Ph.D. Thesis Defense by Richard S. Cowan
Friday, July 12, 2002

(Dr. Ward Winer, advisor)

"Development of Tribological Design Strategies Based on a Thermomechanical Wear Transition Model"


A design methodology is presented, which is capable of assessing the mechanical and thermal behavior of materials in sliding contact. It is based on enhancements made to a thermomechanical wear transition model that uses the total stress field of a sphere-on-flat sliding contact to determine a non-dimensional critical stress that when exceeded initiates material yield or fracture.

Relationships used to define a thermomechanical wear transition are governed by the mechanical and thermal properties of the contacting materials, the distribution of energy that results from the frictional resistance of motion, the time period in which frictional heating occurs, and the operating conditions of load, velocity and bulk temperature. A means to determine the percentage of the total frictional energy allocated to each surface in contact is offered with provisions to consider alternative material failure criteria, and to extend the analysis from a single contact application to an interface of multiple asperity microcontacts.

A fortran computer program, incorporating representative material property and surface profilometry data, is discussed and used to identify those characteristics that promote severe thermomechanical wear. A parametric study using the program reveals that such wear is significantly influenced by the temperature-dependent modulus of elasticity and fracture or yield strength of the materials in contact, surface profile specifications, and the friction coefficient of sliding. For a multiple contact application, the percentage of asperity microcontacts that yield or fracture within an apparent area is of import in defining a transition. Using an asperity failure percentage of 2%, agreement between experimentally based mild-to-severe wear transitions and those predicted by the program is quite good.

Design guidelines are provided that will help one challenged in performing a thermomechanical wear transition analysis due to uncertainties concerning material behavior, the presence of friction, and the area of contact. Methods to determine the influence of fluid film and solid film lubrication on thermomechanical wear are also given, provided the latter is treated as a monolith. These considerations complement a set of applications that illustrate that the thermomechanical wear transition model can be very useful in investigating metal scuffing, ceramic hotspots, solid-coating breakdown and polymer decomposition.