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The University of Southampton
Engineering

Research project: Lubrication of Hydrogen Technology

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Bearing steels suffer from degradation of mechanical properties when atomic hydrogen diffuses into the steel from the contact surface. In rolling contact fatigue tests this can lead to a significant reduction in fatigue life of the specimens as the amount of hydrogen diffused into the steel increases.

Hydrogen is the cleanest fuel available as its combustion product is water. This advantage has enabled hydrogen technology to quickly establish itself as the technology of the future not only for fuel cell vehicles but also for other high energy consumption applications such as power generation, energy storage and transportation, chemical plants and aerospace applications.

A long-standing challenge of hydrogen technology is related to the atomic size of hydrogen which enables it to diffuse readily through the lattice of solid materials and cause catastrophic failure in high strength steels. Embrittlement by hydrogen has been identified as a major consequence of hydrogen uptake and represents an extra challenge for lubricated tribological parts that are normally subjected to high stresses such as rolling element bearings. To mitigate this challenge, various lubricants (synthetic oils of different chemistry, antiwear additives) have been studied in Air, Hydrogen and Argon environments so as to identify their efficiency and mechanism of retarding or preventing hydrogen permeation in steel.

RCF tests were performed in a ball-on-disc setup test rig (Figure 1) using test conditions summarizes in Table 1. The RCF tests are conducted on bearing steel in controlled atmospheres lubricated with synthetic oils and additives. Samples are tested until significant wear or failure is apparent and immediate Thermal Desorption Spectroscopy (TDS) analysis is undertaken for hydrogen content evaluation.

Wear loss is analysed using optical and contact profilometry while the wear track chemistry is determined by XPS, Raman and Auger Electron Spectroscopy.

The oil condition is monitored before and after the RCF tests by FTIR and Karl-Fisher water content analysis.

Sub-surface analysis is performed using traditional sectioning methods alongside a comparative investigation into the prospects of high energy micro-computed-tomography (Micro-CT/µ-CT) as a non-destructive technique for sub-surface damage characterisation (Fig. 2).

The benefit of using CT to understand surface damage in bearing steel, is the prospect of 360° analysis of pore, crack and inclusion networks (Fig. 3) with the bonus of scanned specimens remaining intact for further analysis (albeit the dimensions of the specimens for micro-CT scanning are extremely limited), an impossible task for traditional sectioning and etching analysis.

 

Table 1. RCF test conditions

Temperature 120ºC
Hertzian Pressure 4.8 GPa
Entrainment speed 1500 rpm (3.4 m/s)
Initial composite surface roughness 11 nm
Film parameter Λ 2
Test Gas Air/Argon /H2
Lubricant Synthetic oils: PAO32, PPG, POE
Specimans

JIS SUJ2 steel balls / discs

Rq 10 / 5 nm

Fig. 1. RCF tribometer
Fig. 1. RCF tribometer
Fig. 2. Micro-CT scan set-up
Fig. 2. Micro-CT scan set-up
Fig3
Fig 3
Fig3
Fig 3

Fig. 3.  µ-CT showing a). Base view (left) and Right view (right) exhibiting a multiple pathed crack propagating parallel to the surface of the H2 ball, b). Pores in the H2 race wear track subsurface

Associated research themes

Department of Hydrogen Energy Systems, University of Kyushu Japan

https://kyushu-u.pure.elsevier.com/en/persons/joichi-sugimura

Key Publications

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