The adsorption and desorption of water molecules, which affect the physical and chemical properties of the sliding interface, are critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to an adsorption equation. However, for a confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in a water vapor atmosphere is quantified on the basis of microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first-principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then, the relation between friction and gas pressure was built by employing a fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typical experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. In addition, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that the frictional coefficient of DLC films could be predicted on the basis of fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.

中文翻译：

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通过微观吸附和去除水分子量化类金刚石碳膜的宏观摩擦

水分子的吸附和解吸影响滑动界面的物理和化学性质，对于大气环境中两个固体接触的摩擦行为至关重要。根据吸附方程，吸附在开放表面上的水量是气压的函数。然而，对于有限的滑动界面，尚未弄清被水汽压的气体分子覆盖的表面分数的变化及其对摩擦的诱导作用。在这项工作中，基于水分子的微观吸附和去除，量化了类金刚石碳（DLC）膜在水蒸气气氛中的宏观摩擦。这些研究将吸附在自交联DLC膜界面上的水分子比例与水蒸气压力相关联，以通过第一性原理计算和模型拟合来说明摩擦系数和气压之间的直接关系。计算结果表明，水分子的化学吸附和物理吸附发生在无氢DLC膜（ta-C）和氢化DLC膜（HCF）的表面上。然后，基于线性吸附方程和气体去除率，采用分数覆盖模型建立摩擦与气体压力之间的关系。所得模型与高度氢化的DLC薄膜（HCF）和四面体非晶碳（ta-C）薄膜在各种水蒸气压力下滑动时的稳态摩擦系数的典型实验结果非常吻合。此外，它还给出了表面覆盖率随水蒸气压力变化的曲线。这些结果表明，DLC膜的摩擦系数可以根据表面覆盖率的分数以及表面化学的内在特征来预测。我们建议可以扩展该模型，以理解和预测在低负载和低速度下特定气体压力下DLC膜的摩擦。