Tribological and electric contact resistance properties of pulsed plasma duplex treatments on a low alloy steel

Low friction low cost tribology systems involve most often use of lubricants, which must be periodically refurbished and often present environmental issues. Furthermore, use of liquid lubricants is a non-starter in many critical applications. In this work, the performance of single process duplex tribo-conditioning treatments is explored. The treatments are based on pulsed low-pressure plasma treatments, where nitriding processes are complemented with the deposition of carbonaceous solid lubricating top layers. The duplex treatments are carried out sequentially in the same chamber. The study comprised, among others, exploring the use of different residual gas mixes and pulse frequencies. Roughened blasted 42CrMo4 low alloy steel (AISI 4140 equivalent) was used as substrate. Resulting surface properties were studied with nano-indentation, profilometry and contact angle measurements. Besides, tribological studies were carried out against 100Cr6 steel at different humidities. The evolution of the coefficient of friction and the electrical contact resistance (ECR) was assessed for each test, as well as the wear rates. Results showed tribological outcomes heavily dependent of ambient conditions, and ECR of the wear track surface condition, i.e. the amount of remaining coated surfaces. The use of pulsed plasmas generated comparatively softer, smoother and more hydrophobic surfaces.