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Effects of local high pressures and extreme shears at tribological contact spots

DOI: 10.1016/s0167-8922(01)80126-7
  • Chemistry
  • Mathematics


Load-bearing contact between sliding unlubricated and boundary-lubricated solids occurs at discrete “contact spots” at which asperities meet, typically numbering in the order of 10. Since all of the load is concentrated at them, local pressures routinely approximate the indentation hardness, and heating at the spots can be, but rarely is, severe. However, with relatively rare exceptions, the high local pressure at the contact spots causes plastic deformation that is responsible for both friction and mechanical wear. Realistic modeling of tribology must therefore be done at the scale of the contact spots. Since this is typically large compared to dislocation cells, tribology can mostly be understood in terms of ordinary mechanical behavior, albeit modified through the prevailing large pressures and strains. Accordingly, too, the thickness of “MML's (the frequently observed “mechanically mixed layers” composed of the intimately mixed materials of the two sides and typically including also some of the ambient medium) compares to the size of the largest contact spots. The MMLs' properties are principally due to the discussed local high pressures and extreme shear strains, whereas flash temperatures tend to be intuitively overestimated and mostly are of small importance, especially in metals. This is due to effective high thermal conductivity and the fact that the heat generation occurs in surface layers instead of mathematical interfaces. Because the local pressures and shear strain rates at contact spots are much higher than readily attainable in a laboratory, ordinary macroscopic testing cannot illuminate the conditions of MML formation and the occasional unexpected chemical reactions/transformations in them, up to amorphization. Lately, the above phenomena have therefore been studied through simulations on layered metal foils with and without lubrication, performed in a Bridgman anvil apparatus. Herein large shear strains are imposed at pressures comparable to the hardness of the samples. Using different material combinations and different sample sizes, and utilizing several means of analysis, fundamental insights have been gained. Among others, stress-strain hardening curves were obtained and samples were studied by X-rays, and optical and different forms of electron microscopy. From the workhardening curves the coefficient of friction and the hardness of the contact spots can be inferred. Most importantly, focused ion beam microscopy (FIBM) revealed the mechanism of mechanical mixing in MML's and X-ray studies clarified metal amorphization in wear tracks.

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