Suspension of molybdenum–sulphur complexes in paraffin oil as extreme pressure lubricants

Molybdenyl aryl thiobiurets were evaluated as extreme pressure lubrication additives in a four ball test using steel balls of 12.7 mm diameter. A comparative account of performance of the tested compounds is presented on the basis of various tribological parameters such as initial seizure load, 2.5 s seizure delay load, weld load, mean Hertz load, flash temperature parameter, pressure wear index, friction coefficient (μ) and wear scar diameter (d), etc. All the tested complexes have been found to possess significant Extreme Pressure (EP) efficiency under the experimental conditions of four ball test. The best performance was shown by p-methoxyphenyl and p-chlorophenyl thiobiurets. The surface topography and tribochemistry of wear scar surface of balls in the presence of p-methoxyphenyl derivative at different loads have been studied by scanning electron microscopy and Auger Electron Spectroscopy (AES). The presence of molybdenum, sulphur, oxygen, nitrogen, etc. on the ball surface, detected by AES, suggests that these compounds act as precursors of tribologically active chemical layers formed under extreme pressure conditions.