An Investigation on Wet Tribology of Dispersed Solid Lubricants in The Presence of Surfactant

Abstract

The use of solid additives to enhance the properties of lubricating oil has been gaining immense interest in various automotive and metal working applications, owing to a preference towards the modernization of mechanical components and their compatibility with the environmental friendly additives. These factors help in reducing the friction as well as wear, which in turn, facilitate the enhanced life of the mating parts and improved energy efficiency of systems. Various particles such as MoS2, WS2, Cu, CuO, graphite, hexa-boron nitride (h-BN), soot, graphene, diamond-like carbon (DLC), etc are being tested as potential oil additives. Recently, tribologists have concentrated on tubular structured solid additives such as carbon nanotubes (CNTs) because of their high aspect ratio, flexibility, and unique chemical, optical and mechanical properties. It is worth to mention here that the major obstacle for the usage of micro/nanoparticles in the tribological arrangements is their poor solubility and speedy agglomerate formation, which results in excessive wear and depletion in tribo performance. This problem can be handled by functionalizing the nanoparticles or using the surfactants and dispersants. In this context, it is worthwhile to mention that the target tribo-pair plays a prominent role in the selection and compatibility of particle additives for their potential utilization. To overcome some of the limitations associated with the use of Al-alloys in the tribological applications, the development of AMMCs are getting growing interest amongst the researchers. AMMCs are potential candidate materials for various applications due to their superior properties such as high specific strength and thermal conductivity, better corrosion and wear resistance, low cost, low coefficient of thermal expansion and high toughness. A few examples related to the commercial usage of AMMCs for the tribological application include: Al-SiCp composite material by Duralcan, Martin Marietta and Lanxide for making of pistons; Saffil reinforced AMMC diesel engine pistons by Toyota; AMMC cylinder liners by Honda; Nissan’s Al–SiC connecting rods; and SiC-reinforced aluminium brake rotors. Thus, composite tribocontact such AMMC/steel or AMMC/CI tribopair is an important domain of industrial research in-line with the demand of improved engine tribology, as well as other metal working applications. However, most of the reported studies are related to dry sliding conditions. In the applications where lubricated contact exists, such as between piston-cylinder and metal-working applications, it requires a great deal of research to gain insight on the tribological behaviour of these developed composites. The situations in these cases can be effectively dealt, by tailoring the material properties to suit these applications along with the implementation of an improved lubrication with the help of nano/micro lubricants. It is seen that the antiwear properties of composites are significantly enhanced by the use of various reinforcements, however, antifriction properties of self-lubricating composites are still beyond the reach of the desired level to suit the specific applications, such as that required to overcome the friction loss in IC engine components and surface finish with a MQL lubricated condition in metal-working applications. Studies using base mineral oil have their own importance because such studies set the preliminary insight for an additive to be used as anti-wear (AW), anti-friction (AF) or both. Moreover, the effectiveness of lubricant formulation using the solid particle additives is influenced by particle’s dispersion capability and stability in the suspension. This is often enhanced by functionalizing the nanoparticles or using surfactant and dispersants. Moreover, application of surfactant functionalized solid additives in the development of liquid lubricants involves interesting and intricate features which should be taken care of while developing or formulating such type lubricants for the composite materials, such as AMMCs. However, such studies involving functionalized nanodispersions while improving the tribological behaviour of composite surfaces are also not available in the literature. Thus, a comprehensive study is necessary to get the insight of the tribological behaviour of particle-laden lubricant and to compare with fully formulated commercial oils so that the findings can be suitably accommodated while developing the novel commercial oil package for any specific applications involving composite materials. Along-with the blending of base oil with various additives, great care should be taken in the detailed investigation of the microstructural, morphological and phase analysis on the lubricating contact surfaces. Thus, the present research work will be directed towards the combinatorial approach of utilizing the benefit of composite materials and particle-based lubrication to attain the improved tribological properties. For the present research work, a systematic attempt is made to explore and address some of the fundamental issues related to the formulation and modification of a particle-based lubricant for the lightweight composite material, using the selected solid lubricant and surfactant. Based on this proposed rationale, the authors carried out a comprehensive investigation on the influence of surfactant functionalized particle-based lubricants on the tribological characteristics of h-AMMC/Steel tribopair. For the preliminary screening, three grade-I base oils, two SAE grade fully formulated industrial oils, and four types of particle additives are used for the tribological investigation. Based on the detailed pilot study, a suitable grade of base oil (SN500), a fully-formulated commercial oil (SAE 20W50), and a solid particle additive (MWCNT) are finalized for the comprehensive tribological investigation. Comparative tribological characteristics of both hAMMC/EN31 and hAMMC/CI tribopairs has been analyzed based on the worn-out fractography. This is followed by a detailed investigation on the tribological performances of MWCNT-in-oil and surfactant functionalized MWCNT-in-oil. For the better insight of the lubrication and friction-wear mechanism, thermophysical properties of prepared suspensions and characterization of the worn-out surfaces of h-AMMC are carried out using various techniques such as SEM, EDS, XRD, and Raman spectroscopy. To functionalize the MWCNT particles, Sorbitan mono-oleate (SPAN-80) is used in MWCNT-in-oil suspension. The study reveals some interesting insights on the tribological characteristics of MWCNT laden lubricants in the presence of surfactant, which might be connoted for the future development of the lubricants to be used in the industrial applications. An in-depth investigation of the underlying lubrication mechanism reveals that four primary sub mechanisms are responsible for the improved tribological characteristics of surfactant functionalized MWCNT-in-oil lubrication. Comparative performance analysis of prepared oil lubricants with commercially available formulated oils has been discussed in detail. Wear mechanisms map is also established in the case of functionalized lubricants based on the critical observations. Finally, the conclusions and the findings from the present research work have been summarized and useful recommendations are made for the future scopes of work.