Y. Cai, J. Melendez-Rivera, J.D. Batteas
Texas A&M University,
United States
Keywords: AFM, force spectroscopy, adhesion force, friction, aqueous environments, silica surface
Summary:
Atomic Force Microscopy (AFM) and force spectroscopy were employed to study the adhesion and friction properties of flat-end silica tip and ultra-flat thermal silicon oxide surface on a Si (100) wafer under varying loading forces in water. Our findings suggest the presence of an interfacial water layer on the hydrated silica surface in aqueous environments. For loading stresses below 2MPa, the observed adhesion decreases with increasing loading stress, reaching a minimum at ~2 MPa, after which adhesion slightly increases due to the expansion of the contact area by higher loading force. Friction measurements demonstrate a similar transition point at ~2 MPa. Below this threshold, the coefficient of friction (COF) is approximately 0.17, while decreases can be seen to 0.02 for loading pressures exceeding 2 MPa at pH 7. Control experiments confirm that these transition behaviors are specific to an aqueous environment and hydrophilic silica as the contact surfaces. At loading stresses under 2 MPa, the observed adhesion and friction are dominated by interactions between the interfacial water layers. The interfacial layer ruptures at the stress of 2 MPa, consistent with the compressive strength of this interfacial layer. Only at stress above 2 MPa do the measured values represent the true adhesion and friction between the hydrophilic silica surfaces of the tip and substrate. In solutions with pH values different than 7, all adhesion force versus maximum loading plots follow a similar trend, with a slight shift in the position of the minima. Adhesion decreases with increasing maximum loading force until reaching a minimum, followed by a slight increase in adhesion due to the expanded contact area corresponding to an increase in maximum load. This study aims to provide insights into the chemical and bonding behavior of silica-based materials within a liquid environment.