Review article: Hadal-zone gearbox multi-physics lubrication dynamics: synergistic effects of hydrostatic pressure and thermal suppression

Abstract. Deep-sea gear transmission systems face critical lubrication challenges due to the combined effects of extreme hydrostatic pressure and cryogenic temperatures. These environmental stressors cause exponential increases in lubricant viscosity, leading to poor fluidity, high start-up torque, and lubrication starvation. Seawater intrusion induces lubricant emulsification, additive deactivation, and electrochemical corrosion at meshing interfaces, collectively increasing the risk of lubrication failure and compromising long-term reliability. This study investigates lubrication degradation mechanisms in deep-sea environments and proposes targeted mitigation strategies. Through comprehensive characterization of deep-sea environmental parameters and their effects on lubricant rheological behaviour, we critically evaluate the applicability and limitations of conventional thermal elasto-hydrodynamic lubrication (TEHL) theory under extreme conditions. Our analysis reveals that established TEHL frameworks require substantial modification to accurately capture pressure–viscosity–temperature coupling effects and seawater contamination kinetics. Meshing interface texturing, as an effective strategy for friction reduction and wear mitigation, is examined to understand its mechanisms for enhancing lubricant film formation and tribological performance under starved lubrication conditions. Key findings demonstrate that optimized micro-texture architectures can effectively compensate for viscosity-induced fluidity loss and mitigate against the harmful effects of seawater ingress. Critical knowledge gaps are identified, and future research directions are outlined: (i) multi-physics coupling models integrating thermo-hydrodynamic, chemo-physical, and mechanical degradation processes; (ii) synergistic texture-coating design approaches; (iii) high-pressure, low-temperature experimental validation protocols; and (iv) engineering implementation frameworks for deep-sea gear transmission systems. This review establishes theoretical foundations and provides technical guidelines for robust lubrication design and long-term operational stability of deep-sea transmission equipment.