Speaker
Description
Achieving lubrication at high temperatures and pressures, as well as in oxidative environment, is a challenging problem of modern tribology, relevant for a wide range of applications such as turbomachinery, machining tools and aerospace industry [1, 2]. A promising solution for the conditions of high temperatures/pressures and presence of oxygen is to use a hard and oxidation-resistant coating (Cr-N, Ti-N, Cr-Al-N, Ti-Al-N) containing an additional element, i.e., lubricious agent, that can diffuse to the surface of the coating and form an oxide which reduces friction [3]. Out of several metals used as lubricious agents, vanadium became a standard choice since its oxide melts at considerably low temperatures, hence providing liquid lubrication [4].
Vanadium reacts with oxygen and may form oxides with different stoichiometries. The motivation of this study is to investigate how does the oxidation state of vanadium impact the tribological properties of vanadium oxides. The crucial point is related to the effectiveness of vanadium oxides as lubricants: how low is the coefficient of friction and does it depend on the stoichiometry.
We have performed a reactive molecular dynamics study on the tribological properties of five selected vanadium oxides ($V_xO_y$) under the conditions of elevated temperatures $\{600, 800, 1000\}$ K and pressures $\{1, 2, 3, 4\}$ GPa, in 5 independent runs for each stoichiometry, temperature and pressure, hence totaling in 375 distinguishable simulations. We included the stoichiometries ($V_2O_3, V_3O_5, V_8O_{15}, V_9O_{17}, VO_2$) observed in the experimental studies of vanadium oxide-based coatings [5, 6]. The simulation setup consists of two rigid $V_2O_5$ layers and amorphous vanadium oxide $V_xO_y$ confined between them [7]. The two $V_2O_5$ layers are used to impose the normal load and a constant sliding velocity. Our simulations were implemented using the reactive force field (reaxFF) [8] in the reax/c package [9] of the LAMMPS code [10] and they were run at IT4I’s Barbora supercomputer.
By applying a linear fit on the dependence of the sliding force $F_x$ on the normal load $F_z$:
\begin{equation}
F_x = COF \cdot F_z + F_x^{0}
\end{equation}
we extracted the coefficient of friction $COF$ and the sliding force at zero load $F_x^{0}$. For all stoichiometries the coefficient of friction decreases with the increase of temperature and takes the values $COF < 0.2$, which is consistent with the experimental findings [11]. In the offsets we obtained a clear separation and ordering, depending on the stoichiometry of vanadium oxides. We have explained those results with a structural analysis, i.e., we computed the average coordination number estimating the bonds between vanadium in amorphous $V_xO_y$ and oxygen in rigid $V_2O_5$ layers. As the oxidation state of vanadium in $V_xO_y$ increases (in the order $\{V_2O_3 < V_3O_5 < V_8O_{15} < V_9O_{17} < VO_2\}$), there are less bonds between vanadium atoms of amorphous $V_xO_y$ and oxygen atoms of rigid $V_2O_5$ layers.
The key finding of our study is that each of the considered vanadium oxides provides liquid lubrication under the imposed conditions. This finding represents a valuable information relevant for the design of coatings containing vanadium as the lubricious agent.
