Reaxff Molecular Dynamic Research on Tribochemistry of Si/SiO2 Surface and Role of Water Molecules to Surface Wear Damage

Reactions between SiO₂ surfaces in the presence of water under shear condition are important in interfaces at various length scales: from massive scale earthquakes, movement and collision of tectonic plates, to the nano scale wearing damage and lifecycle of MEMS/NEMS system.  Previous experimental research found evidence of the existence of ice-like and liquid-like water layers, and recent studies revealed that the portion of ice/liquid like water on the surface depends on relative humidity. In addition, several tribological studies and observations reported interesting features of surface wear damage after Si/SiO₂ shearing, under different relative humidity environments, which indicates the relationship between relative humidity, surface water structure, and wear damage. However, the chemical origin of surface damage with water molecules is still unknown. In this research, we performed ReaxFF molecular dynamic simulations to investigate how the existence of water molecule promotes, or prohibits the surface damage and interfacial reactions during the sliding of Si/SiO₂ structure.  Using the Fogarty et al. ReaxFF Si/O/H force field ¹, we prepared an oxidized Si 001 surfaced crystal slab and an amorphous SiO₂ slab. We hydroxylated the surfaces of both slabs until they reached chemical equilibrium, then sheared the slab at 1GPa of contact pressure. Our result indicates that a proper amount of water molecules can assist more dynamic Si-O-Si interfacial connection forming and breaking, which could bring more wear damage than shearing under dry environments. On the other hand, during the shearing simulation with a larger amount of water, water molecules acted as barrier between two surfaces, leading to less surface damage than the cases with a smaller amount of water. These results are compared with experimental findings. Our results supports that the water molecule at the Si/SiO₂sliding interface can help or disrupt the surface reactions and wear damage, and this depends on how much water molecules are present between two plates. 

Reference

(1)     Fogarty, J.; Aktulga, H.; Grama, A. J. Chem. 2010.