Fluid-free lubricant layers for the heavily loaded and unsynchronized operation of dry-running screw machines

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The aim of the research project is to understand friction and wear mechanisms of tailored MoSx(:X) solid lubricants under real contact conditions of dry-running, unsynchronized screw machines at the macro- and microscopic scale and to develop strategies for enhancing the lifetime of the solid lubricant. A self-designed screw-machine test bench reproduces the conditions of real contacts by setting an adjustable surface pressure, temperature, load duration and atmosphere and provides a range of distinct sets of parameters for the situation of the rolling contact. Additionally, the conditioning of HiPIMS sputtered MoSx(:X) thin films adjusts the contact situation. Within this context, the influence of the complex geometry of the screw machine regarding shadowing effects on film-growth mechanisms is evaluated. A variation in the thin film thickness with respect to the roughness of the screw machine, a modification of the thin film with Ag, W or C (X = Ag, W, C) as well as an application of films made from the pure modification element onto the secondary rotor shall control the conditions within the rolling contact.Depending on these contact parameters the question will be elaborated which chemical, physical and structural surface properties determine the functionality of the MoSx(:X) solid lubricants and allow for predicting the material removal and failure at the microscopic scale. The functionality is described by the effect and interaction of elementary processes which include oxidation, material removal, rearrangement of surface strain, phase transformations and material mixing. The elementary processes are characterized by in-situ and operando methods established at the test bench, which consist of light and laser microscopy as well as Raman scattering with an option of laser ablation, and ex-situ techniques. The latter provide spectrally and spatially high-resolved insights into structural and chemical-physical features of the coated and uncoated screw-machine surfaces. The measurement techniques at the test bench dynamically study the tribological elementary processes at the main and secondary rotor of a screw-machine pair.The understanding about the structural and chemical-physical surface properties allows for tailoring the parameters of the contact situation in order to control the elementary processes aiming at enhancing the lifetime of the solid lubricant and reducing the energy input at minimized friction. The correlation of the experimental investigations with a mechanical simulation, which focuses on the material removal and frictional tension in dependence on the duration of load, will explain how and in which operational states of the screw machines a protective tribological layer is formed. Knowledge shall finally be transferred to solid-lubricated alternative components.