Improving Self‐Lubricating Performance of AISI 4140 Steel Composites Reinforced With Nickel‐Coated Multi‐Walled Carbon Nanotubes Under Dry Sliding Conditions

Wear under elevated‐temperature dry sliding substantially limits the durability of steel mechanical components. In this study, an AISI 4140 steel composite reinforced with 1 wt.% nickel‐coated multi‐walled carbon nanotubes (Ni–CNTs) was fabricated by spark plasma sintering. Reciprocating ball‐on‐disc tests were conducted against AISI 52100 steel under contact loads of 20–80 N at 5 Hz and 300°C. Field‐emission scanning electron microscopy (FE‐SEM)/energy‐dispersive x‐ray spectroscopy (EDS), a three‐dimensional surface profilometry, and x‐ray photoelectron spectroscopy (XPS) were employed to characterize the wear response and the resulting tribologically modified surfaces. The Ni–CNT addition increased the hardness by 34.4% and the thermal diffusivity by 4.9%–48.5% compared with unreinforced AISI 4140. The reinforced composite reduced the average friction coefficient by up to 14.5%, whereas the reduction in disc wear volume was strongly load dependent, ranging from approximately 7% to 64%, with the largest reductions obtained at 20 and 80 N. Posttest analyses revealed a heterogeneous tribologically modified layer containing carbonaceous and Ni‐bearing constituents together with Fe‐rich and oxide‐containing wear products. The carbonaceous phase can reduce interfacial shear, whereas the Fe‐rich and oxide‐containing constituents provide mechanical support to the contact. These results demonstrate the load‐dependent potential of Ni–CNT reinforcement for improving the dry‐sliding performance of AISI 4140.