MoS2 formation induced by amorphous MoS3 species under lubricated friction

Abstract

Amide molybdate has been recently introduced as a friction modifier for tribological applications. Combined with zinc dithiophosphate (ZDDP) and fatty amines, it provides an ultralow friction coefficient. The ultimate product of Mo compound transformations in tribological contact, due to frictional heating and shearing, as well as chemical interactions with oil additives, is molybdenum sulfide (MoS₂). Understanding the decomposition of amide molybdate leading to MoS₂ is of primary importance to the optimization of the design of lubricant formulations. This study focuses on the investigation by Raman spectroscopy of amide molybdate decomposition intermediates. Raman spectra of tribofilms, obtained after friction tests under different temperatures and pressures, revealed the formation of an amorphous MoS₃ intermediate coexisting with MoS₂. However, under severe conditions, the tribofilms are mostly composed of MoS₂.

Fig. 1. Lubricant additives: amide molybdate, zinc dialkyldithiophosphate (ZDDP), and fatty triamine.

Fig. 2. Example of an optical microscopy picture of an analyzed section (50 per 100 μm). During the mapping, each square of the image was analyzed.

Fig. 3. Evolution of the friction coefficient as a function of the cycle number under different conditions, as indicated in Table 1.

Fig. 4. Reference spectra of (NH₄)₂MoO₂S₂, (NH₄)₂MoS₄, (NH₄)₂MoS₄ decomposed at 200 °C under N₂, and (NH₄)₂MoS₄ decomposed at 500 °C under H₂.

Fig. 5. Raman mean spectra of the tribofilms obtained under different temperature and pressure conditions. The tribofilm spectra are compared to those of the bulk MoS₂ reference.

Fig. 6. Evolution of the friction coefficient vs. the proportion of spectra containing MoS₂ in the tribofilms. For this measurement, every spectrum containing MoS₂ was counted, independent of the presence of amorphous species. We fixed a threshold value of 1.1, corresponding to the intensity ratio of the MoS₂ A_1g band with respect to the amorphous species band located at 400–550 cm⁻¹, in order to consider the presence of MoS₂.

Fig. 7. Intensity of the A_1g band according to the location on the tribofilm obtained from the mixture of amide-Mo, ZnDTP, and triamine at 90 °C. The intensity increases from dark (no MoS₂) to bright areas (more MoS₂).

Fig. 8. Raman spectra of the tribofilm from the mixture of amide-Mo, ZnDTP, and triamine at 90 °C: (a) mean spectrum for each line (from 0 to 8), and (b) individual spectra of line 6.

Fig. 9. Raman spectrum of the amorphous species from line 6, in comparison with that of the MoS₃ amorphous species made via the decomposition of (NH₄)₂MoS₄ at 200 °C.