X.P. Zhu

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.

Lifetime of anode polymer in magnetically insulated ion diodes for high-intensity pulsed ion beam generation.

Generation of high-intensity pulsed ion beam (HIPIB) has been studied experimentally using polyethylene as the anode polymer in magnetically insulated ion diodes (MIDs) with an external magnetic field. The HIPIB is extracted from the anode plasma produced during the surface discharging process on polyethylene under the electrical and magnetic fields in MIDs, i.e., high-voltage surface breakdown (flashover) with bombardments by electrons. The surface morphology and the microstructure of the anode polymer are characterized using scanning electron microscopy and differential scanning calorimetry, respectively. The surface roughening of the anode polymer results from the explosive release of trapped gases or newly formed gases under the high-voltage discharging, leaving fractured surfaces with bubble formation. The polyethylene in the surface layer degrades into low-molecular-weight polymers such as polyethylene wax and paraffin under the discharging process. Both the surface roughness and the fraction of low molecular polymers apparently increase as the discharging times are prolonged for multipulse HIPIB generation. The changes in the surface morphology and the composition of anode polymer lead to a noticeable decrease in the output of ion beam intensity, i.e., ion current density and diode voltage, accompanied with an increase in instability of the parameters with the prolonged discharge times. The diode voltage (or surface breakdown voltage of polymer) mainly depends on the surface morphology (or roughness) of anode polymers, and the ion current density on the composition of anode polymers, which account for the two stages of anode polymer degradation observed experimentally, i.e., stage I which has a steady decrease of the two parameters and stage II which shows a slow decrease, but with an enhanced fluctuation of the two parameters with increasing pulses of HIPIB generation.

Nonlinear Wear Response of WC-Ni Cemented Carbides Irradiated by High-Intensity Pulsed Ion Beam

Surface hardening on WC-Ni cemented carbides was achieved by high-intensity pulsed ion beam (HIPIB) irradiation, with formation of a binderless, densified, and “hilly” remelted top layer of a few μm in depth and a shock strengthened underlayer down to a hundred μm. The tribological behavior of the samples was studied under dry sliding against GCr15 bearing steel on a block-on-ring tribometer with 98 N and 0.47 m/s. The specific wear rate/wear resistance presented an exponential dependence on the surface hardness, in contrast to the commonly reported linear dependence of the specific wear rate or wear resistance on the hardness of WC based cemented carbides among both WC-Ni and WC-Co systems. The original samples underwent a severe abrasive wear due to the Ni binder micro-abrasion and WC grain fragmentation/pullout, whereas the irradiated samples began with a gradual abrasion of the binderless hard tops, followed by a mild abrasive wear accompanied by local adhesive wear. The wear resistance has been also compared with the reported data concerning the relative hardness of friction pairs in a value range of 2–7 on block-on-ring tribometer tests with the friction pairs of WC cemented carbides and steels in unlubricated condition. The nonlinear wear response is explained by the wear mechanism transition otherwise unobtainable in the case of the reported hardening by either lowering the binder content or refining the WC grains. It is revealed that the interfacial bonding enhancement of the WC/binder and the binder strengthening are pronounced for improving the wear resistance of the cemented carbides, by the effective suppressing of the WC grain fragmentation/pullout and binder micro-abrasion, even though they have limited contribution to the hardness enhancement.

Initial plasma of a magnetically insulated ion diode in bipolar-pulse mode

Initial anode plasma formation in a magnetically insulated ion diode (MID) of bipolar-pulse mode with self-magnetic field is studied for high-intensity pulsed ion beam generation in TEMP-6-type apparatus [X. P. Zhu, M. K. Lei, and T. C. Ma, Rev. Sci. Instrum. 73, 1728 (2002)]. The field emission characteristics on graphite anode, the self-magnetic field, the perveance, and the impedance of the MID are obtained by analysis of the time-dependent diode voltage-current characteristics during the initial negative pulse stage of 100 ns, with peak values of 250 kV and 30 kA, respectively. The anode plasma is initiated by explosive emission on the anode due to a significant field enhancement effect up to four orders of magnitude, and the subsequent merging and movement of the anode plasma are affected by the superimposing space-charge effect and magnetic insulation of the self-magnetic field. The formation process of the initial anode plasma in the MID corresponds to the diode response under the initial negative ...

Microstructural Features of EB-PVD Thermal Barrier Coatings Irradiated by High-Intensity Pulsed Ion Beam

Thermal barrier coatings (TBCs) fabricated by electron-beam physical-vapor deposition (EB-PVD) were irradiated by high-intensity pulsed ion beam (HIPIB) at an ion current density of 100 A/cm2 with a shot number of 1-10. Microstructural features of the irradiated EB-PVD TBCs were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. All the HIPIB-irradiated EB-PVD TBC surfaces present smooth and densified features. The originated intercolumnar channels growing out to the top-coat surface and nanometer-scale gaps inside each single column were sealed after the remelting of TBC surface induced by HIPIB, resulting in formation of a continuous remelted layer about 1-2 μm in thickness. The dense remelted layer can work as a barrier against the heat-flow and corrosive gases, and gives the possibility of improving thermal conductivity and oxidation resistance of the HIPIB irradiated EB-PVD TBC.

Corrosion Behavior of AZ31 Magnesium Alloy with Microarc Oxidation Film Irradiated by High-Intensity Pulsed Ion Beam

The microarc oxidation (MAO) films on AZ31 magnesium alloy were modified by high-intensity pulsed ion beam (HIPIB) at an ion current density of 200 A/cm2 with 1-5 shots. The modified MAO films presented a corrosion resistance superior to that of the original films. Scanning electron microscopy (SEM) observation revealed that a sealing layer was formed on the MAO films by HIPIB irradiation. The corrosion behaviors of the MAO films in 3.5 % NaCl solution were characterized by using electrochemical impedance spectroscopy (EIS). The noticeable improvement in the corrosion resistance of MAO films is attributed to the blocking effect of the sealing layer that hinders the process of electrolyte penetrating the MAO films to the magnesium alloy.

Polymer Surface Textured with Nanowire Bundles to Repel High-Speed Waterdrops

Water drops impacting windshields of high-speed trains and aircraft as well as blades in steam turbine power generators obliquely and at high speeds are difficult to repel. Impacting drops penetrate the void regions of nanotextured and microtextured superhydrophobic coatings, with this pinning resulting in the loss of drop mobility. In order to repel high-speed water drops, we nanotextured polymer surfaces with nanowire bundles separated from their neighbors by microscale void regions, with the nanowires in a bundle separated from their neighbors by nanoscale void regions. Water drops with speeds below a critical speed rebound completely. Water drops with speeds exceeding a critical speed rebound partially, but residual droplets that begin to be pinned undergo a spontaneous dewetting process and slide off. The natural oscillations of residual droplets drive this dewetting process in the interbundle void regions, resulting in a transition from the sticky Wenzel state to the slippery Cassie state without external stimuli.

Features of focusing of a high-intensity pulsed ion beam formed by a diode with a passive anode

The results of investigating the focusing of a high-power ion beam, which is formed by a diode with a semicylindrical geometry and a passive anode, are presented. Two types of focusing diodes were investigated: with external magnetic insulation (one-pulse mode) and self-magnetic insulation of electrons (twopulse mode). Measurements of the energy-density distribution of the ion beam and the ion-current density were performed. It was found that when the diode operates in the two-pulse mode, the region of the maximum ion-beam energy density in the focal plane is displaced relative to the region of the maximum ion-current density by 5–10 mm. It is shown that the effect of a displacement of the focal spot with the maximum energy density is determined by the presence of a large number of accelerated neutral atoms in the ion beam. These atoms are produced as a result of the ion charge-exchange process in the anode–cathode gap of the ion diode during its operation in the two-pulse mode.

Deflection of high-intensity pulsed ion beam in focusing magnetically insulated ion diode with a passive anode

The focused high-intensity pulsed ion beam (HIPIB) of 100 ns order pulse is generated with respect to its spatial stability in two types of magnetically insulated ion diodes (MIDs) with geometrical focusing configuration using the passive anode, i.e., insulation of electrons with an external magnetic-field and a self-magnetic field, respectively. Anode plasma formation for the ion beam generation is based on different processes in the two types of MIDs, as the surface breakdown on the polymer-coated anode operated in the unipolar pulse mode for the external-magnetic field MID and the explosive electron emission on the graphite anode in the bipolar-pulse mode for the self-magnetic field MID. Typical energy density per pulse is in the range of 3–6 J/cm2, at an accelerating voltage of 200–300 kV with a pulse duration of 120–150 ns. The spatial deviations of the HIPIB is evaluated by measuring the energy density distribution by using an infrared diagnostic method considering neutralizing during the ion beam p...

Ion beam enhancement in magnetically insulated ion diodes for high-intensity pulsed ion beam generation in non-relativistic mode

High-intensity pulsed ion beam (HIPIB) with ion current density above Child-Langmuir limit is achieved by extracting ion beam from anode plasma of ion diodes with suppressing electron flow under magnetic field insulation. It was theoretically estimated that with increasing the magnetic field, a maximal value of ion current density may reach nearly 3 times that of Child-Langmuir limit in a non-relativistic mode and close to 6 times in a highly relativistic mode. In this study, the behavior of ion beam enhancement by magnetic insulation is systematically investigated in three types of magnetically insulated ion diodes (MIDs) with passive anode, taking into account the anode plasma generation process on the anode surface. A maximal enhancement factor higher than 6 over the Child-Langmuir limit can be obtained in the non-relativistic mode with accelerating voltage of 200–300 kV. The MIDs differ in two anode plasma formation mechanisms, i.e., surface flashover of a dielectric coating on the anode and explosive...