Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.

Reactively sputtered Mo 2 N/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 2 N, 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 2 N 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 3 N 4 , 440C stainless steel, and Al 2 O 3 . Tribological testing was also conducted at 350 and 600°C against Si 3 N 4 . The coatings and respective wear tracks were examined using scanning electron microscopy (SEM), optical microscopy, profilometry, energy dispersive X-ray spectroscopy (EDX), and microRaman 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 3 N 4 counterfaces. During high-temperature tests, the CoF of single-phase Mo 2 N 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 2 N/MoS 2 /Ag coatings exhibited wear rates that were two orders of magnitude lower compared to Mo 2 N and Mo 2 N/Ag coatings, hence providing a high potential for lubrication and wear prevention of hightemperature sliding contacts.

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