Structure and corrosion behavior of FeCoCrNiMo high-entropy alloy coatings prepared by mechanical alloying and plasma spraying

FeCoCrNiMox composite powders were prepared using the mechanical alloying technique and made into high-entropy alloy(HEA)coatings with the face-centered cubic phase using plasma spraying to address the element segregation problem in HEAs and pre-pare uniform HEA coatings.Scanning electron microscopy,transmission electron microscopy,and X-ray diffractometry were employed to characterize these coatings’microstructure and phase composition.The hardness,elastic modulus,and fracture toughness of coatings were tested,and the corrosion resistance was analyzed in simulated seawater.Results show that the hardness of the coating is HV0.1606.15,the modulus of elasticity is 128.42 GPa,and the fracture toughness is 43.98 MPa·m^(1/2).The corrosion potential of the coating in 3.5wt%NaCl solution is-0.49 V,and the corrosion current density is 1.2×10^(−6)A/cm^(2).The electrochemical system comprises three parts:the electrolyte,the adsorption and metallic oxide films produced during immersion,and the FeCoNiCrMo HEA coating.Over in-creasingly long periods,the corrosion reaction rate increases first and then decreases,the corrosion product film comprising metal oxides reaches a dynamic balance between formation and dissolution,and the internal reaction of the coating declines.

Effects of plasma spraying process on microstructure and mechanical properties of Cr_(2)AlC/410 composite coatings

To investigate the influences of Cr_(2)AlC mass fraction and supersonic plasma spraying process on the microstructure and mechanical properties of Cr_(2)AlC reinforced 410 stainless steel composite coatings,the coatings containing different mass fractions of Cr_(2)AlC were prepared and investigated.The composite coating exhibited low porosity and high adhesion strength.The addition of Cr_(2)AlC significantly enhanced the hardness of the composite coatings through particle strengthening.However,when the mass fraction of Cr_(2)AlC was 20%,the aggregation of Cr_(2)AlC resulted in a strong decrease in the coating preparation efficiency,as well as a decline in adhesion strength.In the supersonic plasma spraying process,the Ar flow rate mainly influenced the flight velocity of the particles,while the H_(2) flow rate and the current mainly affected the temperature of the plasma torch.Consequently,all of them influenced the melting degree of particles and the quality of the coating.The lowest porosity and the highest hardness and adhesion strength could be obtained when the Ar flow rate is 125 L/min,the H_(2) flow rate is 25 L/min,and the current is 385 A.

Numerical simulation and analysis of lithium plasma during low-pressure DC arc discharge

Alkali metal DC arc discharge has the characteristics of easy ionization,low power consumption,high plasma temperature and ionization degree,etc,which can be applied in aerospace vehicles in various ways.In this paper,we calculate the physical property parameters of lithium vapor,one of the major alkali metals,and analyze the discharge characteristics of lithium plasma with the magnetohydrodynamic(MHD)model.The discharge effects between constant current and voltage sources are also compared.It is shown that the lithium plasma of DC arc discharge has relatively high temperature and current density.The peak temperature can reach tens of thousands of K,and the current density reaches 6 x 107 A m 2.Under the same rated power,the plasma parameters of the constant voltage source discharge are much higher than those of the constant current source discharge,which can be used as the preferred discharge mode for aerospace applications.

The mechanism study of low-pressure air plasma cleaning on large-aperture optical surface unraveled by experiment and reactive molecular dynamics simulation

Low-pressure air plasma cleaning is an effective method for removing organic contaminants on large-aperture optical components in situ in the inertial confinement fusion facility.Chemical reactions play a significant role in plasma cleaning,which is a complex process involving abundant bond cleavage and species generation.In this work,experiments and reactive molecular dynamics simulations were carried out to unravel the reaction mechanism between the benchmark organic contaminants of dibutyl phthalate and air plasma.The optical emission spectroscopy was used to study the overall evolution behaviors of excited molecular species and radical signals from air plasma as a reference to simulations.Detailed reaction pathways were revealed and characterized,and specific intermediate radicals and products were analyzed during experiments and simulation.The reactive species in the air plasma,such as O,HO_(2)and O_(3)radicals,played a crucial role in cleaving organic molecular structures.Together,our findings provide an atomic-level understanding of complex reaction processes of low-pressure air plasma cleaning mechanisms and are essential for its application in industrial plasma cleaning.

Microstructure and Properties of Low-pressure Cold Sprayed Al-based Composite Coatings on Magnesium Alloy Surface

Pure Al and Al-30%Al_(2)O_(3)composite coatings are prepared on the surface of AZ31B magnesium alloy by low-pressure cold spraying.The morphology and structure of the coatings are analyzed by scanning electron microscope (SEM),energy dispersive spectroscopy (EDS),and X-ray diffraction (XRD),and the effects of the addition of Al_(2)O_(3)on the microstructure of the Al-based coatings are discussed.The mechanical properties and corrosion resistance of the coatings are fully evaluated by the micro-hardness tester,electronic tensile machine,and electrochemical workstation.The results show that the coating structure is more uniform and denser,and the porosity is significantly reduced after the addition of Al_(2)O_(3).The interfaces between the coatings and the magnesium alloy substrate are distinct,and the coatings and the substrate are mechanically combined.Compared with the pure Al coating,the microhardness of the Al-Al_(2)O_(3)composite coating is increased to 61.1 HV_(0.2),and the bonding strength reaches above 53.1 MPa.The self-corrosion potential of the two coatings is higher than that of the magnesium alloy,and the self-corrosion current density is significantly lower than that of the magnesium alloy substrate.The Al-based coatings prepared by low-pressure cold spraying have high hardness,good bonding strength,and good corrosion resistance,which can be used for the repair and protection of magnesium alloy structural parts.

Simulation of propagation of the HPM in the low-pressure argon plasma

The propagation of the high-power microwave（HPM） with a frequency of 6 GHz in the lowpressure argon plasma was studied by the method of fluid approximation.The two-dimensional transmission model was built based on the wave equation,the electron drift-diffusion equations and the heavy species transport equations,which were solved by means of COMSOL Multiphysics software.The simulation results showed that the propagation characteristic of the HPM was closely related to the average electron density of the plasma.The attenuation of the transmitted wave increased nonlinearly with the electron density.Specifically,the growth of the attenuation slowed down as the electron density increased uniformly.In addition,the concrete transmission process of the HPM wave in the low-pressure argon plasma was given.

Effect of TiO_2 content on properties of Al_2O_3 thermal barrier coatings by plasma spraying

Al2O3 thermal barrier coatings with different TiO2 contents were deposited on 6061 aluminum alloy by plasma spraying. The corrosion resistance, thermal insulation property and phase composition of these coatings were investigated. The results indicate that all the feedstock powders exhibit phase transformation during the spray process. With the increase of the TiO2 content in the powder, the corrosion resistance of the coating is enhanced but the thermal insulation property is decreased. This can be attributed to the higher thermal conductivity but lower melting point and brittleness of TiO2 than those of Al2O3, so it is easy for TiO2 to disperse in the brittle Al2O3 substrate during spraying, in which these dispersively distributed TiO2 play the role of hole sealing, releasing stress and reducing cracks.

Effect of Spraying Condition and Material Properties on the Residual Stress in Plasma Spraying

The thermomechanical behavior and the distribution of residual stresses due to thermal spraying of NiCoCrAlY coating were studied by thermomechanical finite element analysis. The effects of phase transformation due to solidifying process of coating particles, thickness and material properties of coating on the residual stresses were discussed. Results showed that residual stress decreases little with the stress relaxation due to the phase transformation. For the substrates with the same thickness, the residual stress increases with the increase in coating thickness. The state of residual stresses relates to the material properties of coating and substrate closely. The stress-induced failure model of coating is also discussed.

Plasma Spray Forming of Nanostructured Composite Coatings

The nanostructure composite coating is obtained via plasma spraying of Al2O3-13 wt pct TiO2 powder. Brittle and hard lamella results from melted nanostructured powder. Ductile nanostructured matrix forms from unmelted nanostructured particles. Through the adjustment of constituent and nanostructure, hardness/strength and toughness/ductility are balanced and overall properties of the structure composite are achieved.

Preparation of Ni60-WC Coating by Plasma Spraying, Plasma Re-melting and Plasma Spray Welding on Surface of Hot Forging Die

In order to produce the hear-resistant inner layer of hot-forging die, the plasma spraying and plasma re-melting and plasma spray welding were adopted. Substrate material was W6Mo5Cr4V2, including 10%, 20%, 30% tungsten carbide （WC） ceramic powder used as coating material to obtain different Nickel-based WC alloys coating. Micro-structure and micro-hardness analysis of the coating layer are conducted, as well as thermophysical properties for the coating layer were measured. The experimental results show that the coating prepared with 70%Ni60, 30%WC powder has the best properties with plasma spray welding, in which the micro-hardness can achieve 900HV, meanwhile it can improve the thermal property of hot-forging die dramatically.

Morphology and Phase Compositions of Hydroxyapatite Powder Particles Plasma-sprayed into Water

Hydroxyapatite powder particles were plasma sprayed into water, their inner structures and phase compositions were studied by using scanning electron microscope(SEM) and X-ray diffractometer. The results show that the molten HA particles have a central hollow morphology and high crystallinity. The hollow morphology was caused by sublimated P2O5 and H2O, which will have an effect on surface morphology, cohesive and adhesive strength as well as dissolution and degradation of coating. The high crystallinity is attributed to lower cooling speed in water.

Comparative studies on the hot corrosion behavior of air plasma spray and high velocity oxygen fuel coated Co-based L605 superalloys in a gas turbine environment

An improvement in the corrosion resistance of alloys at elevated temperature is a factor for their potential use in gas turbines. In this study, Co Ni Cr Al Y has been coated on the L605 alloy using air plasma spray(APS) and high-velocity oxygen fuel(HVOF) coating techniques to enhance its corrosion resistance. Hot corrosion studies were conducted on uncoated and coated samples in a molten salt environment at 850°C under cyclic conditions. Thermogravimetric analysis was used to determine the corrosion kinetics. The samples were subjected to scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction for further investigation. In coated samples, the formation of Al2O3 and Cr2O3 in the coating acts as a diffusion barrier that could resists the inward movement of the corrosive species present in the molten salt. Coated samples showed very less spallation, lower weight gain, less porosity, and internal oxidation as compared to uncoated sample.HVOF-coated sample showed greater corrosion resistance and inferred that this is the best technique under these conditions.

Deformation Behavior of Nanostructured Ceramic Coatings Deposited by Thermal Plasma Spray

Al2O3-13 wt pet TiO2 coating deposited by direct current plasma spray consists of nanostructured region and micro-lamellae. Bend test shows that the ceramic coating can sustain some deformation without sudden failure. The deformation is achieved through the movement of nano-particles in the nanostructured region under tensile stress.

Ni60-SiC Coating Prepared by Plasma Spraying, Plasma Re-melting and Plasma Spray Welding on Surface of Hot Forging Die

In order to produce the hear-resistant inner layer of hot-forging die, plasma spraying and plasma re-melting and plasma spray welding were adopted. Substrate material was W6Mo5Cr4V2, including 10%, 20%, 30% SiC ceramic powder used as coating material to obtain different Ni-based SiC alloys coating. Micro-structure and micro-hardness analysis of the coating layer were followed, as well as thermophysical properties for the coating layer were measured. The experimental results show that the coating prepared with 70% Ni60, 30% SiC powder has best properties with plasma spray welding, in which the micro-hardness can achieve 1100 HV, meanwhile can improve the thermal property of hot-forging die dramatically.

Microstructure and Tribological Properties of Plasma-sprayed Nanostructured Sulfide Coating

The friction and wear properties of plasma-sprayed nanostructured FeS coating were investigated on an MHK- 500 friction and wear tester under both oil lubrication and dry friction condition. The microstructure, worn surface morphology and phase composition of the coating were characterized by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. It was found that the coating was mainly composed of FeS. a small quantity of Fe1-xS and oxide were also found. The coating was formed by small particles of 50～100 nm in size The thickness of the coating is approximately 150 μm. The friction-reduction and wear-resistance properties of plasma-sprayed nanostructured FeS coating were superior to that of GCr15 steel substrate. Especially under oil lubrication condition, the friction coefficient of nanostructured FeS coating was 50% of that of GCr15 steel, the wear scar widths of the coating were also reduced to nearly 50% of that of GCr15 steel under high load. The failure of the coating was mainly attributed to plastic deformation under both oil lubrication and dry friction condition.

Study of Multi-Function Micro-Plasma Spraying Technology

A multi-functional micro-arc plasma spraying system was developed according to aerodynamics and plasma spray theory. The soft switch IGBT （Insulated Gate Bipolar Transistor） invert technique, micro-computer control technique, convergent-divergent nozzle structure and axial powder feeding techniques have been adopted in the design of the micro-arc plasma spraying system. It is not only characterized by a small volume, a light weight, highly accurate control, high deposition efficiency and high reliability, but also has multi-functions in plasma spraying, welding and quenching. The experimental results showed that the system can produce a supersonic flame at a low power, spray Al2O3 particles at an average speed up to 430 m/s, and make nanostructured AT13 coatings with an average bonding strength of 42.7 MPa. Compared to conventional 9M plasma spraying with a higher power, the coatings with almost the same properties as those by conventional plasma spray can be deposited by multi-functional micro-arc plasma spraying with a lower power plasma arc due to an improved power supply design, spray gun structure and powder feeding method. Moreover, this system is suitable for working with thin parts and undertaking on site repairs, and as a result, the application of plasma spraying will be greatly extended.

Oxidation resistance of Si-coated TZM alloy prepared through combined process of plasma spray and laser surface melting

Plasma thermal spraying ofSi coating layer ontitanium-zirconium-molybdenum （Ti-Zr-Mo）,TZM alloy,was conducted for the surface protection of the Mo substrate that is unstable in air at high temperatures. Although the plasma thermal spraying alone could protect the Mo alloy from oxidation at a high temperature for a short time, the post laser surface melting process further improved the oxidation resistance of Si-coated alloy. In the case of the post laser treated specimen,MoSi compounds, mainly MoSi2 phases, were formed during the additional annealing process, and the oxidation resistance could be even further enhanced. The corrosion behaviors of Si-coated specimens in 3.5%NaCl solution were also investigated;however,nosignificant variations with respect to the post treatment procedure were found.

Numerical simulation of temperature and velocity fields in plasma spray

Based on the turbulence jet model, with respect to Ar-He mixture plasma gas injecting to ambient atmosphere, the temperature filed and velocity field under typical working conditions were investigated. Given the conditions of I=900 A, FAr= 1.98 m^3/h, FEe=0.85 m^3/h, it is found that both the temperature and the velocity undergo a plateau region near the nozzle exit （0-10 mm） at the very first stage, then decrease abruptly from initial 13 543 K and 778.2 m/s to 4 000 K and 260.0 m/s, and finally decrease slowly again. Meanwhile, the radial temperature and radial velocity change relatively slow. The inner mechanism for such phenomena is due to the complex violent interaction between the high-temperature and high-velocity turbulent plasma jet and the ambient atmosphere. Compared with traditional methods, the initial working conditions can be directly related to the temperature and velocity fields of the plasma jet by deriving basic boundary conditions.

Numerical Simulation of Basic Parameters in Plasma Spray

On the basis of energy balance in the plasma gas, a new, simplified but effective mathematical model is developed to predict the temperature, velocity and ionization degrees of different species at the torch exit, which can be directly calculated just by inputting the general spraying parameters, such as current, voltage, flow rates of gases, etc. Based on this method, the effects of plasma current and the flow rate of Ar on the basic parameters at the torch exit are discussed. The results show that the temperature, velocity and ionization degrees of gas species will increase with increasing the plasma current; while increasing Ar flow rate can increase the velocity at the exit but decrease the temperature and ionization degrees of plasma species. The method would be helpful to predict the temperature and velocity fields in a plasma jet in future, and direct the practical plasma spray operations.

Premature failure induced by non-equilibrium grain-boundary tantalum segregation in air-plasma sprayed ZrO_(2)-YO_(1.5)-TaO_(2.5)thermal barrier coatings

ZrO_(2)-YO_(1.5)-TaO_(2.5)(ZYTO)is a promising top-coat material for thermal barrier coatings(TBCs).The bulk properties of ZYTO have been reported by several studies,but its performances as TBCs are less-well understood.In this work,ZYTO TBCs were prepared by air plasma spraying(APS)and their thermal cycling performances were investigated at 1150℃.Despite of the good bulk properties,APS ZYTO TBCs present an extremely short thermal fatigue life.This is attributed to the non-equilibrium grain-boundary segregation of TaO_(2.5) induced by limited solubility and rapid quenching during APS process,resulting in a tetragonal(t)to cubic(c)and metastable-tetragonal(tm)phase transformation in ZYTO TBCs.The volume shrinkage(~0.74vol%)of phase transformation leads to many cracks at the c/tm phase boundaries after deposition.On the other hand,the formation of cubic phase with massive grain-boundary Ta segregation induces a large intergranular embrittlement and a weak bonding strength(~5.3 MPa),resulting in the premature failure of the ZYTO TBCs.