M.K. Lei

Characterization of a high-intensity unipolar-mode pulsed ion source with improved magnetically insulated diode

A magnetically insulated ion diode (MID) with an improved external-magnetic field system has been developed and installed onto a TEMP-6-type high-intensity pulsed ion source in order to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. The external-magnetic field MID is operated in unipolar mode based on dielectric high-voltage flashover, and a double coaxial pulse-forming line (PFL) powered with a Marx generator is used to form the unipolar pulse of nanosecond width. A specially designed cathode has been constructed with a forked connection to two symmetrically installed transformers to improve the effect of the magnetic field and thus increase the stability of generation and propagation of the ion beam. It was found that the efficient generation of HIPIB mainly depended on the magnetic field strength, the gas pressures in reverse and output switches of PFL, and the anode–cathode (A–K) gap of the external-magnetic field MID. A proper magnetic field strength was found...

Surface morphology of titanium irradiated by high-intensity pulsed ion beam

Abstract Surface morphology of pure Ti irradiated by high-intensity pulsed ion beam (HIPIB) with ion current density of 60–250 A/cm 2 and shot number of 1–30 at 220 kV has been investigated by using profilometer, scanning electron microscopy and atomic force microscopy to explore the interaction mechanism between HIPIB and metallic materials. Two kinds of samples were prepared with different initial surface roughness ( R a ), i.e. high-roughness and low-roughness Ti, respectively. A similar change trend of R a was found on the irradiated surfaces for both the two kinds of samples that increase of the R a was obtained with few shot number and then continuous decrease of the R a to a surface smoothing was with multi-shot irradiation. However, the increase of R a on the irradiated low-roughness Ti was greatly limited as compared to the high-roughness case. It is demonstrated that surface smoothing and roughening of irradiated Ti can be realized under optimization of the parameters by combining adjustments of ion current density and shot number. The morphology features on the irradiated high-roughness Ti were craters and waviness for roughened surfaces and some vague or apparent texture with disappearance of the craters for smoothed surfaces. For the low-roughness Ti, no distinct craters were formed except for some obvious texture. The micro-non-uniformity (micro-protrusions) on the irradiated surfaces causes the cratering on pure Ti by inducing a selective ablation under HIPIB irradiation. The locally more intense liquid evaporation and droplet ejection from the irradiated surfaces of different morphologies led to disturbance of the molten surface layer in the different scales, resulting in the surface roughening and smoothing.

Characterization of a high-intensity bipolar-mode pulsed ion source for surface modification of materials

A high-intensity pulsed ion source of TEMP-type series, operating in bipolar mode, has been developed as a unique pulsed energy source to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. To generate the ion beam, a specially shaped bipolar pulse, consisting of a first negative pulse and a second delayed positive pulse both of nanosecond width, is formed by a double coaxial pulse-forming line (PFL) powered with a Marx generator and supplied to a magnetically insulated ion diode (MID) by a self-magnetic field. It is found that the efficient generation of a HIPIB is mainly dependent on the delay time of the bipolar pulse, adjusted by pressure ratio in the two gas switches of a PFL, and the anode–cathode (A–K) gap distance in the self-magnetic field MID. The delay time determines the effective area on the anode surface for plasma generation and the A–K gap distance ensures the stability of the process. A proper delay time and a proper A–K gap distance are obtained by a s...

Pitting corrosion resistance of high nitrogen f.c.c. phase in plasma source ion nitrided austenitic stainless steel

Abstract Plasma source ion nitriding has emerged as a low-temperature, low-pressure nitriding approach for low-energy implanting nitrogen ions and then diffusing them into metal and alloy. In this work, 1Cr18Ni9Ti (18-8 type) austenitic stainless steel was treated at a process temperature of 380°C during a nitriding period of 4 h. A single high nitrogen f.c.c. phase (γ N ) with a high nitrogen concentration of 32 at.% was characterized using Auger electron spectroscopy, electron probe microanalysis, glancing angle X-ray diffraction, and transmission electron microscopy. The pitting corrosion resistance of the γ N phase layer was measured by cyclic polarization in a series of 3% NaCl solutions buffered to pH from 0.4 to 13. In the potential–pH diagram of the γ N phase layer, the extended immunity and perfect passivity zones and the narrowed imperfect passivity and pitting zones were obtained, compared with that of the original austenitic stainless steel. No pitting corrosion resistance was observed for the γ N phase layer in the solutions of pH 4–13. The high supersaturation of nitrogen in the γ N phase led to the improvement in the pitting corrosion resistance.

Plasma-based low-energy ion implantation for low-temperature surface engineering

Abstract This paper summarizes the plasma-based low-energy ion implantation technique, including plasma source ion nitriding/carburizing and plasma source low-energy ion enhanced deposition of thin films, developed from a combination of two techniques based on conventional plasma-based ion implantation and low-energy ion beam implantation for improvement in wear resistance and corrosion resistance for metals and alloys. An electron cyclotron resonance (ECR) microwave plasma source is used to produce the plasma with the high plasma density, electron temperature and ionization degree. The ions are accelerated from the plasma by a low pulsed negative bias of −0.4–−3 kV, which is similar to the cathode potential of conventional plasma thermo-chemical diffusion processing. The low process temperature is in the range from 150°C to 500°C, which corresponds to the upper limit of conventional ion beam implantation and to the lower limit of plasma thermo-chemical diffusion processing, respectively. Low-energy ion implantation and simultaneous indiffusion is the main mass transfer mechanism, and direct thermo-chemical diffusion absorption is an additional mass transfer mechanism for formation of the nitrided/carburized layer and thin film. It has been proved that plasma-based low-energy ion implantation technique has the potential for applications in industry for surface modification of metals and alloys.

Plasma source ion nitriding of pure iron: formation of an iron nitride layer and hardened diffusion layer at low temperature

Abstract Plasma source ion nitriding is a new low temperature, low pressure nitriding approach for the surface modification of iron and steel. A nitriding apparatus based on an electron cyclotron resonance (ECR) microwave plasma source has been developed. Nitrogen ions are accelerated from the ECR microwave plasma by a pulsed negative bias (typically − 2 kV) applied directly to the sample, are implanted, and are finally diffused to a depth below the surface at elevated temperatures which are regulated up to 450 °C by an auxiliary heater. An experimental investigation of plasma source ion nitriding into pure iron is described. The nitrided samples were characterized using optical metallography, microhardness measurement, glancing angle X-ray diffraction and transmission electron microscopy. The effect of the process temperatures on the surface hardness, the hardness-depth profile and the microstructure of nitrided pure iron has been investigated. On the basis of these findings, it has been established that plasma source ion nitriding of pure iron can produce an iron nitride layer and a hardened diffusion zone at process temperatures from 150 to 450 °C.

Crater formation on the surface of titanium irradiated by a high-intensity pulsed ion beam

Abstract Surface morphology and roughness of pure Ti irradiated by a high-intensity pulsed ion beam (HIPIB) have been investigated by using scanning electronic microscopy and profilometry in order to explore the interaction mechanism between HIPIB and metallic materials. Two groups of Ti samples of different initial surface roughness (Ra) were prepared to determine the effect of original surface states on the crater formation as a result of HIPIB irradiation. Particularly, the cratering behavior under various irradiation intensities was clarified by examining large Ti samples of high initial roughness with Ra of 0.18 μm. It is noted that no obvious cratering took place on Ti of low initial roughness. For high-roughness Ti, the Ra significantly increased from an initial value of 0.18 μm to the maximal 0.43 μm at 250 A/cm2 with 1 shot, and then decreased continuously to a final 0.06 μm with 30 shots presenting a planar ablated surface. The similar trend for Ra was also found for the low-roughness Ti, but the maximal value was limited to 0.18 μm from the initial 0.07 μm. The micro non-uniformity of the original surfaces resulted in the cratering on pure Ti by inducing a selective ablation and disturbance of the molten surfaces under HIPIB irradiation. A more uniform ablated Ti surface without crater formation was obtained by multi-shot irradiation due to the gradually decreased non-uniformity at the repetitive ablation.

Plasma source low-energy ion-enhanced deposition of thin films

Abstract Plasma source low-energy ion-enhanced deposition is a new approach for the deposition of thin films on steel and alloys with specific advantages over ion beam-enhanced deposition (IBED) and the complex combined plasma source ion implantation (PSII) and IBED (PSII-IBED). It may be used to develop a thin film process using the plasma-based low-energy ion implantation including plasma source ion nitriding and plasma source ion carburizing. A deposition apparatus based on an electron cyclotron resonance (ECR) microwave plasma source, a magnetron sputtering target and a pulsed low-voltage power supply from 0.5 to 2xa0kV has been developed. The magnetron sputter deposition of Ti metal films and plasma source low-energy nitrogen ion implantation under a high nitrogen ion implantation dose rate of 0.65–1.20xa0mA/cm 2 are used alternately in order to synthesize the stoichiometric TiN films on A3 mild steel and M2 high-speed steel at a deposition rate of 50–110xa0nm/min. The Knoop microhardness of the 1.2–1.6xa0μm thick TiN films on the M2 steel is in the range of 18.0–20.0xa0GPa (0.05xa0N load). The critical loads of the TiN films on the M2 steel measured by the scratch test are 22–35xa0N corresponding to the formation of an intermixed layer of about 40xa0nm thick between the film and the steel substrate. The corrosion resistance of the TiN films in 0.5xa0mol/l H 2 SO 4 solution is evaluated by performing electrochemical polarization measurements. Their superior corrosion resistance is a result of the dense fine-grained microstructure and is independent of the preferential orientation of the films.