Department of Industrial and Manufacturing Engineering, North Dakota State University, Dept 2485, PO Box 6050, Fargo, ND 58108, USA.
Nanoscale Res Lett. 2013 Sep 17;8(1):389. doi: 10.1186/1556-276X-8-389.
Nano/micro-manufacturing under wet condition is an important consideration for various tool-based processes such as indentation, scratching, and machining. The existence of liquids adds complexity to the system, changes the tool/work interfacial condition, and affects material behaviors. For indentation, it may also affect material property measurements. However, little effort has been made to study this challenging issue at nano- or atomistic scale. In this study, we tackle this challenge by investigating nano-indentation processes submerged in water using the molecular dynamics (MD) simulation approach. Compared with dry indentation in which no water molecules are present, the existence of water molecules causes the increase of indentation force in initial penetration, but the decrease of indentation force in full penetration. It also reduces the sticking phenomenon between the work and tool atoms during indenter retraction, such that the indentation geometry can be better retained. Meanwhile, nano-indentation under wet condition exhibits the indentation size effect, while dry nano-indentation exhibits the reverse indentation size effect. The existence of water leads to higher computed hardness values at low indentation loads and a smaller value of Young's modulus. In addition, the friction along the tool/work interface is significantly reduced under wet indentation.
湿条件下的纳/微制造是各种基于工具的工艺（如压痕、划伤和加工）的重要考虑因素。液体的存在增加了系统的复杂性，改变了工具/工件界面条件，并影响了材料的行为。对于压痕，它也可能影响材料性能的测量。然而，在纳米或原子尺度上研究这个具有挑战性的问题的努力很少。在这项研究中，我们通过使用分子动力学 (MD) 模拟方法研究了在水中进行的纳米压痕过程来应对这一挑战。与不存在水分子的干式压痕相比，水分子的存在会导致初始穿透时压痕力的增加，但完全穿透时压痕力的减小。它还减少了压头缩回过程中工作和工具原子之间的粘着现象，从而可以更好地保留压痕几何形状。同时，湿条件下的纳米压痕表现出压痕尺寸效应，而干纳米压痕表现出相反的压痕尺寸效应。水的存在导致在低压痕载荷下计算出的硬度值较高，杨氏模量值较小。此外，在湿压痕下，工具/工件界面处的摩擦显著降低。
