Effects of defects on the friction between film and substrate in a microbalance experiment

The contribution of disorder to the force of friction between a solid film and the substrate in a microbalance friction experiment is calculated using perturbation theory. The disorder gives a much larger force of friction than a defect-free interface if the defect concentration is larger than one part in ${10}^5$. It is also independent of the sliding velocity. Implications for the interpretation of the experimental results are discussed.