Role of Carbon Nanotube Tribolayer Formation on Low Friction and Adhesion of Aluminum Alloys Sliding against CrN

Abstract

This work investigates the role of carbon nanotube (CNT) tribolayer formation in reducing friction and adhesion of an Al-alloy engine block material (Al-6.5% Si, 319 Al) sliding against a common piston ring coating, namely, CrN coated steel, when tested under a boundary lubricated condition. Coefficient of friction (COF) values were determined using pin-on-disk type tests as a function of sliding distance using CNT added to ethanol and ethanol without CNT addition. Boundary lubricated tests that used ethanol with 0.14 wt.% CNT resulted in a steady-state COF of 0.16, and reduced Al adhesion to the CrN due to the formation of CNT tribolayers on the Al-alloy contact surfaces. Raman spectroscopy and high resolution SEM suggested the CNT fibers in the tribolayers were damaged and possibly subjected to plastic deformation, and the carbon bonds were possibly passivated by the -H and -OH dissociated from ethanol as suggested by FTIR. The low friction and adhesion observed when ethanol with 0.14 wt.% CNT was used was attributed to the sliding-induced bending and curling of the CNT tribolayers, leading to the formation of rolled sections of tribolayer with a cylindrical morphology (diameter of ~ 1 µm) that reduced direct contact between Al-alloy and CrN surfaces.