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REVIEW ARTICLE

2D nano‑materials beyond graphene: from synthesis to tribological studies Andreas Rosenkranz1   · Yangqin Liu2 · Lin Yang2 · Lei Chen2 Received: 9 April 2020 / Accepted: 26 May 2020 © King Abdulaziz City for Science and Technology 2020

Abstract Continuously increasing global population and, therefore, energy consumption as well as diminishing resources combined with environmental aspects such as global warming ask for more efficient, sustainable and reliable processes/applications of mechanically moving parts. Especially under harsh conditions, such as high temperatures, vacuum or dry contacts, 2D layered nano-materials used as solid lubricants have demonstrated to be promising candidates to ensure low friction and wear over the entire component’s lifetime. Therefore, this review article aims at summarizing the existing state-of-art regarding solid lubricants with a special emphasis on 2D layered nano-material beyond graphene including graphene oxide, reduced graphene oxide, ­MoS2, ­WS2 as well as ­Ti3C2Tx MXene nanosheets. Initially, experimental approaches allowing for a largescale and layer-dependent synthesis are reviewed for each nano-material. Subsequently, their friction and wear mechanisms at the nano-scale are discussed. Afterwards, the ability to improve friction and wear are reviewed when using the aforementioned 2D nano-materials either as a solid lubricant, lubricant additive under lubricated conditions or reinforcement phase in composite materials. Finally, the existing challenges and shortcomings of each 2D nano-material are discussed before deriving the general conclusions and giving some future research directions. Keywords  2D materials · Tribology · Synthesis · Graphene oxide · MoS2 · Mxenes

Introduction Nowadays, mankind faces significant problems such as global warming as well as continuously increasing energy consumption and decreasing natural resources (Holmberg and Erdemir 2015). All these aspects together ask for more efficient and sustainable processes and applications thus being able to tackle the aforementioned challenges in a short-, medium- and long-term perspective. Looking at the global energy consumption makes evident that about 30 % of the entire energy is only consumed by transportation Electronic supplementary material  The online version of this article (https​://doi.org/10.1007/s1320​4-020-01466​-z) contains supplementary material, which is available to authorized users. * Andreas Rosenkranz [email protected] 1



Department for Chemical Engineering, Biotechnology and Materials, University of Chile, Santiago, Chile



Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Chengdu, China

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(cars, trucks and buses) (Holmberg and Erdemir 2019). The evaluation of the energy efficiency in transportation systems demonstrates that roughly 33 % of the entire energy used in this sector is needed to overcome friction- and wear-related problems (Holmberg et al. 2012, 2014). From a global perspective across different ind