Friction properties of C-containing ceramic coatings on an Mg-Li alloy

Due to its specific strength, superior electromagnetic shielding and excellent processing capabilities, the magnesium-lithium (Mg-Li) alloy is regarded as one of the most promising structural metal materials and has been extensively applied in various fields such as aerospace, offshore engineering, and the communication industry. Unfortunately, inferior tribological behavior, caused by low hardness, a fluctuating friction coefficient, and serious adhesive wear, has severely inhibited large-scale application of Mg-Li alloys in industrial engineering. Therefore, in this study, to enhance the tribological performance of a micro-arc oxidation (MAO)-produced ceramic coating on an Mg-Li alloy, a variety of inorganic particles were tentatively added to MAO electrolytes to prepare composite ceramic coatings with pronounced friction and wear resistance properties. MAO in Na₂SiO₃-KOH electrolytes with graphene additives was used to produce self-lubricating C-containing ceramic coatings on an Mg-Li alloy. The surface morphologies, roughness, hardness, and phase compositions were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), a Vickers hardness test, and X-ray power diffraction (XRD). At room temperature, the tribological properties of the ceramic coatings were evaluated by friction and wear tests. The results indicate that the micro-pores in the C-containing coatings distribute uniformly on the alloy surfaces and a significant decrease in micro-pore size and surface roughness is observed. The surface hardness of the coatings show significant enhancement compared with that of the Mg-Li alloy. The coatings mainly consist of SiO₂, Mg₂SiO₄, and MgO phases; graphene is dispersed throughout via mechanical effects and displayed an antifriction effect. The C-containing coating produced when the volume fraction of graphene in the electrolyte is 1% show good wear resistance and its surface hardness and friction coefficient are 1317.6 HV_{0.1 kg} and 0.09, respectively. Meanwhile, compared with the Mg-Li alloy the wear traces on the coating appears narrower and shallow, and the worn area seems relatively smooth, which indicates that slight adhesive wear occurs on the C-containing coating surface.