Quantitative nanomechanical property mapping of bitumen micro-phases by peak-force Atomic Force Microscopy

It is commonly assumed that the two-phase morphology of bituminous materials at the micrometer length scale is the origin for the material's mechanical response at larger length scales. Usually this morphology, or microstructure, is observed by using Atomic Force Microscopy (AFM), providing height and phase maps of the material's surface. The aim of this work is to quantify the nanomechanical properties of the phases that constitute the microstructure of bitumen, using Peak-Force Tapping mode AFM. The mechanical properties, i.e. modulus, probe-sample adhesion force, dissipated energy and deformation are measured for four well defined bitumen grades from the SHRP library. It is found that the measured mechanical property maps possess similar morphological characteristics as the height and phase images from traditional AFM, i.e. they overlap. For all phases of the four bitumen the averaged mechanical properties could be obtained. After proper averaging, these properties could be compared between the four bitumen grades and with their documented macroscale properties. Such correlations were found to exits, but also anomalies have been found. These anomalies may originate from a different viscoelastic response of the material phases at the experimental loading rate of 2 kHz, while traditional mechanical tests are performed at much lower frequencies. Thus, a systematic study of the nanomechanical properties is presented, with promising results that shall enable a better understanding of the link between micro and macro-scale properties of bituminous materials.