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Effect of the Slide-to-Roll Ratio and the Contact Kinematics on the Elastohydrodynamic Friction in Diamond-Like-Carbon Contacts with Different Wetting Behaviours

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Abstract

In this paper, we show how the slide-to-roll ratio (SRR), the contact kinematics and the surface energy all have important effects on the elastohydrodynamic friction. As reported previously, diamond-like-carbon (DLC) contacts of the type DLC/DLC provide the lowest coefficient of friction, in particular those DLC materials with the lowest surface energies (three different DLC coatings were used in this study). A friction reduction of up to 48 %, compared to a steel/steel contact, was obtained. A surprising new finding from this investigation is that the friction in DLC contacts is significantly reduced for high SRRs, i.e. a difference of up to 27 % was measured in the DLC/DLC contacts when the SRR increased from 0.5 (rolling prevails) to 1.8 (sliding prevails). However, even more surprising is the effect of the SRR on the friction in mixed steel/DLC contacts, which is related to the contact design and its kinematics. We found that if the DLC is coated on the slower surface, a high SRR will not reduce the friction to any significant extent, typically about 5 %, and at most 13 %, compared to steel/steel contacts. However, a significant friction reduction is observed when applying a low-surface-energy DLC coating to the faster surface—on average about 20 % and as high as 33 %. In this case (a high SRR and DLC on the faster surface), mixed steel/DLC surfaces can experience a very similar friction as that seen for DLC/DLC contacts. Qualitatively, the same behaviour was found for both positive and negative SRR values.

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Abbreviations

u e :

Entrainment speed (mm/s)

u s :

Sliding speed (mm/s)

SRR:

Slide-to-roll ratio (–)

h 0 :

Minimum film thickness (µm)

R′:

Reduced radius of curvature (m)

E′:

Reduced Young’s modulus (Pa)

α :

Pressure–viscosity coefficient (m2/N)

W :

Contact load (N)

k :

Ellipticity parameter (–)

η :

Dynamic viscosity (cSt)

A :

Area of ball–disc contact (mm2)

λ :

Tallian (lambda) parameter (–)

R qD :

Root mean square of disc roughness (µm)

R qB :

Root mean square of ball roughness (µm)

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Authors and Affiliations

  1. Laboratory for Tribology and Interface Nanotechnology, University of Ljubljana, Bogišičeva 8, 1000, Ljubljana, Slovenia

    M. Polajnar & M. Kalin

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  1. M. Polajnar
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  2. M. Kalin
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Correspondence to M. Kalin.

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Polajnar, M., Kalin, M. Effect of the Slide-to-Roll Ratio and the Contact Kinematics on the Elastohydrodynamic Friction in Diamond-Like-Carbon Contacts with Different Wetting Behaviours. Tribol Lett 60, 8 (2015). https://doi.org/10.1007/s11249-015-0593-3

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