The performance of gallium oxide (Ga₂O₃) is critically dependent on substrate surface quality, while chemical mechanical polishing (CMP) plays a key role in achieving high-quality finishing of single-crystal Ga₂O₃. However, material removal mechanisms under varying chemical environments remain unclear, thus hampering further optimization. This study investigated the material removal behavior of β-Ga₂O₃ using a tribological approach. Tribochemical interactions between β-Ga₂O₃ and various chemical solutions (deionized water [H₂O], 1% hydrogen peroxide [H₂O₂], and 3% H₂O₂) using different counterbodies (aluminum oxide [Al₂O₃], zirconium dioxide [ZrO₂], and silicon nitride [Si₃N₄]) were systematically analyzed. The results demonstrated a pronounced dependence of tribological behavior on the oxidative aqueous environment. Combined with surface morphology and chemical component analysis, H₂O₂ induced anisotropic selective etching and enhanced surface hydroxylation, leading to an elevated coefficient of friction and wear rate (increasing from ~ 9.74 × 10^-8 mm³/(N·m) in water to ~ 2.27 × 10⁻⁶ mm³/(N·m) in 3% H₂O₂). Furthermore, the tribological performance was governed by the counterbody material. Whereas amphoteric ZrO₂ exhibited mild wear behavior similar to that of Al₂O₃, Si₃N₄ triggered severe mechanical interactions, resulting in the highest wear rate (~ 5.28 × 10⁻⁶ mm³/(N·m)). Cross-sectional microstructural characterization revealed that this interaction leads to the formation of a distinct structural damage gradient in the subsurface. This study provides fundamental insights into the tribochemistry of β-Ga₂O₃, and these findings are anticipated to offer valuable guidance for the future optimization of CMP processes.

Cobalt-based (Co-based) alloys are widely used in aerospace and machinery applications because of their excellent mechanical properties, where extraordinary wear performance is also desirable to ensure stable operation. However, there is still scarce information on the tribological mechanism of the cobalt metal building block, especially under different humidities. Insight into the wear mechanism of Co under different humidities is crucial for studying the tribological performance of Co-based alloys as well as their potential applications under various working conditions. Here, we report an investigation of the effect of humidity on the wear behavior of Co. The results revealed that Co exhibited ultralow wear characteristics in a humid air environment (relative humidity (RH) 70%), with a wear rate of 2.15×10^–7 mm³/(N·m), and dramatically increased by three orders of magnitude to 1.47×10^–4 mm³/(N·m) in the dry environment (~5% RH). Surface analysis revealed that tribochemistry dominated the whole wearing process, with the worn surface almost fully covered by cobalt oxide, Co₃O₄, when subjected to a humid environment, whereas a small number of oxide layers were observed only within the wear grooves under the RH 5% testing condition. The stripe test results revealed the evolution of this protective oxide generation, and the focused ion beam scanning electron microscopy (FIB-SEM) images of the cross-sections at different sliding stages revealed the role of tribochemistry in triggering such self-protection behavior. Our work provides a fundamental understanding of the wear mechanisms of Co metal, and we anticipate that these findings can offer valuable guidance for further improving the wear performance of cobalt-based alloys in the future.