Introduction of hard particles is considered an effective approach to improve alloy wear resistances. However, the wear resistances of Mg alloys could be deteriorated by increasing the hard particle content in many researches. To reveal the underlying negative effect of precipitate on the wear resistance, the wear behaviors of three AZ-Mg alloys (precipitate contents: AZ31: 2.1%, AZ61: 3.8%, AZ91: 5.0%) at the axial loads of 3 and 15 N were investigated. The results indicated that although wear volume of the AZ-Mg alloys decreased with the increasing Mg₁₇Al₁₂ content at 3 N (0.30→0.24→0.20 µm³) and 15 N (1.04→0.88→0.85 µm³), the relative wear resistances of AZ61 and AZ91 to AZ31 decreased with increasing load (AZ61: 1.25→1.17, AZ91: 1.50→1.22) and the reduction was proportional to the precipitates content (AZ61:7%, AZ91:28%). That is because the wear volume of AZ-Mg was mainly attributed to micro-cutting, which was negatively correlated with the precipitate content and tribolayer hardness. However, the wear hardening ability of AZ-Mg alloys was weakened by precipitate for its inhibition on the formation of mechanical twins that the precursors for the tribolayer. Moreover, the inhibition of the precipitate on tribolayer could be amplified by the load, resulting in an increase in tribolayer hardness at 3 N (AZ31: 0.94, AZ61: 1.03, AZ91: 1.10 GPa) but a decrease at 15 N (AZ31: 1.77, AZ61: 1.73, AZ91: 1.62 GPa). Therefore, the formation of twin was inhibited by precipitates, which is detrimental to the wear resistance of Mg alloys. That means the wear resistance could be enhanced by promoting twin formation, which provides a new concept for the design of wear-resistant Mg alloys.

Efficient lubrication of magnesium alloys is a highly challenging topic in the field of tribology. In this study, magnesium silicate hydroxide (MSH) nanotubes with serpentine structures were synthesized. The tribological behavior of AZ91D magnesium alloy rubbed against GCr15 steel was studied under lubricating oil with surface-modified MSH nanotubes as additives. The effects of the concentration, applied load, and reciprocating frequency on the friction and wear of the AZ91D alloy were studied using an SRV-4 sliding wear tester. Results show a decrease of 18.7–68.5% in friction coefficient, and a reduction of 19.4–54.3% in wear volume of magnesium alloy can be achieved by applying the synthetic serpentine additive under different conditions. A suspension containing 0.3 wt.% MSH was most efficient in reducing wear and friction. High frequency and medium load were more conducive to improving the tribological properties of magnesium alloys. A series of beneficial physical and chemical processes occurring at the AZ91D alloy/steel interface can be used to explain friction and wear reduction based on the characterization of the morphology, chemical composition, chemical state, microstructure, and nanomechanical properties of the worn surface. The synthetic MSH, with serpentine structure and nanotube morphology, possesses excellent adsorbability, high chemical activity, and good self-lubrication and catalytic activity. Therefore, physical polishing, tribochemical reactions, and physical-chemical depositions can occur easily on the sliding contacts. A dense tribolayer with a complex composition and composite structure was formed on the worn surface. Its high hardness, good toughness and plasticity, and prominent lubricity resulted in the improvement of friction and wear, making the synthetic MSH a promising efficient oil additive for magnesium alloys under boundary and mixed lubrication.