Among friction modifier lubricant additives, molybdenum dialkyldithiocarbamate (MoDTC) provides excellent friction behavior in boundary lubricated tribocontacts. It is well established that the low friction obtained with MoDTC is as a result of the formation of lattice structure MoS 2 nanosheets. However, the relationship between the molybdenum species quantity, its distribution on the contact surface, and the friction behavior is not yet fully understood. In this work, Raman microscopy and atomic force microscopy (AFM) have been used with the aim of understanding the link between the friction behavior and the MoDTC/ZDDP tribofilm formation and removal. Tribotests were coupled with a collection of ex-situ Raman intensity maps to analyze the MoS 2 tribofilm buildup. Post-test AFM analyses were implemented on the ball wear scar to acquire the average MoDTC/ZDDP tribofilm thickness. In-situ Raman spectra analyses were carried out to detect the MoS 2 tribofilm removal. A good correlation was achieved between the friction coefficient measurements and Raman maps when using a linear relationship between the microscopic friction and the local amount of MoS 2 tribofilm. After a rapid increase, the average MoDTC/ ZDDP tribofilm thickness levels out to a steady state as the friction drop ceases. The removal rate of MoS 2 from tribofilms, obtained at different temperatures, suggests that the MoS 2 tribofilms are much easier to remove from tribocontacts compared to antiwear ZDDP tribofilms. This is the first study that sets out a framework to link MoS 2 amount and coverage to the friction behavior, providing the basis for developing numerical models capable of predicting friction by taking into account tribochemistry processes.
This study investigates the influence of chromium and molybdenum content (Cr: 20–35 wt%; Mo: 5 and 10 wt%) in a cobalt–chromium–molybdenum alloy system on the phases present in the microstructure, the nanomechanical, and electrochemical behavior of the alloys. A linear increase in the hardness is found in relation to the increasing chromium content for both 5 and 10 wt% Mo contents, whereas reduced modulus demonstrates an inflection at 30 wt% Cr, which is influenced by the molybdenum content. The open‐circuit potential, polarization resistivity, and linear sweep voltammetry disclose that passivity of Co–Cr–Mo alloys improves in line with increasing Cr until 30 wt%, with any further increase causing a depreciation in the corrosion properties. X‐Ray diffraction and optical metallographic analysis confirm that as the chromium content transitions through the 25–30 wt% range, the sigma (σ) phase begins to develop within the combined matrix of hexagonal close pack martensite (ε) and face‐centered cubic austenite (γ) phase.
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