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Abstract: In the fields of mechanical engineering, friction is ubiquitous. It is a fundamental cause of energy loss and wear. Another concern is the occurrence of comfort‐relevant friction induced vibrations. Prominent examples of this are NVH phenomena such as brake squeal, which is being investigated at great expense by the German automotive industry and academia, to determine its causes and potential influencing factors. For this purpose, numerous specialized measurements are performed, and models of varying complexity are used. All of these measurements and models have the common trait that the coefficient of friction (COF), defined as the ratio of the tangential force and normal force, has a decisive influence on the systems' stability. To parameterize the friction coefficient in macroscopic models, measurements must be performed. In this case, often an average value over time and over various loading procedures is used. As the measurements reveal, the coefficient of friction is in reality not constant, but is subject to a high degree of dynamics on various time scales, caused by complex processes in the boundary layer. A treatment of the coefficient of friction as a steady‐state parameter, or even as a constant, is thus a major reduction. The large variability of the coefficient of friction causes a corresponding variance in the stability limits of the models considered. This phenomenon is observed in the real world, where squealing seems to have a non‐deterministic behavior. This suggests uncertainties in the modeling of the friction coefficient. Due to the various types of uncertainty (variability, incompleteness and inaccuracy), the entire problem is a matter of polymorphic uncertainty. This paper focuses on the modeling of the friction coefficient, taking into account the various causes of uncertainty. Selections of raw data obtained at the authors institute throughout many years of research on friction phenomena in brake systems will be evaluated and classified with respect to its uncertainty properties. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]