A Comparative Analysis of the Tribological Behavior of Hard Layers Obtained by Three Different Hardened-Surface Processes on the Surface of AISI 4140 Steel

This work compares the tribological behavior of surface layers obtained by three different hardening processes. The layers were formed on the surface of AISI 4140 steel by applying three different thermochemical treatments. Wear resistance was evaluated using a standardized tribological machine for abrasive wear, according to the limits established by the ASTM G65 “Standard Test Method for Measuring Abrasion Using Dry Sand/Rubber Wheel Apparatus”. According to the results, the boride layers exhibited the highest wear resistance, as compared to nitrided and carburized layers. In contrast, the carburized layers presented the highest loss of volume. Scanning electron microscopy (SEM) was used to analyze the worn surfaces to examine the wear mechanisms. Abrasive wear was identified in all the samples, as the main abrasive wear mechanism. The mean values of the coefficient of friction (CoF) of the hardened surfaces were 0.39, 0.55, and 0.65 for carburizing, nitriding, and boriding samples, respectively, indicating that the wear process may not always be related to a low CoF. The results suggest that the highest hardness is normally associated with high wear resistance, but the coefficient of friction could be not directly related to the hardness of the materials. Finally, a statistical study demonstrates the random nature of the layers obtained by three different hardening processes.