The Role of Antioxidants on the Oxidation Stability of Oils with F-ZDDP and ZDDP, and Chemical Structure of Tribofilms Using XANES

Abstract

Oxidative degradation reduces the effectiveness of lubricants, increases their acidity and viscosity, and contributes to the formation of sludge and deposits. Fluorinated zinc dialkyl dithiophosphate (F-ZDDP) has been shown to be a better antiwear additive in comparison to zinc dialkyl dithiophosphate (ZDDP). This study examines the oxidation stability of formulations with ZDDP and F-ZDDP. Oil samples containing F-ZDDP and ZDDP and various types of antioxidants such as alkylated diphenyl amines (ADPA), hindered phenols (HP), methylene bis dithiocarbamate (MBDTC), and molybdenum dithiocarbamate (MoDTC) were oxidized in flowing air. The viscosity and the total acid number were measured and the products of oxidation were quantified using Fourier transform infrared spectroscopy. An oxidation study in base oil indicates that ADPA, HP, and MBDTC assisted in reducing the level of oxidation in the base oil while MoDTC was antagonistic, resulting in acceleration of the oxidation process. The F-ZDDP exhibited oxidation behavior that was similar to ZDDP. Wear tests under boundary lubrication indicated that F-ZDDP exhibited better wear resistance than ZDDP in all cases except in the presence of MoDTC. The tribofilms generated under boundary lubrication conditions were examined using XANES to understand their chemical makeup and the role played by F-ZDDP and ZDDP and different antioxidants.

KEY WORDS:

• Oxidative Stability
• Anti-Oxidants
• XANES
• ZDDP
• Tribofilms

ACKNOWLEDGEMENTS

Support provided by Platinum Research Organization is gratefully acknowledged. XANES studies were conducted at Synchrotron Radiation Center, Madison, Wisconsin, which is supported by NSF under grant number DMR 0537588, and at Canadian Light Source, Saskatoon, Canada. Assistance provided by Dr. Narayan Appathurai, Dr. Lucia Zuin, and Dr. Yongfeng Hu is greatly appreciated. Dr. Ramoun Mourhatch and Mr. BoHoon Kim assisted in plotting some of the XANES data.

Review led by Cyril Migdal