AVS 54th International Symposium
    Advanced Surface Engineering Tuesday Sessions
       Session SE-TuP

Paper SE-TuP1
Protective Coatings of Extensible Biofibers

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Session: Advanced Surface Engineering Poster Session
Presenter: N. Holten-Andersen, University of California, Santa Barbara
Authors: N. Holten-Andersen, University of California, Santa Barbara
F. Zok, University of California, Santa Barbara
J.H. Waite, University of California, Santa Barbara
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As thin film coatings are increasingly finding their way in engineering and biomedical applications, the demand for unique combinations of their material properties is increasing as well. A coatings protection of the underlying substrate against wear relies on its resistance to penetration, i.e. its hardness. Increasing hardness of the coating however, significantly lowers its extensibility. Protecting substrates undergoing large strains against wear is therefore a challenge beyond the reach of current engineering coating applications. By investigating natural coating materials adapted to high- and low-stress environments we have studied nature's solution to this problem. In the protective cuticles of mussel holdfast threads (self-assembled biological fibers), high wear resistance have in wave-exposed species been combined with high extensibility. Here, we demonstrate that the inclusion of deformable micro-particles within a stiff protein matrix allows the cuticle of these holdfast threads to exhibit sufficient hardness against wear without encumbering the elasticity of the thread. This unique composite design results in an impressive damage tolerance by arresting developing micro-cracks thereby preventing catastrophic failure of the coating up until an astounding 70% strain. Cuticles from mussel species in deeper and calmer habitats lack these deformable particles and fracture below 30% strain. This increase in extensibility of the granular composite cuticle parallels the behavior observed with rubber toughened polymers such as High Impact Poly-Styrene. However, whereas the inclusion of rubber particles significantly compromises the hardness of these synthetic composites, our study shows that only a small reduction in hardness is observed in the biological coating composite. Interestingly, the interior morphology of the micro-particles in the cuticle shares a striking similarity with the bi-continuous network morphologies observed in polymer micro-emulsions and block copolymer systems. The superior mechanical properties together with the self-assembling nature of the material, makes this study significant for the broad materials community.