Research project: Nanoadditives for hydrogen technology lubrication Impact of Lubricant Nanoadditives on Hydrogen Tribology

Metal surfaces exposed to hydrogen environments can acts as catalysts to break down the gas molecules into atoms. In this form, hydrogen can penetrate the metal surface and negatively influence its properties. Metal embrittlement has been identified as a major consequence of the hydrogen uptake and it represents an extra challenge for tribological parts which are normally subjected to high stresses.

Previous studies show that the amount of hydrogen that diffuses into the material is directly related to the reduction in service life. However, lubricants can be designed to prevent the permeation of hydrogen and extend the service life of materials. Published research has shown that lubricants (oils and greases) can reduce metal permeation and uptake of hydrogen, and thus protect the tribological parts.

WS2 Nanoadditives

Recently, particular interest has been directed to nanoadditives, because their nano-size facilitates penetration of tribological contacts of the most intricate contact geometries. Nanoparticles fill out the contact asperities, therefore reducing surface roughness, and ultimately form a protective boundary film that is persistent under pressure. The composition of this film is presented in Figure 1.

Amongst lubricant nanoadditives, WS2 nanoparticles have shown the potential to reduce wear and friction significantly better than conventional lubricants. These have weak inter-layer bonds that allow them to slide easily, with very little friction coefficients. WS2 nanoadditives can positively interact with the wear track and reduce hydrogen environment by:

1. the continuous generation of film through the reaction of the 2H-WS2 NPs with the wear track, which impedes the formation of fresh, catalytic surfaces during rubbing and thus prevents the decomposition of oil/water molecules and generation of atomic hydrogen;
2. the tribofilm acting as a physical barrier for hydrogen permeation through the wear track;
3. the lower coefficients of hydrogen diffusion in tungsten compounds, which impede hydrogen permeation;
4. the contribution of hydrogen to the generation of the tribofilm by redox reactions (i.e. reducing the chemical state of tungsten and iron).Current research conducted at the University of Southampton has shown that in high-pressure, high-temperature steel contacts, WS2 nanoparticles form thick solid tribofilms composed of iron and tungsten sulphides and oxides. This is a result of the reaction between the nanoparticles and the substrate.

These effects are summarized in Figure 2.

Using thermal desorption spectroscopy, it can be observed that some additives can reduce the amount of hydrogen which permeates into the bearing steel. WS2 and TOP (trioctylphosphate) show this type of behaviour, acting as antiwear agents at the same time.

Lubricant nanoadditives have the potential to create a barrier against the permeation of hydrogen into steel. Their excellent tribological properties, combined with a direct solution to hydrogen permeation and metal embrittlement, could facilitate the efficient use of hydrogen in fuel cells and lead to a smooth transition towards hydrogen as the main energy carrier of the future.