It becomes highly significant to improve wear resistance and friction properties of mechanical components of automobile, marine equipments, textile manufacturing machine and so on, due to the strong demands of energy saving and the eco-friendly surface engineering with high performance. Diamond-like carbon (DLC) films, as a family of advanced carbon materials, have been considered as strong candidates for the required coating technology because the films not only own high hardness, good wear and corrosion resistance, superior biocompatibility etc,.but also they can be facilely deposited by various eco-friendly PVD/CVD processes over large area uniformity and low cost for mass production.

In the past few years, Prof. Aiying Wang and her group in Key Laboratory of Marine Materials and Related Technologies, CAS, carried out hybrid researches of computations and experiments on the manipulation and development of various DLC films. Their first focus was to find strategy to control the high residual stress of DLC film and understand the correlation of structure-properties-performance of films. By using the home-made hybrid ion beam system, they found that adding third element into DLC film could effectively reduce the residual stress (Figure 1) while without serious deterioration of mechanical and tribological properties of DLC feature. However, due to the variety of doped metal elements and complexity of experimental characterizations, the fundamental mechanism for evolution of structure and properties of Me-DLC films induced by doped metal elements from the atomic scale viewpoint was still lacked; in particular the mechanism of reduced stress was not yet clear. Therefore, the purpose of our research is to gain insight to the relation between structure and properties of Me-DLC films by combined computations and experiments.

Recently, Prof. Aiying Wang and Dr. Xiaowei Li performed ab initio calculations based on density functional theory (DFT) to investigate the structural properties of various Me-DLC films. Ti, Cr, W, Cu and Al were selected as the representative doped metals in terms of strong carbide forming metal and weak carbide ones. It was found that doping Ti, W or Cu into carbon matrix could significantly relax both the distorted bond angles and bond lengths, whereas the Cr or Al doping only relaxed the distorted bond lengths. In spite of the various synergistic effect of distortion, therefore, the reduced fraction of the distorted bond structure and the formation of weak Me-C bond characteristics benefited the residual stress reduction, which consisted with the experimental results and provided the theoretical understanding from atomic scale study (Figure 2). The related results have been published on Journal of Physical Chemistry C (2015) and AIP Advances (2015).

Moreover, utilizing the computation advantages and selecting the Ti, Cr, Cu and Al as the representative elements, they investigated the synergistic effect of binary metal co-doping effect on the structure and property of films. They found that Ti/Al, Cr/Al and W/Al co-doping could further reduce the residual stress with the optimized concentration of doped Ti, Cr, or W to Al. Most importantly, the designed Cu/Cr co-doped material system showing the giant reduction of the residual stress was firstly reported in the literatures to date (Figure 3). Atomic bond structure analysis revealed that the addition of Cu and Cr impurities in amorphous carbon matrix resulted in the critical and significant relaxation of distorted C-C bond lengths. On the other hand, electronic structure calculation indicated a weak bonding interaction formed between Cr and C atoms, while the antibonding interaction was observed for the Cu-C bonds, which played a pivot site for the release of strain energy. As a result, those synergistic interactions of Cu/Cr co-doping combined with the structural evolution accounted for the drastic stress reduction. Noted that if one keeps in mind the designing of selected co-doped metal characteristics, our results present not only a new straight forward strategy to tailor DLC films with low residual stress and other novel physichemical properties, but also foster the theoretical and experimental works of the metal-carbon materials for other functional energy and environmental applications. These results have been published on Applied Materials & Interfaces (2015) and Materials & Design (2016). A Chinese patent has been filed (CN201410528262.5).

Prof. Aiying Wang: aywang@nimte.ac.cn   Dr. Xiaowei Li: lixw@nimte.ac.cn

Research Group Url:

http://wayen.nimte.ac.cn/

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