Most solid surfaces have an adsorbed water layer in nanometer scale. The thickness of this adsorbed water layer increases with surrounding humidity [1]. As a mechanical system is getting smaller, the effect of adsorbed water on friction becomes dominant due to the high surface force to body force ratio. For example, static friction is often blamed for the failure of a Micro-Electro Mechanical System (MEMS) device at low relative humidity (RH) [2]. Most explanations center on how adsorbed water influences the rate of oxidation and then governs the tribological phenomena [3]. However, a research performed in an oxygen-free environment revealed that oxidation itself fails to explain such complex phenomena [4]. A recent unidirectional ball-on-ball micro-sliding test showed that friction was high when the balls were away from each other at low RH as shown in Fig. 1 [5]. This phenomenon is believed to be caused by the negative Laplace pressure exerted by the meniscus formed between asperities of two contact surfaces due to adsorbed water. Since Laplace pressure is a function of the real contact area, which can be manipulated by surface roughness, Ra, this research aims to study the roles of Ra and RH in a ball-on-ball micro-sliding test.
展开▼
