Decomposition Mechanisms of Anti-wear Lubricant Additive Tricresyl Phosphate on Iron Surfaces Using DFT and Atomistic Thermodynamic Studies

Abstract

Tricresyl Phosphate (TCP) is widely used as an anti-wear additive for lubrication and hydraulic fluid technology in aerospace propulsion systems. However, there is no clear description of the chemistry involved in the conversion of TCP to an anti-wear film on metal surfaces. Different reaction mechanisms leading to different product formation have been described in the literature, but no attempts have been made to explain the interfacial chemistry using first principles theoretical methods. In this study, we employed density functional theory with atomistic thermodynamic modeling to study the decomposition mechanisms of TCP on an iron (110) surface. We considered three broad reaction schemes, namely TCP as phosphoric acid reservoir, thermal decomposition of TCP, and TCP decomposition by hydrolysis. Different reaction paths were considered for the three reaction schemes, and the resulting decomposition products were characterized by the change in reaction energy, density of state plots, Bader charge analysis, and work functions. We observed that all three reaction schemes had at least one pathway with decreasing reaction energy, indicating they are likely to proceed in the proposed reaction direction. In the presence of hydrogen, TCP decomposition proceeded through the phosphoric acid reservoir mechanism with the anti-wear film comprised of a phosphate group, an m-tolyl group and 1-methyl-2,5-cyclohexadiene. In the presence of moisture, TCP decomposed through hydrolysis leading to a film comprised of a phosphate group and 3-methyl-2,5-cyclohexadienol. However, in the absence of hydrogen and moisture, film formation proceeded through thermal decomposition, and the resulting film was comprised of a methylphenoxy group, a P=O group, and 3-methyl-3,5-cyclohexadienone. Several of the products formed in the calculations, such as a phosphate group, methylphenoxy group, P=O groups, and di-cresyl phosphate, have been observed experimentally in previous research, but we also observed new products, including 3-methyl-2,5-cyclohexadienol and 1-methyl-2,5-cyclohexadiene.