Towards faster skis: The development of new surface modifications and treatments to reduce overall friction in alpine skiing. A dissertation submitted to The University of Sheffield Department of Chemical and Biological Engineering For the degree of Doctor of Philosophy in Snowsports Engineering Presented by Richard C Coupe October 2013 Abstract Developments in sport technology have been driven by competition and skiing is no exception. Skiing has evolved over many years making use of new materials and post-treatments methods, but in recent years progress has been limited. The low friction exhibited when sliding over snow and ice is governed by the tribological interaction between ski base and snow and the subsequent accumulation of a thin melt water film. The properties of new, innovative, blended ski base materials and post-treatment methods have been investigated using a range of methodologies. The incorporation of fluorinated hydrophobic silica nanoparticles into current ski waxes has been achieved. These nanoparticles have also been used as nano-abrasives to modify the physical surface structure of a ski base composed of ultra-high-molecular-weight polyethylene (UHMW-PE). Such polyethylene samples were also subjected to other physical treatment methods including abrasion with 120 grit, 800 grit sandpapers and 10-15µm silica particles combined with surface planing. The resultant surfaces were examined under a scanning electron microscope, contact angle tested and friction tested. The surface treatments which tended to be both physically and chemically hydrophobic were found to be beneficial at reducing friction in warm and wet snow conditions (snow temperature above -2°C). The addition of carbon black to UHMW-PE was also found to be beneficial in reducing static friction on snow. This was an interesting finding as the unmodified UHMW-PE was shown to be slightly more hydrophobic than UHMW-PE with added carbon from the contact angle evidence. Additionally, the visibly finer surface refinements with micro-scale to nano- scale features were also found to be more beneficial at reducing friction. A hydrophobic fumed silica, polydimethylsiloxane (PDMS), and perfluoropolyether (PFPE) were both blended with UHMW-PE to chemically modify the bulk material in contrast to traditional surface modification with hydrocarbon wax. PFPE, in particular, has been successfully blended with UHMW-PE to produce a novel base material for a downhill ski. Subsequent glide testing of skis, manufactured with this base material, in a range of conditions, demonstrated that the blended white base material closely matched the performance of skis with the current downhill black base material. The new blends also matched a hydrocarbon-waxed surface for hydrophobicity and were also found to be resistance to carbon coating at 5% levels of loading. Combined with the surface planing and nano scale abrasive structuring methods previously described, this novel base material has the potential to either compete or be a compliment to current wax treatment methods in order to achieve the small reductions of friction required to be successful in an alpine race. Abstract |Page: II Table of contents Abstract ................................................................................................................................................ ii Table of contents ..................................................................................................................................iii List of figures ........................................................................................................................................ V List of tables ....................................................................................................................................... VIII List of symbols ..................................................................................................................................... IX Acknowledgements ............................................................................................................................. XI Chapter [ 1 ] Introduction .............................................................................................................. 1 Chapter [ 2 ] Aims and objectives................................................................................................... 4 Chapter [ 3 ] Evolution of the modern alpine ski............................................................................ 5 Chapter [ 4 ] Tribology- the physics of sliding friction .................................................................... 9 4.1. The sliding surface (base) .................................................................................................... 14 4.2. The sliding substrate (snow) ............................................................................................... 25 4.3. Interactions at the interface ............................................................................................... 30 4.3.1. Contact area ................................................................................................................ 33 4.3.2. Solid contact ................................................................................................................ 35 4.3.3. Heat generation .......................................................................................................... 38 4.3.4. Water lubrication ........................................................................................................ 40 4.3.5. Ideal predisposition of the water film ......................................................................... 42 4.4. Hydrophobicity: chemical and physical ............................................................................... 49 4.5. Section summary................................................................................................................. 62 Chapter [ 5 ] Methods .................................................................................................................. 63 5.1. Traditional treatment methods ........................................................................................... 64 5.2. Wax modification ................................................................................................................ 66 5.2.1. Wax modification with particle fillers.......................................................................... 67 5.2.2. Contact angle testing .................................................................................................. 69 5.2.3. Glide testing ................................................................................................................ 71 5.3. Surface modification ........................................................................................................... 75 5.3.1. Samples and preparation ............................................................................................ 75 5.3.2. Scanning electron microscopy ..................................................................................... 75 5.3.3. Inclined plane friction ramp ........................................................................................ 78 Table of contents |Page: III 5.4. Bulk modification ................................................................................................................ 81 5.4.1. PDMS and UHMW-PE .................................................................................................. 81 5.4.2. Particle fillers and UHMW-PE ...................................................................................... 82 5.4.3. PFPE and UHMW-PE .................................................................................................... 82 5.4.4. Subsequent testing procedures................................................................................... 85 Chapter [ 6 ] Results and discussion ............................................................................................. 87 6.1. Wax modification ................................................................................................................ 90 6.1.1. Contact angle .............................................................................................................. 90 6.1.2. Hydrophobic nano-particle fillers ................................................................................ 93 6.1.3. Glide testing .............................................................................................................. 101 6.2. Surface Modification of UHMW-PE ................................................................................... 106 6.2.1. Physical (SEM) and Physiochemical (Contact-angle) analysis. ................................... 107 6.2.3. Influence on friction .................................................................................................. 122 6.3. Bulk modification of UHMW-PE ........................................................................................ 130 6.3.1. Polydimethylsiloxane (PDMS). .................................................................................. 130 6.3.2. Particle Fillers (Hydrophobic Fumed Silica) ............................................................... 131 6.3.3. Perfluoropolyether (PFPE) ......................................................................................... 134 6.3.4. Sintered Blends ........................................................................................................