Preparing of nanostructured Al2O3–TiO2–ZrO2 composite powders and plasma spraying nanostructured composite coating

Nanostructured Al₂O₃–TiO₂–ZrO₂ composite powders for plasma spraying were prepared by spray drying granulation technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. The results show that with increasing the slurry solid content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases, and the flowability of powders increases firstly and then decreases. With increasing the spray drying temperature, the particle size of powders increases, and the bulk density and flowability of powders increases firstly and then decreases. The most appropriate spray drying parameters are the slurry solid content of 40 wt.%, the binder content of 2.0 wt.% and the spray drying temperature of 250 °C. The nanostructured composite coating was successfully prepared by using the as-prepared nanostructured Al₂O₃–TiO₂–ZrO₂ composite powders as feedstocks. The nanostructured coating possessed higher hardness and toughness compared with the conventional microstructured one, which was attributed to the use of the nanostructured composite powders feedstocks.     

Thermisches Spritzen neuartiger SiC und TiC Pulver mit oxidischer Binderphase

Processing of newly developed SiC and TiC Powders with Oxide‐Ceramic Matrix by means of Thermal Spraying Intermediate results of an actual project are presented. The investigations concern newly developed SiC and TiC powders, their processing by means of thermal spraying and the characterization of produced coatings. Development and optimization of powders, thermal spray process and spray parameter optimization are carried out on permanent feedback. The powder production line is spray‐drying, sintering and conditioning. The binder matrix phase is aluminum oxide / yttrium oxide. The produced powders SiC – Al₂O₃/Y₂O₃ and TiC – Al₂O₃/Y₂O₃ show different specific chemical compositions and morphologies each. Carbide contents of >65 % are aimed at. Applied thermal spray processes are atmospherical plasma spraying and high velocity oxygen fuel spraying. The results demonstrated characterize feedstock powders as well as produced coatings. The investigations are done by means of light and scanning electron microscopy.     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.     

Wear behavior of plasma sprayed composite coatings with in situ formed Al2O3

In the present study, the wear behavior of in situ formed Al₂O₃ reinforced hypereutectic Al–18Si matrix composite coatings have been investigated. These coatings were successfully fabricated with mechanically alloyed Al–12Si and SiO₂ powder deposited on aluminum substrates by atmospheric plasma spraying (APS). The produced samples were characterized by means of microscopic examinations, hardness measurements and wear tests. The obtained results pointed out that the amount of in situ formed Al₂O₃ particles increased with increasing spray distance and decreasing in-flight particle velocity and temperature, which was accompanied by an improvement in hardness and wear resistance.     

Synthesis and characterization of Yb and Er based monosilicate powders and durability of plasma sprayed Yb2SiO5 coatings on C/C-SiC composites

Rare-earth silicates such as Yb₂SiO₅ and Er₂SiO₅ are promising environmental barrier coating materials for ceramic matrix composites. In this work, Yb₂SiO₅ and Er₂SiO₅ ceramic powders have been synthesized by solid-state reaction using Yb₂O₃, Er₂O₃ and SiO₂ as starting materials. The fabricated powders were subjected to spray drying treatment for subsequent synthesis of coatings by plasma spraying. The spray drying resulted in well-dispersed and spherical powder particles with good flowability. Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry and differential scanning calorimetry (TGA/DSC) and dilatometry were applied to study the microstructural and thermal characteristics of the powders. Ultra-high purity monosilicate powders formed as a result of heating treatments at 1400 °C in a box furnace for 20 h. TG/DSC revealed the genesis temperatures of the silicate formation (low temperature polymorphs) and also showed that the solid-state reactions to form Yb and Er based monosilicates proceeded without any weight-loss in the tested temperature range. The values of coefficients of thermal expansion (CTE) of the fabricated compounds are found to be 7.1 ppm/°C for Yb₂SiO₅ and 7.5 ppm/°C for Er₂SiO₅ by dilatometric measurements. Besides these studies, coating formation by plasma spraying of spray-dried Yb₂SiO₅ powders on the ceramic matrix composite specimen such as C/C-SiC has also been evaluated. Well-adhered and uniformed coatings result on composite specimens whose durability is tested by thermal cycling from ∼400 °C to 1500 °C in a gas burner rig.     

High dielectric AlN/Al2O3 composite powder using a plasma spray approach for microelectronics application

Increasing demand for higher performance dielectric material for multi-layer ceramics packaging has led to the use of the AlN system due to its very high thermal conductivity and coefficient of expansion compatibility with silicon. This paper reports on a novel process method used to produce an AlN/Al₂O₃ composite powder system which can be subsequently tape cast as a dielectric substrate. The mixture of both Al₂O₃ and AlN was first mechanically alloyed and then spray-dried to obtain a suitable agglomerated powder that was subsequently plasma-sprayed, resulting in a fine micrometer level integrated composite powder. The two main criteria used to ascertain the optimal process parameters during plasma spraying were a high gamma/alpha Al₂O₃ phase ratio, which ensured that all the Al₂O₃ phase had melted during plasma spraying, and a minimal reduction in the AlN/Al₂O₃ ratio to ensure minimal change in the AlN during processing. For the plasma-sprayed composite powders, fully sintered ceramic tapes were produced attaining>99.0% of the theoretical density after sintering at 1650°C for 6 h, which yielded a thermal conductivity value of 32.0 W m^–1 K^–1.     

Mechanical properties of in situ Al2O3 formed Al–Si composite coating via atmospheric plasma spraying

In this study, mechanically alloyed Al–12Si/SiO₂ composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The composite coating consisting of in situ formed Al₂O₃ reinforced hypereutectic Al–18Si matrix alloy was achieved. The produced coatings were extensively analyzed with respect to X-ray diffraction (XRD). The XRD patterns of the coatings include Al, Si and Al₂O₃ phase formation. Mechanical properties of layers were examined by Dynamic Ultra-micro hardness test machine for estimating Young’s modulus due to load–unload sensing analysis. The hardness and Young’s modulus of the composite coatings sprayed at different plasma current and the distance were measured under 200, 400, 600, 800 and 1000 mN of applied peak loads by indentation technique. The effects of spray distance and arc current on the hardness and Young’s modulus have been investigated. Additionally, it was observed that the arc current and spray distance strongly influence the mechanical properties of the coatings.     

Measurement methods for quantifying powder flowability and velocity in cold spray systems

Four powders with varying bulk densities, Al, Al₂O₃, Sn, and Cu, were used to determine quantifiable relationships between powder flowability, mass flow rate, and powder velocity with particle morphology and particle distribution in a cold spray system. High particle density results in good powder flowability, specifically when the powders are spherical relative to irregular morphology. Particle velocity during cold spray, measured with a double disk rotary system, increases non-linearly with an increase of inlet pressure. The increase in mass flow rate from the hopper and the resulting mass output of the cold spray system shows a consequence of good powder flowability. Conversely, a high mass flow rate decreases the particle velocity during the cold spray process, with better flowability leading to decreases on the order of 10% in particle velocity in the cold spray system. The described methods, proposed tools, and findings can be easily made with cost-effective and on-the-spot measurements.     

Corrosion behavior of plasma sprayed ceramic and metallic coatings on carbon steel in simulated seawater

Al₂O₃, ZrO₂ and Ni60 coatings were produced on carbon steels by plasma spray. Ni60 was used as the bond coat in all the cases. The microstructure of these coatings was analyzed by scanning electron microscopy (SEM). The corrosion behavior of the plasma spray coated samples as well as uncoated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in simulated seawater. The results showed that Ni60 coating protected carbon steels against the corrosion and plasma spraying ceramic powders on metallic coating improved the corrosion resistance of the coatings further. The corrosion resistance of the Al₂O₃ coating was superior to that of the ZrO₂ coating due to the relatively few defects in Al₂O₃ coating.     

Microstructural evolutions of nickel-aluminide nanocomposites during powder synthesis and thermal spray processes

The synthesis and microstructural evolutions of the NiAl-15 wt% (Al₂O₃–13% TiO₂) nanocomposite powders were studied. These nanocomposite powders are used as feedstock materials for thermal spray applications. These powders were prepared using high and low-energy mechanical milling of the Ni, Al powders and Al₂O₃–13% TiO₂ nanoparticle mixtures. High and low-energy ball-milled nanocomposite powders were also sprayed by means of high-velocity oxy fuel (HVOF) and air plasma spraying (APS) techniques respectively. The results showed that the formation of the NiAl intermetallic phase was noticed after 8 h of high-energy ball milling with nanometric grain sizes but in a low-energy ball mill, the powder particles contained only α-Ni solid solution with no trace of the intermetallic phase after 25 h of milling. The crystallite sizes in HVOF coating were in the nanometric range and the coating and feedstock powders showed the same phases. However, under the APS conditions, the coating was composed of the NiAl intermetallic phase in the α-Ni solid solution matrix. In both of the nanocomposite coatings, reinforcing nanoparticles (Al₂O₃–13% TiO₂) were located at the grain boundaries of the coatings and pinned the boundaries, therefore, the grain growth was prohibited during the thermal spraying processes.     

Homogeneous dispersion of boron nitride nanoplatelets in powder feedstocks for plasma spraying

Hexagonal Boron nitride nanoplatelet (BNNP) with the combination of excellent mechanical properties, high-temperature thermal stability, and self-lubrication is a promising strengthening agent in plasma-sprayed composite coatings. However, it is a significant challenge to produce plasma sprayable feedstocks with homogeneously dispersed BNNPs owing to the intrinsic agglomeration of BNNPs. In this research, three powder preparation processes (electrostatic interaction, ball milling, and shear mixing) were employed to disperse BNNPs in Ni-Cr-Cr₂O₃ (NCCO) powders. The shear mixing process presents a competitive advantage in powder preparation, providing stable shear force and achieving exfoliated BNNPs homogeneous distribution in the composite powders. The spray drying technique was finally utilized to obtain spherical agglomerated BNNP-NCCO feedstocks with characteristics of relatively smooth surface and uniform size distribution. Hence, shear mixing and spray drying processes are expected to produce large-scale spherical composite powders with a homogeneous distribution of 2D layered nanomaterials for industrial application.     

Optimization of plasma spray parameters on the mechanical properties of agglomerated Al2O3–13%TiO2 coated mild steel

This paper discusses the effect of plasma spray parameters of deposited agglomerated nano Al₂O₃-13%TiO₂ powders on commercial marine-grade mild steels. Prior to the coating work, the nanopowders were subjected to a two-level factorial design of experiment to optimize the operational spray parameters, namely, the primary gas pressure, the carrier gas pressure, and the powder feed rate. These operational spray parameters potentially affect the following responses of the coatings: microhardness, wear rate, and surface roughness. The significant effect on surface roughness is due to the interaction factor with the carrier gas pressure. However, by changing the carrier gas pressure and the powder feed rate, an insignificant effect on the microhardness and wear rate is noted. In general, agglomerated Al₂O₃-13%TiO₂ nanopowder-coated steels, with the lowest primary pressure of 40 psi, carrier gas pressure of 20 psi, and the highest powder feed rate of 3 rpm, are most preferred.     

超音速等离子体喷涂MoSi2涂层工艺研究

涂层技术是C/C复合材料高温抗氧化与抗烧蚀的有效手段,单一的SiC涂层很难为C/C复合材料提供有效的长寿命保护。金属间化合物MoSi2高温时会形成一层致密的SiO2保护膜,具有特别优异的高温抗氧化性能,常作为C/C复合材料的高温抗氧化涂层。本文采用超音速等离子喷涂法在带SiC涂层的C/C复合材料表面制备了MoSi2涂层,主要研究了喷涂功率、主气(Ar)流量对粉料表面温度、飞行速度、沉积率以及对涂层表面微观结构和结合强度的影响。结果表明:喷涂功率在47.5~52.5 kW之间,既能使粒子有较高的速度和温度,还能保证粉末不过熔,在喷涂功率为50 kW时,粉料的沉积率最高,氧化不高,涂层表面致密性好,截面结合紧密,结合强度高;Ar流量为65 L/min时,能够保证MoSi2粉末有较高的表面温度与较快飞行速度,沉积率最高,氧化不高,涂层表面致密,几乎没有孔隙与裂纹。因此,调控超音速等离子体喷涂工艺参数能够在带SiC涂层的C/C复合材料表面得到致密且结合良好的MoSiO2涂层。     

Study of the microstructure of plasma sprayed coatings obtained from Al2O3–13TiO2 nanostructured and conventional powders

The microstructure of coatings obtained from nanostructured or conventional Al₂O₃–13TiO₂ powders and deposited by plasma spraying technique on low-carbon steel was examined by transmission electron microscopy techniques. The dominating phase in both coatings was γ-Al₂O₃ phase. It has been observed that the grains of γ-Al₂O₃ grew in various shapes and sizes, that are particularly visible in the case of coating sprayed from nanostructured powder. The coatings obtained from the fully melted conventional powders exhibited a typical lamellar microstructure, into which the strips of TiO₂ phase were extended. The microstructure of coatings produced from agglomerates of nanostructured particles also revealed the regions consisting of partially melted α-Al₂O₃ powders surrounded by the net-like structure formed from fully melted oxides that improved the coating properties. Along with the observed morphology diversity some changes in the chemical composition on the cross sections of obtained coatings have been also noticed.     

Microstructure of high velocity oxy-fuel sprayed Ti2AlC coatings

The microstructure formation and phase transformations in Ti₂AlC-rich coatings deposited by High Velocity Oxy-fuel spraying of Maxthal 211^? powders is presented. High resolution electron microscopy analysis, using both scanning and transmission electron microscopy with energy dispersive spectrometry and energy filtering, combined with X-ray diffraction reveals that the coatings consist of Ti₂AlC grains surrounded by regions of very small TiC grains embedded in Ti _x Al _y . The composition of the Ti _x Al _y depends on its surrounding and varies with size and distribution of the adjacent TiC grains. Impact of spray parameters on coating microstructure is also discussed. Two spray parameters were varied; powder size distribution and flame power. They were found to greatly affect the coating microstructure. Increasing powder size and decreasing flame power increase the amount of Ti₂AlC, but produces thinner coatings with lower cohesion. Larger powder size will also decrease oxygen incorporation.     

Sol–gel processing and phase characterization of Al2O3 and Al2O3/SiC nanocomposite powders

Monolithic Al₂O₃ and Al₂O₃/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the precipitation of Al(NO₃)₃·9H₂O with NH₄OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al₂O₃. The ²⁷Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al₂O₃ formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al₂O₃) is marked by the abrupt disappearance of tetrahedral Al. Al₂O₃/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the ²⁹Si NMR spectra of the Al₂O₃/SiC powders reveal α-SiC, Al₂O₃·xSiO₂ and SiO₂ phases. Stable α-Al₂O₃ and α-Al₂O₃/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al₂O₃ in the nanocomposites by stabilising the Al₂O₃ phases with concomitant oxidation of SiO₂.     

45S5 bioactive glass coatings by atmospheric plasma spraying obtained from feedstocks prepared by different routes

45S5 bioactive glass powders with the composition of 45 SiO₂, 6 P₂O₅, 24.5 CaO and 24.5 wt% Na₂O were melted and quenched in water to obtain a frit. The frit was milled using two different routes: dry milling followed by sieving to obtain glass particles and wet milling followed by spray drying to obtain a powder comprising porous agglomerates. All feedstocks showed adequate characteristics that make them suitable to be deposited by atmospheric plasma spraying. The powders and coatings were characterised by field-emission gun environmental scanning electron microscope and X-ray diffraction. The roughness and the contact angle of the coatings were also determined. The bioactivity of the powders and coatings was assessed by immersion in simulated body fluid. It was found that bioactive glass prepared from bioglass frit by dry milling exhibited similar bioactivity as that of a commercial bioactive glass. All coatings produced showed good adhesion to the substrate as well as suitable surface properties to ensure efficient contact with body fluid. Regardless of the characteristics of the feedstocks or plasma spray conditions used, all coatings were exclusively made up of an amorphous phase. On the other hand, micrographs revealed that the characteristics of the feedstock strongly impact on the final coating microstructure. The most homogeneous microstructure was obtained when the feedstock was prepared by fine dry grinding of the frit. For this coating, the formation of a bioactive layer was also proved by Fourier transform infrared spectroscopy and X-ray diffraction.     

Architecture and phases composition of suspension plasma sprayed alumina-titania sub-micrometer-sized coatings

Sub-micrometer-sized Al₂O₃-TiO₂ plasma-sprayed coatings exhibit superior performances compared to micrometer-sized ones. Two routes can be implemented to manufacture such finely structured coatings: i) spraying micrometer-sized agglomerates of nanometer-sized particles which results in a two-scale coating architecture and ii) spraying a suspension of sub-micrometer-sized particles (suspension plasma spraying, SPS). SPS was implemented in this study and Al₂O₃-base coatings with 13 and 60 wt.% of titania, respectively, were manufactured by spraying a suspension made of a mixture of Al₂O₃ and TiO₂ particles both of 300 nm, average diameter. Coating structural features and phase contents were studied. Results show that the coatings exhibit a very fine lamellar structure with a homogeneous repartition of Al and Ti. Complex phases, made of intermediate Al, Ti, and O oxides, have been also identified. Indeed, coatings formation results from rapid solidification rates and high transient thermal fluxes imparted by the plasma flow to the substrate due to the short spray distance encountered in SPS (in the order of 30 mm) requested by the small kinetics and thermal inertia of sub-micrometer-sized particles.     

Metallic and ceramic powders for vacuum plasma spraying

All product families, including hard materials, hard metals, hard alloys, cermets, intermetallics and refractories, are usable for vacuum plasma spraying. Fine powders (less than 45 μm) that are narrowly classified within well-defined limits for oversize and undersize are used. The determination of the size distribution is of particular importance.The morphology of powders for vacuum plasma spraying has to be stressed in particular, e.g. the particle shape, the surface area, the density and the composition (agglomerate, primary grain, crystal formation).In addition, the chemical composition, i.e. the main, secondary and trace elements and the gas contents (O₂, N₂, H₂, noble gases), is specified and the lowest specification values have to be met for vacuum plasma spray powders such as M-Cr-Al-Y.Single-phase and refractory compounds with uniformity of structure as well as solid solutions can be vacuum plasma sprayed without any changes and with strict maintenance of phase uniformity and stoichiometry.     

The structure of plasma-prepared Al2O3-TiO2 powders

The morphology and phase constitution of sub-micron Al₂O₃-TiO₂ powders prepared by oxidation of mixtures of Al₂Br₆ and TiCl₄ in an oxygen-argon high-frequency plasma have been studied. The particle size and distribution were consistent with formation of liquid particles by rapid nucleation and surface reaction followed by growth by coalescence of droplets. The particle size of the powders is related to the concentration of reactants in the gas stream and the temperature difference between condensation and solidification. A metastable solution of TiO₂ in -Al₂O₃ was formed in the range 0 to 7 wt% TiO₂, at higher TiO₂ concentrations particles consisted of a dispersion of rutile particles (~ 10 nm) within single crystals of -Al₂O₃. A metastable phase identified as 3Al₂O₃·TiO₂ was also formed in powder with compositions in the range 14 to 40 wt% TiO₂. Over the composition range 40 to 80 wt% TiO₂ the powder consisted predominantly of crystals with a two-phase Al₂TiO₅-rutile structure. Pure TiO₂ consisted largely of anatase and the addition of Al₂O₃ resulted in the formation of rutile as the major phase. The phase constitution of the powders is interpreted in terms of the nucleation kinetics of the various phases.