document: Fig. 7 shows the relationship between the testing time and friction coefficients of various samples under dry conditions. There exist running in and steady wear period in the wear process of uncoated AZ31 and anodizing coating without Al2O3 nanoparticles while there has a steady wear period only in the wear process of composite anodizing coating with Al2O3 nanoparticles. At the same time, the addition of nano-particles to electrolyte led to reduction of friction coefficient. The friction coefficient of composite coating is relatively lower and more stable than what has been reported in literature [24,25] for anodizing coatings. This may be caused by “rolling effect” made by Al2O3 nanoparticles on the surface of oxide coating. Spherical nanoparticles change sliding into rolling, which reduce friction, making the friction coefficient becomes more stable. The friction coefficient of anodizing coating without Al2O3 nanoparticles has large fluctuation maybe for the damage of coating. In contrast to the uncoated AZ31 magnesium alloy, the anodizing coatings show slightly lower friction coefficient. This can be attributed to their higher load-bearing capacity for high hardness.
extractive_keyphrases: al2o3 nanoparticles anodizing coatings coating electrolyte friction coefficient friction coefficients reduce friction “rolling effect” spherical nanoparticles testing time uncoated az31 wear process
abstractive_keyphrases: anodizing coating without al2o3 nanoparticles composite anodizing coating composite coating nano-particles reduction of friction coefficient uncoated az31 magnesium alloy