Ricky Fu

As implanted bone fixation materials, magnesium (Mg) alloys have significant advantages because the density and elastic modulus are closest to those of the human bone and they can bio-degrade in the physiological environment. However, Mg alloys degrade too rapidly and uncontrollably thus hampering clinical adoption. In this study, a highly corrosion-resistant zinc-phosphate-doped micro-arc oxidation (MAO) coating is prepared on the AZ31B alloy, and the degradation process is assessed in vitro. With increasing zinc phosphate concentrations, both the corrosion potentials and charge transfer resistance of the AZ31B alloy coated with MAO coatings increase gradually, while the corrosion current densities diminish gradually. Immersion tests in the simulated body fluid (SBF) reveal that the increased zinc phosphate concentration in MAO coating decreases the degradation rate, consequently reducing the …

In the present research, the NaF assisted plasma electrolytic oxidation (PEO) is designed to fabricate the high-content ZnO nanoparticles doped coating on AZ31B alloy. The microstructure, phase constituents and corrosion behavior of the PEO coatings are investigated systematically. The results reveal that the introduction of NaF promotes the formation of MgF2 nanophases in the passivation layer on Mg alloy, decreasing the breakdown voltage and discharge voltage. As a result, the continuous arcing caused by high discharge voltage is alleviated. With the increasing of NaF content, the Zn content in the PEO coating is enhanced and the pore size in the coating is decreased correspondingly. Due to the high-content ZnO doping, the PEO coating protected AZ31B alloy demonstrates the better corrosion resistance. Compared with the bare AZ31B alloy, the high-content ZnO doped PEO coated sample shows an …

In the present study, a three-dimensional multi-interface structured zinc phosphate particle with large specific surface area was prepared by regulating the zinc phosphate crystal growth process, which was applied to develop a waterborne epoxy resin coating with better corrosion resistance. The results reveal that the multi-interface structured zinc phosphate particles could well coalesce with epoxy resin. Compared with the waterborne coating doped with and without two-dimensional zinc phosphate sheets, the multi-interface structured zinc phosphate particles doped waterborne epoxy resin coating has highest corrosion potential, lowest corrosion current density and the highest low-frequency impedance, which indicates the obvious benefits of multi-interface structured zinc phosphate particles on corrosion resistance. Moreover, the electrochemical properties tests on the waterborne epoxy resin coatings immersed …

Corrosion protection of magnesium alloys is a challenge in industrial applications because of the high chemical reactivity of Mg. In this work, a coating is prepared by plasma electrolytic oxidation (PEO) on the AZ31B Mg alloy and then modified by insoluble zinc phosphate to improve the corrosion resistance. To avoid the influence of the weak acidic Zn-containing electrolyte on the formation of the passivation layer on the magnesium alloy, a two-step process is designed to first produce a passivation layer before introduction of zinc phosphate. Zinc phosphate is mainly distributed on the surface and the sidewalls of the holes in the porous coating and blocks the interactions between the corrosive solution and Mg substrate. The corrosion potential of the optimal coating (Zn-15) increase while the passive current density at the terminal potential decreases, suggesting the significantly improved corrosion resistance …

Coherent grain boundaries (CGBs) has been revealed to improve the hardness and toughness simultaneously, but the density are not enough to increase their mechanical properties. Herein, hetero-interfaces are formed by introducing ion bombardment during magnetron sputtering to increase the amount of CGBs in the CrN coatings. By optimizing the sub-layer thickness, the quantity and width of CGBs from the (111) to (200) orientations increase significantly and consequently, the hardness and toughness of the CrN coating increase to 33.4 GPa and 14.98 MPa m1/2, respectively. At last, the effect of the different CGBs to the coating strengthen is discussed.

Etching of the cathodes in magnetron sputtering determines the plasma discharge properties and deposition efficiency. In high-power and high-ionization discharges, etching becomes more complicated, resulting in inaccurate results if the conventional models are still used. This work aims at establishing an accurate dynamic model for high-power and high-ionization discharges by combining the cellular automata (CA) method and particle-in-cell/Monte Carlo collision (PIC/MCC) method, in which all the interactions pertaining to the etching morphology, plasma density, electric field, and magnetic field are considered. In high-power discharges such as continuous high-power magnetron sputtering (C-HPMS), strong self-sputtering and intense gas rarefaction stemming from the high temperature in the vicinity of the target influence the etching behavior. Compared to the experimental results, the morphology simulated …

Although aluminum alloys are widely used in the aerospace and automotive industry, they are susceptible to local corrosion and wear during long-term service in harsh environment. In this work, micro-arc oxidation (MAO) coatings containing quasi-2D sericite microplates are prepared on the 2024 aluminum alloy, and their corrosion and wear resistance are investigated. The results reveal that the sericite-incorporated MAO sample prepared in the 15 g/L sericite electrolyte (M−15) possesses the best corrosion resistance in 3.5 wt% NaCl solution, as manifested by a corrosion current density of 2.93 × 10−10 A ∗ cm−2 that is 4 orders of magnitude less than that of the 2024 Al alloy substrate. The wear resistance of MAO coating is also improved by the addition of sericite microplates, which is shown by the low wear rate and reduced coefficient of friction (∼0.3) of M−15 against steel ball. Our mechanistic study reveals …

Hydrogen embrittlement has become a hot research topic due to the rapid development of hydrogen energy and coating technology is regarded as the most efficient method to mitigate hydrogen embrittlement. However, the grain gaps in coatings frequently serve as paths for hydrogen permeation thus decreasing the protecting effects. In this work, ion bombardment is performed during magnetron sputtering deposition of dense CrN coatings to decrease the formation of grain gaps. The compactness of the CrN coatings is improved by disrupting the growth of grains using energetic ion bombardment. Hydrogen permeation tests reveal that the apparent hydrogen diffusion coefficient and hydrogen permeability of the dense CrN coating 52.6 and 24.1 times less than those of the unprotected substrate. The tensile test also reveals excellent hydrogen embrittlement resistance compared to the X70 substrate and …

Continuous high-power magnetron sputtering (C-HPMS) is one of the desirable techniques to fabricate coatings with high hardness and a smooth surface to meet the demand by micro-millings for processing/cutting of printed circuit boards (PCBs). However, catastrophic failure can occur on the coatings because of the high residual stress in contact with hard glass fibers in PCBs at a high cutting speed. In this work, CrAlN/CrAl multilayer coatings consisting of thin CrAl interlayers are fabricated and characterized. The thin CrAl layers reduce the residual stress significantly from −6.7 GPa to −1.5 GPa, while the hardness of 35 GPa is preserved. Furthermore, the adhesion strength and wear resistance are improved appreciably by addition of the interlayers. The PCB cutting tests reveal that the CrAl layers mitigate sudden failure of the coatings and improve the cutting stability and service life by near two times.

T-shape structure can make inorganic materials superhydrophobic, but conventional preparation methods based on etching is usually costly and unscalable. In this work, a facile and scalable technique utilizing plasma electrolytic oxidation (PEO) is employed to prepare superhydrophobic inorganic alumina coatings. By optimizing the gradient electric field, the etching behavior of OH– is tunable so that a T-shape micro/nano structured alumina coating can be prepared in one step. Without any organic modification, the coating shows a water contact angle (CA) of 150°±3° and water rolling angle (RA) of 8°±2°. The prepared inorganic ceramic coating remains superhydrophobicity under a harsh environment including 360 days of sunlight, and they have excellent corrosion resistance and can bear long-time flame burning. The preparation technique is simple and economical thereby boding well for great potentials in …

Although aluminum alloys are widely used in the aerospace industry, their relatively poor wear and corrosion resistance have hampered some applications and reduce the lifetime of the components. In this work, by using a zinc-containing phosphate electrolyte, the corrosion resistance, surface hardness, and wear resistance of the coatings produced by plasma electrolytic oxidation (PEO) are simultaneously improved. The micro-arc discharge is optimized by changing the phosphate concentration instead of introducing hard phase cations to improve the crystallinity of the zinc-doped alumina coating. Our results reveal that by using a concentration of (NaPO3)6 of 50 g/L, the hard phase of crystralline alumina and corrosive-resistant phase of Zn3(PO4)2 are produced in concert. In the salt spraying test, the lifetime of the PEO coating is over 5000 h and the electrochemical corrosion current density is reduced by about 2 …

High-power pulsed magnetron sputtering (HiPIMS) can produce high density and high adhesion coatings due to the high ionization. However, industrial application of HiPIMS is limited because of the unstable discharge and small deposition rate. A cylindrical cathode, developed on the basis of hollow cathode effect, can improve the discharge stability. With the development of electromagnetic systems, the plasma transport is improved, and thus increasing the deposition rate significantly. However, the introduction of electromagnetic system leads the strong discharge and large etching area on the target to be incompatibly controlled. In this work, the distribution of the tangential and longitudinal magnetic field on the target surface are improved by adding external magnets, and their effects on the plasma discharge are studied. By optimizing the magnets, the tangential magnetic field on the target surface becomes …

Microarc oxidation (MAO) is a popular surface treatment process to generate oxide coatings with excellent mechanical properties on valve metals. As a plasma discharge technique, the discharge mechanism in MAO is different from that in the bipolar plate. Due to the alternating arcs and multiple electrolytes in MAO, it is difficult to control and optimize the coating properties. Based on the arcing mechanism and ion motion, the boundary conditions of no-arc discharge, alternating arc discharge, and continuous arc discharge are derived, and the relationship between the discharge current and breakdown current and sustained arc current is investigated. For the same electrolyte, the boundary conditions are determined by equivalent concentration and conductivity. The results show that in the stable alternating arc discharge regime, the higher the concentration and smaller the conductivity, the more intense is the …

Titanium alloys are important aerospace metals but suffer the poor mechanical property in high-loading area. Plasma electrolytic oxidation can improve their hardness and reduce friction but the precursor competition of functional phases in PEO electrolyte is inevitable. This work provides a new strategy to avoid ions competition and precipitation by choosing anionic salt (K2ZrF6) and cationic salt (Ce(Ac)3) as the precursors of the hard phase (ZrO2) and lubrication phase (monoclinic CePO4), respectively. As a result, the ions competition is avoided to achieve simultaneous high-content doping of functional phases. The optimized coating shows a maximum hardness of 626 HV, small friction coefficient of 0.12 and a lower wear rate of only 3.1 × 10−5 ± 0.15 mm3 N−1 m−1, indicating excellent long-term self-lubrication property and wear resistance.

Target poisoning is prevalent in reactive magnetron sputtering and causes deleterious effects, especially in deposition using metal targets. Both the deposition rate and the stoichiometry ratio of the coatings will be compromised because of the quicker coverage of the poisoning materials on the target relative to removal by sputtering. Continuous high-power magnetron sputtering (C-HPMS) possesses merits such as more rapid sputtering than high-power impulse magnetron sputtering and better target poisoning resistance. In this work, the discharge and plasma characteristics of C-HPMS for an Al metal target in reactive O 2/Ar atmospheres are investigated by modeling and alumina deposition. At a constant oxygen partial pressure, larger discharge power increases target etching significantly compared to surface combination with O 2, leading to reduced surface poisoning. Besides, a higher temperature is produced …

To obtain both high ionization and high deposition rate, a modified global model for a continuous high-power DC magnetron sputtering (C-HPMS) is established by considering the continuous generation of the hot electrons and the high temperature caused by continuous high-power discharge. The results show that the plasma density is on the order of 10 19 m− 3 for power densities of only 183 W cm− 2 (Al) and 117 W cm− 2 (Cu). The ionization rate exceeds 90% of high-power impulse magnetron sputtering (HiPIMS)(peak power density of 564 W cm− 2) for a DC power density of 180 W cm− 2, and the total diffusion fluxes of the two targets are 26 (Al) and 30 (Cu) times that of conventional HiPIMS, leading to very high deposition rates. The work provides a theoretical basis for the realization of C-HPMS and gives an enlightenment to the development of deposition equipment for continuous high-power discharges.

The self-layered phenomenon has been observed from the hydrothermal processing of magnesium and Mg alloys, but it is still difficult to controllably fabricate a self-layered hydrothermal (SLH) coating due to the unclear formation mechanism and complex influencing factors. In this work, titanium ion implantation is adopted as a trigger process to modify the surface of pure magnesium and induce the formation of self-layered hydrothermal film. Compared to the conventional hydrothermal process that yields a relatively smooth and single-layer coating on pure magnesium, this new method produces a magnesium hydroxide-based SLH coating composed of an inner compact layer and outer nanosheet-based layer. Characterization results show that a Ti-implantation layer with titanium nanocrystals embedded in Mg matrix was formed after Ti-ion implantation. The galvanic effect between Ti nanocrystal and magnesium …

Systematic analysis of discharge processes is needed for a good understanding of the physical mechanism that enables optimal coating deposition, especially pulsed discharges sustained by high voltages and large currents. Owing to the temporal and complex characteristics of the discharge process and relatively simplistic analytical methods, the discharge process and particle evolution in high-power impulse magnetron sputtering (HiPIMS) are still not well understood. In this work, a cylindrical cathode is introduced to restrict the discharge and delay plasma loss, and a global model is established to simulate the discharge on a Cr target in N 2/Ar. Particles with different reaction energies appearing successively produce an asynchronous discharge phenomenon, and a series of inflection points corresponding to different physical processes including excitation, sputtering, ionization, and diffusion are observed from …

Superhydrophobic surfaces with large contact angles (CAs) and small sliding angles (SAs) are used in many applications such as wings of airplanes or engine front cones to reduce air crashing caused by icing. Most anti-icing studies have mainly focused on the icing behavior on surfaces with specific hydrophobic properties but there have been fewer investigations on the dynamic change of hydrophobicity of the materials during the icing process. In this work, micro/nano structures are prepared on stainless steel by laser etching and modified with a fluorine-doped diamond-like carbon (F-DLC) coating to produce a superhydrophobic surface. The dynamic behavior of the wetting state and icing of water droplets on the F-DLC surface during cooling are studied. As the temperature drops, the hydrophobicity of the surface decreases significantly before freezing and the rate of decrease is related to the temperature and …

In spite of advances in integrated circuit technology, dust and corrosion continue to play detrimental roles in electronic and device components such as electrodes and wires, causing possible short circuits and arcing, consequently undermining the long‐time durability of devices. Super‐hydrophobic Cu/CuO films with easy cleaning characteristics and excellent corrosion resistance are prepared by one‐step high‐power magnetron sputtering and tested under high salinity conditions. Self‐regulation of the preferred orientations from Cu (200) to Cu (111) and formation of micro/nano‐rod surface features can be tuned by changing the power density. After exposure for 96 h in air, CuO (111), which has a small surface energy, is produced in situ on the surface of the Cu (111) micro/nano‐rods. Such a super‐hydrophobic surface with a contact angle of 152.5° has attractive properties such as excellent protection against …

In physical vapor deposition (PVD), the energy and incident angle of ions can be adjusted easily by applying an electromagnetic field so that coatings with different structures and properties can be produced. In this work, several high-ionization coating techniques are employed to deposit TiAlN coatings to compare the effectiveness of each technique and resulting coating properties. The continuous high-power magnetron sputtering (C-HPMS) technique shows advantages in the deposition rate and coating performance. In addition, a deposition rate of 400 nm/min is achieved by C-HPMS, whereas those of high power impulse magnetron sputtering (HiPIMS) and arc ion plating (AIP) are 85 nm/min and 225 nm/min, respectively. The coating also shows a particle-free and dense morphology with a smaller surface roughness of 22.4 nm. Owing to efficient ionization, the TiAlN coating deposited by C-HPMS has a high …

Li(NixMnyCoz)O2 cathode materials (NMC) have advantages such as the good Li ion diffusivity, stable reversible capacity, and environmental compatibility in spite of a low actual capacity. Although a double-layer lithium structure can be generated by pre-lithiation, the thickness is very small and the capacity improvement is limited. In this work, a series of Li()O2 nanoparticles are prepared by the sol–gel method and centrifugation and the pre-lithiation process are monitored by transmission electron microscope (TEM). A double lithium structure of about 10 nanometers thick is produced on the NMC materials with different sizes. With decreasing NMC particle size, the proportion of the double-layer lithium structure increases and reaches 48% for a particle diameter of 100nm. The results reveal a viable means to improve the capacity of NMC materials in charging and discharging.

The nose cones of plane engines adjust the airflow direction and guarantee steady operation but they are vulnerable to icing at high altitude leading to potential hazards and accidents. Superhydrophobicity is effective against icing but most organic superhydrophobic materials are not suitable for engineering applications. Herein, a mechanically robust superhydrophobic coating is prepared on stainless steel by incorporating laser-textured micro/nano structures into the plasma-coated multilayered structure of Cr/CrC/DLC/F-DLC. The multilayered coating shows a stable surface contact angle of >150° in addition of robust mechanical properties even in harsh environments such as prolonged ultraviolet light irradiation, wear, and corrosion. Tests conducted under simulated working conditions reveal significantly reduced ice accumulation on the coating because of easy roll-off of droplets consequently giving rise to …

Aluminum alloys are widely used in many industrial applications. However, they are susceptible to corrosion especially in aggressive media such as high salinity which can expedite the corrosion reactions. In this work, zinc phosphate modified plasma electrolytic oxidation (PEO) coatings are fabricated and demonstrated to have excellent corrosion resistance for a much longer duration (>11,000 h) than conventional PEO coatings (<1,000 h). Insoluble zinc phosphate is distributed in the pores of the coating serving as a barrier against the high-salinity medium. A self-healing mechanism is discovered as zinc phosphate migrates from the surface to pores continuously wetting the corrosion droplets on the super-hydrophilic coating until all the zinc phosphate is consumed.

TiAlN coatings are widely used in high-speed cutting, machining, and other high-temperature applications. However, owing to the high cathode temperature, large amounts of metal droplets are produced during fabrication in arc ion plating (AIP) and the shadowing effect introduces particles and voids in the coatings. Continuous high-power magnetron sputtering (C-HPMS) can produce high ionization rates similar to high-power impulse magnetron sputtering (HiPIMS) and high deposition rates similar to AIP. In this work, the morphology, adhesion strength, tribological properties, corrosion resistance, and oxidation resistance of Al-rich TiAlN coatings prepared by C-HPMS are studied systematically. Higher Al and Ti ionization rates and deposition rates are achieved by increasing the power density. The largest deposition rate is 0.45 μm/min and coating hardness is 34.4 GPa. More importantly, no particles are …