Minimizing friction and wear at a rubbing interface continues to be a challenge and has resulted in the recent surge toward the use of coatings such as diamond-like carbon (DLC) on machine components. The problem with the coating approach is the limitation of coating wear life. Here, we report a lubrication approach in which lubricious, wear-protective carbon-containing tribofilms can be self-generated and replenishable, without any surface pretreatment. Such carbon-containing films were formed under modest sliding conditions in a lubricant consisting of cyclopropanecarboxylic acid as an additive dissolved in polyalphaolefin base oil. These tribofilms show the same Raman D and G signatures that have been interpreted to be due to the presence of graphite- or DLC films. Our experimental measurements and reactive molecular dynamics simulations demonstrate that these tribofilms are in fact high-molecular weight hydrocarbons acting as a solid lubricant.

MoS 2 thin films have been deposited onto 52100 steel substrates by aerosol-assisted chemical vapor deposition using the metal-organic precursor tetrakis(diethyl-dithiocarbamato)molybdenum(IV) (1). Analysis of the films indicates growth of an initial, highly crystalline FeS layer exhibiting preferred orientation parallel to the substrate, followed by growth of MoS 2 nanoparticles. Friction coefficients for MoS 2 -coated steel specimens reach 0.10 when tested at 100 °C in air. Tetrakis(tert-butylthiolato)titanium(IV) (2), dissolved with complex 1 in THF solutions, yields nanoparticle films of variable TiO 2 content as a result of solvent decomposition on MoS 2 and subsequent in situ reaction with the titanium precursor.

Mixed lubrication is a mode of fluid lubrication in which both hydrodynamic lubricant film and rough surface asperity contact coexist. Mixed lubrication problems are usually associated with significant surface roughness effect. A common belief is that full-film lubrication occurs when the ratio, defined as average film thickness divided by composite root mean square roughness, is greater than 3.0, while boundary lubrication corresponds to < 0.5-1.0. Mixed lubrication, therefore, is roughly in the range 0.5-1.0 < < 3.0. However, these considerations were established long ago based on early stochastic analyses, which did not adequately consider rough surface asperity interaction and correlation, as well as reduction of asperity heights caused by surface deformation. Recent experimental studies and deterministic numerical simulations suggested that the ratio range of mixed lubrication needs to be re-visited. Actually, when the ratio is greater than 0.6-1.2, little or no asperity contact is found in either experimental results or numerical solutions. If is around 0.05-0.1, there may still be a considerable portion of load, e.g. greater than 10-15%, being supported by lubricant films. It appears that mixed lubrication spans a ratio range roughly from 0.01-0.05 up to 0.6-1.2, according to the numerical simulation results presented in this article. This estimated range is in a reasonably good agreement with experimental observations found in the literature.

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