In-situ vibrational spectroscopic observation for thermally activated structural changes of 100% cellulose nanofiber molding with ultralow friction

IF 2 Q2 ENGINEERING, MECHANICAL Frontiers in Mechanical Engineering Pub Date : 2024-07-09 DOI:10.3389/fmech.2024.1422412

Hikaru Okubo, Tomori Ishikawa, H. Hashiba, Toru Inamochi, Ken Nakano

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Abstract

This paper reports the thermally activated ultralow friction of 100% cellulose nanofiber (CNF) molding. The mechanism of friction reduction was investigated using a laboratory-built in-situ Raman tribometer. Our experimental results showed that a CNF molding exhibited an ultralow friction coefficient of below 0.04 in a CNF ring and steel disk tribopair under high-temperature conditions (T > 100°C). The results of the temperature-rise friction test showed that the friction coefficient of the CNF molding strongly depended on the temperature and decreased linearly with increasing temperature. The in situ tribo-Raman monitoring results, during friction, indicated a change in the structure of the CNF molding. Therefore, the crystallinity indices and lengths of the CNF fibers gradually changed as the temperature increased. Moreover, transfer tribofilms were observed on the counter-steel surface against the CNF rings. When the CNF molding exhibited thermally activated ultralow friction, the tribofilm was mainly composed of cellulose and graphitic carbon. Our results suggest that the thermal and friction-activated structural transformations of CNF molding and CNF-derived transfer film formation are pivotal factors contributing to the ultralow friction phenomenon observed in CNF molding at high temperatures.

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原位振动光谱观测超低摩擦 100%纤维素纳米纤维成型的热激活结构变化

本文报告了 100%纤维素纳米纤维（CNF）成型的热激活超低摩擦。我们使用实验室自制的原位拉曼摩擦仪研究了摩擦力降低的机理。实验结果表明，在高温条件下（T > 100°C），CNF 模塑在 CNF 环和钢盘摩擦对中表现出低于 0.04 的超低摩擦系数。温升摩擦试验结果表明，CNF 模塑的摩擦系数与温度密切相关，并随温度升高呈线性下降。摩擦过程中的原位三波长拉曼监测结果表明，CNF 模塑的结构发生了变化。因此，CNF 纤维的结晶度指数和长度随着温度的升高而逐渐发生变化。此外，在与 CNF 环相对的对钢表面上观察到了转移三膜。当 CNF 成型表现出热激活超低摩擦时，三膜主要由纤维素和石墨碳组成。我们的研究结果表明，CNF 模塑的热激活和摩擦激活结构转变以及 CNF 衍生转移膜的形成是导致高温下 CNF 模塑出现超低摩擦现象的关键因素。

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