Influence of carbon coatings on lubricating properties of boundary layers

Abstract

The friction behaviours of two types of diamond-like carbon coating (amorphous and monocrystalline) were studied under boundary lubrication. As lubricants, liquid paraffin with and without surfactant additives (stearic and oleic acids) were used as well as base mineral oil with viscosity corresponding to SAE 30. Tests were conducted on a ball-on-disc tribometer and on a tribometer where two rings were rubbing against each other. It was found that in all types of test the monocrystalline coating ensures minimal values of friction coefficient in a wider range of temperatures, whereas the amorphous coating has maximum values of friction coefficient compared with the monocrystalline coating and with the reference uncoated steel pair. This effect takes place for all lubricants studied. The observed experimental data can be explained by the strong orienting effect of the monocrystalline carbon coating on the structural order of molecules in boundary layers and correspondingly on their lubricating ability. The effect of the amorphous coating on molecular orientation in boundary layers is rather poor.

Keywords

friction coefficient boundary layers lubricating ability diamond-like carbon coatings molecular orientation structural order epitropic liquid crystals

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