Effect of base oil chemistry on ZDDP film formation:

Reducing hydrogen permeation in bearing steel

Vlad Bogdan Niste¹, Hiroyoshi Tanaka^1,2, Monica Ratoi,³ Joichi Sugimura^1,2

¹ International Institute for Carbon-Neutral Energy Research, Kyushu University, Japan

² Research Centre for Hydrogen Industrial Use and Storage, Kyushu University, Japan

³ National Centre for Advanced Tribology at Southampton, University of Southampton, UK

INTRODUCTION

High strength steels are particularly susceptible to hydrogen embrittlement and therefore rolling element bearings can experience premature failure that drastically reduces their operation life. Although the exact mechanisms of embrittlement are still disputed, the permeation of atomic hydrogen into the substrate is known to be the initial step. Recent studies have suggested that in lubricated conditions, hydrocarbon molecules or water in the oil can decompose on the nascent surfaces produced during rolling/sliding and generate atomic hydrogen. Lubricants can affect this process by interacting with the metal interface during operation. This study is investigating the effect of ZDDP based oils on the generation and permeation of hydrogen in the material, offering further knowledge regarding optimum operating conditions of rolling elements.

Lubricated rolling contact tests were conducted using various lubricants containing ZDDP in a hydrogen atmosphere and the concentration of hydrogen in the samples after the tests was measured using desorption spectroscopy. The chemical composition at the interfaces was investigated using Auger electron spectroscopy.

METHODS

Rolling contact tests were carried out in air or hydrogen gas, in a ball-on-disc setup test rig. Six 6.35 mm balls are loaded against a flat disc specimen (4.8 GPa) and the contact undergoes rolling through the rotation of the upper shaft. The ball and disc material is JIS SUJ2 steel, equivalent to AISI 52100. The contact is subjected to mixed lubrication conditions.

Figure 1 - Diagram of the rolling contact test rig.

The concentration of hydrogen in the steel samples was determined using TDS. The technique involves heating the sample in a vacuum chamber in order to induce the desorption of gaseous species, which are then analysed by a mass spectrometer. During the TDS analysis, the specimens were heated from room temperature to 1073 K at a rate of 60 K min-1 for the disc and 10 K min-1 for the ball.

The depth and morphology of the wear tracks on the disc specimens were investigated by optical profilometry. The elemental distribution across the wear track was investigated using an Auger electron spectroscope with a 10 keV electron beam.

RESULTS

The amount of hydrogen (in ppm) present in the disc specimens after the 10 hour RCF tests is shown in Figure 2. Results show that the concentration of hydrogen in the material is related to the affinity of the base oil to the wear track, and its susceptibility to degenerate in the contact and produce atomic hydrogen. Tests performed in hydrogen gas show a larger variation of hydrogen in the material as compared to tests conducted in air, indicating that the environmental gas is an important factor.

Figure 2 - Hydrogen content (ppm) in the disc specimens after the RCF tests (10 h) measured by Thermal desorption spectroscopy.

Base oil 1 is a polyester and is attracted to the surface of the metal easily. This provides competition to any chemical reactions that would allow the generation of atomic hydrogen. As a result, the concentration of hydrogen in the material does not increase substantially during operation.

Base oils 2 and 3 are fluorinated oils and their affinity towards the interface is reduced. However, the decomposition of these molecules creates various chemical species that can corrode the interface and thus influence hydrogen generation. In this case the amount of hydrogen measured in the samples after the tests varies substantially.

The morphology and chemical composition on the wear tracks was also investigated in order to determine the chemical reactions that take place in the tribological contact in each case.

Understanding the behaviour of steel components in hydrogen environments and designing technologies capable of improving their performance and life cycle constitutes the next important challenge towards a clean and efficient hydrogen based economy in the future.

REFERENCES

1. Tanaka, Niste, Abe, Sugimura, Tribol. Lett., 65, 94, (2017),

2. Niste, Tanaka, Ratoi, Sugimura, RSC Adv., 5 (51), 40678-40687, (2015),

3. Tanaka, Morofuji, Enami, Hashimoto, Sugimura, Tribol. Online, 8(1), 90–96, (2013).