The Load-Carrying Properties of Metal Dialkyl Dithiophosphates: The Effect of Chemical Structure

Abstract

The extreme pressure (e.p.) and mixed lubrication or antiwear (a.w.) properties of a range of metal dialkyl dithiophosphates have been examined using the four ball machine. The results show that the nature of the metal atom is a major factor in determining these properties, and that the factors governing e.p. and a.w. activities are different. Thus the order of decreasing e.p. activity is Bi(III) > Ag(I), Pb(II) > Sb(III), Sn(II) ≥ Cd(II), Fe(III), Ni(II) > Zn(II), whilst the order of decreasing a.w. activity is Cd(II), Zn(II), Ni(II) ≥ Fe(III) > Ag(I) > Pb(II) > Sb(III), Sn(II) > Bi(III) for a series of 4-methylpentyl-2 and n-hexyl salts. The nature of the alkyl group has a lesser effect on either e.p. or a.w. properties. Possible explanations for these results are presented.