Discharge Characteristics Of A Area-controlled Hollow Cathode And Its Effect On The Performance Of Si-DLC Films

The failure of the surface of large workpieces is an important bottleneck restricting the development of bearings,automobiles,machinery manufacturing and other industries.Deposition of functional films on the surface of the workpiece is an effective method to extend the working life and improve performance of the workpiece.Diamond-like carbon（DLC）film,as a kind of functional film,are widely concerned for their advantages such as low friction coefficient,high wear resistance,good biological properties,and corrosion resistance.The deposition of DLC films on the surface of large workpieces lacks a suitable plasma source and cannot be used to deposit films regionally according to actual needs.Among the many DLC film preparation methods,the cage-type hollow cathode not only retains the advantages of hollow cathode,such as high plasma density and fast deposition rate,also applied bias voltage between the cage and the workpiece to improv the performance of DLC films.However,it is difficult to completely cover the large workpiece in the cage.The workpiece will affect the discharge in the hollow cathode,resulting in abnormal discharge,and all surfaces must be deposited with thin films regardless of whether they are needed or not.The closed structure is also not conducive to the application of bias on large workpieces.According “subplantation model” for film growth,bias voltage is favorable for the formation of sp³ bonds in DLC films.Bias voltage adjusts the energy of bombardment ions,which can control and improve the structure and performance of DLC films.Bias voltage is the key parameter.Bias voltage adjusts the energy of bombardment ions,which can control and improve the structure and performance of DLC films.To solve those problems,a area-controlledhollow cathode discharge deposited DLC film technology is proposed,which is suitable for the regional customized film on the surface of large workpieces.The discharge characteristics of area-controlledhollow cathode and the performance and uniformty of Si-DLC deposited at various bias voltages are researched.Expect to solve the problem of lack of plasma source coating on the surface of large workpieces,and achieve the goal of controllable coating areaStructure of area-controlledhollow cathode was designed,the influence of the gap between cage and workpiece,cage height and grid size on hollow cathode discharge were studied.The bigger gap between cage and workpiece led to a hard overlap of negative glow region when the pressure was constant,and the higher ignition voltage was needed.The discharge current decreased with the increasing of gap.The experimental results show that under the premise of ensuring the insulation of the cage and the workpiece,the gap of 5mm is the optimized parameter.The increasing of cage height broadened the area that emitted electrons,the diacharge in hollow cathode was enhanced.The discharge current and the thickness of Si-DLC film got the maximum when the cage height was 200 mm.When the cage height exceeded 200 mm,the negative glow area of cathode was gradually separated from each other and the electrons oscillations was weakened,the discharge current of hollow cathode decreased.A smaller size grid was benefit to improve the discharge current of hollow cathode,while too small grid madean unstable jitter discharge at low pressure.The grid size of 1.0 mm×1.0mm was the optimized parameters.The mechanism of the influence of cage height on the hollow cathode discharge was explained.During Argon（Ar）discharge,under the condition of low bias（<-100 V）,the embedded consumption of space carriers by bias voltage led to the dropping of discharge current.When the bias voltage was higher than-100 V,the self glow discharge of the workpiece was enhanced to promote the hollow cathode discharge.Differ from the Ar discharge,the discharge current of Acetylene（C₂H₂）and Ar/C₂H₂/Tetramethylsiloxane（TMS）hybrided gas increased with the bias voltage.When the Ar flow rate is fixed,adding TMS promoted the hollow cathode discharge,the promotion effect of TMS was not obvious when the flow rate of TMS got a threshold.And the threshold of the TMS increased with the Ar flow rate.Fixed the flow rate of TMS,the Ar/C₂H₂ flow ratio increased,and the spectral intensity and discharge current change were parabolic.Cage current got the maximum when the flow rate ratio of Ar/C₂H₂ was 30 sccm/100 sccm.The uniformity of the Si-DLC thin film deposited with the area-controlledhollow cathode was studied.When the bias voltage was 0 V,the deposition rate and microstructure of Si-DLC films deposited by area-controlledhollow cathode discharge showed the ununiformity: the proformances of Si-DLC films along horizontal direction（xy plane）and vertical direction（z axis）showed the trend of middle large and edges small.The sp³/sp² value and hardness of Si-DLC films deposited at different position changed with the locations.To improved the uniformity of Si-DLC films,-200 V,-300 V and-500 V bias voltage were used to deposited Si-DLC films.Compared with the situation of 0 V bias voltage,the thickness,hardness and the wear resistance of the Si-DLC films deposited with bias voltage were improved,and the uniformity of Si-DLC films increased dramatically.However,all the performance of Si-DLC films improved slightly when the bias voltage over-200 V,and worsen the wear resistance of Si-DLC film deposited at-500 V bias voltage,the wear rate increased.The-200 V bias voltage is an optimization parameter that improved the uniformity.The mechanism of bias to improve the uniformity of Si-DLC films was explained.Si-DLC films were prepared with vrious bias voltages,and the effect of bias voltage on morphology,structure and properties were systematically studied.The deposition rate and surface roughness of the Si-DLC films showed a trend of decreasing first and then increasing with increasing of bias voltage.The deposition rate was the lowest at-200 V bias voltage,but still reached 6.88 μm/h.The bombardment of positive ions on the surface of workpiece were improved by the bias voltage,which would sputter away the particles which without firmly bonded,and then made the films densified.The hardness of Si-DLC films increased first and decreased slightly.The improving of hardness mainly dued to the dense structure of the film and the higher sp³ bond content.And the H content in the Si-DLC films decreased with the bias voltage.The samples with-200 V bias voltage showed the higher sp³ content,better wear resistance and lower coefficient of friction as well as wear rate.