This work shows that a soft, thin film comprising randomly aligned carbon nanotubes (CNTs) can reduce surface wear more effectively than a homogeneous thin film because of enhanced elastic recoverability and contact stress relief originating from its mesh structure. To investigate the wear characteristics of the mesh structure compared to those of the homogeneous thin film, multi-walled CNTs (MWCNTs) and diamond-like carbon (DLC) thin films were prepared to conduct nanoscale tribological experiments using the atomic force microscopy (AFM). The MWCNT thin film showed unmeasurably low wear compared with the DLC thin film under a certain range of normal load. To demonstrate the wear reduction mechanism of the MWCNT thin film, its indentation and frictional behaviors were assessed. The indentation behavior of the MWCNT thin film revealed repetitive elastic deformation with a wide strain range and a significantly lower elastic modulus than that of the DLC thin film. The permanent deformation of the MWCNT thin film was observed through frictional experiments under relatively high normal load conditions. These results are expected to provide insights into the design of highly wear-resistant surfaces using nanostructures.

A one-step method was developed to create a highly biocompatible micropatterned surface on a diamond-like carbon (DLC) through irradiation with a nitrogen ion beam and thus enhance the biocompatibility of osseointegrated surfaces and biotribological performance of articular surfaces. The biocompatibility and biotribological mechanisms were analyzed in terms of the structure and morphology of DLC. It was demonstrated that a layer enriched in sp³ C–N bonds was formed on the surface of the DLC after nitrogen ion beam irradiation. Moreover, with an increase in the radiation dose, the content of sp³ C–N on the DLC surface increased significantly, and the biocompatibility was positively correlated with it. The adhesion of the MC3T3 osteoblasts increased significantly from 32% to 86% under an irradiation dose of 8 × 10¹⁵ ions/cm². In contrast, the micropattern had a significant negative effect on the adhesion of the osteoblasts as it physically hindered cell expansion and extension. The micropattern with a depth of 37 nm exhibited good friction properties, and the coefficient of friction was reduced by 21% at relatively high speeds.

Polymer gears are used extensively in various applications. However, durability issues have been emerging because of friction at gear tooth contact areas. To extend the lifetime of polymer gears, a low-frictional coating has been considered as a possible strategy. In this study, a finite element simulation method was performed to investigate the contact stress between a pair of coated polymer gears. The simulation included various friction coefficients (COFs) for studying the effects of friction during the operation. Numerical results revealed that the friction causes the contact stress to shift over the roll angle, which is attributed to the direction of the sliding friction based on a free-body diagram. We also investigated the effects of coating and found that a thin coating has little effect on the bulk deformation behavior of the gear. Moreover, the stress distribution in the coating at the pitch point was investigated as the COF increased. Under zero friction, three notable stress regions were observed: 1) the center of the surface, 2) the end of the contact, and 3) the overall contact area. As COF was increased in the micro-slip region of the contact interface, both tensile and compressive stresses in the coating increased. This study provides significant aid to engineers for understanding the stress response of the coating applied to polymer gears to achieve an optimal design.

The benefits of reinforcing polyimide (PI) films with boron nitride (BN) particles and boron nitride nanosheets (BNNSs) were assessed with the aim of enhancing their thermal, optical, and mechanical properties for flexible device applications. BNNSs were prepared from BN particles using a liquid-phase exfoliation method assisted by an ultrasonic probe-type sonicator and centrifugator. PI-based composite films blended with BNNSs and BN particles were fabricated at various concentrations via mechanical stirring and spin coating. The transparency of the PI/BNNS composite films remained almost the same as that of pure PI films up to 3 wt.% whereas the transparency of the PI/BN composite films decreased with increasing concentration of the BN fillers at 550 nm. The thermal stability improved significantly with increasing concentrations of both BN and BNNS relative to that of pure PI films. The temperature for 5% weight loss of the PI/BNNS composite film was higher than that of the PI/BN composite film at the same filler concentration. The composite films with 2 wt.% BN or BNNS showed the lowest wear rate, and the PI/BNNS composite films showed more stable frictional behavior compared to the PI/BN composite films. In addition, bending tests showed that the PI/BNNS composite films exhibited excellent flexibility compared to the PI/BN composite films. Overall, the results indicate that the BNNS can be effectively used as a filler that can enhance the thermal and mechanical properties of polymer materials for flexible device applications.

Friction and wear phenomena encountered in mechanical systems with moving components are directly related to efficiency, reliability and life of the system. Hence, minimizing and controlling these phenomena to achieve the desired system performance is crucial. Among the numerous strategies developed for reducing friction and wear, coatings have been successfully utilized in various engineering applications to mitigate tribological problems. One of the benefits of coatings is that they may be fabricated using a variety of materials in several different forms and structures to satisfy the requirements of the operating conditions. Among many types, coatings that are comprised of a combination of materials in the form of a multilayer have been gaining much interest due to the added degree of freedom in tailoring the coating property. In this paper, the properties and development status of multilayer coating systems for tribological applications were reviewed with the aim to gain a better understanding regarding their advantages and limitations. Specifically, focus was given to Ti-based and Cr-based coatings since Ti and Cr were identified as important elements in multilayer coating applications. Emphasis was given to materials, design concepts, mechanical properties, deposition method, and friction and wear characteristics of these types of coatings.