High temperature quasistatic and dynamic mechanical behavior of interconnected 3D carbon nanotube structures

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCarbon nanotubes (CNTs) are one of the most appealing materials in recent history for both research and commercial interest because of their outstanding physical, chemical, and electrical properties. This is particularly true for 3D arrangements of CNTs which enable their use in larger scale devices and structures. In this paper, the effect of temperature on the quasistatic and dynamic deformation behavior of 3D CNT structures is presented for the first time. An in situ high-temperature nanomechanical instrument was used inside an SEM at high vacuum to investigate mechanical properties of covalently interconnected CNT porous structures in a wide range of temperature. An irreversible bucking at the base of pillar samples was found as a major mode of deformation at room and elevated temperatures. It has been observed that elastic modulus and critical load to first buckle formation decrease progressively with increasing temperature from 25 degrees C to 750 degrees C. To understand fatigue resistance, pillars made from this unique structure were compressed to 100 cycles at room temperature and 750 degrees C. While the structure showed remarkable resistance to fatigue at room temperature, high temperature significantly lowers fatigue resistance. Molecular dynamics (MD) simulation of compression highlights the critical role played by covalent interconnections which prevent localized bending and improve mechanical properties.142291299CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOSem informaçãoSem informação2013/08293-7S.O. acknowledges financial support from a LANL Director's Postdoctoral Fellowship. LDM, and DSG acknowledge the Brazilian Research Agencies CNPq, CAPES, and FAPESP for financial support. DSG also acknowledges the Center for Computational Engineering and Sciences at Unicamp through the FAPESP/CEPID Grant No. 2013/08293-7, for computational and financial support. N.M.P. is supported by the European Commission H2020 under the Graphene Flagship Core 2 grant no. 785219 (WP14, Composites) and under the FET Proactive ("Neurofibres. no. 732344), as well as by the Italian Ministry of Education, University and Research (MIUR) under the "Departments of Excellence" grant no. L.232/2016 and by Fondazione Caritro under "Self-Cleaning Glasses" no. 2016.0278. R.A.B. acknowledges financial support from Fondazione Caritro

Oscillating adhesive contacts between micron-scale tips and compliant polymers

Adhesion of micron-scale probes with model poly(dimethylsiloxane), PDMS, elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. For contacts between diamond indenters (radius R=5 or 10 μm) and PDMS, force-displacement curves were highly reversible and consistent with Johnson-Kendall-Roberts (JKR) behavior. However, our experiments have revealed striking differences between the experimental measurements of tip-sample interaction stiffness and the theoretical JKR stiffness. The measured stiffness was always greater than zero, even in the reflex portion of the curve (between the maximum adhesive force and release), where the JKR stiffness is negative. This apparent paradox can be resolved by considering the effects of viscoelasticity of PDMS on an oscillating crack tip in a JKR contact. Under well described conditions determined by oscillation frequency, sample viscoelastic properties, and the Tabor parameter (with variables R, reduced elastic modulus, E*, and interfacial energy, Δγ), an oscillating crack tip will neither advance nor recede. In that case, the contact size is fixed (like that of a flat punch) at any given point on the load-displacement cycle, and the experimentally measured stiffness is equal to the equivalent punch stiffness. For a fixed oscillation frequency, a transition between JKR and punch stiffness can be brought about by an increase in radius of the probe or a decrease in PDMS modulus. Additionally, varying the oscillation frequency for a fixed E*, R, and Δγ also resulted in transition between JKR and punch stiffness in a predictable manner. Comparisons of experiments and theory for an oscillating viscoelastic JKR contact are presented. The storage modulus and surface energy from nanoscale JKR stiffness measurements were compared to calculated values and those measured with conventional nanoindentation and JKR force-displacement analyses. © 2005 Elsevier Inc. All rights reserved

Surface damage of yttria-tetragonal zirconia polycrystals and magnesia-partially-stabilized zirconia in single-point abrasion

Commercially available 3Y-TZP and Mg-PSZ flats mere abraded by a 150 degrees diamond cone at -196 degrees, 25 degrees, 200 degrees, and 400 degrees C. The coefficient of friction, the track width, and the morphological features of the track were recorded. Raman spectroscopy mas used to record the tetragonal-to-monoclinic phase transformation (t --> m) as a function of distance away from the track. The study was undertaken to establish the influence of tangential traction on phase transformation and surface damage