The influence of lubricant additives and surface roughness and hardness of material on the damage behavior of gears

This study investigates the influence of lubricant additives, surface roughness, and material hardness on gear damage behavior under boundary lubrication conditions. We conducted both the Short-term Test and the Standard Test using an FZG gear test machine to evaluate how lubricant additives and gear surface roughness influence damage progression when the surface roughness exceeds the oil-film thickness. Acid phosphate ester effectively suppressed micropitting through surface smoothing but led to severe damage such as pitting and scuffing during prolonged use. In contrast, sulfurized fatty oil promoted mild wear, delaying catastrophic failures and extending gear life. Higher surface roughness accelerated wear, while increased hardness reduced deformation but it expanded damage areas. The study found that initial surface roughness and its progress during load stages strongly correlate with gear durability. Measurement of arithmetic mean roughness after sufficient running-in under actual load conditions proved useful for predicting long-term performance. These findings highlight the importance of selecting lubricant formulations tailored to specific gear operating environments and damage modes. Understanding the interplay between lubrication chemistry and material properties enables the design of more durable gear systems. • Lubricant additives differently affect gear wear, micropitting and damage modes. • Acid phosphate ester suppressed micropitting but can cause severe long-term damage. • Sulfurized fatty oil delays major failures by enabling controlled wear progression. • Additive chemistry shifts damage evolution from micropitting to other mechanisms. • Gear durability depends on tailored additives to resist specific damage types.