Plasma preparation method and tribological properties of diamond-like carbon coating on magnesium alloy AZ31 substrate

Magnesium alloys are light weight and exhibit good recyclability but suffer from low hardness and wear resistance. In this study, the hardness and wear resistance of AZ31 magnesium alloy were improved by depositing diamond-like carbon (DLC) films as hard protective coatings using ion-beam-enhanced deposition with various CH4/H-2 ratio, gas flow rates and accelerating voltages. The supporting effect of the magnesium alloy was enhanced by the production of a graded interfacial layer which is composed of film atoms and substrate atoms. The composition and mechanical properties of the DLC coatings were characterized using scanning electron microscopy (SEM), Raman spectroscopy, Rockwell test and nano-indentor. The tribological properties of the coating were also investigated using a frictional surface microscope with an in situ observation system and friction force measurements. The DLC films were characterized by a lower intensity ratio of the D-peak to G-peak (I-D/I-G), higher hardness, and improved tribological properties when deposited at a lower accelerating voltage (6 kV). At the CH4/H-2 ratio of 1:99 and 6 sccm/6 kV, minimum I-D/I-G values of 0.62, relatively low friction force value of 0.12 N, and a maximum hardness of 4056HV were attained respectively. In addition, the DLC film exhibited improved wear resistance and a shallower wear track at this condition.