Tribological Properties and Action Mechanism of Complex Compounds of Sn(II) and Sn(IV) in Lubrication of Steel

Abstract

A systematic study has been made to evaluate the lubricating properties of metal-organic chelate structures containing tin bonded to aliphatic dicarboxylic acids. These tin compounds were synthesized find used, as additives in paraffin oil. Their tribological behavior was investigated on three different sliding test machines.

Both the valence state of the tin, i.e., Sn(II) or Sn(IV), and the chain lengths of the dicarboxylic acids affected the results. The seizure loads increased with increases in the acid chain length. Sn(IV) complex compounds, particularly stannic sebacate, had the best antiwear and antiseizure properties.

Using a number of analytical, techniques, it was found that after reaction of the tin compounds with the steel, tin diffuses into the steel substrate in the sliding zone. The diffusion process depends on the kind of compound being investigated, and on the load applied. The basic difference in the behavior of stannic oxalate and stannic sebacate consists of tin appearing in the surface layer to a greater extent in the latter case. Most probably, it results from the formation of intermetallic compounds in the oxidation-reduction reaction: Sn(IV) + Fe(O) ⇌ Sn(II) + Fe(II). The hypothesis of Sn-Fe bimetallic system formation in the friction zone is confirmed by results of X-ray microanalysis and microhardness tests of surface layers.

On the basis of the results of tribological and chemical investigations, a four-stage lubrication mechanism of these additives has been proposed. The mechanism provides one explanation for all of the facts discovered through experiments.

Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Tribology Conference in Pittsburgh, Pennsylvania, October 20–22, 1986

Notes

Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Tribology Conference in Pittsburgh, Pennsylvania, October 20–22, 1986