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- Hammerström, Lars, et al.
(author)
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Designed high-friction surfaces : influence of roughness and deformation of the counter surface
- 2008
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 264:9-10, s. 807-814
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Journal article (peer-reviewed)abstract
- The present paper experimentally investigates the prospects of using surfaces with carefully designed topography to design contacts with a high level of static friction. All tests are run under boundary lubricated conditions. Specifically, very high static coefficients of friction (up to mu approximate to 1.2) are demonstrated for surfaces covered with sharp pyramids. The test surfaces were manufactured using micromechanical techniques based on photolithography and etching of silicon followed by deposition of CVD diamond. The technique results in exceptionally well-defined surface textures with very sharp and durable diamond pyramids. The possibilities of using such surfaces for gripping and various types of coupling applications are discussed in some detail. A good correlation between the achieved results and theoretical predictions of the ploughing component of friction is demonstrated. The technique showed to be very robust with only minor influence of surface roughness and counter surface deformation.
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- Hammerström, Lars, 1972-
(author)
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Mechanisms and Phenomena in Braking and Gripping
- 2006
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Doctoral thesis (other academic/artistic)abstract
- Applications relying on a high static friction include various types of fixtures, couplings, bolted joints, torsion joints, etc. The common characteristic of these applications is that they rely on the friction force to maintain the relative position of two mating surfaces. Also applications relying on high dynamic friction are common, the main example being brakes, where a low friction could be devastating.The plateau model for the friction of brakes has been refined. By using advanced electron microscopy, it has been shown that during braking a partly amorphous friction film, comprising nanosized iron oxide agglomerates, dissipates the friction energy. The film is only about 100 nm thick. It is separated from the underlying less mobile material by a thin boundary. The actual braking power is thus localised to this very thin film, leading to remarkably high power densities. In a typical case it was estimated to 40 GW/dm3.Squeal and vibrations are critical problems for brakes. The present work has shown that a textured disc pattern may counteract squeal efficiently. The most successful pattern has spiral shaped arms in which wear resistant ceramic particles are embedded. The different wear characteristics of treated and untreated disc surface lead to an elevation of the patterned area above the rest of the disc. In a related experiment, laser technique was used to inject the particles deeper into the disc surface, and thus prolonging the time of silence. Textured diamond surfaces have been used to study the influence of load, repeated scratching and surface roughness on the static coefficient of friction. It was shown that these surfaces were remarkably stable at high friction levels. A maximum load limit was found above which the coefficient of friction falls. This and a number of other factors were found important for the successful design of high-friction joints.
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