类聚合物碳薄膜的制备及其摩擦学研究进展

A Survey on Deposition and Tribological Study of Polymer-like Carbon Films

空间润滑技术是支持航天工程的基础性关键技术,与航天工程的成败直接相关,对有效载荷的使用性能具有重要影响。近年来,我国航天事业的快速发展对空间飞行器提出了超长寿命、超高精度、高稳定度、大转矩、低功耗、低振动、低噪音、小型化、轻量化等新要求。这些新要求将迫使空间润滑由传统润滑向超润滑(摩擦系数低于0.01的润滑状态)的方向发展。类聚合物碳(PLC)薄膜因在高真空下具有超润滑性能而被视为一种潜在的新型空间固体润滑材料。PLC薄膜是一种具有高氢含量(40%(原子分数,下同)以上)、低硬度(10 GPa以下)、宽光学带隙(1.7～4 eV)等类似碳氢聚合物特征的含氢非晶碳(a-C:H)薄膜。与其他a-C:H薄膜一样,PLC薄膜的制备也是采用基于等离子体放电的真空气相沉积技术,但是PLC薄膜的生长需要在低离子能量条件下进行。因此,PLC薄膜的生长由表面吸附机理主导,这使得PLC薄膜具有较高的氢含量和较大的自由体积。基底偏压是控制离子能量的主要沉积参数:低的基底负偏压对应于低的离子能量。常见的PLC制备技术有:反应磁控溅射、电感耦合等离子体化学气相沉积、微波辅助射频等离子体化学气相沉积等。目前研究者提出了两种摩擦机理来解释PLC薄膜在高真空下的超润滑行为:氢钝化机理和网络结构弛豫机理。这两种机理分别从化学和机械的角度解释PLC薄膜的超润滑行为。氢钝化机理强调氢原子对PLC摩擦界面处碳悬键的钝化作用,该机理已经被许多实验和理论研究验证。网络结构弛豫机理突出自由体积增强PLC网络结构弛豫能力,进而减弱摩擦界面处微凸体间碰撞阻力的作用。尽管有一些实验结果可以佐证网络结构弛豫机理,但是目前还缺少在原子尺度上对该机理的进一步证实和阐述。本文综述了PLC薄膜的真空沉积技术及其摩擦学性能与机理的研究进展,指出了低离子能量是沉积PLC薄膜的关键,并凸显了"自由体积"在PLC薄膜摩擦磨损中的角色。最后,对未来PLC薄膜的研究发展方向进行了展望,指出PLC薄膜的纳米复合化和多层化有望成为实现长寿命超润滑与环境自适应超润滑的技术突破口。

As a fundamental key technology supporting the aerospace engineering,space lubrication directly impacts the success or failure of space engineering and determines the performance of payloads.In recently years,the rapid advance of Chinese aerospace industry has put forward new demands for the spacecrafts,including ultra-long service lifetime,ultra-high precision,higher stability,larger torque,lower power consumption,lower vibration and noise,miniaturization,lightweight,etc.These new demands will force the development of space lubrication from traditional lubrication to superlubrication(with the friction coefficient less than 0.01).Due to the super-lubricating property in high vacuum,polymer-like carbon(PLC)films have been regarded as new potential solid lubricants for space applications.PLC film is a kind of hydrogenated amorphous carbon films with similar characteristics to hydrocarbon polymers,including high hydrogen content(above 40 at%),low hardness(below 10 GPa),and wide optical band gap(1.7-4 eV).Like other a-C:H films,PLC films are also prepared by vacuum vapor deposition technology based on plasma discharge,but the growth of PLC films should be conducted under low ion energy conditions.Hence,the growth of PLC films is dominated by surface adsorption mechanism,which results in higher H content and larger free volume in a PLC film.Substrate bias is a main deposition parameter controlling of ion energy,and a low substrate negative bias corresponds to a low ion energy.Common deposition techniques of PLC films include reactive magnetron sputtering,inductively coupled plasma chemical vapor deposition,microwave assisted RF plasma chemical vapor deposition,etc.Two frictional mechanisms,namely the hydrogen passivation mechanism and the network structure relaxation mechanism,have been proposed explain to the superlubricating behavior of PLC films in high vacuum from the chemical and mechanical perspectives,respectively.The hydrogen passivation mechanism emphasizes the passivation of dangling bonds of carbon atoms at the frictional interface by hydrogen atoms,which has been demonstrated by many experimental and theoretical studies.The network structure relaxation mechanism highlights the role of free volume in improving of the relaxation ability of the PLC network and hence lowering the collision resistance between micro-asperities at the frictional interface.Although some experimental results can support the network structure relaxation mechanism,there is still a lack of further verification and elaboration of the mechanism on atomic scale.In this article,the progress in vapor deposition of PLC films and their tribological performance and mechanism studies are reviewed.It is pointed out that low ion energy is the key to the deposition of PLC films,and the role of free volume in friction and wear of PLC films is highlighted.Finally,the prospective development of PLC films is proposed,suggesting that preparing nanocomposites and multilayered PLC films are expected to be the technical breakthrough to achieve long-lifetime and environment-adaptive superlubricity.