Investigation of humidity-dependent nanotribology behaviors of Si(1 0 0)/SiO2 pair moving from stick to slip

Highlights: ► The effect of humidity on the motion behavior of Si(1 0 0)/SiO2 pair was clarified. ► With increase in humidity, adhesion force increases slowly firstly, then sharply. ► With increase in humidity, friction force increases sharply firstly, then slowly. ► The wear degree of Si is relative to the physical state of absorbed water film. ► The tribochemical reaction of Si(1 0 0) in humid air was verified by ToF-SIMS. - Abstract: With an atomic force microscopy, the humidity-dependent nanotribology behaviors of Si(1 0 0) against SiO2 microsphere were investigated while the relative movement translated from stick to slip. The relative humidity RH of air exhibits a strong effect on the motion behavior of Si(1 0 0)/SiO2 pair. With the increase in RH, relative movement of Si(1 0 0)/SiO2 pair is easier to keep into stick state, namely, the relative slip becomes more difficult to occur in a higher humidity range. The adhesion Fa will increase with the increase in RH in the given humidity range. In the low RH range (<20%) where the adsorbed water layer forms the 'solid-like' structure, due to the absence of water meniscus, Fa increases very slowly. However, in relative higher RH range (>20%), Fa increases very sharply once 'liquid-like' adsorbed water layer forms, because it increases the capillary force. The initial friction forces Ft of Si(1 0 0)/SiO2 pair also increase with the increase in RH in the given humidity range. However, different from Fa, it increases sharply in the low RH range (<30%) and slightly in the higher RH range (>30%). During the cyclic friction process, under the higher RH, relative stable tangential force is easier to be observed at higher displacement amplitude, here, the relative movement usually keeps into stick state. With the increase in RH, the surface damage of Si(1 0 0) transforms from mechanical deformation (forming hillock) to tribochemical wear (material removal). The tribochemical wear is sensitive to the absorbed water film with 'solid-like' structure, here, the wear volume increases drastically in this RH range (<20%); further increase of wear is small in higher relative humidity regime where the 'liquid-like' water layer is formed. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis verifies the water molecules participate into tribochemical reaction to rupture the Si-Si and Si-O network bonds on Si(1 0 0) substrate.