含氢非晶碳膜宏观超滑行为与卷曲结构的形成机制

Superlubricity, proposed firstly by Hirano in 1990s, is a significant mean to essentially deal with the friction and wear, which refers that the friction between two surfaces is close to zero. Diamond like carbon (DLC) film is one of the most promising superlubricity coatings, for it can considerably reduce friction (μ≤0.01), and extend lifetime of moving parts. Common wisdom holds that low friction of DLC film is attributed to shear induced tribofilm formation at rubbing interfaces, whereas the soft graphitic tribofilms with low shear resistance, which contributed to the superlubricity performance. However, similar to friction induced graphitic structures, general graphite materials are multi-cristal and the coefficient of friction varies from 0.1 to 0.6. Accordingly, it will be self-supported that the coefficient of DLC can be as low as that of graphite. Howbeit, in some cases, DLC had been proved to be with superlow frictional behavior, and the friction coefficient can be below 0.05, which is much lower than graphite (μ›0.1). This phenomenon hints that the superlow friction is related to graphitic transformation but not limited to this, since the sp2 carbon has many allotropes with diverse structures and friction properties. Our recent study showed that graphene nano scrolls were developed in the tribofilms consisting of outer graphene shell and inner amorphous core, which is the critical factor to the superlubricity. In order to give a integrate explanation in theory to the superlubricity of DLC films, based on the existing experimental basis and theoretical analysis, we are planning in this project to investigate the formation mechanism of graphene nano scrolls. Hope to verify the impact of load and sliding velocity on the nano scrolls of various DLC films through experiments, to uncover the key factors of superlubricity of DLC films in different conditions, and to reveal the roles of nano scrolls and interface structure in the friction mechanism.

碳基薄膜的超滑机理，通常基于薄膜的结构、成分、转移膜及摩擦表面钝化分析，忽略了磨屑纳米结构演化对超滑的贡献，无法给出准确的摩擦机理。基于现有实验基础和理论分析，我们认为碳膜的超滑行为与磨屑中卷曲结构的形成和演化相对应，其过程是应力和热诱导形成类石墨烯摩擦膜，摩擦膜在切向力作用下石墨烯可卷曲成卷状结构，形成非公度和滚动的摩擦界面使摩擦下降。同时，摩擦界面的接触状态发生相应演变，由薄膜/对偶过渡到薄膜/摩擦膜/卷曲结构/摩擦膜/对偶面的接触方式。因此，本项目拟基于磨屑结构和摩擦界面结构分析，研究卷曲结构演化的整个过程与摩擦行为的对应关系、探究碳膜超滑的关键界面结构因素。通过理论计算、模型再现等手段，揭示碳膜超滑行为的物理起源。

含氢非晶碳膜（a-C:H）是最有希望实际应用的超滑材料，然而其摩擦机理在学术界一直存在争议。卷曲结构是碳膜超滑的关键结构因素，认识碳膜摩擦卷曲结构的形成过程，理解卷曲结构对碳膜超滑行为的贡献机制，是揭示a-C:H薄膜摩擦机理，突破a-C:H薄膜超滑行为本质的前提和必要基础。通过本课题的研究: 实现了系列超滑薄膜的制备技术突破，形成了超滑体系构筑的基本原则和方法，解析了碳基润滑材料摩擦界面结构演化的动态过程，归纳了超滑界面的关键结构因素，指出了超滑状态下原子尺度的摩擦学规律，揭示了卷曲结构的形成过程，建立了卷曲结构与碳膜超滑行为的内在关系，总结了卷曲结构对宏观摩擦行为的影响规律; 构建了卷曲结构形成的动态过程，发现了卷曲结构的基本推动力，研究了表面化学态对卷曲形成的影响机制。结果表明，完美的石墨片层在磨擦过程无法卷曲，表现出较高的摩擦力。当石墨片层存在缺陷时，摩擦过程中可形成卷曲结构，同时缺陷密度越大越容易卷曲。卷曲结构的形成可以使界面摩擦力由16nN下降到小于1nN，缺陷是石墨烯卷曲的推动力，形成卷曲结构可使摩擦力成数量级降低；基于卷曲结构原位构筑的思路：突破了金属-金属界面（μ＜0.008）的超滑难题；系统开展了MoS2二维原子晶体H-DLC异质结超滑体系构筑及机理研究，通过滴铸法构筑了MoS2+H-DLC复合结构薄膜，在纯氮气、空气、二氧化碳和氧气气氛中，实现了稳定的超滑。发现在摩擦过程中，气氛中的氧气能够有效地将MoS2转移膜和Al2O3对偶球结合在一起，同时界面形成卷曲结构，形成摩擦膜/卷曲结构/摩擦膜的非公度摩擦界面，从而显著地延长超滑的寿命。