Microstructure and corrosion resistance of Fe/Mo composite amorphous coatings prepared by air plasma spraying

Fe/Mo composite coatings were prepared by air plasma spraying （APS） using Fe-based and Mo-based amorphous and nanocrys- talline mixed powders. Microstructural studies show that the composite coatings present a layered structure with low porosity due to adding the self-bonded Mo-based alloy. Corrosion behaviors of the composite coatings, the Fe-based coatings and the Mo-based coatings were in- vestigated by electrochemical measurements and salt spray tests. Electrochemical results show that the composite coatings exhibit a lower polarization current density and higher corrosion potentials than the Fe-based coating when tested in 3.5wt% NaC1 solutions, indicating supe- rior corrosion resistance compared with the Fe-based coating. Also with the increase in addition of the Mo-based alloy, a raised corrosion re- sistance, inferred by an increase in corrosion potential and a decrease in polarization current density, can be found. The results of salt spray tests again show that the corrosion resistance is enhanced by adding the Mo-based alloy, which helps to reduce the porosity of the composite coatings and enhance the stability of the passive films.

Effect of Thermal Treatment on the Grain Growth of Nanostructured YSZ Thermal Barrier Coating Prepared by Air Plasma Spraying

A nanostructured thermal barrier coating is prepared by air plasma spraying using the 8wt% Y_2O_3 partially stabilized zirconia nano-powder with an average grain size of 40 nm. The microstructure and phase composition of feedstock nano-powder and coating are investigated using SEM, TEM and XRD. It is found that the as-sprayed zirconia coating has an average grain size of 67 nm and mainly consistes of metastable tetragonal phase, together with some monoclinic phase and tetragonal phase. Thermal treatment results show that the grains of the nanostructured coating grow slightly below 900℃, whereas over 1000℃ the gains grow rapidly and monoclinic phase noticeably appeares.

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.

Preparation and characterization of nanostructured Al_2O_3-13wt.%TiO_2 ceramic coatings by plasma spraying

Nanostructured and conventional Al2O3-13wt.%TiO2 ceramic coatings were prepared by plasma spraying with nanostructured agglomerated and conventional powders, respectively. The microstructure and microhardness of the coatings were investigated using scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, and microhardness measurement. Meanwhile, the friction and wear behaviors were analyzed and compared using a ball-on-disk tribometer. The results show that the conventional coating has lamellar stacking characteristic and has some pores. However, the nanostructured coating shows a bimodal microstructure, which is composed of both fully melted regions and partially melted regions. According to the microstructural difference, the partially melted regions can be divided into liquid-phase sintered regions （a three-dimensional net or skeleton-like structure： Al2O3-rich submicron particles embedded in the TiO2-rich matrix） and solid-phase sintered regions （remained nanoparticles）. The microstructural characteristics of the liquid-phase sintered region are formed due to the selective melting of TiO2 nanoparticles during plasma spraying. On the other hand, the TiO2 and Al2O3 nanoparticles of the solid-phase sintered regions are all unmelted during plasma spraying. Due to the existence of nanostructured microstructures, the nanostructured coating has a higher microhardness, a lower friction coefficient, and a better wear resistance than the conventional coating.

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.

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.

High-temperature frictional wear behavior of MCrAlY-based coatings deposited by atmosphere plasma spraying

Al2O3-r2O03/NiCoCrAIYTa coatings were prepared via atmosphere plasma spraying （APS）. The microstructure and phase com- position of the coatings were analyzed by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, laser confocal scanning microsco- py （LSCM）, and transmission electron microscopy （TEM）. The dry frictional wear behavior of the coatings at 500℃ in static air was inves- tigated and compared with that of 0Cr25Ni20 steel. The results show that the coatings comprise the slatted layers of oxide phases, unmelted particles, and pores. The hot abrasive resistance of the coatings is enhanced compared to that of 0Cr25Ni20, and their mass loss is approxi- mately one-fifteenth that of 0Cr25Ni20 steel. The main wear failure mechanisms of the coatings are abrasive wear, fatigue wear, and adhe- sive wear.

APPLICATION OF UNIFORM PRECRIPTION DESIGN IN STUDYING OF WEARREISTANT COATING IN PLASMA SPRAYING

Plasma spraying technics, a method of strengthening surfaces, shows advantages in wear-resistant, reducing friction, bearing corrosion and enduring higher temperature. While for the coating material, it is important to improve the ability of the functional coating. In this paper, a coating material of wear-resistant, bearing corrosion and enduring higher temperature in middle and low speed diesel is studied quantificationally with a uniform prescription design method, which reduced the test period as the test points distribute uniformly. An optimized combination is put forward of each part in the coating and a method to develop a new coating material is supplied.

Investigation on in-flight particle velocity in supersonic plasma spraying

In-flight particle velocity and flux distribution were measured using CCD thermal spray monitor system during supersonic plasma spray processing with nano-structured Al_2O_3-TiO_2 feed stocks. According to the results of particle flux measurement, the largest radian of the divergent particle stream is about 0.2. Within the measuring range, top speed of in-flight particles reached 800m/s. Particle acceleration was accomplished within 4cm down stream of the nozzle. Average particle velocity (about 450m/s) exceeded local sound speed (340m/s) even at a mean standoff distance of 17cm. With increasing mean standoff distance, average velocity of in-flight particle decreased according to a parabolic rule approximately. Image diagnosis showed that the result of in-flight particle velocity measurement is credible.

Characterization of NbSi_2–Al_2O_3 nanocomposite coatings prepared with plasma spraying mechanically alloyed powders

The present study characterized NbSi2-Al2O3 nanocomposite powders plasma-sprayed on Ti-6Al-4Vsubstrates. The powders were agglomerated to obtain suitable particle sizes for spraying. The agglomerated powders were then plasma-sprayed using atmospheric plasma spraying. The structural transformations of the powders along with the morphological and mechanical changes of the coatings were examined by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, and hard- ness testing. The results showed that after plasma spraying, the grain size increased, and the lattice strain decreased. However, the grain size of this compound after spraying was still in the nanometer range. The coating was uniform and exhibited good adhesion to the substrate. The microhardness and fracture toughness of the nanocomposite coating were higher than those of a nanostructured NbSi2 coating.

Synthesis of NiCr_2O_4 spinel coatings with high emissivity by plasma spraying

Air plasma spraying （APS） was used to produce high emissivity coatings with a NiCr204 spinel structure. The relationship between the emissivity and the crystal structure was investigated. X-ray diffraction （XRD） analyses show that NiCr204 spinel has been fabri- cated with the space group Fd3m. Scanning electron microscope （SEM） photographs show that the coating consists of a laminated structure with homogeneous grains and high porosity because of the unique feature of plasma spraying. The emissivity measurement and Fourier transformation infrared radiation （FT-IR） spectra show that NiCr204 has a high emissivity of about 0.91 because of its special spinel structure APS is a suitable method to produce high emissivity coatings.

A new application field of plasma spraying technique to environmental depollution

To expand the application of plasma spraying technique, TiO2 coatings were prepared using agglomerated anatase TiO2 powder to solve the environmental problems. The composition and photocatalytic activity of plasma sprayed TiO2 coatings were investigated systematically. The content of anatase TiO2 in the sprayed coatings was approximate to 7%- 15%, which was influenced by the melting state of TiO2 powder in plasma spraying process. The surface of sprayed coating was very rough and the arithmetical mean deviation of the surface profiles (Ra) was in the range of 5.7 - 8.8 μm. Under lower arc current, the surface of the coating became rougher. The anatase to rutile phase transformation temperature of agglomerated anatase TiO2 powder was approximate to 1 173 K. The TiO2 coating sprayed under the arc current of 400 A had good photocatalytic activity for the relative high content of anatase phase in it. It is concluded that the application of plasma spraying technique to environmental field has been developed.

Nanostructured Sulfide Composite Coating Prepared by Atmospheric Plasma Spraying

Nanostructured FeS-SiC coating was deposited by atmospheric plasma spraying（APS）.The microstructure and phase composition of the coating were characterized with SEM and XRD,respectively.In addition,the size distribution of the reconstituted powders and the porosity of the coating have been measured.It was found that the reconstitiuted powers with sizes in the range of 20 to 80 μm had excellent flowability and were suitable for plasma spraying process.The as-sprayed FeS-SiC composite coating exhibited a bimodal distribution with small grains（30～80nm） and large grains（100～200nm）.The coating was mainly composed of FeS and SiC,a small quantity of Fe1-xS and oxide were also found.The porosity of the coating was approximately 19%.

Fluctuation Phenomenon Analysis of an Arc Plasma Spraying Jet

The effects of three factors,including the power supply,the arc behaviour in the arc channel and the fluid dynamic process of the jet,on a plasma spraying jet have been experimentally detected by means of spectroscopic diagnostic techniques.The fast Fourier transform method has been applied to the analysis on the arc voltage and spectral line intensity of the jet.The three factors have been studied and distinguished from each other.

Preparation and Properties of Plasma Spraying Cu-Al_2O_3 Gradient Coatings

In order to overcome the limitations of low adhesion strength and poor thermal-shock resistance of pure ceramic coatings, Cu-Al2O3 gradient coatings were fabricated by plasma spraying. The microstructure and distribution of Cu-Al2O3 gradient coatings were analyzed. The adhesion strength, thermal-shock resistance and porosity of the coatings were tested. The results show that the composition of the gradient coatings has a gradient distribution along the thickness of coatings. As copper has a relatively low melting point and the molten copper has good wettability on the surface of Al2O3, it can be melted sufficiently and could fill the interstices and pores among the spraying particles effectively, thus improves the adhesion strength, thermal shock resistance and reduces the porosity. The adhesion strength of the gradient coating is 15.2 MPa which is two times of that of the double-layer structure coating.

Nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying

Nanostructured yttria partially stabilized zirconia coatings were deposited by air plasma spraying with reconstituted nanosized powder. The microstructures and phase compositions of the powder and the as-sprayed nanostructured coatings were characterized by transmission electron microscopy(TEM), scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results demonstrate that the microstructure of as-sprayed nanostructured zirconia coating exhibits a unique tri-modal distribution including the initial nanostructure of the powder, equiaxed grains and columnar grains. Air plasma sprayed nanostructured zirconia coatings consist of only the nontransformable tetragonal phase, though the reconstituted nanostructured powder shows the presence of the monoclinic, the tetragonal and the cubic phases. The mean grain size of the coating is about 42 nm.

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.

Effect of Auxiliary Gas Flow Parameters on Microstructure and Properties of Ta Coatings Prepared by Three-Cathode Atmosphere Plasma Spraying

Based on the three-cathode plasma spraying system,tantalum(Ta)coatings were pre-pared on the substrate of CuCrZr alloy.The effects of different auxiliary gas(helium)flow rates on the microstructure,phase composition,mechanical and wear resistance properties of Ta coatings were studied.The results showed that the oxidation degree of the coatings decreases first and then increases with the increase of the auxiliary gas flow.When the auxiliary gas flow rate is 70 L·min-1,the oxidation degree of the coating is the lowest,minimum value of the porosity is 0.21%,and the bonding strength reaches the maximum,59.3 MPa.At this time,the coating wear rate is 0.0012 mm^(3)·N^(-1)·m^(-1)with the best wear resistance.This indicates that the auxiliary gas flow has an important influence on the quality and surface mechanical properties of tantalum coating.

Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiCf/SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb2Si2O7) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiCf/SiC composite substrate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are investigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO2, leading to an early spallation of the coating.

Microstructure and Properties of Plasma Spraying Boron Carbide Coating

Microstructure of plasma spray boron carbide coating was studied by SEM and TEM. Its physical, mechanical and electrical properties were measured. The results showed that high microhardness, modulus and low porosity of B4C coating were manufactured by plasma spray. It was lamellar packing and dense. The B4C coating examined here contained two principal structures and two impurity phase besides major phase. The relatively small value of Young's modulus, comparing with that of the bulk materials, is explained by porosity . The Fe impurity phase could account for the relatively high electrical conductivity of boron carbide coating by comparing with the general boron carbide materials.