Lubricants

There is great interest in using hydrogen as a gaseous fuel in combustion engines to eliminate CO₂ emissions.  Unfortunately, hydrogen gas is a poor lubricant for most engineering metals and an effective lubrication solution for pumping and injecting hydrogen is required. This study explores the possibility of additivating hydrogen with a low concentration of a lubricious gas to reduce friction and wear. We find that unsaturated hydrocarbon gas additives form protective carbon-based tribofilms, while gaseous ammonia and amine additives form nitrogen-based films on steel surfaces during rubbing in additivated hydrogen. Gaseous amines are particularly effective in reducing friction and wear, even at concentrations as low as 100 ppm mole/mole. This demonstrates that the addition of a small concentration of lubricious gas is a feasible way to improve the lubricity of gaseous hydrogen.

Keywords: Gas-phase Lubrication, Hydrogen, Tribofilm formation, Gaseous Additives

Many previous researchers have reported the formation of carbonaceous tribofilms from organic lubricants on rubbing metallic surfaces. This paper shows that a very important factor in the formation of such tribofilms is the presence or absence of molecular oxygen. When steel surfaces are rubbed in saturated hydrocarbon lubricants in the absence of oxygen, for example in nitrogen or hydrogen gas, carbonaceous films form very readily, resulting in low friction and wear. However, when a significant amount of oxygen is present, as is the case in air, carbonaceous tribofilms are not generally formed, so friction and wear are very high, with values comparable to those seen when no lubricant is present. In situ Raman analysis combined with gas-switching experiments show that the carbonaceous films formed during rubbing when no oxygen is present are rapidly removed during rubbing in air, while tests in which lubricant is removed during a test in N₂ indicate that the films are quite weak. This suggests that these carbonaceous films are being continually removed and replenished during rubbing in oxygen-free conditions. It is proposed that these carbonaceous films are formed from hydrocarbyl free radicals that are generated mechanochemically from hydrocarbon molecules during rubbing. In the absence of oxygen, these free radicals then react together to form a carbonaceous film. However, when oxygen is present, the hydrocarbyl free radicals react extremely rapidly with oxygen molecules to produce hydroperoxyl free radicals and so are no longer available to generate a carbonaceous tribofilm.

Keywords: Carbon film, Free radical, Propane, Hexacane, Isooctane, Oxygen, Tribooxidation, Lubricant Inerting