Search for: All records

Abstract Structural superlubricity (SSL) at layered material interfaces is an exciting and vibrant field of research, offering vast opportunities to achieve ultralow friction and wear with numerous potential technological applications. At increasing length‐scales, new physical and chemical energy dissipation pathways emerge that threaten to push the system out of the superlubric regime. Physical inhibitors of SSL are primarily associated with in‐plane elasticity, out‐of‐plane corrugation, moiré superlattices, grain boundaries, and lattice defects. Chemical mechanisms that may suppress superlubric behavior include interlayer bonding, wear, and external contaminants. In this article, these and other challenges are reviewed facing the scaling‐up of structural superlubricity, as reflected in recent experimental and theoretical studies. Further perspectives are offered on future directions for realizing and manipulating macroscale superlubricity, outlining technological opportunities that it entails. 

Electric vehicles (EVs) represent a new paradigm for a sustainable transportation future with the potential to offer unparalleled energy security, environmental cleanliness, and economic prosperity for all humanity. However, rapid development and adaptation of this new transportation approach depend on addressing multiple challenges, including the development of new materials and coatings that can meet the more stringent thermal, electrical, and tribological requirements of EV drivetrains. Specifically, the operating conditions of moving mechanical assemblies (i.e., bearings, gears, among others) in EVs differ radically from those found in conventional internal combustion (IC) engines—thus giving rise to notable reliability issues. In particular, as the function of bearings and gear systems shift from mostly load-carrying (in IC engines) to the torque-transferring mode in EVs, durability concerns of these assemblies due to severe deformation, wear, micro-pitting, fatigue, and scuffing may worsen—as electric motors can generate maximum torque at near zero speeds but maximum efficiency at around 90% speed (this is opposite of conventional IC engines, which generate high torques at high speeds). These conflicting requirements require a different set of lubricant-material combinations to circumvent such problems under high loads and low-speed conditions of EVs. Therefore, new materials, coatings, and lubrication strategies need to be developed and implemented for future EVs to become tribologically viable and reliable. Accordingly, EV drivetrains can potentially benefit from advanced functional coatings that have already significantly improved the functionality of moving components of IC engines. The main objective of this article is to draw attention to some of the tribological issues in EVs and how advanced functional coatings can help resolve these issues due to their unique thermal, electrical, mechanical, and tribological properties, ultimately making EVs more durable and reliable. 

Photocatalytic self-cleaning coatings with a high surface area are important for a wide range of applications, including optical coatings, solar panels, mirrors, etc. Here, we designed a highly porous TiO2 coating with photoinduced self-cleaning characteristics and very high hydrophilicity. This was achieved using the swelling-assisted sequential infiltration synthesis (SIS) of a block copolymer (BCP) template, which was followed by polymer removal via oxidative thermal annealing. The quartz crystal microbalance (QCM) was employed to optimize the infiltration process by estimating the mass of material infiltrated into the polymer template as a function of the number of SIS cycles. This adopted swelling-assisted SIS approach resulted in a smooth uniform TiO2 film with an interconnected network of pores. The synthesized film exhibited good crystallinity in the anatase phase. The resulting nanoporous TiO2 coatings were tested for their functional characteristics. Exposure to UV irradiation for 1 h induced an improvement in the hydrophilicity of coatings with wetting angle reducing to unmeasurable values upon contact with water droplets. Furthermore, their self-cleaning characteristics were tested by measuring the photocatalytic degradation of methylene blue (MB). The synthesized porous TiO2 nanostructures displayed promising photocatalytic activity, demonstrating the degradation of approximately 92% of MB after 180 min under ultraviolet (UV) light irradiation. Thus, the level of performance was comparable to the photoactivity of commercial anatase TiO2 nanoparticles of the same quantity. Our results highlight a new robust approach for designing hydrophilic self-cleaning coatings with controlled porosity and composition. 

Protective coatings are important for enhancing tribological behavior, preventing surface degradation, and reducing friction-induced energy losses during the operation of mechanical systems. Recently, tribocatalytically driven formation of protective carbon films at the contact interface has been demonstrated as a viable approach for repairing and extending the lifetime of protective coatings. Here, we study the effect of catalytic metals, specifically their composition and amount, on the tribocatalysis process. To achieve this, we test the tribological performance of electro-deposited amorphous CoNiP and CoCuP coatings in different hydrocarbon-rich environments. Our results indicate that the tribocatalytic repair of wear-induced damage is optimal when Ni and Cu are included in the Co-P matrix at 5 wt% Ni and 7 wt% Cu, respectively. Characterization of the wear tracks suggests that among the considered samples, the tribofilms formed on the surface of Co7CuP have the highest concentration of graphitic carbon, leading to a more significant reduction in the COF and wear rate. The carbon tribofilm formation was more pronounced in decane and synthetic oil than in ethanol, which is attributed to the difference in the length of the hydrocarbon molecules affecting viscosity and the lubricant film thickness during boundary lubrication sliding. 

The conformal nanoporous inorganic coatings with accessible pores that are stable under applied thermal and mechanical stresses represent an important class of materials used in the design of sensors, optical coatings, and biomedical systems. Here, we synthesize porous AlOx and ZnO coatings by the sequential infiltration synthesis (SIS) of two types of polymers that enable the design of porous conformal coatings—polymers of intrinsic microporosity (PIM) and block co-polymer (BCP) templates. Using quartz crystal microbalance (QCM), we show that alumina precursors infiltrate both polymer templates four times more efficiently than zinc oxide precursors. Using the quartz crystal microbalance (QCM) technique, we provide a comprehensive study on the room temperature accessibility to water and ethanol of pores in block copolymers (BCPs) and porous polymer templates using polystyrene-block-poly-4-vinyl pyridine (PS75-b-P4VP25) and polymers of intrinsic microporosity (PIM-1), polymer templates modified by swelling, and porous inorganic coatings such as AlOx and ZnO synthesized by SIS using such templates. Importantly, we demonstrate that no structural damage occurs in inorganic nanoporous AlOx and ZnO coatings synthesized via infiltration of the polymer templates during the water freezing/melting cycling tests, suggesting excellent mechanical stability of the coatings, even though the hardness of the inorganic nanoporous coating is affected by the polymer and precursor selections. We show that the hardness of the coatings is further improved by their annealing at 900 °C for 1 h, though for all the cases except ZnO obtained using the BCP template, this annealing has a negligible effect on the porosity of the material, as is confirmed by the consistency in the optical characteristics. These findings unravel new potential for the materials being used across various environment and temperature conditions.