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Halloysite nanotube-filled epoxy composites were fabricated using spray-coating methods. The halloysite nanotubes (HNTs) were aligned by the hydrodynamic flow conditions at the spray nozzle, and the polymer viscosity helped to preserve this preferential orientation in the final coatings on the target substrates. Electron microscopy demonstrated a consistent trend of higher orientation degree in the nanocomposite coatings as viscosity increased. The nanoindentation mechanical performances of these coatings were studied using a Hysitron TriboIndenter device. Composites showed improvements up to ∼50% in modulus and ∼100% in hardness as compared to pure epoxy, and the largest improvements in mechanical performance correlated with higher alignment of HNTs along the plane-normal direction. Achieving this nanotube alignment using a simple spray-coating method suggests potential for large-scale production of multifunctional anisotropic nanocomposite coatings on a variety of rigid and deformable substrates.

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The influence of nanoparticle orientation on wear resistance of transparent composite coatings has been studied. Using a nozzle spray coating method, halloysite nanotubes (HNTs) were aligned in the in-plane and out-of-plane directions and in various randomly oriented states. Nanoscratching, falling sand, and Taber Abrasion tests were used to characterize the wear resistance at different length scales. Composites consistently displayed better wear resistance than pure epoxy. Samples with out-of-plane particle orientations exhibited better wear-resistant behavior than those with in-plane particle distributions. In nanoscratching tests, the out-of-plane orientation decreases the normalized scratch volume by as much as 60% compared to pure epoxy. In the falling sand and Taber Abrasion tests, out-of-plane aligned halloysite particles resulted in surfaces with smaller roughness based on stylus profilometry and SEM observations. The decrease in roughness values after these wear tests can be as large as 67% from pure epoxy to composites. Composites with higher out-of-plane particle orientation factors exhibited better light transmittance after sand impingements and other wear tests. This study suggests a useful strategy for producing material systems with enhanced mechanical durability and more durable optical properties.

We report the piezoresistive and mechanical characteristics of three-dimensional (3D) graphene foam (GF)–polydimethyl­siloxane (PDMS) nanocomposites processed by a facile two-step approach. A polyurethane (PU) foam with graphene embedded (and aligned) in the pore walls is pyrolyzed and then impregnated with PDMS to form a GF–PDMS nanocomposite, resulting in a slitlike network of graphene embedded in the viscoelastic PDMS matrix. The interconnected graphene network not only imparts excellent electrical conductivity (up to 2.85 S m–1, the conductivity of PDMS is 0.25 × 10–13 S m–1) to the composite but also enables ultrasensitive piezoresistive behavior. For an applied compressive strain of 10% we report a 99.94% reduction in resistance, with an initial gauge factor of 178, and note that this value is significantly higher than those reported in the literature. Cyclic compression–release tests conducted at different strain amplitudes demonstrate that both the mechanical and piezoresistive responses of the GF–PDMS are fully reversible up to a maximum strain amplitude of 30%. The facile processing, recoverable, and reversible response over 1000 cycles, good hysteresis performance over a range of strain rates, and energy absorption characteristics open new opportunities for GF–PDMS nanocomposites in various applications such as soft robots and human–machine interface technologies.