聚合物材料在摩擦设计上具有广泛的应用。对聚合物进行改性, 增强聚合物表面与水合离子的结合能力, 利用水合效应减小摩擦是聚合物摩擦副减摩设计的重要手段。但聚合物材料的弹性模量通常比较低, 对这种低弹性模量材料在水合效应作用下的接触情况, 目前还没有构建出很好的数值模型。该文构建了考虑水合效应的低弹性模量聚合物材料接触数值模型, 分析了表面力和弹性模量对接触状态的影响。发现低弹性模量材料接触压力小, 接触区域的各处分布压力可能完全由水合层承担, 形成纳米级间隙。研究还表明, 表面形貌对低弹性模量下考虑表面力的接触计算结果影响较小, 对高弹性模量下的则影响较大。此外, 弹性模量对模型收敛性有显著影响, 低模量情况下收敛难度显著增加。该成果为后续聚合物材料在表面力作用下的混合润滑数值计算研究奠定了基础。

Objective: Polymer materials exhibit promising prospects in various applications, such as ship bearings and artificial joints, due to their low density as well as high toughness, corrosion resistance, and foreign-matter tolerance. However, in engineering applications, there is often a need to reduce friction. Modifying polymers to enhance their surface binding capacity with hydrated ions and utilizing the hydration effect to reduce friction, represents an important strategy in the tribological design of polymer friction pairs. Polymer materials typically possess a low elastic modulus, and the hydration effect introduces nonlinearity into iterative algorithms. Consequently, a robust numerical model for simulating the contact behavior of such low-elastic-modulus materials under the hydration effect remains elusive. Methods: This paper presents a numerical model for simulating the contact behavior of low-elastic-modulus polymer materials under the hydration effect. Taking ultra-high-molecular-weight polyethylene (UHMWPE) and sapphire as an example, a rough surface with a Gaussian-distributed topography is constructed to analyze the influence of surface forces and material elastic modulus on material contact behavior. Results: The findings reveal that low-elastic-modulus materials exhibit low contact pressures, with pressure distribution across the contact area potentially fully supported by the hydration layer, resulting in the formation of nanoscale gaps. Calculations indicate that when the maximum pressure in the contact area is less than the maximum repulsive pressure provided by surface forces, the two surfaces can be completely separated. Conversely, when a part of the pressure in the contact area exceeds the maximum repulsive force, partial separation occurs, with regions where the contact pressure is less than the maximum surface force remaining fully separated. A low elastic modulus promotes the reduction of pressure in the contact area, facilitating surface separation and friction reduction. This study shows that surface topography has a minor effect on contact calculations considering the surface forces of low-elastic-modulus materials but has a significant effect in the case of high-elastic-modulus materials. Furthermore, the elastic modulus significantly affects model convergence: model convergence becomes challenging at low elastic moduli. Conclusions: First, under static contact conditions, the surfaces of low-elastic-modulus materials can be completely separated by surface forces. Calculations demonstrate that contact pressures in low-elastic-modulus materials are sufficiently low and smaller than the maximum inter-surface force, resulting in complete separation of the surfaces and the entire load being borne by the hydration layer, forming a continuous nanoscale gap between the surfaces. In addition, due to a large contact area and low elastic modulus, polymer materials are less prone to plastic deformation. Second, the surface topography of low-elastic-modulus polymer materials has a limited influence on the effect of surface forces and the distribution of contact pressure. Contrasting results are obtained for high-elastic-modulus materials. Therefore, for friction pairs based on the hydration effect used in hard materials, attention must be paid to surface finishing; this requirement is less critical for soft materials. For low-elastic-modulus materials, rough secondary surfaces can enhance the load-bearing capacity of the hydration layer. Finally, low elastic moduli significantly increased the difficulty of convergence in iterative algorithms. This study found that secondary details on the surface of low-elastic-modulus polymer materials do not significantly affect the contact calculation results, enabling the use of low computational mesh densities. However, material elastic modulus significantly affects the convergence of iterative algorithms used for calculating surface forces, with low elastic moduli leading to great convergence challenges and necessitating strict parameter constraints to achieve convergence. This study lays the foundation for subsequent numerical studies on mixed lubrication in polymers under the action of surface forces.