Investigation of third body role in dry contacts. Experimental procedure to dissociate the effects of substrate and interface layer on the contact pair frictional response

This paper presents an experimental approach for the evaluation of the third body role in dry contacts. The ultrasonic cleaning is applied to remove the third body layer from a contact sample pair, allowing to perform frictional tests with and without the interface layer. The comparison between the observed behaviors evidences a strong influence of the third body on the overall frictional response. The cleaned samples have been used as the substrate to introduce an external third body. The frictional tests pointed out the predominant role of the interface layer, rather than the substrate, on the frictional performances. This procedure allows to test external (eventually artificial) third bodies, investigating the role of different features (morphology, composition) on the frictional response

Mechanical Characterization of Polymer Nanocomposites and the Role of Interphase

Mechanical characterization of four polymer nanocomposite systems and two pure polymer reference systems was performed. Alumina (Al2O3) and magnetite (Fe3O4) nanoparticles were embedded in poly(methyl methacrylate) (PMMA) and polystyrene (PS) matrices. Mechanical testing techniques utilized include tensile testing, dynamic mechanical analysis (DMA), and nanoindentation. Consistent results from the three techniques proved that these nanocomposite systems exhibit worse mechanical properties than their respective pure polymer systems. The interphase, an interfacial area between the nanoparticle filler and the polymer matrix, was investigated using two approaches to explain the mechanical testing results. The first approach utilized data from thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) to predict the structure and density of the interphase for the four nanocomposite systems. The second approach analyzed the bonding between the polymer and the nanoparticle surfaces using Fourier Transform Infrared Spectroscopy (FT-IR) to calculate the density of the interphase for the two PMMA-based nanocomposite systems. Results from the two approaches were compared to previous studies. The results indicate that Al2O3 nanoparticles are more reactive with the polymer matrix than are Fe3O4 nanoparticles, but neither have strong interaction with the polymer matrix. The poor interaction leads to low density interphase which results in the poor mechanical properties.M.S.Committee Chair: Rina Tannenbaum; Committee Member: Hamid Garmestani; Committee Member: Karl I. Jaco

Identification of dynamic contact instabilities generated by braking materials

The occurrence of unstable friction-induced vibrations is a major issue for braking manufacturers, as they lead to annoying noise, structure vibrations and brake surface degradation. Understanding the underlying causes of frictional instabilities, arising during the sliding between two bodies, is necessary for developing solutions and countermeasures. For this purpose, in this work, an experimental and numerical investigation of contact instabilities has been performed. Mode coupling and negative friction-velocity slope instabilities have been numerically investigated by both lumped-parameter and finite element models. As well, an experimental campaign has been carried out for recovering the frictional and vibrational response of braking materials under different boundary conditions. The comparison between numerical and experimental results allows validating a new methodology, based on the study of the phase shift between the tangential and normal vibrational responses, in order to distinguish the different types of contact instabilities