The Effect of a Friction Modifier Additive on Micropitting

Previous studies have shown that many anti-wear additives have a tendency to aggravate micropitting damage in hard steels subjected to rolling sliding contact at low lambda values. This is apparently because anti-wear additives suppress the gradual smoothing of the rough surfaces that takes place when a pure base stock is used under mild conditions. However, in practice, it is common to combine anti-wear and friction modifier additives in formulated oils, a combination that leads to reduced boundary friction. In the work described in this article, we added a common friction modifier agent, a molybdenum bis-diethylhexyl dithio-carbamate (MoDTC), to an oil containing an anti-wear additive, a secondary zinc dialkyl-dithio-phosphate (ZDDP) in a mineral base-stock. We examined micropitting behavior using a disc tester. The oil with both MoDTC and ZDDP showed initial micropitting that gradually disappeared with continued running, whereas ZDDP alone led to continuing severe damage. Analysis of the counter-discs after the test showed the presence of MoS2 deposits on the asperity crests. Reduced boundary friction was confirmed using a tribometer test. It is speculated that the improved micropitting behavior resulted from the effect of a reduction in local tensile stress due to reduced asperity friction. This may have reduced the opening of the surface cracks and inhibited their extension.

[1]  N. Spencer,et al.  Surface chemistry in tribology , 2002 .

[2]  Keith D. Bartle,et al.  A review of zinc dialkyldithiophosphates (ZDDPS): characterisation and role in the lubricating oil , 2001 .

[3]  A. Olver.  The Mechanism of Rolling Contact Fatigue: An Update , 2005 .

[4]  H. Spikes,et al.  Chemical and Physical Analysis of Reaction Films Formed by Molybdenum Dialkyl-Dithiocarbamate Friction Modifier Additive Using Raman and Atomic Force Microscopy , 2001 .

[5]  Andrew V. Olver,et al.  Effect of lubricants on micropitting and wear , 2008 .

[6]  A. Gangopadhyay,et al.  Friction and wear of tribofilms formed by zinc dialkyl dithiophosphate antiwear additive in low viscosity engine oils , 2005 .

[7]  Hugh Spikes,et al.  The History and Mechanisms of ZDDP , 2004 .

[8]  Sandrine Bec,et al.  Synergistic Effects of MoDTC and ZDTP on Frictional Behaviour of Tribofilms at the Nanometer Scale , 2004, 0704.0338.

[9]  Andrew V. Olver,et al.  An experimental study of micropitting, using a new miniature test-rig , 2003 .

[10]  C. J. Aylott,et al.  Martensite decay in micropitted gears , 2005 .

[11]  Da Hofmann,et al.  The effect of the white layer on micro-pitting and surface contact fatigue failure of nitrided gears , 1999 .

[12]  Andrew V. Olver,et al.  Direct observations of a micropit in an elastohydrodynamic contact , 2004 .

[13]  Hugh Spikes,et al.  Wear in rolling contacts , 1986 .

[14]  Andrew V. Olver,et al.  The Durability of Gear and Disc Specimens—Part II: Post Failure Examination and Gear-Disc Correlation , 1999 .

[15]  J. H. Green,et al.  ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio , 2006 .

[16]  Béatrice Vacher,et al.  Mechanisms of MoS2 formation by MoDTC in presence of ZnDTP: effect of oxidative degradation , 2005 .

[17]  Maurice Godet,et al.  Micropitting in Hertzian Contacts , 1980 .

[18]  Andrew V. Olver,et al.  The Durability of Gear and Disc Specimens — Part I: The Effect of Some Novel Materials and Surface Treatments , 1999 .