Lubrication characteristics and wear mechanism mapping for hybrid aluminium metal matrix composite sliding under surfactant functionalized MWCNT-oil

https://doi.org/10.1016/j.triboint.2019.106152Get rights and content

Highlights

  • Preparation of tribologically and thermophysically effective surfactant functionalized MWCNT-in-oil lubricant.

  • Development of a wear mechanism mapping for h-AMMC/steel pair sliding under SPAN80-MWCNT-oil lubrication.

  • Identifying the underlying friction-wear mechanism by fractography of worn-out pins.

  • Role of tribolayer and MMLs behind the favorable lubrication effect.

Abstract

The friction-wear response of surfactant functionalized MWCNT-in-oil suspension under hybrid aluminium metal matrix composite/EN31 contact is presented. Composites were fabricated, characterized and tribological tests were performed with a pin-on-disc tribometer. Prior to tribological testing, thermophysical and stability analysis of the prepared lubricants were carried out. The operating regimes of the lubricants were predicted based on the estimated film thickness and friction responses. It is observed that the film thickness ratio for surfactant-MWCNT-oil is increased by 47% which helped in the shifting of lubrication towards the mixed regime. The findings of the study reveals that there is a remarkable enhancement in tribological behavior when surfactant functionalized MWCNT-in-oil is used. Finally, a wear map of the underlying wear mechanisms is also presented.

Introduction

Application of solid particle additives in formulating liquid lubricants has been more demanding because of the diminishing effect of frictional forces and wears which causes harms in the various automobile components such as cylinder liners, piston rings or bearings. Various authors had experimentally tested different solid additives as potential oil additives such as metal dichalcogenides (MoS2/WS2), Cu, graphene, diamond-like carbon, etc.

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 [[1], [2], [3], [4], [5], [6]]. Some noteworthy literature reports are available on steel-steel tribo-contacts [[7], [8], [9], [10]]. Tribo-material surface play an eminent role in the selection and compatibility of potential solid additives. Particulate reinforced metal matrix composites (PRMMCs) are better in contrast to conventional monolithic materials owing to their high stiffness and strength, low-density, low cost of manufacturing via liquid metallurgy route [11]. Many authors have tribologically tested different reinforced composites for dry sliding conditions. Various manufacturers such as Duralcan, Martin Marietta and Lanxide had used aluminium-silicon carbide (Al-SiCp) composite material for making pistons [12]. Hence, Al-SiCp/steel tribopair is important for improved engine tribology.

Sliding wear behavior is characterized by different regimes such as mild, transition and severe [13]. Severe wear conditions are characterized by severe surface damage and a huge amount of material transfer from the surface, whereas the magnitude of surface damage is lesser in the mild regime of operation. Thus, the “safe wear operational region” refers to the mild wear regime and the severe wear regime is counted as an “unsafe wear operational region” [14]. Hence, the transition of these two regimes is important for tribological studies. A wear mapping method can be adopted to characterize the wear transition and identify the dominating mechanisms with respect to applied load and speed conditions.

The Taguchi approach based on design of experiments (DOE) and use of statistical software such as Minitab is a powerful contrivance for planning the data in an organized form and to study the effect of processing parameters on particular responses [15,16]. This approach has been effectively utilized for the examination of wear performance in metal matrix composites (MMCs) [17]. Regression analysis and contour plotting using the experimental data gives valuable inputs for wear mechanism mapping.

However, the tribological behavior of composites under the oil lubricated sliding condition is a less explored arena. The underlying lubrication mechanisms in such contacts are relatively complicated owing to the interaction of solid additive based liquid lubricants with composite surfaces. Moreover, the effectiveness and performance of lubricants using solid additives are tailored by using surfactants or dispersants. To the best of the author's understanding, some research reports are available on the tribological behavior of single-particle reinforced composite contacts under oil lubrication, with and without the use of additives [[18], [19], [20]].

Carbon-based materials such as graphite or multi-walled carbon nanotubes (MWCNTs) are characterized using Raman spectroscopy. Enormous information regarding structural defects, functionalization involved and tribolayer formation mechanism can be investigated by Raman spectroscopy by extracting the information from the peak bands such as D, G, 2D and their ratios i.e., ID/IG and I2D/IG [21].

In the present study, MWCNT based lubricant is prepared using the two-step method in the presence of a surface-active agent. Sorbitan mono-oleate (SPAN-80) amphiphile is selected to make particle suspensions. The thermophysical properties of prepared suspensions are also studied for a vivid understanding of lubrication mechanisms. For a clear understanding of the friction - wear mechanism, a wear map of the underlying wear mechanisms is also developed based on the statistical and wear track analysis. The worn-out hybrid aluminium metal matrix composites (AMMC) surfaces were characterized by various tools such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD) and Raman spectroscopy. The improved tribological properties of surfactant assisted MWCNT-oil dispersions connoted for its applications in advanced industrial routes.

Section snippets

Selection and characterization of materials

In the present investigation, Aluminium alloy-Al6061 (density = 2.7 g/cm3) is used as the base matrix and silicon carbide (SiC) with 10% weight fraction (average particle size = 1–100 μm and density = 3.2 g/cm3) along with of graphite (Gr) with 3% weight fraction (average particle size = 1–60 μm and density = 2.2 g/cm3) are used as reinforcing materials for the fabrication of hybrid composite (Al6061–SiC-Gr) [22,23]. The selection of materials was based on cost and the inherent mechanical

Rheological and thermal studies

From the rheological study (Fig. 3) of stable dispersions, it is noticed that viscosity of MWCNT based oils and surfactant assisted MWCNT based oils is considerably enhanced, as compared to fresh oil. This was due to a high exposure of surface area and the filament-like morphology structures of CNTs, their aggregation and large asymmetric geometric distributions in oil which led to restrict the movement of fluid layers. However, the quantitative increase in the dynamic viscosities of nanofluid

Conclusions

The main motive of the current investigation is to study the influence of surfactant-assisted MWCNT-oil suspensions on the tribological performance of hybrid composite (Al6061–SiC-Gr) sliding against steel pairs. Selection of suitable tribopair is performed by comparing the friction-wear performance of characterized hybrid AMMC (Al6061/SiC/Gr), AMMC (Al6061/SiC) and Al6061 pins sliding against EN31. Thermo physical properties of fresh oil and oil dispersions are also investigated. Tribological

Author Contributions Section

Harpreet Singh: Conceptualization, Methodology, Software, Data curation, Writing- Original draft preparation, Investigation, Analysis, Validation

Hiralal Bhowmick: Supervision, Resources, Reviewing -Editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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