Discharge Characteristics In Ag/C Heterogeneous Targets Hipims Process And Fabrication Of Ag-DLC Films

Owing to the excellent mechanical properties and biocompatibility,DLC films have been widely used in aerospace,medical devices,photoelectric devices and other fields.The doping of Ag elements has been applied to reduce the high internal stress,increase the adhesion strength of DLC films,and improves the optical and biomedical properties.However,due to a series of problems in traditional deposition methods,such as complicated equipments and uncontrolled composition of the film,the application of doped DLC films was extremely limited.To solve these problems,heterogenous targets high power impulse magnetron sputtering(Heter-Hi PIMS)metood was proposed in this research.In this study,Ag and C targers were employed,and series of positive and negative pulses were applied between this two targers.The magnitudes of different pulses were controlled by two direct current supplies,which stabilized the discharge process of Ag/C targets and increased flexibility of the processing parameters.Through Heter-targets Hi PIMS,the fabrication and structure regulation of Ag-DLC films were realized.The discharge behavior of dual-Hi PIMS with Ag/C targets has been studied by using electrical waveforms,ion currents on sample substrate and mass spectroscopy diagnosis of the plasma.With a constant pulse duration,the recycling mechanism of the sputtering particles was decided by discharge characteristics of targets material and the sputtering voltage.The discharge mode of graphite target was dominated by gas sputtering,while that of Ag target shifted from gas sputtering towards self-sputtering with the increasing of target voltage.According to the results of time-averaged and time-resolved diagnosis of the discharge plasma,it can be found that the high energy tail of ion energy distribution function(IEDF)was generated in the initial stage of the positive pulse,and along with the pulse-on time,the high energy ions were gradually replaced by the low energy ones which dominated the plasma in the range of 0～10 e V.For the dual targets sputtering system,due to the low sputter yield and ionization rate of C,the sputtering and ionization of Ag during the negative pulse was significantly suppressed when the discharge of graphite target was enhanced.Based on the plasma diagnosis for the discharge behavior of dual-targets Hi PIMS,the composition of Ag-DLC films were designed,and the Ag content of deposited samples ranged from 19.1 to 32.0 at.%.The surface morphology and structure of the Ag-DLC films were analyzed by SEM,XPS,XRD,Raman and TEM.With the rising of Ag content,the size of nanoparticles on the film surface gradually increased,and the agglomerates composed by nanoparticles could be found.When a higher bias voltage was applied,the sizes of nanoparticles were reduced,and the formation of agglomerates was also suppressed.No chemical bond was found between Ag and C,and Ag nanocrystals were dispersed in the amorphous carbon matrix.The doping of Ag improved the proportion of sp2 bonds and promoted the graphitization of carbon matrix,while the proportion of sp3bonds could be improved by using a higher bias voltage.During the deposition process,the nucleation and growth of Ag nanocrystals followed the Ostwald ripening mechanism,and nanocrystals nucleated in early time have larger grain sizes,and thus,the size of Ag nanocrystals gradually decreased along with the direction of film growth.For Ag-DLC films deposited with the same sputtering voltage of Ag target,the size of Ag nanocrystals near the surface region could be reduced by using a higher bias voltage.When the bias was raised from-50 V to-100 V,the size of Ag nanocrystals decreased from 5～12 nm to 2～5 nm.To better understand the relationship between the antibacterial performance and film structure,the surface roughness and nanoparticle size was measured by AFM,and the capability of releasing Ag~+was evaluated by ICP-MS.The inhibition zone test and survival test were both performed for E.coli and S.aureus.The galvanic corrosion between Ag nanoparticles and carbon matrix promoted the release and dissolution of Ag~+.With a certain Ag content,by using a higher bias voltage,the specific surface area of Ag nanoparticles on the film was increased,improving the capability of releasing Ag~+,and the Ag~+concentration after 24 h was up to 56.7 ppb.By increasing the quantity of Ag nanoparticles as well as reducing the particle size,inhibition area and antibacterial efficiency could be raised.The antibacterial efficiency of all samples was more than 70%.For the samples prepared with a bias of-100 V,whose Ag content ranged from 25 to 30at.%,the antibacterial efficiency was over 99.5%.