Experimental analysis of tribological properties of lubricating oils with nanoparticle additives

Polymer-assisted fabrication of nanoparticles and nanocomposites

Roles of nanoparticles in oil lubrication

The need for energy efficiency is leading to the growing use of additives that reduce friction in thin film boundary and mixed lubrication conditions. Several classes of such friction modifier additive exist, the main ones being organic friction modifiers, functionalised polymers, soluble organo-molybdenum additives and dispersed nanoparticles. All work in different ways. This paper reviews these four main types of lubricant friction modifier additive and outlines their history, research and the mechanisms by which they are currently believed to function. Aspects of their behaviour that are still not yet fully understood are highlighted.

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Friction Modifier Additives

Nanoparticles of WS2 and MoS2 with a closed cage structure (fullerene-like) that are termed IF phases are synthesized in large amounts in a pure form. These nanoparticles were shown to play a favorable role as solid lubricants under severe conditions where fluids are unable to support the heavy load and are squeezed away from the contact area. Various tribological scenarios are presented for these superior solid lubricants, demonstrating the large scale potential for applications of these materials. The mechanism of action of these solid lubricants is briefly discussed. Various other potential applications of IF phases for nanocomposites with high impact resistance; in rechargeable batteries and in optical devices are discussed in short.

Applications of WS2(MoS2) inorganic nanotubes and fullerene-like nanoparticles for solid lubrication and for structural nanocomposites

Behavior of fullerene-like WS2 nanoparticles under severe contact conditions

Recent experiments showed that the addition of inorganic fullerene-like (IF) WS 2 or MoS 2 solid lubricant nanoparticles in oil, grease or impregnated into porous matrix provides remarkable lubricating properties of friction pairs in a wide range of operating conditions. Inorganic nanoparticles filled polymer composites show low friction and high wear resistance. This work reports the first attempt to fill polymers with hollow solid lubricant nanoparticles. It was found that application of the IF solid lubricant nanoparticles leads to a considerable improvements in the tribological behavior of the nanocomposites. The morphology of the surface layers and the mechanism of the friction reduction by the IF nanoparticles are discussed.

Polymer nanocomposites with fullerene-like solid lubricant

Sedimentation of IF-WS2 aggregates and a reproducibility of the tribological data

Recently, the behavior of inorganic fullerene-like (IF) WS2 nanoparticles in the interface of steel-on-steel pair has been analyzed. It was shown that originally when the gap between the contact surfaces is smaller than the size of the IF nanoparticles, there is no effect of the nanoparticles on the friction force. During the test stiff IF nanoparticles can plough the surface of hard steel samples and penetrate into the interface under friction. Molecular sheets of WS2 from the delaminated IF nanoparticles, which reside in the valleys of the rough surfaces cover the contact spots and thus decrease the number of adhered spots at the transition to seizure. The goal of the present work was to study the behavior of IF nanoparticles in the interface of ceramic surfaces. The friction tests were performed using a ball-on-flat device. A silicon nitride ball was slid against an alumina flat with maximum contact pressure close to 2 GPa. SEM, TEM and AFM techniques have been used in order to assess the behavior of IF nanoparticles in the interface. The behavior of IF nanoparticles in the much harder ceramic interfaces was found to be appreciably different from the steel pair. The pristine IF nanoparticles are damaged in the inlet of the contact during the first few cycles and thin shells of broken nanoparticles gradually cover the middle range of the contact surface. Different modes of deformation and destruction of the IF nanoparticles are exhibited when going from the middle to edge area of the contact. While aggregates of the pristine nanoparticles are formed at the edge of the contact, thin shells of broken IF nanoparticles are observed in the middle area where contact pressure is maximum. Mechanical stability and damage of IF nanoparticles in the ceramic interface are discussed.

Friction and wear of fullerene-like WS2 under severe contact conditions: friction of ceramic materials

For a number of polymers with a variety of chemical structures and different properties, we have performed scratch-resistance tests and investigated the profiles of the grooves formed using a profilometer. Three main kinds of material response are seen: plowing; cutting; and densification. The cross-sectional areas of the grooves include the groove and side top-ridge areas. The latter are smaller than the former, an indication of densification at the bottom and the sides of the groove; the effect can be connected to molecular dynamics simulations of scratch testing. The sum of the groove and top-ridge areas is the highest for Teflon, thus providing another measure of its poor scratch resistance. The Vickers hardness of the polymers was also determined. An approximate relationship exists between the hardness and the groove area. An unequivocal relationship between the hardness and the total cross-sectional area of the material displaced by the indenter is found. The resulting curve can be represented by an exponential decay function.

A. Verdyan

Papers

Behavior of fullerene-like WS2 nanoparticles under severe contact conditions

Polymer nanocomposites with fullerene-like solid lubricant

Sedimentation of IF-WS2 aggregates and a reproducibility of the tribological data

Friction and wear of fullerene-like WS2 under severe contact conditions: friction of ceramic materials

Grooves in scratch testing