Abstract
Biomimetic design of new materials uses nature as antetype, learning from billions of years of evolution. This work emphasizes the mechanical and tribological properties of skin, combining both hardness and wear resistance of its surface (the stratum corneum) with high elasticity of the bulk (epidermis, dermis, hypodermis). The key for combination of such opposite properties is wrinkling, being consequence of intrinsic stresses in the bulk (soft tissue): Tribological contact to counterparts below the stress threshold for tissue trauma occurs on the thick hard stratum corneum layer pads, while tensile loads smooth out wrinkles in between these pads. Similar mechanism offers high tribological resistance to hard films on soft, flexible polymers, which is shown for diamond-like carbon (DLC) and titanium nitride thin films on ultrasoft polyurethane and harder polycarbonate substrates. The choice of these two compared substrate materials will show that ultra-soft substrate materials are decisive for the distinct tribological material. Hierarchical wrinkled structures of films on these substrates are due to high intrinsic compressive stress, which evolves during high energetic film growth. Incremental relaxation of these stresses occurs by compound deformation of film and elastic substrate surface, appearing in hierarchical nano-wrinkles. Nano-wrinkled topographies enable high elastic deformability of thin hard films, while overstressing results in zigzag film fracture along larger hierarchical wrinkle structures. Tribologically, these fracture mechanisms are highly important for ploughing and sliding of sharp and flat counterparts on hard-coated ultra-soft substrates like polyurethane. Concentration of polyurethane deformation under the applied normal loads occurs below these zigzag cracks. Unloading closes these cracks again. Even cyclic testing do not lead to film delamination and retain low friction behavior, if the adhesion to the substrate is high and the initial friction coefficient of the film against the sliding counterpart low, e.g. found for DLC.
Highlights
Skin is the heaviest organ of all animals
Spinosum, germinativum, papillary and reticular dermis and wrinkle depth and density is adapted to the required deformability hypodermis): The 10 to 25 μm thick stratum corneum layer with an and tribological resistance: The thicker the skin and the larger its elastic modulus between 50 and 400 MPa
We broadened our bio-inspired research of structure formation and mechanical properties of nano-wrinkled thin hard films on polymers towards the tribological behavior under ploughing and sliding conditions: Based on the mechanical behavior of human skin under compressive and tensile loads and the results of tribological tests, we established a biomimetic material model for hard surfaces with low friction coefficients on soft, highly elastically deformable substrates
Summary
Skin is the heaviest organ of all animals As described above and carbon (a-C:H), etc.) on different polymers (polycarbonate (PC), shown for tribological contact in [21], wrinkles elastically compact thermoplastic polyurethane (PU), polyamide, polyimide, etc.) Such under compressive loading in front of a slider and smooth out under conditions generally lead to high intrinsic compressive film stresses tensile loading behind the slider. Higher strains result in possess high friction coefficients Their high elasticity causes large fracture under tensile stresses, whereby the cracks run zigzag on the surface deflections during compressive loading, e.g. found in sliding micrometer scale along wrinkle grooves (the areas of lowest strength) contacts: Hard, stiff, and sharp counterparts (like e.g. mineral grains) and follow rather perpendicular to the tensile stress direction on the are deeply incising, ploughing, and scratching the polymer surface. Pass 1 was automatically subtracted from pass 2 and 3 in order to eliminate roughness influence to improve the determination of critical loads Lc1 and Lc2
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