Liquid Metal Lubricant Tames Molybdenum Friction From Room Temperature to 300 Degrees

Gallium-based liquid metals have quietly become one of the most intriguing candidates for lubricating the machines that nobody can afford to see fail: precision sliding bearings inside high-energy radiation devices and high-power X-ray sources. A new study in the journal iScience now shows that these shimmering metallic fluids can protect molybdenum surfaces across a remarkable temperature window, cutting wear by as much as 99.4 percent compared with dry sliding — but only when the surrounding atmosphere cooperates. The finding overturns the intuitive assumption that oxidation is always the enemy of a lubricant, revealing instead that a controlled dose of oxygen is precisely what makes the system work.

The research team, led by Qiaona Wang and Yuan Yu, set out to address a stubborn engineering problem. Conventional oils and greases evaporate, decompose, or oxidize under high-temperature or vacuum conditions, leading to unstable viscosity, carbonaceous deposits, and catastrophic film failure. They also conduct heat and electricity poorly, which is a serious handicap in bearings that must simultaneously manage thermal loads and survive intense irradiation in confined spaces. Traditional liquid metals such as lead and mercury offer superb thermal and electrical conductivity, but their toxicity severely limits their use. Gallium-based alloys, particularly the ternary mixture of gallium, indium, and tin known as Galinstan or GIS, sidestep these objections with a low melting point near room temperature, negligible vapor pressure, and comparatively low toxicity.

What makes the new work distinctive is the choice of rubbing materials. Previous liquid-metal lubrication studies mostly examined copper, aluminum, and steel combinations, but precision bearings in radiation equipment are typically built from molybdenum and its alloys, which combine high melting points, high hardness, excellent high-temperature strength, and low thermal expansion. The researchers therefore tested a self-mated molybdenum pair — a pin rotating against a disk of the same metal — lubricated with GIS from room temperature up to 300 degrees Celsius, in both air and argon. Under dry sliding in air, the bare molybdenum surfaces performed poorly, with friction coefficients hovering between 0.51 and 0.65 and severe adhesive wear, plowing, and spalling visible across the worn tracks.

Introducing the liquid metal in air transformed the picture. The average friction coefficient dropped to 0.31 at room temperature and 0.32 at 100 degrees, remaining below the dry-sliding values across the entire range. More striking still was the wear suppression: the wear rate of the molybdenum disk plummeted from 135.2 to just 0.8 thousandths of a cubic millimeter per newton-meter at room temperature, and stayed between 0.8 and 2.7 in those units all the way to 300 degrees — a reduction of 90.9 to 99.4 percent. Electron microscopy of the worn surfaces confirmed the visual difference: instead of deep tearing and spalling pits, the lubricated surfaces appeared shallow and smooth, with only fine abrasive traces.

The surprise came when the same tests were repeated in argon, an inert atmosphere where oxidation is suppressed. At room temperature, the liquid metal actually performed slightly better in argon than in air, achieving a friction coefficient of 0.27. But as the temperature climbed, the inert-atmosphere results deteriorated dramatically. Friction rose to 0.54 at 100 degrees, 0.64 at 200 degrees, and 0.70 at 300 degrees — approaching or exceeding the dry-sliding values — while the wear rate at 300 degrees reached 15.6, roughly nine times higher than in air. A lubricant that thrives in a vacuum-like environment is normally the goal of space tribology; here, the absence of oxygen turned a winning lubricant into a failing one.

The explanation lies in the delicate chemistry of the interface. Using X-ray photoelectron spectroscopy, the team detected both metallic gallium and oxidized gallium on the worn surfaces, with the oxidized Ga3+ component dominating in air. As temperature increased, the proportion of gallium oxides grew, and the XPS Ga3+/Ga0 ratio was 3.30 in air compared with only 2.86 in argon, confirming that the inert atmosphere partially throttled gallium oxidation. Notably, the Mo 3d spectra showed only metallic molybdenum, with no evidence of gallium-molybdenide compounds forming up to 300 degrees — the protective film is not a reaction product of the two metals but a Ga-O-rich layer built on the liquid metal itself.

Cross-sectional analysis of the samples tested in argon revealed why the failure was so severe. Energy-dispersive mapping showed gallium concentrated near the worn surface and, more troublingly, extending into the mechanically damaged subsurface region of the molybdenum. Nanoindentation measurements in these gallium-affected zones recorded lower hardness and reduced resistance to plastic deformation compared with the pristine substrate. In other words, when oxygen is unavailable to lock the gallium into a stable oxide film, the liquid metal seeps into surface defects and softens the near-surface material, promoting cracking, layered delamination, and shear instability under the repeated stresses of sliding.

Temperature also produced a curious non-monotonic pattern in air, with the wear rate under liquid-metal lubrication rising from room temperature to a peak at 200 degrees before falling again at 300 degrees. The researchers attribute this to an intermediate state: at 200 degrees, gallium oxidation is still insufficiently extensive, so the Ga-O-rich tribofilm remains patchy and delamination features appear on the worn surface. By 300 degrees, more thorough oxidation produces a continuous, protective film that separates the surfaces effectively. The team proposes an overall mechanism in which the liquid metal first spreads into surface asperities and reduces direct metal-to-metal contact, then the oxygen-driven oxide film forms and reforms under frictional shear, repeatedly rupturing and replenishing while shielding the molybdenum from direct exposure to the softening gallium.

Load-dependence tests at 7 and 13 newtons confirmed that the 10-newton condition used in the main experiments delivered the lowest friction and wear among the tested loads, and the wear measurements were repeated at least three times for statistical reliability. The authors are candid about the study’s limits: the tests used fixed operating conditions, so stability under broader loads, speeds, and long-duration service remains unverified, and the absence of pristine-liquid-metal XPS controls and nanoscale transmission electron microscopy means the relative contributions of native, thermal, and friction-assisted oxidation cannot yet be fully separated.

Even so, the study delivers a genuinely counterintuitive design principle with wide implications. For molybdenum bearings destined for X-ray sources, nuclear installations, and medical equipment, the trick is not to exclude oxygen but to embrace a moderate, regulated amount of it, allowing engineers to co-design the lubricant and the atmosphere as a single system. Gallium-based liquid metals, with their metallic conductivity and vapor-free fluidity, could anchor a new generation of functional lubricants in which the environment itself becomes part of the machinery — provided the oxygen supply is tuned just right.

Subject of Research: Atmosphere-regulated lubrication of molybdenum friction pairs by gallium-based liquid metal across a wide temperature range

Article Title: Oxidation-regulated wide-temperature tribological behavior and mechanism of a Mo/Mo tribopair lubricated by Ga-based liquid metal

Article References: Wang, Q., Liu, H., Zhang, G., Li, T., Cui, H., Feng, X., Yu, Y., Ma, Z., Wang, X., Qiao, Z., & Tang, H. (2026). Oxidation-regulated wide-temperature tribological behavior and mechanism of a Mo/Mo tribopair lubricated by Ga-based liquid metal. iScience, 29(11), Article 117805. https://doi.org/10.1016/j.isci.2026.117805

Image Credits: AI Generated

DOI: Not provided

Keywords: gallium-based liquid metal, tribology, molybdenum, lubrication, Ga-O tribofilm, high-temperature friction, oxidation, wear reduction, Galinstan, precision bearings, XPS, surface chemistry