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Research Papers: Mixed and Boundary Lubrication

# Effect of Friction Modifiers and Antiwear Additives on the Tribological Performance of a Hydrogenated DLC Coating

[+] Author and Article Information
T. Haque, A. Neville

Institute of Engineering Thermofluids, Surfaces, and Interfaces (iETSI), School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK

A. Morina1

Institute of Engineering Thermofluids, Surfaces, and Interfaces (iETSI), School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UKa.morina@leeds.ac.uk

1

Corresponding author.

J. Tribol 132(3), 032101 (Jun 16, 2010) (13 pages) doi:10.1115/1.4001650 History: Received December 07, 2009; Revised April 20, 2010; Published June 16, 2010; Online June 16, 2010

## Abstract

There has been a lot of attention on the effect of lubricant additives on the friction at carbon coated surfaces. But only few papers have addressed the effect of additives on the durability of some diamondlike carbon DLC coatings. This paper presents a systematic study assessing the additive/additive and additive/surface interactions, and their influences on the durability of a low hydrogen-containing ($15 at. %$ hydrogen) metal-free DLC coating (a-C:15H). In this study, lubricating oils containing a zinc dithiophosphate (ZDDP) antiwear additive and/or organomolybdenum friction modifiers (moly dimer and moly trimer) were used. Tribological tests were carried out in a pin-on-plate tribometer under boundary lubrication conditions. To understand the effect of additives, tribofilms formed on the wear tracks were analyzed using surface sensitive analytical techniques such as atomic force microscope, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. Results showed that the ZDDP formed a zinc phosphate containing an ultrathin antiwear tribofilm, which offered excellent durability/wear protection to the a-C:15H coating. However, the antiwear performance of this additive was compromised when it was used with moly dimer or moly trimer. Surface analysis revealed that unlike steel surfaces, $MoS2$ formed on the DLC surfaces had negligible influence on friction, while the low friction DLC wear debris had strong influence on friction. Abrasive wear was found to be the dominating wear mechanism in the cases when additives showed poor wear protection on the a-C:15H coating.

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## Figures

Figure 1

Structure of lubricant additives—(a) moly dimer or molybdenum dithiocarbamate (MoDTC), (b) moly trimer, and (c) zinc dialkyl dithiophosphate (ZDDP). The radical group (R) designates alkyl or aryl groups.

Figure 2

Schematic representation of the arrangement of samples and lubricant in the pin-on-reciprocating plate apparatus

Figure 3

(a) Friction coefficients as a function of time for all oils in a-C:15H/CI system; (b) mean steady state friction for various oils in a-C:15H/CI and UC steel/CI systems

Figure 4

Wear coefficients of a-C:15H coated plates and CI pins

Figure 5

SEM images of the wear scars formed on the a-C:15H coating using oils (a) A, (b) B1, (c) C1, (d) B2, and (e) C2. The arrows on the left side of the images show the sliding directions.

Figure 6

AFM images of (a) the bare surface of the a-C:15H coating, and tribofilms/worn coating formed by oils (b) A, (c) B1, (d) C1, (e) B2, and (f) C2. The average roughness of tribofilm/bare DLC coating are also shown on the respective image.

Figure 7

Examples of the XPS curve fitting. Mo 3d, S 2p, O 1s, and Zn 2p peaks recorded from the tribofilms formed by Oil C1 on a-C:15H coating

Figure 8

Depth profiling of ZDDP-derived antiwear tribofilms formed by Oil A: (a) UC steel/CI and (b) a-C:15H/CI system. The CPS in the vertical axis is the abbreviation of “counts per second”

Figure 9

Raman spectroscopic analysis on the samples obtained using Oil B2. Hd and Hg are the intensity height of D and G peaks, respectively. The colored boxes on the optical image of the CI pin and the SEM image of a-C:15H coating show the sites where the Raman analyses were carried out.

Figure 10

Friction coefficients as functions (a) of the Mo-sulphide/Mo-oxide ratio for UC steel/CI and a-C:15H/CI systems and (b) of wear for a-C:15H/CI system

Figure 11

Schematic diagram showing the dominance of the low friction species present at the interfaces in (a) UC steel/CI and (b) a-C:15H/CI systems. The other species present in the tribofilms are not shown here to avoid complexity.

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