According to Holmberg and Erdemir [1,2], 23% of global energy consumption arises from tribological contacts. The impact of friction is six times higher than wear concerning energy losses and carbon dioxide emissions. This evidence emphasizes the importance of surface engineering in developing new tribological technologies that achieve wear resistance associated with friction reduction. 

This study investigates the development of surfaces that combine high hardness, wear-resistance, and solid lubrication properties using single low-temperature plasma carburizing treatments associated with the metal dusting phenomenon. The surfaces consisted of a bilayer system formed by a regular cementite (Fe₃C) layer and a film composed by vertically aligned carbon nanotubes (VACNT) grown over the carburized layer (Fig. 1a). For that, specimens of AISI 1005 low-carbon steel were used as a substrate. A fractional factorial design of experiments was conducted to identify the best processing conditions to achieve the required Fe₃C-VACNT surfaces. Morphological and structural aspects of the surfaces were assessed by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Reciprocating dry sliding tests in a ball-on-flat configuration were used to evaluate the friction coefficient and wear rate of the tribosystems. 

The results show that the processes where carburizing was performed between 600 and 700 °C and with plasma power densities ≥ 0.30 W/cm² were effective for developing the bilayer surfaces (Fig. 1b). A signature of that processes was a temporal increase of the plasma current. According to a created statistical model, the most influential experimental parameters in the average power plasma density were output voltage, %CH₄ in the gas mixture and duty cycle. The Fe₃C-VACNT surfaces were able to reduce the friction coefficient to values down to 0.08 and decrease the wear rate in 25 to 70%, depending on their morphological and structural characteristics.

References

[1] K. Holmberg and A. Erdemir Friction, 2017, 5, 263 – 284.
[2] K. Holmberg and A. Erdemir. Tribology International, 2019, 135, 389 – 396.