J. David Cogdell

Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel

Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness λ) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide–reinforced amorphous hydrocarbon, and silicon-incorporated diamond-like carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high λ with increased hydrocarbon content in the coating. Coating micro-texture and composition were believed to influence traction as λ became small.

Composite Skewness for Two Rough Surfaces in Contact

Typically, the contact of two rough bodies is transformed into an equivalent rough surface contacting a flat. Due to the mathematics involved, this was usually conducted for surfaces assumed to be Gaussian, where distribution of peaks and valleys are equal. These assumptions were acceptable when dealing with ground surfaces, which are typically Gaussian; however, other finishing techniques often result in skewed surface distributions. As such, an equation was developed numerically within, based on the Weibull distribution, to estimate the composite skewness of contacting rough bodies. The assumption of using the Weibull distribution to describe the surface variation was validated with measured data from different finishing techniques, as was the equation for composite skewness developed within. Presented at the STLE Annual Meeting in Las Vegas, Nevada, May 15-19, 2005 Review led by Alan Lebeck

Investigating Friction Modification and Potential Wear Reduction in the Railroad Wheel to Rail Contact

Reduced friction with top of the rail friction modifiers continues to be investigated for improved energy efficiency and reduction in lateral forces between railroad car wheels and the rail. Another benefit often not considered is the potential reduction in wear of both the wheel and the rail surfaces. This paper details the results of fundamental laboratory test work to compare dry contact condition with one where a “friction modifier” has been applied, to define the difference in the surface deterioration and wear. The basics of this wear testing are described, along with information on the materials used for the testing. The results show a very significant difference in friction coefficients and the wear characteristics, suggesting substantial benefit potential in both reduced rail wear and wheel tread wear. Selected wheel wear tests are discussed and historical wheel wear information is provided. Wheel life data for two North American coal freight car fleets are reviewed to point out the average mileage of wheels in service. The potential for increasing wheel wear life, and therefore potential cost savings, is highlighted. Finally, recommendations for future work are offered.Copyright

Traction of Lubricated Rolling Contacts Between Thin Film Coatings and Steel

Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness λ) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide‐reinforced amorphous hydrocarbon, and silicon-incorporated diamondlike carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high λ with increased hydrocarbon content in the coating. Coating micro-texture and composition were believed to influence traction as λ became small.

Archard Wear Coefficients for Ruby and Silicon Nitride Scanning CMM Probe Tips

Small (1 mm diameter) probe tips are used on sub-micron accuracy scanning coordinate measuring machines. One of the larger contributions to measurement uncertainty is micron-scale probe tip wear. Wear was generated on ruby and silicon nitride probe tips under precisely controlled dry sliding conditions on ground, carburized and hardened steel surfaces. Volumetric wear was measured using an interferometric microscope. Wear volume data was then expressed in terms of Archard wear coefficients. The measured wear rate of silicon nitride sliding on steel was approximately twice that of ruby.Copyright