Tribo-induced evolution of nodular supporting structures providing high-performance solid lubricants in carbon nanotube coated films

Abstract

Carbon nanotubes (CNTs) exhibit a high degree of sp2-bonded carbon and form long tubular structures that are one-dimensional and multi-walled. Pristine CNT films show poor lubrication performance when sliding against different counterparts due to their loose and porous carbon structures. However, heat treatments, particularly the 600 °C annealing process in a vacuum, have greatly improved their wear resistance. The 600 °C annealed film has a slightly high coefficient of friction (COF) than the unannealed (pristine) film but possesses significantly enhanced durability. During the 600 °C annealing treatment, numerous nodular structures appear on the surface of the steel substrate and beneath the CNT film. When the CNT film is worn out, these nodules support the periodic reciprocating counterpart and enable a lower COF to be remained for an extended lifespan. This is due to a new tribolayer that forms on both sides of the tribocouples where the tubular CNT transforms into amorphous carbon with continuous friction testing. Ultimately, the annealed CNT films and the as-formed nodules provide exceptional synergetic protection for the durable friction process.