The study of mechanical properties in porous materials is challenging. The inhomogeneous distribution of porous with sizes ranging from micro to nanoscale strongly influences the measured mechanical properties. For porous materials, the mechanical properties are then scale dependent and difficult to correlate with macroscale measurements. The atomic force microscope can be used to measure materials surfaces with high spatial resolution allowing the identification of pores and other surface features. The microscope can also be used to measure local mechanical properties on materials surfaces.
In this work, we present the use of the AFM (Nx-10, Park Systems) in the force modulation and acoustic modes to characterize the surface of samples with a wide distribution of pores, with diameters ranging from ~ 5 nm to tens of micrometers. Mapping of the elastic modulus were performed with the use of an external acoustic piezoelectric ceramic between the microscope XY scanner and the sample. The acoustic piezo was dithered by an external function generator with frequencies ranging from 300 kHz to 5MHz while the microscope tip movement was measured with an external lock-in amplifier. Rectangular silicon cantilevers with resonances of ~ 145 kHz and tips with radius ranging from ~ 10 nm to ~ 200 nm were used. The elastic modulus images were then correlated with the materials topography and pore distribution, confirming the high heterogeneity of the samples and its influences on the mechanical properties. The quantitative determination of the elastic modulus, at specific surface sites, was also obtained by measuring the first and second resonance frequencies of the AFM cantilever tip while in contact with the surface. The mechanical properties of the samples were additionally measured by nanoindentation (triboscope, Hysiotron). The elastic modulus measured by AFM was then compared with the values obtained using nanoindenters.