ICMCTF 2022 Session E1-1-TuA: Friction, Wear, Lubrication Effects, and Modeling I

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, have evolved as competitive materials and fillers for developing composites and hybrids for different applications. MXenes are denoted by a chemical formula of M_n+1X_nT_x (n = 1 to 4), where M represents n+1 layers of early transition metals (groups 3 – 6 of the periodic table) which are interleaved by n layers of X, where X represents carbon or nitrogen. In addition, T_x represents surface terminations bonded to the outer M layers of MXenes, where T are generally a mixture of –O, –F, –(OH), or –Cl surface groups. MXenes are synthesized via a top-down topochemical etching of their precursor carbides and nitrides, such as MAX phases. MXene flakes have strong mechanical properties (330 ± 30 GPa and 386 ± 13 GPa for Ti₃C₂T_x and Nb₄C₃T_x, respectively), which make MXenes the stiffest solution-processable 2D nanomaterials to date. The current forty synthesized MXene compositions paired within-depth ability to control their composition and structure makes MXenes a unique family of 2D materials with unlimited number of compositions and tunable properties. In this talk, we provide an overview of different MXenes compositions, their synthesis, and MXenes’ mechanical properties and discuss the effects of MXenes’ compositions, synthesis, and processing steps on their mechanical properties.

Keywords: MXenes, 2D materials, carbides, mechanical properties, composites.

We show evidence of low friction in BCC metals through molecular dynamics simulations and ultra-high vacuum experiments. This is shown to be correlated with grain boundary sliding (GBS) as the primary mechanism of deformation. Specifically, when grain sizes at the sliding interface are smaller than a critical, material-dependent value (on the order of 10-30 nm), a crossover occurs from dislocation mediated plasticity and Hall-Petch strengthening to GBS and interfacial softening. Results from simulations and experiments are quantitatively compared to a new predictive model of shear strength.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US Department of Energy’s National Nuclear Security Administration under Con- tract No. DE-NA0003525. This work was funded by the Laboratory Directed Research and Development (LDRD) program. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the US Department of Energy or the United States Government.

M. Dockins¹, A. Ayyagari¹, S. Srilliviputhur¹, S.P. Berkebile², D. Berman¹, A.A. Voevodin¹, S.M. Aouadi¹

¹Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA

² US DEVCOM Army Research Laboratory, Aberdeen Proving Ground, MD, United States

Degradation of sliding surfaces creates a significant problem for mechanical assemblies. In the case of next generation fuel delivery systems, steel on steel contacts in low viscosity fuel environments have been shown to experience scuffing failure after extended operation under rough conditions, as well as poor general tribological performance. To prevent this scuffing-induced failure, various surface modification techniques were implemented, such as the application of carbide- and nitride-based protective coatings via PVD and plasma spray methods. These surface modification techniques were evaluated in various fuel environments and counterbody compositions using a high frequency reciprocating rig tribometer in a sliding velocity range of 0.2 to 0.6 m/s and contact pressures ranging from 500 to 1200 MPa. Their relative performance was evaluated according to their wear rates, average coefficient of friction, and oxidation as determined via SEM/EDS. These results were explained theoretically through density functional theory calculations that quantify the effect of surface interactions with the fluid.