Research Papers: Tribochemistry and Tribofilms

MoDTC Tribochemistry in Steel/Steel and Steel/Diamond-Like-Carbon Systems Lubricated With Model Lubricants and Fully Formulated Engine Oils

[+] Author and Article Information
Cayetano Espejo

Institute of Functional Surfaces,
School of Mechanical Engineering,
University of Leeds,
Leeds LS2 9JT, UK
e-mail: c.espejoconesa@leeds.ac.uk

Benoît Thiébaut, Frédéric Jarnias

TOTAL, Centre de Recherche de Solaize,
Chemin du Canal,
Solaize BP 22-69360, France

Chun Wang, Anne Neville, Ardian Morina

Institute of Functional Surfaces,
School of Mechanical Engineering,
University of Leeds,
Leeds LS2 9JT, UK

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received January 29, 2018; final manuscript received July 20, 2018; published online August 24, 2018. Assoc. Editor: Xiaolei Wang.

J. Tribol 141(1), 012301 (Aug 24, 2018) (12 pages) Paper No: TRIB-18-1046; doi: 10.1115/1.4041017 History: Received January 29, 2018; Revised July 20, 2018

This work focuses on the tribochemistry of molybdenum dithiocarbamate (MoDTC) oil additive to improve friction behavior of diamond-like-carbon (DLC) coated systems lubricated in boundary regime. Raman microscopy has been used to investigate surface tribolayers formed on coated (hydrogenated a-C:H and non-hydrogenated ta-C) and steel surfaces when lubricated with model lubricants and commercial engine oils. The effect of the additive package and the type of DLC played a crucial role in the development and composition of the tribolayer and the friction performance. The additive package contained in the fully formulated (FF) oils limited the friction reduction capabilities of MoDTC additive for every material pair. Accelerated a-C:H coating wear related to MoDTC tribochemistry was found. For the first time, it has been shown that a distinctive MoS2-containing tribolayer can be formed on the ta-C surface, leading to a coefficient of friction lower than 0.04. The underlying mechanisms of MoDTC/surface interactions and their effect on friction and wear are discussed.

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Fig. 1

The schematic of the tribometer and dimensions of pin-on-disk parts

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Fig. 2

Molybdenum dithiocarbamate additive structure

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Fig. 3

Average steady-state friction for all the material-lubricant combinations used in this study

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Fig. 4

Friction coefficient versus sliding time in (a) steel/steel, (b) steel/a-C:H, and (c) steel/ta-C contacts

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Fig. 5

Wear coefficient for all the material-lubricant combinations used in this study. The dashed bars represent the disk wear. Disk wear was measurable only for the a-C:H disk when lubricated with BO+MoDTC lubricant.

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Fig. 6

Disk wear track micrographs

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Fig. 7

Contact profilometry on a-C:H coated disk samples tested using BO, BO+MoDTC, and FF+MoDTC lubricants

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Fig. 8

Raman microscopy on different surfaces tested using BO as a lubricant: (a) steel/steel, (b) steel/a-C:H, and (c) steel/ta-C

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Fig. 9

Raman analysis of (a) steel ball and (b) steel disk in BO+MoDTC lubricated test

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Fig. 10

Raman analysis of (a) steel ball and (b) a-C:H disk in a BO+MoDTC lubricated test

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Fig. 11

Raman analysis of (a) steel ball and (b) ta-C coated disk in a BO+MoDTC lubricated test

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Fig. 12

Raman microscopy on different steel balls tested using FF as a lubricant: (a) steel/steel, (b) steel/a-C:H, and (c) steel/ta-C

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Fig. 13

Raman microscopy on different steel balls tested using FF+MoDTC as a lubricant: (a) steel/steel, (b) steel/a-C:H, and (c) steel/ta-C

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Fig. 14

Proposed model for accelerated wear on a-C:H coating and friction-reducing tribolayer formation on ta-C coating using BO+MoDTC and a ferrous counterpart



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