Abstract
In this work, an experimental investigation of the nanomechanical properties of flame-formed carbonaceous particle layers has been performed for the first time by means of Atomic Force Microscopy (AFM). To this aim, carbon nanoparticles with different properties and nanostructures were produced in ethylene/air laminar premixed flames at different residence times. Particles were collected on mica substrates by means of a thermophoretic sampling system and then analyzed by AFM. An experimental procedure based on the combination between semi-contact AFM topography imaging, contact AFM topography imaging and AFM force spectroscopy has been implemented. More specifically, a preliminary topological characterization of the samples was first performed operating AFM in semi-contact mode and then tip-sample interaction forces were measured in contact spectroscopy mode. Finally, semi-contact mode was used to image the indented surface of the samples and to retrieve the projected area of indents. The hardness of investigated samples was obtained from the force–distance curves measured in spectroscopy mode and the images of intends acquired in semi-contact mode. Moreover, the Young’s modulus was measured by fitting the linear part of the retraction force curves using a model based on the Hertz theory. The extreme force sensitivity of this technique (down to nNewton) in addition to the small size of the probe makes it extremely suitable for performing investigation of mechanical properties of materials at the nanoscale. The experimental procedure was successfully tested on reference materials characterized by different plastic behavior, e.g., polyethylene naphthalate and highly oriented pyrolytic graphite. Both hardness and Young’s modulus values obtained from AFM measurements for different soot particle films were discussed.
Highlights
The modelling and experimental activities performed in the last decades to study the formation process of soot particles in combustion systems has led to substantial progresses [1,2]
The nanomechanical properties of nanostructured films made of flame-formed carbonaceous particles have been investigated for the first time by means of Atomic Force Microscopy nanoindentation
The hardness H was obtained from the force–distance curves measured in spectroscopy modes and the images of intends acquired in semi-contact mode, while the Young’s modulus E was measured by fitting the linear part of the retraction force curves using a DMT model based on Hertz theory
Summary
The modelling and experimental activities performed in the last decades to study the formation process of soot particles in combustion systems has led to substantial progresses [1,2]. The mechanical behavior and the physical nanostructure of soot particles proved to be useful in understanding tribological effects, such as the enhanced wear of mating surfaces covered with soot particles [24] This mechanism is relevant in internal combustion engines which use soot-contaminated oil and lubricants [25] and it could be applied for designing diesel particulate filters and their regeneration processes. An experimental investigation of the mechanical properties of flame-formed soot nanoparticles collected as nanostructured films could be a useful addition to the literature, as well as a piece of work of great relevance from a material science point of view. This work represents a first attempt to overcome a lack of experimental information about the mechanical properties of soot layers and to furnish direct experimental measurements of hardness and elastic modulus of nanostructured films of flame-formed carbon particles
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