Tribological behavior and graphitization of carbon nanotubes grown on 440C stainless steel

Vertically aligned carbon nanotube (CNT) arrays were directly grown onto 440C stainless steel substrates by plasma-enhanced chemical vapor deposition. Tribological properties of both short and long CNTs samples were studied under normal loads of 10 g, 25 g and 100 g. The CNTs had a steady-state friction coefficient of about 0.2 in humid air. In dry nitrogen, a friction of 0.2 was measured under a load of 10 g while high friction was measured at 25 g and 100 g loads. No significant variation of tribological behavior was measured between the short and long CNTs samples. SEM observations showed that rubbing caused the CNTs to align or lay down along the wear scar. They formed aggregates and were compressed by rubbing, which resulted in layer-structured graphite formations. SEM observation of the wear scars revealed loss of CNT structures accompanied by the appearance of dark areas. Micro Raman spectroscopic studies demonstrated that the dark areas were graphitized CNTs. Shear stress aligned the basal planes of the small graphene sheets in the CNT layers to the low friction orientation and eventually caused formation of more ordered graphite. The tribological formation of interfacial carbon layers increased with increasing stress from higher loads.