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Research Papers: Applications

Wear and Friction of Diamondlike-Carbon Coated and Uncoated Steel Roller Bearings Under High Contact Pressure Oil Lubricated Rolling/Sliding Conditions

[+] Author and Article Information
P. A. Dearnley

Engineering Sciences,
nCATS,
University of Southampton,
Southampton SO17 1BJ, UK
e-mail: p.dearnley@soton.ac.uk

A. M. Elwafi

Faculty of Engineering,
Sohar University,
Sohar 31, Oman
e-mail: allilwafi@yahoo.co.uk

R. J. Chittenden

School of Mechanical Engineering,
University of Leeds,
Leeds LS2 9JT, UK
e-mail: r.j.chittenden@leeds.ac.uk

D. C. Barton

School of Mechanical Engineering,
University of Leeds,
Leeds LS2 9JT, UK
e-mail: d.c.barton@leeds.ac.uk

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received May 27, 2013; final manuscript received November 17, 2013; published online January 20, 2014. Assoc. Editor: Dong Zhu.

J. Tribol 136(2), 021101 (Jan 20, 2014) (11 pages) Paper No: TRIB-13-1109; doi: 10.1115/1.4026078 History: Received May 27, 2013; Revised November 17, 2013

Diamondlike-carbon (DLC) coatings have received a lot of research attention by physicists and engineers, especially in the past 25 years. Attempts to use such materials in tribological applications have achieved variable success. The rationale for this work was to investigate the wear durability of three types of DLC coatings applied to hardened and tempered bearing steel and subject them to realistic high pressure cyclic loading under oil lubricated conditions for long duration. A thrust bearing design was deployed for this purpose. The wear and friction behavior of the DLC coated materials relative to uncoated materials was compared when using base (additive free) oils and typical autoengine formulated oils. The type of oil used made no difference to the dynamic friction and oil temperature for all the material and oil combinations used. Durability of the coated and uncoated roller bearings was determined by the type of material. For the uncoated bearings, life was limited after very many test cycles (approaching a billion) via classical rolling contact fatigue pitting. For all the DLC coated rollers life was governed by wear of their coatings. In the case of the tungsten doped DLCs (a-C:H:W), these were worn progressively and uniformly via microabrasion, whereas the nondoped ta-C and a-C DLC coatings were principally worn via delamination and tearing. The latter effect was relatively rapid and was considered to be initiated by blistering of the coating, a process that was probably driven by the high elastic energy/internal stress within the nondoped coating materials. The durability to delamination and tearing of the ta-C coatings was slightly lowered in formulated oil compared to tests made in base oil. Overall, for the test conditions used here, there was no apparent benefit in using DLC coatings.

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Figures

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Fig. 1

Details of thrust bearing test components: (a) disassembled thrust bearing showing roller bearings (only three rollers actually used per test), housing bearing ring/washer and driven shaft ring/washer; (b) schematic section through the test head/housing; and (c) plan view showing rolling and sliding kinematics for a single roller

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Fig. 2

Mean (a) dynamic friction coefficients and (b) test head temperatures for uncoated and carbon coated 100Cr6 rollers tested against uncoated 100Cr6 bearing rings in base and formulated oils. Pmax was 1.5 GPa in all cases. Measurements for uncoated and a-C:H:W coated 100Cr6 rollers determined over the first 50 h of testing, while those for the a-C and ta-C coated rollers determined over the first 2 to 6 h after which most tests were stopped due to coating delamination.

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Fig. 3

Example s of worn uncoated 100Cr6 housing bearing ring surfaces: (a) original surface; (b) after testing against a-C coated rollers in base oil 2; (c) after testing against ta-C coated rollers in base oil 2; (d) after testing against a-C:H:W coated rollers in base oil 2; (e) after testing against ta-C coated rollers in formulated oil 2; and (f) after testing against a-C:H:W coated rollers in formulated oil 2. Smooth wear attributed to microabrasion in all cases. Pmax was 1.5 GPa in all cases.

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Fig. 4

Collated S-N data for coated and uncoated 100Cr6 rollers tested against uncoated 100Cr6 bearing rings in BASE oils. The line shows the linear regression fits for the ta-C coated roller data set. Right angled arrows indicate nonfailures of uncoated rollers tested at 1.5 GPa.

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Fig. 5

Collated S-N data for coated and uncoated 100Cr6 rollers tested against uncoated 100Cr6 bearing rings in formulated oils. The lines show linear regression fits for the a-C (dashed line) and ta-C (continuous line) coated roller data sets. Right angled arrows indicate nonfailures of uncoated rollers tested at 1.5 GPa.

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Fig. 6

Example of a large rolling contact fatigue (RCF) macropit, produced in the surface of an uncoated steel roller after testing in formulated oil 1 for 670 × 106 cycles at a maximum Hertz contact pressure of 1.5 GPa

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Fig. 7

Examples of tested uncoated 100Cr6 rollers—revealing RCF cracking (encircled) prior to onset of macropitting

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Fig. 8

Surfaces of: (a) original and (b) smoothly worn a-C coated 100Cr6 rollers; (c) original and (d) smoothly worn ta-C coated 100Cr6 rollers; and (e) original and smoothly worn (f) a-C:H:W coated 100Cr6. The wear is attributed to microabrasion. Tests carried out in base oil 2.

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Fig. 9

Example of micropitting on the surface of an a-C:H:W coated 100Cr6 roller after approximately 500 × 106 stress cycles at a maximum Hertz contact pressure of 1.5 GPa, after testing in: (a) base oil 1 and (b) formulated oil 1. Note the smooth polished surface of the nonpitted areas, attributed to microabrasion.

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Fig. 10

FIB section revealing the depth of a micropit of an a-C coated 100Cr6 after several million test cycles in formulated oil 1. The pit (arrowed—beneath dashed line) has not reached the coating/substrate interface. (Note the outer white Pt layer was applied during sectioning to assure edge integrity during ion milling.)

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Fig. 11

Advanced state of tearing and delamination a-C coated 100Cr6 rollers after testing in formulated oil 1: (a) macroimage and (b) field emission gas SEM image—arrow denotes exposed substrate (numerous micropits are also visible in the coating to the right side of the arrow)

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Fig. 12

Collated S-N data for ta-C coated 100Cr6 rollers tested against uncoated 100Cr6 bearing rings in base and formulated oils. The lines show linear regression fits for both data sets.

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Fig. 13

FIB section showing coating/substrate interface cracking (arrow) of an a-C coated 100Cr6 adjacent to a major tear, after several million test cycles

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Fig. 14

Schematic of blister and micropit formation (sequence (a) to (c))

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Fig. 15

Schematic depiction of the progression of microdelamination and tearing of a DLC coating on a100Cr6 roller bearing substrate: (a) initial blister distribution; (b) sheared blisters and micropits (substrate exposure); (c) initial tearing of DLC coating in direction of rolling; and (d) pronounced tearing and coating loss

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