Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

Abstract

MoS 2 -Sb 2 O 3 -C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a ''chameleon'' coating. Coatings used in this report were applied by burnishing mixed powders of MoS 2 , Sb 2 O 3 and graphite. The solid lubricant MoS 2 and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb 2 O 3 was used as a dopant because it acts synergistically with MoS 2 , improving friction and wear properties. The MoS 2 -Sb 2 O 3 -C composite films showed lower friction and longer wear life than either single component MoS 2 or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS 2 -Sb 2 O 3 -C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb 2 O 3 , which is not lubricious, was covered by the active lubricants (MoS 2 -dry, C -humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb 2 O 3 particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.

Transmission Electron Microscopy Analysis of Mo–W–S–Se Film Sliding Contact Obtained by Using Focused Ion Beam Microscope and In Situ Microtribometer

“…It implies that the surface material modification occurred by atomic rearrangements in the sliding interface and resulted in the recrystallization and reorientation of the film. This data from the sliding contact sample without counterpart separation is consistent with prior cross-sectional TEM [24,25,[31][32][33] and X-ray diffraction [48] measurements on separated contact samples, and undoubtedly validates the prior results. The tribological film formation could be confined in a few atomic layers in terms of the recrystallization and reorientation of (002) basal planes.…”

“…Cross-sectional TEM showed that the hexagonal MoS 2 -basal planes aligned parallel to the wear scar surface. This was observed in magnetron sputtered MoS 2-x films [24], ion beam sputtered MoS x films [25], dual ion beam deposited Pb-Mo-S films [31], burnished MoS 2 /Sb 2 O 3 /C composite films [32], and PLD MoS 2 /Sb 2 O 3 multilayer films [33]. The post-test analyses inevitably involved the separation of sliding contact surfaces as part of the sample preparation procedure.…”

A Possible Link Between Macroscopic Wear and Temperature Dependent Friction Behaviors of MoS2 Coatings

“…All of the normal and frictional forces are measured using a multiaxial load-cell that is rigidly mounted to the pin sample. The stage and stepper motor assembly is used to position the sample under the microscope objective develop a thin layer of MoS 2 basal planes oriented parallel to the surfaces during running-in, after which the wear process is a minimum [29]. The established interfacial surfaces relative motion is then likely governed by surface potential interactions of MoS 2 sliding against itself.…”

Synthesis of MoS2-Au nanocomposite films by sputter deposition.

“…A number of metal or oxide dopants in MoS 2 have also been successfully tried. Notable examples of dopants include: Ti [17][18][19][20][21], Al [22,23], Ni [24][25][26], Au [8,24,26], Pb [24,27,28], PbO [24,29,30], Sb 2 O 3 [25,[30][31][32][33][34][35]. The presence of these dopants can lead to increased coating density, hardness, and oxidation resistance.…”