Plasma Ion Implantation / Deposition In Slender Bores Using Hollow Cathode Discharge

Plasma ion implantation / deposition of the inner wall of cylindrical components is always a difficult task in the plasma modification field, especially for tubes with small diameter or high ratio of length/diameter. In this thesis, a novel hollow cathode plasma source to treat the inner wall of tube is utilized. The plasma characteristics of the hollow cathode plasma soruce and surface modification of flat specimens have been investigated. Moreover, the plasma dynamic of ion implantation into inner wall of tubes and internal deposition of diamond-like-carbon (DLC) films and nitrogen ion implantation have been focused on in detail.The hollow cathode plasma system with composite grounded electrode for surface modification of inner walls of tube has been developed based on hollow cathode glow discharge. Stable plasma can be sustained in the tube with dimension such asФ20×500mm, and film deposition and nitrogen ion implantation can also be implemented by this hollow cathode plasma system.With free hollow cathode discharge configuration, the radial current density at cathode nozzle is decreased with increasing radial distance, and high RF power can obtain large current density. As for the confined hollow cathode discharge configuration (e.g. in tube), the plasma density decreases from hollow cathode outlet along the axial direction of tube, and plasma density exhibits nonlinear relationship with increasing gas flow and gas pressure. The tubes with large inner diameter and short length are beneficial for achieving uniform plasma density along axial direction of tube. In addition, the inner diameter of hollow cathode has great effect on plasma density. Small inner diameter hollow cathode is responsible for higher plasma density. Compared with floating potential of tube, the grounded tube also gains higher plasma density. The discharge breakdown is difficult when the shielding grounded electrode is employed. More specifically, with increasing distance between the outlet of shielding grounded and hollow cathode nozzle, the plasma density increases using the same RF power. The optimal inner diameter of shielding grounded electrode can give rise to the highest plasma density, and smaller or larger inner diameter appears to affect adversely the plasma density. Plasma density is higher with grounded electrode compared to non-shielding grounded electrode hollow cathode system.Deposition of TiN and DLC film, and nitrogen ion implantation are successfully performed on stainless steel sample with 20mm in diameter. The XRD result indicates TiN films trends to (111) preferred orientations with increasing sample bias. The atomic force microscope (AFM) results reveal low surface roughness dominated by an island-like morphology with similar crystal size on the entire surface, and the film thickness is also quite uniform across the sample surface. All the DLC films have good wear resistance. In addition, the highest sp3 bond fraction, the largest thickness, and the highest hardness of film are also achieved at a bias voltage of -50V. Nitrogen ion implantation in stainless steel sample leads to the substantial improvement of corrosion resistance. The sample treated at 15kV exhibits the higher hardness and the better wear resistance.The dynamic of ion implantation in tube based on hollow cathode discharge has been simulated by particle-in-sell (PIC) model. The radial electrical field is induced by inserting central grounded electrode, which improves the radial velocity of incident ions and gains good modification effectiveness. Longer central grounded electrode and smaller inner diameter shielding grounded electrode are beneficial for achieveing higher radial velocity of incident ions. Our results also suggest that the higher voltage applied to tube, the higher radial velocity of incident ions, but ion implantation dose doesn't change obviously. The higher plasma density may lead to the reduction of radial ion velocity in spite of higher ion flux.Inner surface modification of tube with 20mm- 40mm in inner diameter, 140mm in length, through DLC films deposition and nitrogen ion implantation has been accomplished by hollow cathode plasma source with composite grounded electrode. The film thickness varies from 1 to 2μm for a given treated mode, and the highest film depositon rate can reach to 65.7nm/min. Raman spectra results show that all the films have two peaks at around 1590cm-1, commonly labeled as G (graphite) peak and at around 1370cm-1, labeled as D (disorder) peak. The DLC film thickness displays nonlinear trend with increasing sample bias and gas pressure. The samples treated by 4kV and 6kV have low Id/Ig value and good wear resistance. The sample treated at 0.8Pa and 1.0Pa display better wear resistance when different gas pressure is employed. As for the whole tube DLC deposition, thick films with good wear resistance can be gained by reducing inner diameter of treated tube and prolonging treated time. The XPS results show that the metallic nitrides, such as FeN, CrN are formed in the nitrogen ion implanted layer, and sample surface has high nitrogen concentration. The wear resistance of ion implanted samples depends on the processing parameters, such as RF power, inner diameter of tube, implantation voltage and frequency. All the treated samples have substantial improvement of corrosion resistance.