Transitions in the Friction Coefficients, the Wear Rates, and the Compositions of the Wear Debris Produced in the Unlubricated Sliding of Chromium Steels

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Abstract
Steels containing up to 12 percent of chromium have been used in unlubricated pin-and-disk continuous sliding wear experiments. Variations in wear rates with load followed the pattern already reported for carbon steels, namely, a transition from severe to mild wear at a particular transition load. Chromium contents greater than three percent have a strong and unexpected influence upon the wear rates at loads above the transition load. The variations of the friction coefficients with increasing load tended to decrease in a continuous fashion for chromium contents less than three percent. Above this percentage, however, the friction showed a transition at the same load as for the wear rate transitions. It is shown that the wear transitions are probably due to changes of phase in the surface structure from that of the rhombohedral oxide to that of the spinel oxide as the load is increased. The proportionate amounts of spinel and rhombohedral oxide in the wear debris after the transition load has been exceeded is shown to be strongly connected with the wear rate. Comparison with static oxidation results reveals that the transition from mild to severe wear probably occurs at a contact temperature of about 300 C. Finally, it is shown that the decrease in wear rate with increasing chromium content is compatible with an increase in the activation energy needed for oxidation to occur at the real area of contact between the sliding surfaces.

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