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Carbon/Carbon and Carbon/Oxide Coupled Interfacial Electron Density Diagrams
The research group of the Low-dimensional Materials Tribology of the Lanzhou Chemical Physics Research Institute of the Chinese Academy of Sciences has made new progress in the study of the friction mechanism of diamond-like carbon-based thin films under high vacuum conditions. Research work has been published in the recently published ACS Appl. Mater. Interfaces (2013, 5, 5889–5893) and Carbon (2014, 66, 259-266).
Diamond-like carbon films (DLCs) with excellent tribological properties are solid lubricants that are widely used in micro-scale micro-electromechanical systems to macro-scale engineering components. However, due to the lack of in-depth understanding of the friction mechanism of DLC, it is difficult to control the tribological behavior of DLC in different environments, thus greatly limiting its practical application. Researchers from different perspectives put forward the DLC anti-friction wear mechanism, including chemical adsorption passivation theory, the graphitic theory of the sliding interface and the transfer film theory, but so far has not proposed a universally accepted friction mechanism.
How to find the key factors affecting the DLC friction and thus propose a unified friction mechanism is an important scientific problem faced by DLC tribology research today. Researchers at the Lanzhou Institute of Materials Research have found that significant graphitization occurs at low rotational speeds and high friction through the study of DLC friction coefficients at different vacuum levels and friction speeds.
In addition, studies have found that P/v (gas pressure/rotational speed) influences the passivation state of the carbon dangling bonds at the friction interface, thereby controlling the friction coefficient of the DLC. In the same P/v situation, the influence of rotating speed on the friction in vacuum environment is mainly through influencing the passivation of the gas to the friction interface, rather than affecting the degree of graphitization of the friction interface, ie the high degree of graphitization is not low friction. The reason for the coefficient.
This result is totally different from the traditional graphitization theory, and it challenges the traditional graphitization leading to the low-friction DLC friction mechanism. It is proposed that the frictional interface interaction is the key factor of the DLC friction behavior. By inhibiting the formation of the carbonaceous transfer film on the dual surface, the generation of a pure carbonaceous sliding interface is avoided, thereby avoiding the strong carbon-carbon cross-interface interaction and achieving low friction, long life of the low-hydrogen DLC in a high vacuum environment. .
The researchers used Al2O3 and ZrO2 metal oxide ceramics as dual materials for weak interaction with carbon. SiC and Si3N4 were selected as materials with strong interactions with carbon. First-principles calculations and simulations of relevant interface electronic structures were conducted. The highly localized or overlapping electrons between the interfaces are the essential reason for the low friction or high friction of the DLC. Through the above research, the DLC friction mechanism of "highly localized and overlapping friction interface electrons" was established. The introduction of the separation work theory verified that the weak bonding of the carbon film/dual interface is the essential factor to improve the high vacuum friction life of the film. The analysis of the electronic interactions based on the frictional interface confirms that the weak coupling between the carbon-to-dual interface and the highly localized electrons between the interfaces cause a small amount of electron transfer between the interfaces.
The above research work was funded by the National Natural Science Foundation of China (11172300) and the Excellent Youth Fund (51322508).
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