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    Where is Wear….. and Why.

    Material loss (wear, abrasion loss) when two surfaces rub together, follows a similar pattern to friction.  One mechanism, ‘adhesion loss’, consists of transfer of material from one surface to another. Adhesion loss is related to the chemical attraction between the two surfaces.  In the second mechanism, ‘abrasion loss’, material is removed by a rupture process when nanoscale ‘peaks’ interact at the interface. This is related to the relative mechanical properties of the materials.  Adhesion loss is more important in dry sliding conditions with smooth surfaces, low velocity and low load. Abrasion loss predominates with rough surfaces, at high speeds and high loads.

    Gear wear

    Against smooth steel bearings, POM and HDPE have low friction and low wear. However, this increases significantly for rougher surfaces, at higher speeds and higher loading. Additives such as PTFE and silicone, create lubricating films at the interface and greatly reduce wear for all thermoplastics.  Fibre filler also greatly reduces wear.  Carbon fibre filled polyamide 6 has high friction but very low wear for a wide range of conditions.

    Materials with a high ability to absorb work (high area under a stress strain curve) suffer less wear than more brittle materials. In a paper mill, a small steel cog, driven by a very large steel gear wheel, continually had to be replaced. Introducing polyamide for the small cog reduced the wear but the larger, more expensive, gear wheel had then to be replaced.

    Most of the published data refers to thermoplastics rubbing against steel and values for wear between two thermoplastic are rare.  Data for plastics against steel refers to a limited set of conditions (speed, load, temperature etc) and designers and users have to resort to their own ad hoc tests to predict wear in their product.  When I was in the flooring industry, the quickest way to evaluate wear in a floor covering under development, was to install it round a barber’s chair.

    Finally, more information on this topic may be available on the excellent British Plastics Federation website.

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