Tribology of Bio-Inspired Nanowrinkled Films on Ultrasoft Substrates

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.

Effect of deposition parameters on surface roughness and consequent electromagnetic performance of capacitive RF MEMS switches: a review

“…Deposition is the most commonly applied process during the fabrication of thin films. According to the thin-film forming mechanism, deposition can be classified into chemical vapor deposition (CVD), such as low pressure CVD [38] and plasma enhanced CVD (PECVD) [39], and physical vapor deposition such as sputtering deposition [40][41][42], ion beam sputtering [43,44], thermal evaporation [45,46], pulsed laser deposition (PLD) [47,48], etc. This review mainly focuses on sputtering deposition and some other deposition processes are also proposed.…”

Nano-wrinkling of diamond-like carbon thin films on amorphous polycarbonate and semicrystalline polyethylene terephthalate

“…Intrinsic film growth stresses were found in the range of -0.8 (MS) and -2.34 GPa (DD) on Si [18,19]. Reasonably, relaxation of such high compressive stresses occurs continuously during thin film growth in vacuum by a common deformation of film and substrate ("nano-wrinkling") [6][7][8]. These wrinkled topographies for DLC on PET and PC are well visible in Fig.…”

“…We found recently wrinkling mechanisms during deposition of hard thin films on soft substrates due to high intrinsic growth stresses in such films. Wrinkling reduces the film stresses by geometric deformation forming nano-to micrometer scaled topographies with a wavelength dependent on the film thickness [6][7][8].…”

Roughness Influence On Macro- And Micro-Tribology Of Multi-Layered Hard Coatings On Carbon Fibre Polymer Composite

“…1c and e). However, this sub-micrometer wrinkling is mechanically limited by stiffening of the surface by the growing film thickness, which either may crack or enlarge the wrinkle wavelength for stress release (see detailed explanations in [20]). Most of formed cracks are able to heal during ongoing deposition, resulting in elongated tops [7], and only a few are visible after deposition.…”