Lubrication in Desert Environments: Oil-Soluble Organo-Silver Molecules Designed for In-Situ Deposition of Metallic Silver at High Temperatures (2016)

The quest for improved engine performance and reduced emissions drives the design of increasingly sophisticated lubrication technologies. Lubricating oils and greases are engineered to function over a broad range of temperatures and loading conditions. Modern engines operate at higher temperatures, speeds and pressures than previous engines, and therefore require lubricants capable of handling harsher conditions. Reliable performance in extreme conditions is also necessary in emergency and combat situations. Thus, a major challenge for next-generation lubrication technology is to improve performance at extreme temperatures exceeding the thermal degradation limits of conventional engine oils. In automotive engines, the surface temperature of critical tribological components can easily reach 200°C, while asperity contacts can generate ‘flash temperatures’ up to 1000°C. These extreme pressures and temperatures in the contact zones can lead to plastic deformation, wear away mating surfaces, and catalyze chemical reactions which damage the surfaces and lubricant. Tests carried out on PAO4 and 15W40 motor oils show that they decompose at 275°C, irreversibly losing viscosity and generating oil-insoluble acids and salts that corrode surfaces and form sludges. Surface coatings, such as diamond-like carbon, and texturing can be used to reduce friction at temperatures which lead to motor oil thermal degradation. However, such treatments are costly for large components, and these coatings cannot be replenished without dismantling the treated machinery. Soft metal ductility can also be utilized in lubrication. The low shear-strengths of metallic films can form smooth “glaze layers” on tribosurfaces which lubricate sliding contact. Noble metals have oxidative stability, enabling lubricious performance at extreme temperatures. Silver-coated contact surfaces exhibit reduced friction and wear from 25–750°C. However, a method is required to dissolve metallic silver precursors in base oil for deposition at high temperatures. Silver nanoparticles are known to increase surface fatigue life, decrease friction, and wear, and work synergistically with other lubricant additives. However, silver nanoparticles are expensive, difficult to suspend in nonpolar media, and typically require a surfactant to prevent agglomeration. An alternative, described here, is to use a silver-containing molecular precursor. Organic ligands impart solubility to silver atoms and control the organosilver complex decomposition temperature to deposit silver only when and where it is needed. Controlled silver deposition is arguably more economical than full protective coatings. Also, a lubricant additive can be replenished during oil changes to provide more lubricious silver to high asperity engine contact regions. We report here the synthesis, characterization, and tribological implementation of a silver-pyrazole complex, silver 3,5-dimethyl-4-n-hexyl-pyrazolate (HPzAg)3. This complex is oil-soluble and undergoes clean thermolysis at ∼310°C to deposit lubricious, protective metallic silver on mechanical surfaces. Temperature controlled tribometer tests show that an optimized 2.5 wt% (HPzAg)3 loading reduces wear by 60% in PAO4 (poly-α-olefin lubricant) and 70% in a commercial fully-formulated motor oil (military grade 15W40). This organosilver complex also imparts sufficient friction reduction that the tribological transition from oil as the primary lubricant through its thermal degradation, to (HPzAg)3 as the primary lubricant, is experimentally undetectable.