| AVS 54th International Symposium | |
| Tribology | Friday Sessions |
| Session TR4+SE-FrM |
| Session: | Friction and Wear of Engineered Surfaces Macro- to Nanoscale Approaches |
| Presenter: | F. Stevens, Washington State University |
| Authors: | F. Stevens, Washington State University S.C. Langford, Washington State University J.T. Dickinson, Washington State University |
| Correspondent: | Click to Email |
Commercial silicon nitride tips for atomic force microscopy (AFM) were used as model asperities to study the mechanical and tribochemical wear of a Si(110) surface. Aqueous sodium hydroxide and tetraethyl ammonium solutions were employed as chemical etchants. Under these conditions, tip wear is a significant issue; a new tip was employed for each wear measurement. Changes in tip contact stress were estimated by characterizing tip shapes before and after wear. In basic solution, the native oxide can be patterned by the AFM tip to expose the more vulnerable underlying Si to the chemical etchant.2 Features 20 nm deep, with lateral dimensions less than 100 nm, are readily produced. The rate of oxide wear is a strong function of force applied to the AFM tip; even at low contact forces, scanning significantly accelerates oxide wear. Initial penetration of the native oxide is not uniform and produces deep pits—presumably at pinholes or similar defects in the oxide. Once the oxide is fully penetrated in the scanned region, subsequent tribochemical etching produces depressions with flat, smooth bottoms. For a given contact force and solution, the final wear depth relative to the surrounding, chemically etched material depends only on the number of times the AFM tip has passed over the surface; changing the tip velocity has no significant effect. Thus the tribochemical component of wear is not limited by chemical reaction rates under the conditions of this work. To characterize mechanical wear apart from chemical effects, hydrogen-terminated Si surfaces were scanned in inert solutions and atmospheres; images after wear in argon show wear debris, consistent with abrasive wear. The observed abrasive wear is sufficient to account for 5-10% of the Si wear observed in the presence of basic solution. Tribochemical effects during Si wear at a contact force of 300 nN in 0.1 M NaOH can easily enhance the total etch rate by a factor of two over the sum of the chemical and mechanical wear rates.
1This work was supported by the US National Science Foundation under Grant CMS-0409861.
2S. Miyake and J. Kim, Nanotechnology 16, 149 (2005).