24th International Colloquium Tribology - January 2024 103 Lubricant Inerting - a New Era in Lubrication Technology Jie Zhang 1 , Janet Wong 1 , Hugh Spikes 1* 1 Tribology Group, Imperial College London * Corresponding author: h.spikes@imperial.ac.uk 1. Introduction The concept of blanketing an operating lubricant in nitrogen gas to prevent its oxidative degradation, so-called “inerting”, was first considered by NASA in the 1960s in the context of high temperature aerospace transmission lubrication [1,2]. The principle is quite simple. From the lubricant oxidation cycle as shown in Fig, 1, it is evident that oxygen plays the key role, both in the initiation phase and in sustaining the highly damaging chain-branching hydroperoxide cycle. If there is little or no oxygen present, then there can be little or no hydrocarbon oxidation. Fig. 1: Hydrocarbon oxidation cycle Throughout the 1960s considerable efforts were made to explore this inerting possibility and it was found possible to operate aerospace transmissions lubricated with synthetic hydrocarbons and ester-based oils in prolonged tests at up to 370-°C, with negligible lubricant degradation [2]. Also, to some surprise, it was found that many lubricants showed lower friction and wear in a nitrogen atmosphere than in air, a phenomenon that was eventually ascribed to “oxidative wear” in which oxygen promotes the formation of high friction, easily worn oxides [3]. Eventually it was accepted by the late 1960s that this inerting approach was simply not practicable for aerospace application at that time. High levels of nitrogen leakage from the operating gearboxes could not be prevented and so would necessitate a continual supply of the inert gas during use. However, the storage of nitrogen in aircraft in the quantities needed was simply not feasible [2]. Matters have now changed. In the last decade, low cost, and in some cases portable, membrane-based nitrogen and oxygen concentrators have become commercially available that are able to separate nitrogen and oxygen from air to provide high flow rates of either gas. Oxygen concentrators are now widely used in hospitals, homes and while travelling to support patients with breathing difficulties. Nitrogen concentrators are being employed, for example, in food packaging, laboratory synthesis and for instruments such as mass spectrometers that require an inert gas flow. These can deliver more than 10 litres/ min of almost pure nitrogen, have power consumptions of typically 10 W, and useful life between membrane changes of several years. In principle this technology now makes it quite feasible to operate stationary and mobile, closed lubricated components in an almost pure nitrogen atmosphere, with nitrogen being supplied as needed, e.g. when the machine is operating or when it exceeds a certain temperature. The potential benefits are evident as indicated in Table 1 and clearly offer considerable improvements in sustainability at several levels. Table 1: Potential benefits of lubricant inerting • Much longer lubricant lives (fill for life) • Higher component operating temperature (>100-°C hotter than at present). Reduced cooling needs. • Much wider application of bio-based lubricants • Reduced acid formation and consequent yellow metal corrosion Initial applications envisaged are those that operate at high temperatures, such as hydraulics and steel rolling bearings, and those where lubricant replacement is costly, for example in wind turbine transmissions. However, once the technology is established a much broader range of inerted applications is likely. For lubricant inerting to be introduced safely and robustly, two research questions must be addressed... a. Can lubricants that have been deigned to operate in air function effectively in preventing friction/ wear/ scuffing etc. when little or no oxygen is present? If not, how should lubricants be modified? It should be noted that the solubility of nitrogen in base oils is similar to that of oxygen, so there are no implications in term of hydrodynamic or EHD lubrication of replacing an atmosphere that already has 80% nitrogen to one of almost pure nitrogen. b. What are the predominant lubricant degradation mechanisms in atmospheres containing little or no oxygen, and thus what extension of useful lubricant life can be expected? For example, if we reduce oxygen concentration from 21% (as present in air) to 2%, can we expect a ten-fold increase in lubricant life? The research outlined in this presentation aims to address both of the above questions and thus facilitate the introduction of lubricant inerting technology. 2. Test Methods For both friction and wear tests and high temperature lubricant degradation tests, a PCS high frequency reciprocating rig (HFR) was located in a sealed Perspex chamber with inflow and outflow gas ports and containing an oxygen-level 104 24th International Colloquium Tribology - January 2024 Lubricant Inerting - a New Era in Lubrication Technology sensor (Fig. 2). Base oils used were hexadecane and PAO10 (SpectraSyn 10). Fig. 2: HFR in atmosphere-controlled chamber 3. Friction and Wear Results As was seen in the 1960s, in the absence of lubricant additives, friction and wear of base oils were both found to be considerably lower in N 2 than in air. However, we have shown that this does not originate from oxidative wear, as previously suggested, but rather from the formation of protective carbon-based tribofilms in the absence of oxygen [4]. When O 2 is present these carbon films do not form since oxygen immediately reacts with hydrocarbyl free radicals generated mechanochemically by rubbing and thus prevent the radicals forming tribofilms. In the latter’s absence, friction and wear are dominated by adhesion of metals and metal oxides. Fig. 3 compares the friction of solutions of two friction modifiers in hexadecane at 60- °C, MoDTC and glyceryl monooleate (GMO). MoDTC gives almost identical performance in the two atmospheres, suggesting that molecular oxygen does not participate in its friction-reducing tribofilm formation. However, GMO consistently shows lower friction and wear in nitrogen than in dry air. We believe that this is because GMO adsorbs more effectively on a carbon film than on iron or iron oxide. 4. Lubricant Degradation Results For degradation tests, a high temperature version of the HFR, able to reach 400-°C, was employed. At the end of two hours tests the lubricant was extracted and its viscosity measured. Because a rubbing contact is present, this test measures the ability of the lubricant to withstand tribodegradation. This is recognised to be more severe than autooxidation as measured in glassware-based tests, perhaps because catalytic ferrous ions are generated during rubbing and/ or hydrocarbyl free radicals that initiate oxidation are produced in rubbing contacts. Typical results are shown in Fig. 4. This compares the endof-test viscosity at 25-°C from PAO10 base oil degradation tests in (i) cylinder N 2 (0% O 2 ), (ii) 0.5% O 2 in N 2 (from a nitrogen concentrator) and (iii) cylinder dry air (20% O 2 ). It is evident that there is negligible viscosity increase in N 2 and 0.5% O 2 , even at 300-°C, while in dry air substantial degradation occurs. For the test in dry air at 300-°C, a solid-like brown residue, whose viscosity could not be reliably measured, was left in the lubricant bath at end of test. Fig. 3: Friction of MoDTC and GMO solutions Fig. 4: End of test viscosity of PAO10 tested in cylinder N 2 , dry air and 0.5% O 2 from nitrogen concentrator Conclusion We are currently exploring the use of nitrogen concentrators to blanket operating lubricants in a low or zero oxygen environment to greatly reduce lubricant degradation. Results to date illustrate the overall feasibility of the approach both in terms of both tribology and lubricant life. Acknowledgement We thank ExxonMobil for supply of SpectraSyn TM 10. References [1] Rhoads, W. L. Supersonic transport lubrication system investigation. NASA CR-72424, Sept 1968. [2] Loomis W. R., Townsend, D. P, Johnson, R. L., “Lubricants for inerted lubrication system in engines for advanced aircraft, NASA TN D-5420 1969. [3] Appeldoorn, J. J., Goldman I. B., Tao, F F., Corrosive wear by atmospheric oxygen and moisture, ASLE Trans., 12: 2, 140-150, (1969). [4] Zhang, J., Campen, S., Wong, J. S. S, Spikes, H. A., Oxidational wear in lubricated contacts - or is it? Trib. Intern. 165, 107287, (2022).