Afm Study Of Wear Process Of Sio₂ Micro-sphere For Colloidal Probe Application

In recent years atomic force microscope (AFM) has been widely used for surface force studies. By gluing microsized spherical particles onto AFM cantilevers, surface force can be measured more reliably. However, during force measurement, microspheres can be worn and contaminated after contacting repeatly with substrates, thus influencing the results of surface force measurement. AFM users just began to pay attention to the wearing of microsphere for colloidal probe application. However, no systematic research has been carried out on the wear process of microspheres against various substrates as well as the underlying mechanisms.A single SiO2 microsphere with 5μm in diameter was glued, via the wireless method, onto AFM cantilevers to prepare contaminant free colloidal probes. the standard Piranha solution was used to clean glass and Al₂O₃ substrates to remove surface contaminants. The AFM reverse imaging technique was intensively used to study the wear process of SiO2 microsphere on substrates of mica, glass, Al₂O₃. The change in morphology of the microspheres after wear was found changing the measured surface force against these substrates.Under the controlled experimental conditions, the wear of SiO2 microsphere on three substrates of mica, glass, Al₂O₃ which are different in hardness and surface roughness was systematically studied with AFM reverse imaging technique. It was found that the wearing of microspheres depended on not only surface roughness, but also the hardness of substrates. At loading force of 800nN, scanning size of 1μm×1μm, wearing time of 120min, the morphology of microspheres changed most significantly with glass, with which the surface roughness is highest. For Al₂O₃, which shows the highest hardness, the morphology change was less significant. For the least hard and smoothest substrate - mica, the morphology change was only marginal. Force measurements were carried out, immediately after wearing step, to study the relationship between the morphology change and adhesion force of microspheres on the substrates. The general trend is that adhesion force increased with the tested substrate and with mica substrate (as a standard substrate) as well. From start to 60min of wearing, adhesion force was found rising quickly, then the increasing rate of adhesion force became slower after 60min. Topography of worn microspheres revealed that the increased adhesion force was due to the form of a platform, with increasing diameter as wearing continued, on the top of SiO2 microsphere caused by the wear against the substrates. Therefore, effective contact area between the microsphere and substrates increased, leading to higher adhesion force. Force curves obtained on glass and mica separately after wear on glass sample. Comparative studies were conducted, regarding adhesion force measurements before and after wering, on both glass and mica substrates. It was found that, after 120 min wearing for both substrates, the adhesion force was 6.37 times higher for glass substrate, 55.52 times higher for mica substrates. Similar findings were obtained for Al₂O₃ and mica substrates after comparative studies .