Evaluation of tribological performance of oxide nanoparticles in fully formulated engine oil and possible interacting mechanism at sliding contacts

Abstract

The oxide nanoparticles have been claimed as the lubricant additive, which are capable of alleviating wear and reducing friction for the engine components. However, how oxide nanoparticles speficially behave together with the representative additive (Zinc dialkyl dithiophosphate, ZDDP) in the formulated engine oil, is still poorly understood. The results well revealed that the addition of oxide nanoparticles provided the improved lubricity of engine oil and significant resistance to wear. The Al₂O₃ nanoparticle specifically rendered superior tribological capability compared with other nanoparticles. Furthermore, the differentials of wearing mechanism, tribofilm and interface hierarchy were clarified by means of the morphological observation and compositional characterization. This study allowed us to understand the interacting mechanism when nanoparticles being involved within commercial engine oil was presenting between the sliding contact.

Introduction

By reducing mechanical failures in association with excessive wear and friction, significant energy saving and less materials loss occur in machinery equipment. As estimated in the engine system, the energy assigned to the mechanical friction accounts for as much as 33% of the efficiency loss, which is responsible for 25% of CO₂ emissions in the road transportation [1]. Existing oil lubricants for engine always contain different additives, which work together to deliver the better performance of machine components. In particular, ZDDP has been involved in engine oil formulation, and it functions against the wear and oxidation under the boundary lubrication condition [2,3]. In this study, the oxide nanoparticles are primarily considered as the supplementary additives to justify the potential capacity in reducing friction and wear of commercially formulated oil for engine.

Considerable literatures on formulating oxide nanoparticles as the colloidal additives into base oil have been published, which enables improving the heat transfer capability and thermal stability of engine oil [4], reducing friction and wear between the sliding contacts [5,6], even reducing the fuel consumption significantly [7]. However, theirs friction-reducing and anti-wear capabilities are determined by different ambiances (temperature, load, sliding speed, morphology, size, and concentration of nanoparticles, etc.) [8,9]. It has been found that the nanoparticles have the spherical geometry with the size less than 100 nm, which permit them being sufficiently entrapped into the contact regions, desired friction and wear performance happens between the rubbing contact under the boundary/mixed lubrication regime. In addition, the presence of nanoparticles can avoid unexpected seizures between the sliding contacts. Various working mechanisms, the rolling effect mostly from smooth and spherical particles [10], a protective film from the deposition of nanoparticles as a result of tribochemical reactions [11], the mending effect of the minimal size less than 100 nm [12], the polishing effect [13], and/or the combinations of above mentioned [9], have been proposed to reveal how the nanoparticles behave in the sliding contacts. It is commonly concluded that the ultimate behavior in tribology is closely associated with the physics and chemistry of nanoparticles, as well as theirs interaction with the surrounding media [8,9]. For the nanoparticle additives in engine oil, because the complex interaction and compatibility of nanoparticles along with existing additives present, either synergy or antagonism happens. Sgroi's group has attained the impressive values from the engine- and vehicle-level tests of lubricant oil with MoS₂ nanoparticles, e.g. a significant friction reduction by 50% in tribological lab-scale experiments was achieved [14], further benefits in terms of lower particulate emission and no additional SO₂ from the decomposition of MoS₂ was provided [15]. Moreover, low-dimension carbon materials, e.g. graphene and carbon nanotube, have been applied as the lubricant additives for oil, they demonstrated the unique capability of reducing friction and lessening wear [16,17]. However, some researchers claimed that the addition of carbon nanoparticles with a high amount would deteriorate the polyphosphate tribofilm [8,9]. As for the effect of concentration of nanoparticles blended within lubricating oil on resulting tribological properties, it has been discussed in Reference [18]. The addition of nanoparticles, either too much or too little, would result in the unexpected friction increase and wear deterioration. The optimum concentration of nanoparticle for the lubricating oil varies with the changing condition, thus is the system specific. Until now it is worth arguing that whether oxide particles perform positively in combination with ZDDP and other types of additive in engine oil, and there exist only a few reports concerning the synergy or antagonism of oxide nanoparticle/ZDDP tribofilm on wear mitigation and effective lubrication.

In this study, the objective is to experimentally investigate the friction and wear of oxide nanoparticles within fully formulated engine oil, where Al₂O₃, TiO₂, SiO₂, and ZrO₂ nanoparticles are blended into commercially formulated engine oil (SAE 10W30) without any modification. The interaction of nanoparticles with resulted polyphosphate/sulfide matrix and the interfacially structural transition was characterized by the SEM/EDS and FIB/TEM/EDS techniques. Furthermore, the interacting mechanism between oxide particles and zinc polyphosphate was correspondingly speculated for the determination of friction and wear properties and, for clarifying how adding particles inlfuences the tribological interface.