World Tribology Congress 2013 Torino, Italy, September 8 – 13, 2013
Effect of Organic Lubricant Additives on the Tribological Performance and Tribofilm Formation in Diamond-Like-Carbon Coatings Hongyuan Zhao1)*, Joe, Lanigan1), Ardian Morina1), Anne Neville1), Thomas Polcar2), Albano Cavaleiro2) and Frederic Meunier3) 1)
iETSI, School of Mechanical Engineering, University of Leeds, Leeds, UK FCTUC - Universidade de Coimbra, Portugal (Current postal address for Thomas Polcar is University of Southampton, UK) 3) Sulzer SOREVI SAS, Limoges, France * Corresponding author:
[email protected]
1. Introduction Diamond-like-carbon (DLC) coatings have been very popular for material engineers and researchers in the tribological world for the last decade. DLCs are used in powertrain parts, valvetrain, fuel-injection components, piston- and cam-systems by more and more automotive manufacturers. Hydrogenated DLC has shown good low-friction and wear performance under different test conditions [1-3]. Other studies reported improved tribological performance and additive adsorptions on the hydrogenated DLC surface by adding W-dopant [3, 5, 6]. Also, increase of Si content in the DLC is shown to improve coating tribology performance [4]. However, considering that most of the engine oil formulations available in the market are optimised to lubricate ferrous surfaces, their effectiveness in lubricating DLC coated systems is still not fully clear. In the current study, an organic additive has been added into the engine oil formation in order to assess the effects on the friction and wear modifications on DLCs. Results showed that the organic additive has varied influence on different DLCs.
2. Methodology Hydrogenated DLC, W- and Si-doped DLC films were deposited on steel substrates. Coated samples were tested against steel pins using a TE-77 tribometer under boundary lubrication conditions. Current diesel engine oil formulation with extra organic additive (Oil AC1) was used as the test lubricant. The original oil formulation was also tested as a reference (Oil A). Friction was measured during the test. Wear and tribofilm chemical composition were measured by interferometer, Time-of-Flight Secondary Ion Mass Spectrometry (TOF SIMS), and X-ray Photoelectron Spectroscopy (XPS).
3. Results and Discussion Below are shown only few representative results
from this study, while the paper to be presented in the WTC contains detailed discussion of the complete results. 3.1. Friction and Wear Results Figure 1 shows the friction and wear performance of all tested materials with both lubricants. Coated systems showed lower friction than uncoated in all material/lubricant combinations. Not surprisingly, the extra organic friction modifier effectively reduced friction of uncoated lubricated system. The friction performance on coated systems seems to be affected by the nature of the coating but generally showing that the organic friction modified used in this study is as effective in friction reduction in coated systems as it is in uncoated systems. H-DLC had the lowest wear in this series of tests. The wear was barely measurable after 2 hour test, with no significant effect of the lubricant used. 0.14
Coeffcient of Friction, μ
2)
W‐DLC
0.12 Si‐DLC
0.1 0.08
Steel
H‐DLC
0.06 0.04
Oil A
0.02
Oil AC1
0 0
0.2
0.4
0.6
Wear Volume (e‐6 μm^3)
Figure 1 Friction and wear results of the DLC/steel plates.
In contrast, Si-DLC showed higher wear in Oil AC1 than in Oil A, indicating that the extra friction modifier did not provide better tribological performance to Si-DLC compared with the original Oil A. The surface analysis results on the tribofilm is used to study the mechanism. It is noted that Si-DLC experienced highest
wear in the three DLC surfaces in this study.
3.2. Surface Analysis Results - XPS Surface analysis results (included in the WTC paper) show that the Oil AC1 and Oil A results in formation of Ca, P, S and Zn tribofilm on the wear track. One key observation was higher C 1s concentration seen in Oil AC1 wear track. It is noted that the extra additive in Oil AC1 is organic, which is most likely to provide higher carbon-related molecules adsorption at the surface. On the other hand, the higher concentrations of Ca, N, P, S and Zn of H-DLC Oil A wear track indicate that the P, S and Zn containing additives in the original formulation are effective on the tribofilm formation on DLC. Oil A also provided good tribological performance to H-DLC as shown in Figure 1.
properties of DLC/DLC contacts with ZDDP. Wear, 2013. 298-299(1): p. 109-119. 4. Ikeyama, M., et al., Effects of Si content in DLC films on their friction and wear properties. Surface and Coatings Technology, 2005. 191(1): p. 38-42. 5. Kalin, M., et al., Metal-doped (Ti, WC) diamond-like-carbon coatings: Reactions with extreme-pressure oil additives under tribological and static conditions. Thin Solid Films, 2010. 518(15): p. 4336-4344. 6. Yue, W., et al., Influence of W content on microstructural, mechanical and tribological properties of sulfurized W-doped diamond-like carbon coatings. Surface and Coatings Technology, 2013. 7. Ban, M., et al., Tribological characteristics of Si-containing diamond-like carbon films under oil-lubrication. Wear, 2002. 253(3-4): p. 331-338.
Si-DLC also showed higher C 1s concentration in the Oil AC1 wear track than in Oil A, suggesting that the organic-rich tribofilm does not work as good as the inorganic-rich tribofilm provided by Oil A on Si-DLC surface. In this paper, it is found that P, S and Zn-containing additives provide better friction performance than the organic additive on Si-DLC surface. The complete results will be discussed in the WTC paper.
4. Summary The focus of this paper is to understand how the organic lubricant additive work on different DLC surfaces compared with the original engine oil additive package. This is important because most of the engine oil formulations now are designed for ferrous surfaces. The results in this paper show that different DLCs may have different performance when the organic additive/friction modifier is used.
5. References 1. Jahanmir, S., A.Z. Hunsberger, and H. Heshmat, Load capacity and durability of H-DLC coated hydrodynamic thrust bearings. Journal of Tribology, 2011. 133(3). 2. Keunecke, M., et al., CrC/a-C:H coatings for highly loaded, low friction applications under formulated oil lubrication. Surface and Coatings Technology, 2012. 207: p. 270-278. 3. Vengudusamy, B., et al., Influence of hydrogen and tungsten concentration on the tribological `
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