Spectroscopic analysis of structural and chemical properties of solid thin film lubricants

Abstract

Upon tribological loading, especially at elevated temperatures or in humidity, chemical transitions and structural processes are known to take place in solid thin film lubricants like amorphous carbons (a-C) and molybdenum disulphide (MoS2) and to critically limit their performance. To accommodate those restrictions, a selection of element-modifications is applied on a regular basis. General aim is to analyse structural and chemical properties in a-C and MoS2 thin films. The understanding of those is a prerequisite for the scientific conception around tribo-film formation, which is identified as critical part of the tribological system. For this, Raman microscopy is augmented by a setup for optical temperature tuning via a pulsed laser. This work summarises the Raman-accessible film properties and the changes after element-modification and thermal impact. In a-C studies of optical temperature tuning, two ordering mechanisms are found, which are affected by element-modification. As initial reaction upon heating, graphitic clusters within an amorphous matrix increase in number and, subsequently, in size upon further heating. A Five Stage-model of structural relaxation in a-C is proposed. For MoS2, it is found that the formation of tribo-films is strongly dependent on the working environment, the tribo-film gains temperature resistance over untouched thin film material, and tribo-film formation is affected by element-modification. In studies of optical temperature tuning, an initial ordering process of amorphous MoS2 and subsequent chemical reaction to distinct oxides were found. The results are useful for identification of defect mechanisms and possibly for gauging the status of wear in a-C and MoS2 thin films. Element-modification in a-C with previously unknown effects may now be interpreted within a found Five Stage-model; similarly, the behaviour of MoS2 spectra upon heating can now be evaluated.