Development of a ReaxFF Reactive Force Field for the Investigation of Thermochemical, Thermophysical and Oxidation Behavior of Titanium Diboride

Abstract

To predict the thermochemistry and thermophysical properties and oxidation mechanisms of titanium diboride (TiB2), we have developed a ReaxFF reactive force field in which the parameters are trained against a set of quantum mechanics data, including the heats of formation and elastic properties for various titanium boride phases, as well as heats of formation of various titanium oxide and boron oxide phases. The developed ReaxFF accurately reproduces the formation energies, relative stability, and elastic properties of these phases. To demonstrate the applicability of our developed ReaxFF force field, we performed ReaxFF-based molecular dynamics simulations to evaluate thermal expansion and melting behavior and validated the results against published experimental data. The thermal expansion simulations reflected the experimentally observed anisotropic behavior, demonstrating greater expansion along the c-axis compared to the a-axis, and showed good agreement in volume expansion measurements. The predicted bulk melting temperature of 3120.87 K is consistent with experimental values (3063–3498 K), while a lower surface melting point of 3049.20 K was observed due to reduced atomic coordination. Moreover, the oxidation behavior of TiB2 was investigated using molecular dynamics simulations. To accelerate the oxidation process within the accessible MD time scales, we employed elevated oxygen concentrations and high-temperature conditions. Additionally, replica exchange molecular dynamics (REMD) simulations were conducted at a temperature of 1970 K, corresponding to experimental oxidation conditions. The simulations revealed that oxidation was initiated with the formation of titanium oxides, consistent with the lower formation energy of TiO2 compared to B2O3. Notably, the titanium oxide formed during the REMD simulations mostly resembled the TiO2 phase in terms of crystallinity. Smaller boron oxide species were also observed through REMD simulations. These results confirm that the developed ReaxFF potential reliably models both the thermochemical and thermophysical behavior of TiB2, as well as complex oxidation mechanisms, making it an accurate and computationally inexpensive tool for simulating high-temperature ceramic materials.