Chen Xingyu, Yang Juntao, Yasuda Koji, Koga Nobuaki, Zhang Hedong
Department of Complex Systems Science, Graduate School of Informatics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
Langmuir. 2022 Mar 15;38(10):3170-3179. doi: 10.1021/acs.langmuir.1c03203. Epub 2022 Mar 2.
Organic friction modifiers (OFMs) added to lubricating oils to reduce friction and wear are crucial for reducing energy loss and CO emissions. In our previous studies, we have developed -(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as CTEMPO, as a new type of OFM and experimentally demonstrated its superior performance to conventional OFMs of stearic acid and glycerol monooleate. However, the behavior of CTEMPO adsorbing onto solid surfaces from base oil during sliding, which largely dictates the lubrication performance, is yet to be elucidated. Here, we performed molecular dynamics simulations for confined shear of a CTEMPO solution in poly-α-olefin between hematite surfaces. Unlike conventional OFMs, which typically have one functional group or multiple functional groups of the same type, CTEMPO features two functional groups of different types: one amide and one terminal free oxygen radical. The results showed that adsorbed boundary films with a double-layer structure form stably during sliding, owing to double- or single-site surface adsorption and interlayer hydrogen bonding via the two functional groups. Additionally, some molecules in each of the first and second layers also form intralayer hydrogen bonding. Such multitype adsorption is unique and favorable for enhancing the strength of boundary films to withstand heavily loaded and prolonged sliding. The velocity distribution indicates that the first and second layers are solid- and liquid-like, respectively. The second layer could act as a buffer for the first layer, which is the last barrier to prevent solid-solid contact, against shear. We also found that the second layer can act as a reservoir to rapidly repair the once depleted region in the first layer because of the interlayer hydrogen bonding. The combination of the high strength and self-repair ability of the CTEMPO boundary films can rationally explain the experimentally observed properties of high load-carrying capacity, excellent antiwear effect, and high stability of friction over time.
添加到润滑油中以减少摩擦和磨损的有机摩擦改进剂（OFM）对于降低能量损失和一氧化碳排放至关重要。在我们之前的研究中，我们开发了 -(2,2,6,6-四甲基-1-氧基-4-哌啶基)十二烷酰胺，简称为CTEMPO，作为一种新型的OFM，并通过实验证明了其相对于硬脂酸和甘油单油酸酯等传统OFM的优越性能。然而，CTEMPO在滑动过程中从基础油吸附到固体表面的行为，在很大程度上决定了润滑性能，尚未得到阐明。在这里，我们对赤铁矿表面之间的聚-α-烯烃中的CTEMPO溶液进行了受限剪切的分子动力学模拟。与通常具有一个官能团或多个相同类型官能团的传统OFM不同，CTEMPO具有两种不同类型的官能团：一个酰胺和一个末端游离氧自由基。结果表明，由于通过两个官能团的双位点或单位点表面吸附和层间氢键作用，在滑动过程中稳定地形成了具有双层结构的吸附边界膜。此外，第一层和第二层中的一些分子也形成层内氢键。这种多类型吸附是独特的，有利于增强边界膜的强度以承受重载和长时间滑动。速度分布表明，第一层和第二层分别呈固体状和液体状。第二层可以作为第一层的缓冲层，第一层是防止固体-固体接触的最后一道屏障，以抵抗剪切力。我们还发现，由于层间氢键作用，第二层可以作为一个储存库，迅速修复第一层中曾经耗尽的区域。CTEMPO边界膜的高强度和自修复能力相结合，可以合理地解释实验观察到的高承载能力、优异的抗磨效果和随时间变化的高摩擦稳定性等性能。
