A low-alloy steel, AISI 5140, was surface hardened by plasma nitriding. Plasma nitriding process variables included time, temperature, and nitrogen partial pressure of the nitriding atmosphere. The plasma-nitrided samples were characterized using both light optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness techniques. Significant surface hardening was observed with all the nittided samples. Investigation of the structures showed that the compound layers produced by plasma nitriding mainly consisted of gamma'- and epsilon-nitride phases. Both case depth and compound layer thickness increased with increasing process temperature and time. A pin-on-ring wear testing machine was utilized to assess the wear resistance of the treated samples. The resulting wear loss was monitored as a function of sliding distance. Worn surfaces and collected wear debris were analyzed and dominant wear mechanisms were identified using microscopy techniques. It was found that the wear resistance improved considerably after the plasma nitriding process. However, for maximum resistance to wear, process parameters should be chosen to minimize the compound layer thickness and maximize the surface hardness and case depth. (C) 2003 Elsevier Science Inc. All rights reserved.