COMMUNICATION 1802717 (1 of 11) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com Bioinspired Wear-Resistant and Ultradurable Functional Gradient Coatings Zhengzhi Wang,* Kun Wang, Houbing Huang,* Xiao Cui, Xiaoming Shi, Xingqiao Ma, Bei Li, Zuoqi Zhang,* Xuhai Tang, and Martin Y. M. Chiang Prof. Z. Wang, K. Wang, Prof. Z. Zhang, Prof. X. Tang School of Civil Engineering Wuhan University Wuhan, Hubei 430072, China E-mail: zhengzhi.wang@whu.edu.cn; zhang_zuoqi@whu.edu.cn Prof. H. Huang Advanced Research Institute of Multidisciplinary Science Beijing Institute of Technology Beijing 100081, China E-mail: hbhuang@bit.edu.cn DOI: 10.1002/smll.201802717 mechanical strength, stiffness, and wear- resistance compared with the underneath substrate. The mismatched mechanical and/or thermal properties between the coating and the substrate inevitably generate stress concentrations within the interfacial regions upon stressing/ straining, which deteriorate significantly the integrity and durability of the coating/ substrate system. [1,2] Such a conflict between surface mechanics and interfa- cial integrity can be well circumvented by constructing a functional gradient inter- phase layer that widely adopted in natural materials. [3,4] Many load-bearing biolog- ical materials such as squid beak, [5] spider fang, [6] sandworm jaw, [7] toe pad of tree frog, [8] etc., utilize hard and wear-resistant outermost skin layer to resist external forces while gradually softened interlayer to bridge with the interior tissues and to maintain the integrity of the whole struc- ture. [9–11] Inspired by this design principle of nature, functional gradient coatings (FGCs) with the material compositions and mechanical properties gradually var- ying along the coating thickness direction have been developed. [12–16] With improved mechanical compatibility between the coating and the substrate, FGCs can achieve simultaneously good interfacial integrity and high sur- face hardness/strength and have been applied extensively for thermal, wear, corrosion, and damage barriers. [17–20] For mechanically protective coatings, the coating material usually requires sufficient stiffness and strength to resist external forces and meanwhile matched mechanical properties with the underneath substrate to maintain the structural integrity. These requirements generate a conflict that limits the coatings from achieving simultaneous surface properties (e.g., high wear-resistance) and coating/substrate interfacial durability. Herein this conflict is circumvented by developing a new manufacturing technique for functional gradient coatings (FGCs) with the material composition and mechanical properties gradually varying crossing the coating thickness. The FGC is realized by controlling the spatial distribution of magnetic- responsive nanoreinforcements inside a polymer matrix through a magnetic actuation process. By concentrating the reinforcements with hybrid sizes at the surface region and continuously diminishing toward the coating/ substrate interface, the FGC is demonstrated to exhibit simultaneously high surface hardness, stiffness, and wear-resistance, as well as superb interfacial durability that outperforms the homogeneous counterparts over an order of magnitude. The concept of FGC represents a mechanically optimized strategy in achieving maximal performances with minimal use and site-specific distribution of the reinforcements, in accordance with the design principles of many load-bearing biological materials. The presented manufacturing technique for gradient nanocomposites can be extended to develop various bioinspired heterogeneous materials with desired mechanical performances. Protective Coatings The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201802717. X. Cui, Dr. X. Shi, Prof. X. Ma Department of Physics University of Science and Technology Beijing Beijing 100083, China Prof. B. Li School of Materials Science and Engineering Wuhan University of Technology Wuhan, Hubei 430070, China Dr. M. Y. M. Chiang Biosystems and Biomaterials Division National Institute of Standards and Technology Gaithersburg, MD 20899, USA For protective coatings applied in various engineering fields such as aeronautics, automobiles, constructions, electronics, etc., the coating materials usually exhibit much higher Small 2018, 1802717