流化床喷雾制备磷酸二氢钙成粒条件的实验研究

在底部直径为200 mm的锥形流化床中,研究了喷雾造粒制备粒状磷酸二氢钙的成粒条件,实验考察了喷淋密度、床层温度及流化气速等因素对成粒过程的影响,分析了磷酸二氢钙颗粒层式生长与团聚生长的机理,并给出了两种生长成粒的操作条件。     

Lanthanum cerate thermal barrier coatings generated from thermal plasma synthesized powders

Lanthanum cerate (La₂Ce₂O₇, LC) is one of the promising advanced thermal barrier coating (TBC) materials due to its high melting point, no phase transformation between room temperature and operating temperature, low thermal conductivity, comparable coefficient of thermal expansion (CTE) with metallic substrate. The present study investigates plasma transferred arc synthesis of LC powder, its subsequent spheroidization in a thermal plasma jet and plasma spray deposition. The PTA-synthesized LC powder, spheroidized as well as the plasma sprayed coatings was found to possess excellent phase stability; the single phase cubic fluorite structure of LC was found to be retained even after prolonged arc-melting, corroborating that the material was stable from room temperature up to its melting point. It was observed that PTA melting for longer duration resulted in small deviation from stoichiometry, although the phase structure of LC was retained. Spheroidization efficiency was found to increase with the input power of the torch. Very good adherent LC coatings could be deposited on nickel super alloy with reasonably good deposition efficiency.     

Failure of the plasma-sprayed coating of lanthanum hexaluminate

Lanthanum magnesium hexaluminate (LaMgAl₁₁O₁₉, LMA) is an attractive material for thermal barrier coatings (TBCs), and the failure of its coating was studied in this work by thermal cycling, X-ray diffraction, dilatometric measurement and thermal gravimetric-differential thermal analysis. The dilatometric measurement indicates that even though the bulk material of LMA has a higher sintering-resistance than the typical TBC material, i.e. yttria-stabilized zirconia (YSZ), the plasma sprayed coating of LMA has two serious contractions due to the re-crystallization of LMA and phase transitions of alumina. LMA has similar thermal expansion behaviour with alumina, leading to a good thermal expansion match between LMA and the thermally grown oxide layer. On the other hand, the plate-like structure of LMA not only results in a low thermal conductivity, low Young's modulus, but also a high stress tolerance, and these are believed to be the reasons for the long thermal cycling life of LMA coating.     

Investigation of the crystallinity of suspension plasma sprayed hydroxyapatite coatings

Hydroxyapatite coatings were deposited on stainless steel substrates. The arc current was varied to study its effect on the coating crystallinity. The crystallinity was calculated according to the XRD patterns via Jade 6.0 software and the full width at half maximum (FWHM) of Raman peak at 962 cm⁻¹. The FE-SEM images showed that HA coatings had rod-like nanostructures and agglomerated into microspheres. The XRD patterns indicated that the as-sprayed coatings were composed of HA and some decomposition phases. Micro-Raman spectroscopies demonstrated that the main phase in the coatings was HA. The results showed that the crystallinity was increased from 68.68% to 76.84% while the FWHM varied from 9.74 to 6.38 cm⁻¹, when the arc current increased from 400 A to 600 A. The selected area electron diffraction (SEAD) patterns were used to analyze the crystallinity qualitatively, and the results agreed with the conclusions of XRD and FWHM of Raman peak.     

Preparation of spherical WC-Co powder by spray granulation combined with radio frequency induction plasma spheroidization

In this paper, spray granulation and radio frequency plasma spheroidization were used to obtain spherical WC-Co powder for laser powder bed fusion from mixed WC and Co powder. The effects of solid content and polyvinyl alcohol content of the slurry on granulated powder were studied. Then the spheroidization effects of different granulated WC-Co powders were investigated and compared. Results show that the granulated powder obtained from the slurry with solid content of 65 wt% and polyvinyl alcohol content of 2.5 wt% has the best performance after plasma spheroidization, whose flowability and apparent density are 10.20 s/50g and 6.76 g/cm³, respectively. Moreover, W and C distribute uniformly in the spheroidized WC-Co powder while the Co element is mainly distributed in the gaps of tungsten carbide. It is also noted that W₂C, free carbon and Co₃W₃C appear in the spheroidized WC-Co powders due to the decomposition of WC during the plasma spheroidization process. Furthermore, the decomposition of WC-Co powder particles with smaller size is more serious, which leads to the presence of black and white particles with significantly different carbon content distribution in the spheroidized powder.     

Thermal and mechanical properties of crack-designed thick lanthanum zirconate coatings

Vertical cracks are beneficial in thermal barrier coatings due to enhanced thermo-mechanical compliance. Accordingly, an aqueous nitrate based precursor solution was atomized on stainless steel substrates by spray pyrolysis to deposit thick crack-designed lanthanum zirconate coatings. Coatings with designed crack patterns were deposited and characterized by electron microscopy, tribology, Vickers indentation, and thermal diffusivity. The crystallization of the coatings was investigated by in situ high temperature X-ray diffraction. The green coatings crystallized from 600 °C and the pyrochlore structure was formed after heat treatment at 1000 °C. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, lower thermal diffusivities, and higher thermal conductivities compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings, ∼28 mm²/s at room temperature, was similar to the values reported for yttria-stabilized zirconia plasma sprayed coatings.     

Environmental barrier coatings using low pressure plasma spray process

Yb₂Si₂O₇/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb₂Si₂O₇/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb₂Si₂O₇ with ~15.5v% Yb₂SiO₅ phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb₂Si₂O₇/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H₂O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7-based dense EBCs by impeding diffusion and ingression of water vapor/O₂.     

Deposition of graded SiO2/SiC coatings using high-velocity solution plasma spray

Carbon/carbon (C/C) composites are widely used in structural components, particularly in the aerospace and aeronautics sectors. However, the application of C/C composites is limited by low oxidation resistance at high temperatures. In order to overcome this problem, graded SiO₂/SiC coatings were deposited on C/C composites by a high-velocity solution plasma spray (HVSPS) process. Graded coatings were formed by reactions between the Si(OH)₄ sprayed liquid precursor and the C/C substrate; these reactions were promoted by the high temperature of the plasma torch. The morphologies, microstructures, and chemical compositions of the coatings were investigated by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. By altering the deposition time, the coating thickness was controlled, therefore demonstrating SiC formation and realizing graded SiO₂/SiC coatings.     

Influence of phase composition on microstructure and thermal properties of ytterbium silicate coatings deposited by atmospheric plasma spray

In this study, ytterbium silicate coatings with different compositions were designed by controlling the Yb₂O₃/ SiO₂ ratio and fabricated by atmospheric plasma spray. The microstructure and thermal properties of these coatings were characterized. Results showed that the Yb₂O₃-rich coatings contained Yb₂O₃ and Yb₂SiO₅ phases, which were characterized by Yb₂O₃ columnar grains, obvious interfaces between splats and many microcracks. The SiO₂-rich coatings were composed of Yb₂SiO₅ and Yb₂Si₂O₇ phases, which were composed of well bonded splats with many spherical pores. The Yb₂O₃-rich coatings had higher coefficient of thermal expansion values and lower thermal conductivities than the SiO₂-rich coatings. The SiO₂-rich coatings presented much better thermal cycling resistance than the Yb₂O₃-rich coatings. The relationship among phase composition, microstructure and thermal properties of ytterbium silicate coatings was analyzed. The results of this study may provide some clues for designs and applications of rare-earth silicates as environmental barrier coatings.     

A comparative study of MoSi2 coatings manufactured by atmospheric and vacuum plasma spray processes

In this work, MoSi₂ coatings were manufactured by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS) technologies, respectively. Phase composition and microstructure of the coatings were characterized by X-ray diffraction and scanning electron microscopy. Microhardness, void and oxygen content of the coatings were also determined. Oxidation behavior of the coatings at high temperature was examined. The results showed that the surface of VPS-MoSi₂ coating was dense and homogeneous. However, there were many microcracks formed on the surface of APS-MoSi₂ coating. The VPS-MoSi₂ coating also had lower void and oxygen contents, higher Vickers hardness compared with those of APS-MoSi₂ coating. Besides, oxidation resistance of the VPS-MoSi₂ coating was better than that of APS-MoSi₂ at 1500 °C.     

溶液注入等离子喷涂——一种制备热障涂层的新技术

介绍了溶液注入等离子喷涂技术(SPPS)在制备热障涂层领域的应用,分析了SPPS热障涂层的结构和性能特点,将该技术与其它工艺过程进行比较,指出了该技术的优势和未来发展方向。     

Microstructure and thermal properties of inflight rare-earth doped thermal barriers prepared by suspension plasma spray

Rare-earth doped yttria-stabilized zirconia (YSZ) coatings with lower thermal conductivity have been fabricated via suspension plasma spray by dissolving rare-earth nitrates into YSZ powder-ethanol suspensions prior to plasma spraying. The effect of dopant concentration and dopant type on properties of the coatings was determined by comparing two coatings containing different concentrations of the same dopant pair (Nd₂O₃/Yb₂O₃), and three coatings having similar concentrations of different dopant pairs (Nd₂O₃/Yb₂O₃, Nd₂O₃/Gd₂O₃, and Gd₂O₃/Yb₂O₃). The porosity content of the coating was found to increase with increased total rare-earth dopant concentration but did not significantly change with dopant pairs. The cross-sectional morphology of every coating displayed a cauliflower-like structure. However, the most heavily doped coating exhibited a larger surface roughness and feathery features in the columnar structures. The thermal conductivity measurement showed that the thermal conductivity decreased with increased Nd₂O₃/Yb₂O₃ concentration. Among coatings containing different dopant pairs, the Gd₂O₃/Yb₂O₃ doped coating exhibited lowest conductivity.     

Protecting the MoSi2 healing particles for thermal barrier coatings using a sol-gel produced Al2O3 coating

A successful sol-gel process to encapsulate molybdenum di-silicide MoSi₂ particles with a closed and thermally stable Al₂O₃ layer using aluminium tri-sec-butoxide as a precursor is presented. The processing conditions such as precursor selection and temperature were optimized through analysing the interaction of the MoSi₂ particles with the sol. The application of the sol-gel based coating was followed by calcining the coated particles at temperatures between 900 and 1200?°C in Ar. The shell composition and the mechanical stability of the microcapsule were analysed by means of X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Upon calcining at 1200?°C in Ar, the MoSi₂ core remains intact as it is, covered by an α - alumina shell with a thickness of about 0.6?μm. The stability tests proved that the encapsulate particles are about five times more oxidation resistant than the uncoated MoSi₂ particles.     

Model on thermal conductivity prediction of quasi-columnar structured coating by plasma spray physical vapor deposition

Firstly, the yttria-stabilized zirconia (YSZ) coating and gadolinium zirconate (GZO) coating with the quasi-columnar structure were manufactured by plasma spray physical vapor deposition. At the same time, a novel three-dimensional geometrical model was established that could satisfactorily reflect such quasi-columnar structural characteristics. Then, based on this model, the three-dimensional spatial distribution of pores and porosity of coatings and the thermal resistance behaviors of the quasi-columnar structured coating were analyzed. Later on, the thermodynamic model was established to estimate the thermal conductivity of the quasi-columnar structured coatings at different temperatures. Finally, a model for predicting the effective thermal conductivity of the GZO/YSZ double-layer coating with quasi-columnar structure was validated to account for the effects of the variable thickness ratios of GZO top layer to YSZ inner layer.     

Architecture designs for extending thermal cycling lifetime of suspension plasma sprayed thermal barrier coatings

Suspension plasma spraying (SPS) as a relatively new spraying technology has great potential on depositing high performance thermal barrier coatings (TBCs). In some cases, however, columnar SPS TBCs show premature failure in thermal cycling test. To explain the reasons of such failure, a failure mechanism for columnar SPS TBCs was proposed in this work. The premature failure of TBCs might be related to the radial stresses in the vicinity of top coat/bond coat interface. These radial stresses were introduced by the thermal misfit and the roughness of bond coat. According to this mechanism, two architecture designs of SPS TBCs were applied to improve the thermal cycling lifetime. One was a double layered top coat design with a lamellar atmospheric plasma sprayed (APS) sub-layer and a columnar SPS top-layer. The other one was a low roughness bond coat design with a columnar SPS top coat deposited on a low roughness bond coat which was grinded before the spraying. With both designs, lifetimes of SPS TBCs were significantly extended. Especially, a lifetime even better than conventional APS TBCs was achieved with the double layered design.     

Thermal conductivity of air plasma sprayed yttrium heavily-doped lanthanum zirconate thermal barrier coatings

The yttrium heavily doped La₂Zr₂O₇ solid solutions coatings, with a Y to La molar ratio of 1:1, have been successfully prepared by air plasma spraying technique. The evolution of phase composition, phase structure and thermal conductivity of such coatings with annealing at 1300?°C has been investigated. The results show that, a single pyrochlore structure can be retained for coating after annealing up to 48?h, beyond which the fluorite phase begins to precipitate out. By comparing thermal conductivities to those undoped counterparts at a similar porosity level, we find a considerably flat thermal conductivity versus temperature (k-T) curve, suggesting the existence of a strong phonon scattering source, which is inferred as rattlers. In addition, after the segmentation of the fluorite phase, the thermal conductivity of corresponding coatings rises considerably, indicating that the fluorite phase has a higher thermal conductivity than that of pyrochlore phase. Moreover, while the as-sprayed coatings show a clear indication of radiative thermal conduction beyond 1000?°C, the thermal conductivity of annealed coatings do not show such an uprising trend after 1000?°C, suggesting that the radiative thermal conduction has been greatly suppressed. The reason is proposed as the formation of local dipoles due to local enrichment of certain elements influences the propagation of electromagnetic waves and thus suppresses the radiative thermal conduction.     

Preparation of crystalline ytterbium disilicate environmental barrier coatings using suspension plasma spray

In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb₂Si₂O₇, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb₂Si₂O₇ coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.     

Effect of splat-interface discontinuity on effective thermal conductivity of plasma sprayed thermal barrier coating

The thermal barrier coating obtained by atmospheric plasma spraying (APS TBCs) has a distinct lamellar microstructure, in which the splats discontinuous interfaces running parallel to the metal/ceramic interface contribute largely to the reduction in the effective thermal conductivity of APS TBCs. The dependency of such contribution on the topological structure of the interface discontinuity is investigated in the present work. Firstly, the concept of discontinuity of splats interfaces was defined to quantify the splats discontinuous interfaces revealed by microscopic observations. Then, the microstructure model with a random distribution of discontinuous interfaces was established by utilizing the finite element simulation method to investigate the effect of interlayer discontinuity on thermal conductivity of the APS TBCs. Finally, an optimal topological structure of the interface discontinuity was found to be responsible for the lowest effective thermal conductivity of the APS TBCs and typical parametrical tendencies demonstrated.     

Preparation and characterization of nanostructured La2Zr2O7 feedstock used for plasma spraying

The nanostructured La₂Zr₂O₇ (LZ) feedstock with high density, suitable size distribution and nearly spherical morphology which can be used for plasma spraying was prepared by spray drying in this study. The spray drying process was discussed. In addition, the formation mechanism of feedstock with hollow shell structure was discussed by finite element method in this paper. The double ceramic layer (DCL) LZ/YSZ (yttria stabilized zirconia) thermal barrier coatings were prepared using the as prepared LZ feedstock. The average grain size computed by Scherrer formulation, the observation of powder size by Transmission Electron Microscope (TEM) and “single splat” deposition experiment indicate that the as prepared LZ feedstock is nanostructured feedstock.