Performance and anti-wear mechanism of CaCO3 nanoparticles as a green additive in poly-alpha-olefin

doi:10.1016/j.triboint.2009.02.012

Tribology International

Volume 42, Issue 7, July 2009, Pages 1029-1039

https://doi.org/10.1016/j.triboint.2009.02.012 Get rights and content

Abstract

This paper reports the tribological behavior of CaCO₃ nanoparticles as a green additive in poly-alpha-olefin (PAO) base oil under variable applied load, sliding speed, sliding duration, and temperature. The tribological properties and the electrical contact resistance between the tribo-pairs lubricated with PAO alone, and PAO containing CaCO₃ nanoparticles, were determined using an Optimol-SRV 4 oscillating friction and wear tester (SRV). The morphology and wear volume of the worn scar were measured simultaneously using a surface profilometer. The results showed that CaCO₃ nanoparticles can dramatically improve the load-carrying capacity, as well as the anti-wear and friction-reduction properties of a PAO base oil. In addition, higher applied load, moderate frequency, longer duration time, and lower temperatures are beneficial to the deposition of CaCO₃ nanoparticles accumulating on rubbing surfaces. X-ray photoelectron spectroscopy (XPS) reveals a boundary film composed of CaCO₃, CaO, iron oxide, and some organic compounds on the worn surfaces.

Introduction

The reduction of wear depends on interfacial conditions such as normal load, geometry, relative surface motion, sliding speed, surface roughness, lubrication, and vibration [1]. In addition, anti-wear properties, load-carrying capacities, and friction-reduction are mainly controlled by the chemical additives in lubricating fluid under boundary lubrication conditions. Since stabilization of nanoparticles has been resolved by the addition of a dispersing agent or the use of a surface modification preparation technique, inorganic nanoparticles have received considerable attention in the lubrication field. In particular, the tribological properties of metal oxides [2], [3], [4], [5], rare earth compounds [6], [7], [8], metals [9], [10], [11], [12], metal borates [13], [14], [15], metal sulphides [16], [17], [18] and silicon dioxide [19] used as lubricate additives have been investigated. Battez et al. [5] showed that the anti-wear mechanism of a metal oxide nanoparticulate additive was tribo-sintering of nanoparticles on the wear surfaces. That process reduced the metal-to-metal contact and created a load-bearing film. Wu et al. [20] summarized the mechanisms of the friction-reduction and anti-wear of nanoparticles in lubricants as the result of colloidal effect, rolling effect, protective film, and third body. Huang et al. [21] described their proposed mechanisms of nanoparticle additives using the ball effect, tribochemical reactions, and adsorption film theory. The results of these investigations indicate that nanoparticles used as a lubricating oil additive can improve the tribological properties of the base lubricant. However, most of the reported nano-additives contain either heavy metals, sulphur, or phosphorus as active atoms, which can be potential threats to the environment.

In the present work, we have synthesized a stable CaCO₃ nanoparticle with an eco-friendly or “green” modifier (triethanolamine monooleate) which gives very good dispersibility in organic solvents. In order to estimate the ranges of application of CaCO₃ used as an oil additive, it was necessary to investigate its tribological behavior under increasingly severe contact conditions. As a result, the tribological properties of CaCO₃ nanoparticles as a “green” oil additive in poly-alpha-olefin (PAO) under variable load, sliding speed, sliding duration, and temperature were evaluated. In addition, a dynamic dripping test was designed and used to discuss the possible lubrication and anti-wear mechanism.

Section snippets

Materials

A commercial PAO (SpectraSyn™ Poly-alpha-olefins, ExxonMobil) was used as the base oil without further treatment. The properties of the PAO are listed in Table 1.

The CaCO₃ nanoparticles were prepared via the following chemical co-precipitation method, in which triethanolamine monooleate was used as a surface modifier. An aqueous Na₂CO₃ solution (0.05 M) was added to aqueous CaCl₂ (0.05 M) with vigorous stirring, whereupon a white reaction mixture was formed. The mixture was allowed to stand for

Duration dependence

According to traditional tribological concepts, material wear loss of the tribo-pairs is often accelerated due to high friction [22]. However, the wear volume did not continue to increase with increasing test duration when CaCO₃ nanoparticles were used as the lubricating additives. When the PAO containing 1 wt% CaCO₃ nanoparticles was used as lubricant, the wear volume initially increased slowly then decreased after prolonged rubbing. When the duration of sliding was 30 min, the wear volume was

Conclusions

The lubricating ability of the CaCO₃ nanoparticles as an additive in the PAO depends on the experimental conditions. Friction-reduction, high load-carrying capacity, and anti-wear can be achieved if the CaCO₃ nanoparticles are present in the PAO under increasingly severe conditions such as extended duration, high load, high sliding frequency, and high temperature. The mechanism is attributed to the synergistic effect of the lubricant viscosity, and the deposition of the CaCO₃ nanoparticles

Acknowledgements

The authors acknowledge the financial support of this work by the China National 973 Program (2007CB607606), the Program of the Light in China's Western Region, and the Nature Science Foundation of Gansu (3YS051-A25-003).

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Performance and anti-wear mechanism of CaCO3 nanoparticles as a green additive in poly-alpha-olefin

Abstract

Introduction

Section snippets

Materials

Duration dependence

Conclusions

Acknowledgements

Wear

Wear

Tribol Int

Wear

Wear

J Rare Earths

Mater Sci Eng A

Trans Nonferrous Met Soc China

Wear

Wear

Tribol Int

Mater Res Bull

Wear

Performance and anti-wear mechanism of CaCO₃ nanoparticles as a green additive in poly-alpha-olefin