Interacting mechanisms of organic sulfides with metallic surfaces and their importance for problems of friction and lubrication

Boundary and Extreme Pressure Lubrication

“…It is known that a major feature of nascent surface is its elevated catalytic activity compared to quiescent, oxidized metal [17,[119][120][121]. At high temperatures found in a heavily loaded EHL film, whenever asperity contact occurs, i.e., when the EHL film thickness becomes comparable to the combined surface roughness of the contacting surfaces, the exposure of nascent surfaces worn by asperity interaction may directly affect the base oil of the lubricant which may have severe consequences for the lubricant film.…”

“…Nascent surface typically catalyzes decomposition reactions of organic compounds found in oil to release low molecular weight products that are often gaseous [119]. Such catalysis can have a destructive effect on the lubricating capacity of an oil.…”

Model of scuffing based on the vulnerability of an elastohydrodynamic oil film to chemical degradation catalyzed by the contacting surfaces

“…A hot nascent surface has been observed to catalyze the decomposition of fatty acids into very low molecular weight species [12]. At lower temperatures, nascent iron surface was found to cause decomposition of organo-sulphur extreme pressure additives to products with lower molecular weight [13]. Catalytic activity of nascent metallic surfaces can last for up to several seconds in a partial vacuum [26] and is more likely to be limited by oxidation outside of the contact [23].…”

“…This notion may explain why there is a sudden collapse in lubricating oil film thickness at the onset of scuffing [16] and is also supported by increased surface roughening with intensified asperity interaction causing more extensive plastic deformation of the asperities. Plastic deformation is a cause of mechanical activation which significantly raises the catalytic activity of a nascent metal surface [13,55]. Scuffing would therefore more likely occur with wearing surfaces which exhibit stronger catalytic activity.…”

“…A wide variety of decomposition products may be formed. These might be low molecular weight products formed in the absence of oxygen [12,13] or high molecular weight material resembling sludge that is formed by oxidative cross-linking [7]. Whatever the specific composition of the decomposition products, they represent non-optimal lubricants that do not possess the physical capacity to sustain the extremely high shear rates prevailing.…”