碳纳米管薄膜是一种新兴的宏观片状薄膜多孔材料。它继承了碳纳米管的优点，具有卓越的机械、电气和热性能，在微电子器械、生物材料、避雷材料等众多前沿领域有潜在应用。然而，由于对该类材料的摩擦变形以及性能调控机制缺乏充分认识，相关材料的优化设计和应用受到了限制。本文建立了包含交联、考虑断键等特征的碳纳米管薄膜模型，采用粗粒化分子动力学模拟系统地研究了表面接触状态以及正压力、摩擦速度和交联密度等因素对摩擦性能的影响，取得以下三方面成果：

（1）球形压头在碳纳米管薄膜表面的接触状态。通过特征尺寸和特征压力定义压头在碳纳米管薄膜上的接触状态。对于一定的薄膜孔隙尺寸，在压入过程中，当特征压力超过某一临界值时，压头的接触模式由“浅接触”转变为“深接触”；

（2）碳纳米管薄膜的摩擦变形和破坏模式。碳纳米管薄膜在“浅接触”和“深接触”状态下呈现不同的摩擦变形模式。在“浅接触”状态下，摩擦力由两部分构成，即碳纳米管本身形状引起的较小阻力和薄膜表面凸起引起的较大阻力。在“深接触”状态下，摩擦力特征类似，但较大阻力由滑动引起的表面重构导致；

（3）正压力、摩擦速度和交联密度对碳纳米管薄膜材料摩擦性能的影响。碳纳米管薄膜在“浅接触”和“深接触”状态下对正压力呈现不同的响应。“浅接触”状态下的“局部表面黏附”机制使得摩擦力对正压力不敏感，摩擦系数随其增大而降低，摩擦力与滑动速度线性相关。“深接触”状态下的“非局部纤维堆积”机制使得摩擦力随正压力线性增加，摩擦系数基本不变。存在一个临界速度使碳纳米管发生塑性变形和断裂，摩擦力对摩擦速度非线性响应。此外，碳纳米管薄膜的摩擦系数随交联密度的增加而降低。交联密度的增加使得表面接触状态由“深接触”转变为“浅接触”。压头滑动过程中，低交联密度下的薄膜无法有效传递受力并限制形变。而高交联密度使应力几乎被全体碳纳米管耗散，薄膜不出现纤维堆积，摩擦系数显著减小。

上述结果为提升和优化碳纳米薄膜的物理力学性能提供科学依据，对具备类似结构的纤维多孔材料的性能优化以及工程应用具有重要的指导意义。

Carbon nanotube films (CNT films) is an emerging porous material of macroscopic sheet films. It inherits the advantages of carbon nanotubes, with excellent mechanical, electrical and thermal properties. It has potential applications in many cutting-edge fields, such as microelectronic devices, biomaterials, and lightning protection materials. However, the optimal design and application of related materials are limited due to the lack of full understanding of their friction deformation and regulation mechanism. In this paper, a carbon nanotube film model including cross-linking with consideration of bond breaking is well established. A coarse-grained molecular dynamics simulation (CGMD) system is used to study the influence of the surface contact state, normal pressure, friction speed and crosslinking density on the friction performance, followed with three achievements:

(1) Contact state of the indenter on the surface of the carbon nanotube film. The contact state of the indenter on a carbon nanotube film is defined by the characteristic size and the normalized pressure. For a certain size of pore, the contact mode changes from shallow to deep contact when the normalized pressure exceeds a certain critical value during the indentation.

(2) Frictional deformation and destruction mode of carbon nanotube films. The CNT film exhibits different friction deformation modes in both shallow and deep contact states. In the shallow contact state, the friction force consists of two parts, namely, small resistance caused by the shape of the carbon nanotube itself and large resistance caused by the bulge of the film surface. In the deep contact state, the frictional features are similar, but the surface reconstruction cause the larger friction force.

(3) Effects of positive pressure, friction velocity and cross-linking density on the friction properties of carbon nanotube film. CNT film shows different responses to positive pressure in both shallow and deep contact states. The mechanism of "local surface adhesion" in the shallow contact state makes the friction force insensitive to the positive pressure. The friction coefficient decreases and the friction force is linearly associated with the sliding velocity. The mechanism of "non-local fiber accumulation" in the deep contact state makes the friction increase linearly with the positive pressure, while the friction coefficient is basically constant. There is a critical velocity that makes the carbon nanotubes break, to which the friction force responds nonlinearly. Moreover, the friction coefficient of CNT film decreases with increasing crosslinking density. The increase of crosslinking density shifts the surface contact state from deep to shallow contact. CNT film with low cross-linking density fails to effectively dissipate force and limit deformation. However, CNT film with high cross-linking density makes the stress almost fully dissipated, lead to a significantly reduced friction coefficient.

All these results provide a scientific basis for improving and optimizing the physical and mechanical properties of carbon nanotube film, and have a profound guiding value for the performance optimization and engineering applications of porous materials with similar structures.