Study Of Enhanced Discharge Of Graphite Cathode Arc And Deposition Of Ta-C Coatings

The ta-C coatings have many excellent properties comparable to those of diamond,such as good chemical stability,high hardness and excellent wear resistance,good biomechanical properties and biocompatibility.There can be an order of magnitude difference of the thermal expansion coefficient between the ta-C coating and high speed steel.If the ta-C coatings are deposited directly on high speed steel,the great difference of hardness,elastic modulus and expansion coefficient will lead to high stress between the coating and the substrate,and the adhesion strength is poor.In order to improve the adhesion strength,the bottom layer and transition layer are usually deposited between the substrate and the ta-C coatings to achieve the gradient change of physical properties such as hardness,modulus of elasticity and coefficient of expansion.In this paper,the discharge characteristics of graphite cathode arc were studied by changing the experimental parameters such as coil current,DC arc current,working pressure and average pulse current.The substrate/Cr/CrC/ta-C gradient film structure was designed to deposite ta-C coating,and the ta-C coating was prepared on M2 high speed steel and single crystal Si wafer by vacuum cathode arc ion plating technology.The effects of different parameters on the microstructure,phase structure and the mechanism of the coatings were investigated,and the effects of different parameters on the adhesion strength,friction and wear properties and hardness of the coatings were studied.The discharge test results indicated that the processing parameters had great influence on the discharge characteristics of graphite cathode arc.The substrate current increased with the increase of arc current,and the arc voltage was almost constant;the substrate current increased with the increase of the coil current,and the arc voltage also increased;with the increase of Ar gas pressure the substrate current increased first and then decreased,and the arc voltage was almost constant;the substrate current waveform was similar with the pulse current waveform,and the arc voltage obviously increased during the pulse-on period.The structure analysis of ta-C coating indicated that when the deposition time was over 30 min,the ta-C coating started to fall off,and the ID/IG value increased with time and the sp3 bond content decreased.With the increase of coil current,the sample was bombarded more strongly,and the overall trend of the ID/IG value increased.The ID/IG value decreased first and then increased with the increase of arc current,and the sp3 bond content got the maximum at 60 A.The properties analysis of ta-C coating indicated that the adhesion strength between the coating and substrate decreased gradually with time,and the surface of 50 min coating appeared wrinkling.When the coil current increased,the bonding strength between the film and the substrate decreased and then increased.With the increase of arc current,the bonding strength decreased,but all of them were over the HF2 level.When the current was equal,the bonding strength of the pulse current was obviously lower than that of DC.The maximum hardness of the sample in the coil current series experiment was 33.5GPa with the coil current of 0.1A.The maximum hardness of the sample in the pulse current series experiment was 45 GPa with the pulse average current of 80 A.Hardness and the graphitization are two major factors that affect the friction coefficient.