The Cerium Oxide nanoparticles developed by Nanum Nanotecnologia can be used as an additive for lubricants used in metal-mechanical applications. The addition of Cerium Oxide nanoparticles results in reduction of friction and wear.
The lubricant on the market that demonstrates the best performance with the addition of nanosized Cerium Oxide particles is Lithium Grease, most used today in aerospace applications and machinery in general.
The Department of Mechanical Engineering, at the Anyang Institute of Technology, China, studied the tribological behavior of lithium-based greases with different levels of Cerium Oxide nanoparticles, also considering the influence of temperature on the tribological properties and compared the grease with and without CeO2 nanoparticles additive.
Results have confirmed that lithium grease with CeO2 nanoparticles present a better friction reduction and wear performance than grease without CeO2 nanoparticles additive, that result a 28% reduction of friction coefficient, in optimal operating condition of 50°C and grease addition of 0.6% of CeO2 nanoparticles additive by weight.
Characteristics of the Grease used in the study:
Density (20 °C): 950 Kg/m3
Spill point: 380°C
Viscosity degree: 3
Kinematic viscosity (40°C): 380 mm2/s
The tribological test of the study was carried out in a Universal Friction and Wear Test Machine MMW-1A (Zhongke Kaihua Technology Development Co., Ltd., Lanzhou, China). According to the study, the upper ball and the three lower fixed ones were all made of GCr15 steel (diameter 12.7 mm and hardness 59~61 HRC). During the test, the upper ball was pressed against the three lower ones (fixed).
Influence of temperature on greases
With the increase in temperature, the hardness of the GCr15 steel tends to decrease, which can lead to an increase of wear of the parts used in the tests. Therefore, there is an optimal temperature for the performance of friction test, that best balances the grease lubricating properties and the hardness of the steel. The best performance was obtained at 50°C.
During the performance of friction test (at 75°C) with the incorporation of CeO2 nanoparticles to lithium grease, the Ce and O elements overlay on the worn surface of GCr15 steel. It means that occurred the Cerium Oxide is incorporated by adsorption (or deposition) creating a protective surface, reducing the friction and wear of the tribological pairs during the test.
In the study, the particle size of CeO2 used was of 500 nm (maximum). The non-uniform size distribution and irregular morphology of CeO2 increases the probability of smaller and irregular particle formation to fill and repair the worn surface under the grinding action of friction pairs. Particles on a nanoscale can easily adsorb or deposit on the metal surface.
Thus, when CeO2 nanoparticles are added to lithium grease, they can be easily transferred to the metal surface and even to the steel worn area during the friction process. Under lubricating conditions, CeO2 additives can share part of the compression stress and thus form a solid state self-laminated protective film to micro-polish and self-repair the friction surface. The nanoparticles rescan the rough areas and fill the valleys. Therefore, the Ce and O elements could enrich on the worn surface of the materials.
In the study, the average nanoparticles size is 500 nm, while the ones of Nanum range from 5 to 10 nm in size and, when dispersed, reach up to 40 nm, presenting even better results than that of the study in question.
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