The effect of heat treatment, carburizing and plasma nitriding on the material-removal mechanisms and wear behavior of steels under boundary-lubricated conditions has been studied. A controlled procedure using mild abrasives under a light load was adopted in order to avoid long-term effects from severe wear in the running-in stage of pin-on-disc testing. Electron microscopy of the sliding surfaces showed that wear of the untreated steel took place by abrasive and adhesive mechanisms. Heat treatment by austenitizing, quenching and tempering (through-hardening) reduced the wear rate and carburizing produced a further reduction. The principal wear mechanisms in the both through-hardened and carburized steels was abrasive and delamination wear with adhesive wear being unimportant. Electron microscopy on cross-sections through the sliding surfaces of the carburized and through hardened steels showed extensive plasticity, cracks, and delaminated wear phenomena. The elimination of adhesive wear as a major wear mechanism is attributed to the influence of hardness on junction growth and the emergence of delamination wear to the effect of nano-crystalline carbides on fatigue life. Plasma nitriding resulted in an additional reduction in wear rate and the effective elimination of delamination wear. Electron microscopy on the plasma-nitrided steel revealed the presence of a dispersion of white layer regions, which raised the yield strength but also - resulted in micro-pitting due to a deformation mismatch with the matrix.Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 8–12, 1997