Sliding wear, viscoelasticity, and brittleness of polymers
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Moshe Narkis Department of Chemical Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel; and Department of Chemical Engineering, Shenkar College of Engineering and Design, Ramat-Gan 52526, Israel (Received 11 March 2006; accepted 23 June 2006)
We have connected viscoelastic recovery (healing) in sliding wear to free volume in polymers by using pressure-volume-temperature (P-V-T) results and the Hartmann equation of state. A linear relationship was found for all polymers studied with a wide variety of chemical structures, except for polystyrene (PS). Examination of the effect of the indenter force level applied in sliding wear on the healing shows that recovery is practically independent of that level. Strain hardening in sliding wear was observed for all materials except PS, the exception attributed to brittleness. Therefore, we have formulated a quantitative definition of brittleness in terms of elongation at break and storage modulus. Further, we provide a formula relating the brittleness to sliding wear recovery; the formula is obeyed with high accuracy by all materials including PS. High recovery values correspond to low brittleness, and vice versa. Our definition of brittleness can be used as a design criterion for choosing polymers for specific applications.
I. INTRODUCTION
In his already classic book on friction and wear of materials, Rabinowicz1 talks about the 1966 Jost report to the British government, which evaluated losses due to ignorance of tribology at M£ 515/per year. He says: “At the time the Jost Report appeared it was widely felt that the Report greatly exaggerated the savings that might result from improved tribological expertise. It has now become clear that, on the contrary, the Jost Report greatly underestimated the financial importance of tribology. The report paid little attention to wear, which happens to be (from an economical point of view) the most significant tribological phenomenon.” For many reasons there is an ongoing process of replacement of metal parts by polymeric ones. More mileage per gallon in cars and airplanes due to lighter weight, for example, is one of the desired effects. In the case of metal parts and components, a simple—and fairly successful—procedure to mitigate wear consists in using external lubricants.1 However, because polymers often swell in contact with liquid lubricants, the approach used for metals cannot be simply transferred. We have shown
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0300 2422 J. Mater. Res., Vol. 21, No. 9, Sep 2006 http://journals.cambridge.org Downloaded: 02 Jun 2014
before2 that carbon black results in lower friction in polymer blends, irradiated as well as un-irradiated ones. However, in various applications lack of transparency and black color are excluded. The gradual replacement of metals by polymers or materials with polymer matrices (PBMs, polymer-based materials) has not been accompanied by a sufficient growth in understanding of their tribology. PBMs i
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