Adaptive Mo 2N/MoS 2/Ag tribological nanocomposite coatings for aerospace applications
Reactively sputtered Mo 2N/MoS 2/Ag nanocomposite coatings were deposited from three individual Mo, MoS 2, and Ag targets in a nitrogen environment onto Si (111), 440C grade stainless steel, and inconel 600 substrates. The power to the Mo target was kept constant, while power to the MoS 2 and Ag targets was varied to obtain different coating compositions. The coatings consisted of Mo 2N, with silver and/or sulfur additions of up to approximately 24 at%. Coating chemistry and crystal structure were evaluated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), which showed the presence of tetragonal Mo 2N and cubic Ag phases. The MoS 2 phase was detected from XPS analysis and was likely present as an amorphous inclusion based on the absence of characteristic XRD peaks. The tribological properties of the coatings were investigated in dry sliding at room temperature against Si 3N 4, 440C stainless steel, and Al 2O 3. Tribological testing was also conducted at 350 and 600 °C against Si 3N 4. The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry, energy dispersive X-ray spectroscopy (EDX), and micro-Raman spectroscopy. During room temperature tests, the coefficients of friction (CoF) were relatively high (0.5-1.0) for all coating compositions, and particularly high against Si 3N 4 counterfaces. During high-temperature tests, the CoF of single-phase Mo 2N coatings remained high, but much lower CoFs were observed for composite coatings with both Ag and S additions. CoF values were maintained as low as 0.1 over 10,000 cycles for samples with Ag content in excess of 16 at% and with sulfur content in the 5-14 at% range. The chemistry and phase analysis of coating contact surfaces showed temperature-adaptive behavior with the formation of metallic silver at 350°C and silver molybdate compounds at 600°C tests. These adaptive Mo 2N/MoS 2/Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo 2N and Mo 2N/Ag coatings, hence providing a high potential for lubrication and wear prevention of high-temperature sliding contacts. © 2007 Springer Science+Business Media, LLC.
Publication Source (Journal or Book title)
Aouadi, S., Paudel, Y., Luster, B., Stadler, S., Kohli, P., Muratore, C., Hager, C., & Voevodin, A. (2008). Adaptive Mo 2N/MoS 2/Ag tribological nanocomposite coatings for aerospace applications. Tribology Letters, 29 (2), 95-103. https://doi.org/10.1007/s11249-007-9286-x