Carbon nanomaterials and carbon coatings in lubrication - "MINIMA MAXIMA SUNT"

Controlling the friction and wear properties of mechanical contacts under high loads by liquid lubrication enables energy savings, provides better efficiency and increases component lifetime. Lubricant performance can be improved by use of additives, for example nanomaterials or protective coatings. Carbon has many allotropic forms which include diamond with sp3-structure, graphite with sp2-structure or amorphous carbons with mixed sp2/sp3 structures, among others. These allotropes have properties which are potential for lubrication purposes such as high hardness and strength, high thermal conductivity, low friction surfaces and chemical activity. In this work nanodiamonds (NDs), graphene oxide (GO) and silica/graphene oxide (SiO2/GO) composites were used to improve the friction and wear performance of high-load steel-steel and carbon-steel contacts in ethylene glycol (EG) or water lubrication by Pin-On-Disc (POD). The SiO2/GO composites used as lubricant additives were reported in this thesis for the first time. The carbon nanomaterials were dispersed into the lubricants or deposited directly on contact surfaces by spray or agglomeration methods. The effect of carbon coatings and steel surface finishing methods on friction performance was studied in oil lubrication by Twin Disc (TD) testing. The coatings used for TD testing were tetrahedral amorphous carbon (ta-C) and tungsten containing carbon (WC-C). The carbon nanomaterials in EG reduced friction coefficient 19 % (GO), 31 % (NDs), and 38 % (SiO2/GO composite), respectively, in comparison to pure EG in steel-steel contacts. All nanomaterials in EG reduced the wear rate of steel surfaces. GO dispersions in water reduced friction 57 % in comparison to pure water in ta-C-steel contact. Optimized combination of surface roughness and WC-C coatings in oil lubrication reduced the friction coefficient 27 % in comparison to reference steel surfaces. According to the results, the improved friction and wear performance of carbon nanomaterials was due to incorporation of particles into the tribolayer and embedding onto the contact surfaces, respectively. Furthermore, carbon coatings deposited on steel with optimized surface roughness both prohibited metal asperity contacts and improved lubrication by acting as lubricant reservoir. The results here suggest that the use of carbon nanomaterials and carbon coatings in liquid lubrication can significantly improve the friction and wear performance of high-load mechanical contacts and provide longer lifetime for many industrial applications such as gears and bearings.