important to understand the process involved and to be able to accurately calculate the friction. The existing empirical data is very inaccurate and do not consider such parameters as surface roughness, mechanical properties, surface energy, etc. that have strong effect on friction.

The purpose of this work is to develop a model for contact and static friction between two metallic flat rough surfaces. Static friction in this work is interpreted as a failure of the asperities that form the contact between the surfaces, contrary to previous work that related friction to normal load using an empirical static friction coefficient. This is done by first analyzing contact, adhesion and failure under combined normal and tangential loading of a single asperity and then incorporating the results in a statistical multi-saperity model that simulates the rough surfaces.

For a single asperity contact the evolution of the plastic region at the asperity tip was monitored from fully elastic to fully plastic contact. In the elastic regime the adhesion force was found to be significant with respect to the contact load. However, in the elastic-plastic regime a maximum normal load was found, above which the adhesion is negligible. A most interesting finding is that with a single asperity a limiting normal load exists above which the asperity cannot resist any additional tangential load and thus exhibits a zero friction behavior. Two different failure modes were identified depending on the nature of the normal preloading.

It was shown that the classical Coulomb friction law is a specific case of the more general model presented in this work. As a by product of this research, one can estimate the mean separation, real area of contact and adhesion between rough surfaces that are extremely important in small scale mechanisms, such as magnetic storage devices.