Siming Ren

Study of tribological mechanisms of carbon-based coatings in antiwear additive containing lubricants under high temperature

With the development of low-emission and low-consumption engines, diamond-like carbon (DLC) coatings have been considered as promising surface coatings of engine parts in terms of friction and wear performance. However, the performance of DLC-coated parts depends on the compatibility and interaction between the coating and lubricant additives. To obtain high-performance coatings in the engine environment, the tribological behaviors of non-doped and metal-doped carbon-based coatings (a-C and a-C/WC, respectively) were studied systematically in poly-alpha-olefin (PAO) oil at 180 °C (operating temperature of engine oil) with and without antiwear additive zinc dialkyldithiophosphate (ZDDP) under various applied loads. Energy dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed on a friction surface to understand tribological mechanisms. Results showed that ZDDP-derived tribofilms were able to suppress the surface graphitization of carbon-based coatings. The combination of ZDDP-derived tribofilms and tribochemical products WS2/WO2, which formed under the joint actions of heat and contract pressure, led to improved tribological behavior of a-C/WC coating in PAO with ZDDP additive.

Multilayer Regulation of Atomic Boron Nitride Films to Improve Oxidation and Corrosion Resistance of Cu

The boron nitride (BN) monolayer (1L) with high impermeability and resistivity seems to hold promise as a long-term corrosion barrier for Cu under ambient condition, which is supported by recent researches. Here, we perform a complete study of the alternating temperature tests (the sample is exposed in air for 30 days and subsequently heated at 200 °C for 2 h) and electrochemical measurements on 1L and multilayer BN-coated Cu foils. Results imply that the BN-coated Cu foils are less oxidized than uncoated Cu foils after alternating temperature tests, regardless of the layers of BN. Particularly, the oxidation process proceeds slowly in multilayers because most of the underlying defects are covered with BN layers to suppress the oxygen diffusion in the vertical direction and the oxidation mainly occurs on the wrinkled region of BN films. Electrochemical analyses reveal that the BN layers provide an effective physical barrier against the corrosive medium and inhibit the electron diffusion because of their high electrical insulating behavior and the corrosion resistance of the samples increases with increasing BN layers. These findings indicate that BN films with adequate layers are good candidates for oxidation and corrosion protection at the atomic level, which is vital to many industrial and academic applications.

A feasible multilayer structure design for solid lubricant coatings in a lunar environment

Solid lubricant coatings have received considerable research attention in space applications owing to their remarkably improved tribological characteristics. But their service life is seriously restricted by the harsh environment, such as high vacuum and abrasive wear. In this paper, a novel design of carbon-based multilayer (MoS2/DLC multilayer) coatings was reported to clarify the friction and wear behavior in high vacuum conditions with and without simulated lunar-dust (SLD). Compared with pure DLC or MoS2 coatings, the multilayer coating showed excellent tribological performance with a low friction coefficient of 0.02 and a wear rate of ∼6.5 × 10−6 mm3 N−1 m−1. What is particularly interesting is that the wear volume of MoS2/DLC multilayer coatings with the increase of time is in accordance with the Archard linear law, regardless of the condition with or without SLD. Moreover, the surface morphology and composition of wear tracks and scars reveal that the long life of carbon-based multilayer coatings cannot be explained solely by excellent mechanical performance, and is also attributed to the formation of ridge layers as third body reservoirs and a tribo-induced composite transfer layer containing SLD nanoparticles and coating materials.

Non-covalent functionalized hexagonal boron nitride nanoplatelets to improve corrosion and wear resistance of epoxy coatings

In the present study, a non-covalent method was employed to modify hexagonal boron nitride (h-BN) nanoplatelets through π–π interaction of amine-capped aniline trimer (AT), which resulted in a stable dispersion of h-BN nanoplatelets in organic solvents. Meanwhile, epoxy coatings containing different content of h-BN nanoplatelets were fabricated, and corresponding corrosion protection performance and wear resistance were investigated. Results showed that the 1.0 wt% h-BN/epoxy coating has better corrosion protection performance than other coatings, which was due to the good barrier properties from stably dispersed h-BN nanoplatelets and the passivating effect from AT. Besides, the addition of h-BN nanoplatelets also contributed to the improvement in wear resistance of epoxy the coating.

N-doping of graphene: toward long-term corrosion protection of Cu

Despite the extraordinary impermeability of graphene, graphene fails to prevent the oxidation of Cu substrates under long-term air exposure owing to their high conductivity. Previous investigations showed that N-doped graphene could modulate the electrical properties of graphene while there has been no experimental example to investigate the protective performance of N-doped graphene to date. Here we demonstrate the synthesis and characterization of N-doped graphene with large-area atomic layers, and then evaluate its protective properties by both short- (electrochemical tests) and long-term (air exposure) corrosion tests. The results show that both short- and long-term corrosion are inhibited because incorporating nitrogen in graphene can decrease the conductivity of graphene and thus prevent the electron transport between Cu/N-doped graphene interfaces. Our finding provides a new protection method of graphene and will promote the basic research and applications of graphene.