Paper 12: Mild Wear of Unlubricated Hard Steels in Air and Carbon Dioxide

Abstract

A study has been made of the wear of various hard steels sliding unlubricated in dry air and in dry carbon dioxide at temperatures up to 500°C. Over the range of loads and speeds employed the wear is of a mild character and the wear debris is of oxide particles, no metallic debris being formed. The characteristics of the wear process depend on the type of oxide formed under the particular conditions of specimen temperature and surrounding atmosphere. In dry air, the wear rate increases with temperature to a maximum which depends on the type of steel, and then decreases at higher temperatures. Below the maximum, α-Fe₂O₃ is formed and is swept clean of the surface by the sliding process. The oxidation rate determines the wear rate which is independent of the load at constant specimen temperature. Above the maximum the spinel (Fe₃O₄) type oxide is formed which is adherent and wear protective. In dry carbon dioxide, spinel oxide is formed at all temperatures in the range 20–500°C and the wear rate is low, except at high temperatures when the steel becomes too soft to support the protective layer.