Bismuth Sulfide (Bi₂S₃) as the Active Species in Extreme Pressure Lubricants Containing Bismuth Carboxylates and Sulfur Compounds

Abstract

A series of bismuth carboxylates were tested with active and inactive sulfur compounds as extreme pressure (EP) lubricant additives. Wear and load measurements from four-ball tests are reported. Bismuth 2-ethylhexanoate (Bi 2EH), bismuth neodecanoate (Bi NDA) and bismuth naphthenate (Bi Nap) showed excellent EP activity with both active and inactive sulfur compounds (≥ 620 kg four-ball weld load). The mechanism of the EP lubricity was studied using thermogravimetric analysis (TGA), high-temperature X-ray diffraction (HTXRD), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The results of these studies support the formation of bismuth sulfide during thermolysis of the bismuth carboxylate-polysulfide mixtures. The structure of bismuth sulfide suggests that interlayer shear occurs and provides lubricity.

KEY WORDS:

• Extreme Pressure Additives
• Bismuth Carboxylates
• X-Ray Diffraction (XRD)
• Thermogravimetric Analysis (TGA)
• Scanning Electron Microscopy (SEM)

ACKNOWLEDGMENTS

The authors thank Holly Skiba for experimental assistance, Neil Canter of Chemical Solutions for helpful discussions, and the Shepherd Chemical Company for allowing publication.

Review led by Cyril Migdal