Humidity effects on the determination of elastic properties by atomic force acoustic microscopy

We have investigated how ambient humidity can affect quantitative measurements of elastic properties on the nanoscale. Using an emerging technique called atomic force acoustic microscopy (AFAM), two samples were examined: a thin film of fluorosilicate glass and a section of borosilicate glass. When experimental results were analyzed using a simple model of the atomic force microscope cantilever dynamics, values of the tip–sample contact stiffness $k*$ increased approximately linearly with relative humidity. The effect is believed to be due to the presence of a humidity-dependent layer of water on the sample. To account for this, the data analysis model was extended to include viscoelastic damping between the tip and the sample. A damping term proportional to the relative humidity was used. The revised values for $k*$ showed virtually no dependence on humidity. Thus, the subsequent calculations of the indentation modulus M from $k*$ yielded similar values regardless of measurement humidity. These results indicate that environmental conditions can influence quantitative nanoscale measurements of elastic properties, at least in some materials.