Speaker
Description
Providing effective lubrication at high temperatures/pressures and in oxidative environments is relevant for various industrial applications, such as turbomachinery and cutting tools [1,2]. Promising solutions for such conditions are oxidation-resistant hard coatings consisting of binary or ternary films (Cr-N, Ti-N, Cr-Al-N, Ti-Al-N) doped with an additional element which can diffuse to the surface of the coating and form an oxide layer that serves as a lubricant. Vanadium became a popular dopant since its oxides melt at considerably low temperatures, hence providing liquid lubrication.
The amount of oxygen present in an oxidative environment can be taken as a study parameter, leading to the consideration of different vanadium oxide stoichiometries. This study aims to explore the tribological performance of under-oxidized vanadium lubricants, selected in accordance with available experimental studies [3].
We present a reactive molecular dynamics study on the tribological properties of five vanadium oxide stoichiometries {$V_{2}O_{3}$, $V_{3}O_{5}$, $V_{8}O_{15}$, $V_{9}O_{17}$, $VO_{2}$} at elevated temperatures {$600$, $800$, $1000$} [K] and pressures {$1$, $2$, $3$, $4$} [GPa]. Our tribological system consists of two rigid $V_{2}O_{5}$ layers, modeling two solid surfaces in a tribocontact, and a vanadium oxide with stoichiometry labeled as $V_{x}O_{y}$, confined between them. Under the imposed working conditions, all studied vanadium oxides were amorphous.
We have employed an atomistic model within the ReaxFF (reactive force field) potential to describe the interactions of vanadium and oxygen atoms. Sliding simulations were implemented in the reax/c package of the LAMMPS code [4].
By applying a linear fit on the dependence of the sliding force $F_x$ on the normal load $F_z$:
$$F_x = COF \cdot F_z + F_x^{0},$$ we extracted the coefficient of friction $COF$ and the sliding force at zero load $F_x^{0}$ (adhesion component of the friction force). At a fixed temperature, we did not notice significant changes of the friction coefficient with stoichiometry. The values which we obtained for the $COF$ (∼0.2 at 600 K, ∼0.15 at 800 K and ∼0.1 at 1000 K) are in good agreement with the previously determined results for amorphous $V_{2}O_{5}$ lubricant at the same temperatures [5]. We concluded that all considered $V_{x}O_{y}$ stoichiometries (i.e., under-oxidized vanadium) are going to be an effective lubricant. The friction coefficient $COF$ decreases with the increase of the temperature. We observed the increasing trend of the adhesion-related offset of the friction force $F_x^{0}$ with the decrease of the oxygen content in $V_{x}O_{y}$ lubricants and explained it by the more-pronounced tendency of vanadium atoms from $V_{x}O_{y}$ to bond with oxygen atoms from $V_{2}O_{5}$ in oxygen-poorer environments.
Our study on vanadium oxide lubricants provides a reference which is relevant for the design of vanadium doped oxidation-resistant hard coatings.
References
1. P. A. Papi, C. Mulligan, and D. Gall, Thin Solid Films 524, 211 (2012)
2. A. A. Voevodin, C. Muratore, and S. M. Aouadi, Surf. Coat. Technol. 257, 247 (2014)
3. F. Fernandes, J. Morgiel, T. Polcar, and A. Cavaleiro, Surf. Coat. Technol. 275, 120 (2015)
4. S. Plimpton, J. Comput. Phys. 117, 1 (1995)
5. I. Ponomarev, T. Polcar, and P. Nicolini, Tribol. Int. 154, 106750 (2021)
