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Research Papers: Lubricants

Rheological and Film Forming Behavior of the Developed Nanocomposite Greases Under Elastohydrodynamics Lubrication Regime

[+] Author and Article Information
Jayant Singh, N. Tandon

Industrial Tribology, Machine Dynamics and
Maintenance Engineering Centre (ITMMEC),
Indian Institute of Technology Delhi,
New Delhi 110016, India

Deepak Kumar

Industrial Tribology, Machine Dynamics and
Maintenance Engineering Centre (ITMMEC),
Indian Institute of Technology Delhi,
New Delhi 110016, India
e-mail: dkumar@itmmec.iitd.ernet.in

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received May 25, 2018; final manuscript received August 22, 2018; published online October 24, 2018. Assoc. Editor: Wenzhong Wang.

J. Tribol 141(2), 021804 (Oct 24, 2018) (10 pages) Paper No: TRIB-18-1206; doi: 10.1115/1.4041304 History: Received May 25, 2018; Revised August 22, 2018

Performance of grease lubricated point contact under elastohydrodynamics lubrication (EHL) regime is critical in many engineering applications. The present work deals with the evaluation of rheological, film forming characteristics and elastic recovery of newly developed nanocomposite greases. The nanocomposite greases are formulated by dispersing different nano-additives to bare grease (BG). The nano-additives include reduced graphene oxide (rGO) nanosheets, CaCO3 and α-Al2O3 nanoparticles. The microstructure evaluation of the nano-additives and different greases is done using high-resolution transmission electron microscopy (HRTEM). Estimation of the rheological parameters (storage and loss moduli) is done using rotational rheometer. The film forming behavior is recorded using elastohydrodynamic (EHD) rig for range of speed at different temperatures and constant load. The results indicate that change in microstructure due to nano-additive incorporation improves the responses of different greases. Based on rheological response, CaCO3 doped grease seems better but rGO doped grease is able to bear high shear stresses. Further, based on film forming characteristics and reflow or recovery behavior, rGO doped grease is better. The rGO-based grease registers approximately 90% elastic recovery followed by 75% for CaCO3-based grease, 65% for BG and 10% for α-Al2O3-doped grease.

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Figures

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Fig. 1

Schematic representation of grease lubricated point contact formed between steel ball and glass disk

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Fig. 2

High-resolution transmission electron microscopy images of (a) rGO nano-sheets, (b) CaCO3, and (c) α-Al2O3 nano-particles, with their respective size distribution in toluene ((d)–(f)) and the HRTEM morphology of fibrous network on their dispersion in BG (g) resulting in nanocomposite greases ((h) BG + rGO nanosheets (i) BG + CaCO3 and (j) BG + Al2O3 nanoparticles) after extracting the oil content [14]

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Fig. 3

Frequency sweep of the storage (G′) and loss moduli (G″) of different grease samples

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Fig. 4

Frequency dependence of the loss tangent (tan δ) of BG and optimized concentrations of the nano-additives in the BG

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Fig. 5

Variation of storage (G′) and loss moduli (G″) with respect to shear stress (τ) under the linear viscoelastic region of BG and optimum concentrations of the nano-additives in BG. X denotes the cross-over point.

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Fig. 6

Complex modulus (G*) decay for BG and nanocomposite greases (doped with optimum concentrations of nano-additives) at a frequency of 1 Hz, undergoing different shear stress from linear to nonlinear viscoelastic region, with recovery when former shear stress is induced again (10 Pa–1000 Pa–10 Pa)

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Fig. 7

Film thickness for BG and nanocomposite greases (doped with optimum concentrations (w/w %)) with a variation of rolling speed varying from 0.001–4 m/s at ((a), (c), and (e)) initial stage and ((b), (d), and (f)) matured stage at 25 ˚C, 60 ˚C, and 120 ˚C, respectively. Black dashed rectangle ((e) and (f)) denotes shifting of the lubrication regime toward the lower-speed side.

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Fig. 8

Variation of the film thickness with time at a constant rolling speed of 1 m/s at 25 ˚C. Stages 1–3 represent 60 min running with the interval of 20 min each. Also, the circular dot represents 5 min halt after the completion of the stage, for which the film thickness measurements are again resumed. The dotted rectangle represents the photographic images of the rolled tracks for different greases ((a) and (b) for BG, (c) and (d) for BG + 0.4% rGO, (e) and (f) for BG +5% CaCO3, (g) and (h) for BG +0.8% Al2O3) after completion of the experiment just after stage 1 and track replenishment after 5 min halt, respectively.

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Fig. 9

High-resolution transmission electron microscopy morphologies of used samples of (a) BG, (b) BG + 0.4% rGO, (c) BG + 5% CaCO3, and (d) BG + 0.8% Al2O3 after the completion of the experiment of film thickness for 1 h of run at 120 ˚C

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Fig. 10

Schematic representation explaining the effect of optimality of rGO nanosheets in the grease structure (a) at lower concentration and (b) at higher concentration

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