Ablation Modification On Surface Irradiated By High-intensity Pulsed Ion Beam

The experimental and theoretical investigation of ablation behavior on graphite and ZrO₂ coatings irradiated by high-intensity pulsed ion beam(HIPIB) is performed in TEMP-6 and ETIGO-II HIPIB apparatuses with the ion energy of 300 keV and 1 MV,the pulse width of 50-80 ns,the ion current density of 100-1500 A/cm² and the energy density of 1-90 J/cm².The temperature field and stress field of the ablated surface are confirmed to explore the ablation effects mechanism of HIPIB irradiation on materials.Based on ablation effects of HIPIB, modification of microarc oxidation film on AZ31 magnesium alloy is achieved by forming a continuous and compact remelted layer on the surface of magnesium alloy to enhance the protective effect of microarc oxidation film(MAO) and further improve the corrosion performance of magnesium alloy.The experimental and theoretical investigation on the temperature field and stress field of ablated surface of graphite and ZrO₂ coating irradiated by HIPIB shows that the elevated temperature rate and temperature gradient of the irradiated surface could reach to 10¹²-10₁₃ K/s and 10⁹ K/m under the high energy density,respectively.The stress wave transmits in the irradiated materials periodically with a compressive stress and tensive stress alternately and the maximal value of compressive stress is obtained to 300 MPa.The phase transformation form graphite to diamond like carbon(DLC) on ablated graphite surface and the remelting and recrystalization of ZrO₂ columnar crystal could happen under HIPIB ablation effects.The remelting,evaporation and ablation on the irradiated surface induced the removal of the surface layer and the uniformity and compaction of the sublayer on irradiated materials,as well as a nonequilibrium surface structure was formed on the ablated surface under impulse stress and temperature gradient.The surface ablation modification is mainly attributed to thermo-mechanical coupling effects of HIPIB irradiation.The MAO films on magnesium alloy irradiated by HIPIB were performed at 100-350 A/cm² up to 10 shots.Surface remelting of MAO films with double layer structure of inner and outer sublayer becomes intense with the increase of ion current density and irradiation shot number,the apparent remelting characteristic could be observed on the irradiated surface at 200 A/cm².The thickness of remelted layer increased and then decreased and the maximal value of 10μm was obtained at 200 A/cm² with 5 shots,the porosity of the completely remelted sublayer decreased and inner sublayer became more compact.Surface roughness of the ablated surface increased and then decreased with the increase of shot number at 200 A/cm².The surface roughness for the original MAO film is about 2.10μm,it decreased to 1.18μm with 1 shot irradiation and then increased to 4.13μm with irradiation shots up to 10. Correspondingly,the surface energy of the ablated surface augmented,resulting in the tedious decrease of static contact angle from 145.9 for original film to 49.7 for the film with 10 shots.The phase structure of the ablated surface still consisted of Mg₂SiO₄ and MgO,which are the same as that of the original ones.The ablation modification enhanced the continuity and compaction of the MAO films on AZ31 magnesium alloy.Potentiodynamic polarization measurements and electrochemical impedance spectrum (EIS) in 3.5%NaCl solution were performed to test the corrosion performance of MAO films on AZ31 magnesium alloy.The corrosion process of the ablated surface transformed from the active solution to passivation-pitting breakdown.The passivation-pitting breakdown voltage increased and then decreased with increasing ion current density and shot number.With the increase of ion current density from 100 A/cm² to 350 A/cm² at 5 shots,the breakdown voltage increased from-1240 mV(SCE) to-800 mV(SCE),and then down to -1240 mV(SCE). On the contrary,the passivation current density show a tendency of decrease followed by an increase,which is described by an decrease from 1x10^-8A/cm² to 4×10^-9 A/cm²,and then up to 1x10^-7A/cm².With the increase of shot number from 1 to 10 at the ion current density of 200 A/cm²,the breakdown voltage increased from -1420 mV(SCE) to -800 mV(SCE). Correspondingly,the passivation current density decreased from 3×10^-7A/cm² to 4x10^-9 A/cm².The corrosion potentials of MAO films irradiated by HIPIB were all enhanced compared with those of original films.The corrosion potential was enhanced from -1580 mV(SCE) for original MAO films to maximal value of -1350 mV(SCE) for the irradiated films at 200 A/cm² with 5 shots.The anode polarization performance of the ablated MAO film was pronounced improved by HIPIB irradiation.The(EIS) in 3.5%NaCl solution was measured,in order to characterize the effect of modified continuity and compaction of the ablated MAO film on AZ31 magnesium alloy by HIPIB on its corrosion properties.The Nyquist plots for EIS shows typical capacitive arc and inductive loop,the phase-frequency curves show typical high-frequency(HF)and middle-frequency(MF)capacitive arcs and low-frequency(LF) inductive loop.The diameters for capacitive arc and inductive loop are both increased and then decreased with increasing the ion current density and irradiation shot number,the maximal value for diameters obtained at 200 A/cm² with 5 shots at immersion time of 5 h was achieved to 6x10⁷Ωand 1.5x10⁵Ω,respectively.No apparent changes were observed on the two capacitive arcs and inductive loop for Bode plots except for the width of MF capacitive arc.With immersion time augment, the diameters for capacitive arc and inductive loop of Nyquist plots both are decreased,and phase angle for HF capacitive arc and width for MF capacitive arc of Bode plots decreased, either.The diameter for the capacitive arc of Nyquist plots for ablated MAO films at 200 A/cm² with 5 shots decreased to 4.5x10⁵Ωwith immersion time up to 48 h,correspondingly, the LF inductive loop just appeared on Bode plots,the diameter for capacitive arc decreased continuously and no obvious changes observed on LF inductive loop with further increasing the immersion time.The EIS for ablated MAO films were fitted by Zsimpwin software,which indicated that the corrosion resistances for inner and outer sublayer are all improved by HIPIB irradiation. The corrosion resistance for inner and outer sublayer at 200 A/cm² with 5shots all increased from 1.52x10⁶Ωand 5.3x10⁴Ωto 7.31x10¹⁰Ωand 3.5x10⁵Ω,respectively,compared with the original ones at immersion time of 5 h.The enhancement in corrosion property of ablated MAO films is mainly caused by improvement of continuity and compaction of films irradiated by HIPIB.The ablation modification on MAO films on AZ31 magnesium alloy irradiated by HIPIB affords a new and reliable surface modification technique with pulse ion beam.