重型燃气轮机用La_2(Zr_(0.7)Ce_(0.3))_2O_7/YSZ双层热障涂层热循环性能研究

重型发电燃气轮机低排放、高效率的发展目标,要求燃气轮机透平的进气温度不断提高。热障涂层作为保护金属热端部件的重要手段,是实现这一目标的关键技术之一。目前世界上最先进的J级燃气轮机的透平进口温度已经达到1600℃,热端部件表面温度超过1250℃,传统的YSZ热障涂层已经不能满足这一发展需求,因此迫切需要开发温度更高、热导率更低的新型热障涂层。本文对La_2(Zr_(0.7)Ce_(0.3))_2O_7(以下简称LZ7C3)热障涂层在重型发电燃气轮机中的应用进行了初步验证。结果表明,LZ7C3涂层具有比常规YSZ热障涂层更低的热导率(0.79～0.48Wm~(-1)K~(-1),相对传统YSZ涂层下降30%以上),且在1250℃的高温火焰台架试验中表现出优良的热循环寿命( 7370次),具有良好的应用前景。     

Effect of Y2O3 stabilized ZrO2 coating with tri-model structure on bi-layered thermally grown oxide evolution in nano thermal barrier coating systems at elevated temperatures

Bi-layered thermally grown oxide(TGO) layer plays a major role in the spallation of Y2 O3 stabilized ZrO2(YSZ) layer form the bond coat in the thermal barrier coating(TBC) systems during oxidation. On the other hand, bi-layered TGO formation and growth in the TBC systems with nanostructured YSZ have not been deeply investigated during cyclic oxidation. Hence, Inconel 738/NiCrAlY/normal YSZ and Inconel 738/NiCrAlY/nano YSZ systems were pre-oxidized at 1000 °C and then subjected to cyclic oxidation at 1150 °C. According to microstructural observations, nanostructured YSZ layer over the bond coat should have less micro-cracks and pinholes, due to the compactness of the nanostructure and the presence of nano zones that resulted in lower O infiltration into the nanothermal barrier coating system, formation of thinner and nearly continuous mono-layered thermally grown oxide on the bond coat during pre-oxidation, lower spinels formation at the Al2 O3 /YSZ interface and finally, reduction of bi-layered thermally grown oxide thickness during cyclic oxidation. It was found that pre-heat treatment and particularly coating microstructure could influence microstructural evolution(bi-layered TGO thickness) and durability of thermal barrier coating systems during cyclic oxidation.     

Microstructure and properties evolution of plasma sprayed Al_2O_3-Y_2O_3 composite coatings during high temperature and thermal shock treatment

Al_2O_3-Y_2O_3 composite powder with TiO_2 additive was plasma sprayed to prepare Al_2O_3-Y_2O_3 composite coatings.The micro structure and properties evolution of the Al_2O_3-Y_2O_3 coatings during high temperature and thermal shock resistance were investigated.The results show that the micro structure of the Al_2O_3-Y_2O_3-TiO_2 coating is more uniform than that of the Al_2O_3-Y_2O_3 coating.Meanwhile,amorphous phase is formed in the two coatings.The Al_2O_3-Y_2O_3(-TiO_2) coatings were heat treated for 2 h at temperatures of 800,1000 and 1200℃,respectively.It is found that the microstructure and properties of the two coatings have no obvious change at 800℃.Some of the amorphous phase is crystallized at1000℃,and meanwhile Y_2O_3 and Al_2O_3 react to form YAG phase and YAM phase.At 1200℃,all of the amorphous phases are crystallized.After heat treatment,the micro hardness of the two coatings is increased.The thermal shock resistance of the Al_2O_3-Y_2O_3 system coatings can be improved by using TC4 titanium alloy as substrate and with NiCrAlY bonding layer.Moreover,the Al_2O_3-Y_2O_3-TiO_2 coating exhibits better thermal shock resistance due to the addition of TiO_2.     

The influence of milling parameters on the characteristics of milled and spray-dried NiCoCrAlY–Al2O3 composite powders

Spray-drying process was selected to agglomerate ball milled NiCoCrAlY–Al₂O₃ composite powders. The effect of the starting alloy powder size on the morphology of composite powder was studied. The parameters of milling were optimised by orthogonal experiment to improve the powder’s flowability and apparent density. Then the optimised powder was sprayed by air plasma spray to prepare NiCoCrAlY–Al₂O₃ composite coating. The results showed that the size distribution of starting particles decided the deformation of alloy particles and the characteristics of agglomerated powders eventually. With the decreasing size range of the starting alloy particles, the sphericity of agglomerated powders increased. The optimised milling parameters were as follows: solid content, 60?wt-%; BPR, 4:1; the rotating speed, 350?rev?min^?1; and milling time, 5?h. And the contribution of solid content was the largest. The Al₂O₃ splats showed good adhesion with alloy matrix when the composite powder melted in good condition.     

Adhesion and Thermal Shock Resistance of Al2O3 Thin Films Deposited by PACVD for Die Protection in Semi‐solid Processing of Steel

In this work thin Al₂O₃ films were deposited on hot working steel AISI H11 by plasma assisted chemical vapour deposition (PACVD). The effect of the AlCl₃/O₂ ratio in the gas mixture and the substrate temperature on the film hardness and constitution of the deposited films was investigated by nanoindentation and X‐ray diffraction, respectively. Within the investigated process parameter window thin films containing either the γ‐Al₂O₃ or the α‐Al₂O₃ phase were grown. The performance of these Al₂O₃ coatings for the semi‐solid processing of steel was studied with respect to hardness, adhesion and resistance to thermal shock. The maximum critical load of 50 N as determined by scratch testing was achieved after plasma nitriding of the substrate. No cohesive or adhesive coating failure could be detected after 1000 thermal shock cycles at a contact temperature of 1170±30 °C. Based on the here presented adhesion and thermal shock data it can be concluded that the Al₂O₃ thin films are suitable candidates for the die protection during semi‐solid processing of steel.     

基于正交实验设计方法的APS喷涂Al_2O_3涂层性能的研究

采用正交实验方法研究了大气等离子喷涂工艺参数主气流量、喷涂功率和送粉量对Al2O3陶瓷涂层结合强度和显微硬度的影响,通过X射线衍射仪和扫描电子显微镜研究了粉末和涂层的组织结构,采用WDW-50微机控制电子万能试验机、HV-1000维氏硬度计测量了涂层的结合强度、截面硬度,结合涂层组织结构并应用极差和方差分析方法对实验结果进行了分析,得到了优化后的工艺参数。结果表明,涂层由熔融和未熔融区域混合而成,截面形貌凹凸咬合,以机械结合为主。影响大气等离子喷涂Al2O3涂层性能工艺参数的主次顺序依次为:喷涂功率、主气流量、送粉量,随着功率和主气流量的升高,涂层的结合强度和硬度均出现先增加后降低的趋势。大气等离子喷涂Al_2O_3最优工艺参数为喷涂功率44KW,主气流量40L/min,送粉量40g/min,制备的涂层结合强度52MPa,显微硬度1219.4HV0.3,孔隙率值为4.00%。     

Refractory/steel/inclusion interactions in Al-deoxidised valve spring steel treated with Na2CO3

Ca treatment is generally used in the production of Al-deoxidised steel to prevent nozzle blockage by modifying high melting point inclusions such as MgO·Al₂O₃ (Melting point: 2408?K). However, the successful application of Ca treatment can be quite difficult due to the requirements for stringent control of the amount of calcium that is added. In the present paper, a novel and effective method of inclusion modification by Na₂CO₃ treatment is developed and the evolution process of inclusions in Al-deoxidised steel with Na₂CO₃ treated is studied, which is in good agreement with FactSage calculation results. In addition, the relevant refractory/steel/inclusion interaction mechanisms are also discussed. The experimental results show that MgO–Al₂O₃ inclusions can be transformed into Na₂O–MgO–Al₂O₃–SiO₂ inclusions with low-melting points by Na₂CO₃ treatment. After Na₂CO₃ addition, both the dissolved [Na] and [Si] start to reduce MgO and Al₂O₃ in the generated MgO–Al₂O₃ inclusions from outside to inside, accompanied with the reduction of MgO–Al₂O₃ reaction layer at the steel-refractory interface by [Na] and [Si] in the steel.     

Performance of Pd/CeO2-ZrO2-Al2O3 catalyst for motorcycle

The Pd-only catalysts for motorcycle were prepared by impregnating CeO2-ZrO2-Al2O3 and CeO2-ZrO2+Al2O3 with PdCl2 aque-ous solution and characterized by X-ray diffraction (XRD), oxygen storage capacity (OSC) and H2-temperature-programmed reduction (H2-TPR) methods. The XRD result indicated that the CeO2-ZrO2-Al2O3 compound prepared by co-precipitation formed a single solid solu-tion and had good thermal stability, and Pd phase was not observed in all catalysts. The TPR results showed that the reduction temperature of Pd/CeO2-ZrO2-Al2O3 catalyst was lower than that of Pd/CeO2-ZrO2+Al2O3 catalyst whether they were fresh or aged catalysts. The Pd/CeO2-ZrO2-Al2O3 exhibited high three-way catalytic activity at low temperature, high thermal stability, and wide working window, sug-gesting a great potential for applications.     

Theoretical consideration on composite oxide scales and coatings

The present paper discussed some fundamental aspects on composite oxide scales and coatings for protection of alloys from high temperature oxidation, the related thermodynamic conditions, special mechanical characteristics and a sealing mechanism. It was proposed that the oxide scales and coatings with a composite structure should possess superior mechanical properties than that with a single phase oxide. It also showed that the Al2O3 scales or coatings doped with Y2O3 and ZrO2 (or YSZ)-Al2O3 composite coatings possessed superior properties at high temperatures. In such composite oxide scales and coatings, the fracture resistance of the scales was increased by the toughening effect, the thermal stress was decreased owing to the increase of thermal-expansion coefficients, and Al2O3 phase could seal the alloy substrate well. In addition, the kinetic equation of thermal growth oxide on alloy covered with composite oxide coatings was derived.     

Development of electroless composite coatings by using <Emphasis Type="Italic">in-situ</Emphasis> co-precipitation followed by co-deposition process

This article presents the studies carried out on the in-situ coprecipitation followed by codeposition of Al₂O₃ and ZrO₂ solids along with Al₃Zr within electroless Ni-P matrix. The present pioneered route for synthesis of composite electroless coating, Ni-P-X (X=ZrO₂-Al₂O₃-Al₃Zr), is developed and studied as a function of concentration of coprecipitation reactants and bath loading factors on coating rate. The coating has been found to grow laterally and vertically to form a continuous layer, and fine second-phase particles, from the coprecipitation reaction, are seen to get entrapped in the Ni-P matrix. The coating rate of the composite coating has been observed to be higher than that of Ni-P alloy coatings for the identical experimental conditions. The morphology, phase analysis, thermal response, and hardness studies of the composite coatings have been reported. This article also includes the characterization of coprecipitated powder obtained. Different analysis methods used in this study include scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDXA) and differential thermal analysis (DTA).     

Effect of CaO/Al2O3 ratio on viscosity and crystallisation behaviour of mould flux for high Al non-magnetic steel

In order to facilitate the development of CaO–Al₂O₃ based mould flux for casting high aluminium, non-magnetic steel, the effect of CaO/Al₂O₃ ratios from 0.6 to 3.2 on viscosity and crystallisation characteristics were investigated with the aid of a rotational viscometer, Fourier transform infrared spectroscopy, a single hot thermocouple technique and X-ray diffraction analysis. The results showed that, at temperatures above 1543?K (1270°C), the viscosity first decreased and then became stable with increase in the CaO/Al₂O₃ ratio. At temperatures below 1543?K (1270°C), the viscosity again first decreased but then increased, with the CaO/Al₂O₃ ratio. This viscosity behaviour can be attributed to changes in the network structure characteristics and the precipitation of solid particles within the liquid flux. Increase in the CaO/Al₂O₃ ratio also first inhibited and then enhanced crystallisation as demonstrated by the changes in initial crystallisation temperatures and incubation times. The X-ray diffraction results confirmed that, at both low and high CaO/Al₂O₃ ratios, the dominant precipitates were compounds with high melting points. On the other hand, with CaO/Al₂O₃ ratios in the midrange, the dominant precipitates were compounds with relatively low melting temperatures. It is concluded that mould flux with a CaO/Al₂O₃ ratio in the range 1.1–1.6 is the most appropriate for casting high aluminium, non-magnetic steels.     

Phase diagram assessment of CaO–Al2O3–La2O3 system

Considering the significance of CaO–Al₂O₃–RE₂O₃ (RE?=?rare earth) phase diagrams, the Redlich–Kister expression was employed to evaluate the binary phase diagrams of CaO–Al₂O₃, La₂O₃–Al₂O₃ and CaO–La₂O₃ systems in the present work. Also, the activities of CaO and Al₂O₃ in the CaO–Al₂O₃ binary system were calculated. The phase diagram of the CaO–Al₂O₃–La₂O₃ ternary system was first assessed by Kohler extrapolation method. The accuracy of calculated phase diagrams was validated by comparing with existing experimental points and the calculation results from the literature. The obtained diagrams will lay a foundation for Al-killed steelmaking process, metallurgical flux and other fields’ industrial application.     

Evaluation of B2O3 as replacement for CaF2 in CaO–Al2O3 based mould flux

In order to develop low fluoride or fluoride free CaO–Al₂O₃ based mould flux for casting high aluminium steel, an investigation was carried out to study the effect of substituting CaF₂ with B₂O₃ on heat transfer and crystallisation behaviour of CaO–Al₂O₃ based mould flux by employing a heat transfer simulator of mould flux and a single hot thermocouple technique. The results showed that addition of CaF₂ promoted heat transfer of CaO–Al₂O₃ based mould flux, which was opposite to the effect of CaF₂ on heat transfer in conventional CaO–SiO₂ based mould flux. Addition of CaF₂ inhibited crystallisation of CaO–Al₂O₃ based mould flux by lowering the start crystallisation temperature and prolonging the incubation time of crystallisation. B₂O₃ showed similar effects to CaF₂ on heat transfer and crystallisation of CaO–Al₂O₃ based mould flux, but its ability to promote heat transfer and suppress crystallisation was stronger than CaF₂. Ca₃B₂O₆ (melting temperature 1480°C) was found as the primary crystalline phase in fluoride free CaO–Al₂O₃ based mould flux compared with the primary crystalline phase Ca₂Al₃O₆F (melting temperature 1507°C) in fluoride bearing (20% CaF₂) CaO–Al₂O₃ based mould flux.     

Mechanical and Wear Properties of Al–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Metal Matrix Composites Fabricated by the Combined Effect of Stir and Squeeze Casting Method

The effects of nano particles on double shear strength and tribological properties of A356 alloy reinforced with Al₂O₃ nano particles of size 30 nm were investigated. The percentage inclusions of Al₂O₃ were varied from 0.5 to 1.5 wt%. The particles were added with stirring at 400 rpm and squeeze casting at 750 °C and pressure of 600 MPa in a squeeze casting machine. Comparison of the performance of as cast samples of A356/Al₂O₃ nano composite was conducted. The tribological properties of the samples were also investigated by pin-on-disk tests at 10, 30 and 50 N load, sliding speed 0.534 m/s and sliding distance 1100 m in dry condition. SEM images of microstructure analysis of the composite, Al₂O₃ (0.5 and 1 %) particles were well dispersed in the A356 alloy matrix. Partial agglomeration was observed in metal matrix composite with higher (1.5 %) Al₂O₃ particle contents. The nano dispersed composites containing 0.5 and 1 wt% of Al₂O₃ nano particles exhibited the highest double shear strength, lesser wear loss and coefficient of friction.     

Preparation of Al2O3/Y2O3 composite coating for deuterium permeation reduction

A tritium permeation barrier is required in fusion blankets for the reduction of fuel loss and radiological hazard. In this study, an Al₂O₃/Y₂O₃ composite coating was prepared on 316L stainless steel by radio-frequency magnetron sputtering in order to improve the tritium permeation resistance. The microstructure and the phase composition of the Al₂O₃/Y₂O₃ composite coating are observed by scanning electron microscopy, transmission electron microscopy and grazing incidence X-ray diffraction. Moreover, Auger electron spectroscopy was used to characterize the depth profiles of Al, Y and O elements. The results clearly indicate that the Al₂O₃/Y₂O₃ composite coating is fully dense and the total thickness is approximately 340 nm. The Al₂O₃/Y₂O₃ coating consists of an amorphous Al₂O₃ and the cubic Y₂O₃, in which Al, Y and O elements are homogeneously distributed in the vertical base direction. Furthermore, the deuterium permeation property of the Al₂O₃/Y₂O₃ composite coating was measured by the gas phase permeation method. The results show that the introduction of an interface and the existence of a tiny amount of micro-defects improve the deuterium resistance of the Al₂O₃/Y₂O₃ coating, and its deuterium permeation reduction factor is 536–750 at 873–973 K. Therefore, it is concluded that the Al₂O₃/Y₂O₃ composite coating as deuterium permeation barrier can significantly enhance the deuterium permeation resistance property.     

Al2O3 含量对 YSZ 热障涂层性能的影响

Al_2O_3等氧化物对YSZ热障涂层的高温使用性能有一定的影响。本文用HVOF喷涂Ni Co Cr Al Y合金粘结层,APS喷涂YSZ陶瓷面层,制备了Al_2O_3含量为0.01~0.64wt%的YSZ涂层。比较了不同Al_2O_3含量的YSZ涂层在1100℃下的热震性能和抗烧结性能,并探讨Al_2O_3对涂层的影响机理。结果表明相较于高纯YSZ涂层,随着涂层中Al_2O_3含量升高,涂层的抗热震性能降低,且Al_2O_3促进YSZ涂层的烧结。Al_2O_3含量在小于0.01wt%-0.12wt%区间内时,对涂层抗热震和抗烧结性能有显著影响,含量继续增加至0.64%时,对性能影响减缓。显微组织观察与EDS检测结果表明涂层中Al_2O_3并未在熔融颗粒界面处偏聚,但在颗粒内部有局部偏析。由此推测,含Al_2O_3的YSZ涂层热震失效的原因可能是Al_2O_3在YSZ颗粒内部偏析,并影响涂层的烧结性能,导致裂纹容易萌生和扩展。     

Viscous characteristics and modelling of CaO–Al2O3-based mould flux with B2O3 as a substitute for CaF2

Following a series of laboratory studies on the development of fluoride-free or low-fluoride CaO–Al₂O₃-based mould flux for continuous casting of high-aluminium steel, the viscosity properties of CaO–Al₂O₃-based mould flux with B₂O₃ as a substitute for CaF₂ were investigated using the rotating cylinder method. It was found that B₂O₃ behaved in a similar manner to CaF₂ in changing the viscosity of the flux system. From calculations of the viscous activation energies, it was found that similar changes in the concentrations of CaF₂ and B₂O₃ resulted in almost the same changes in activation energy. A viscosity prediction model was developed using the optical basicity concept as an indicator of melt behaviour. Predicted viscosity values showed good agreement with measurements reported in the literature.     

The effects of MgO and Al2O3 behaviours on softening–melting properties of high basicity sinter

Because the behaviours of MgO and Al₂O₃ during slag formation of high basicity sinter are not clearly understood, the effects of MgO and Al₂O₃ on softening-melting properties are always arguable. In this paper, four kinds of sinter containing different MgO and Al₂O₃ content are investigated. Some observations are obtained. The mechanism of the influence of MgO and Al₂O₃ on softening properties of sinter are different. Al₂O₃ has priority over MgO to enter into slag phase and forms low-melting point phase while MgO remains unslagged state and mainly exists in wustite as FeO–MgO solid solution. When sinter melts, the viscosity of the slag generated from sinter containing high MgO and Al₂O₃ content is low, which could result in low pressure drop. As MgO and Al₂O₃ content increase, the main minerals of residual slag change from 2CaO?SiO₂ to merwinte and melilite. The changes of the minerals in slag phase can well explain the trend of softening-melting characteristic temperatures.     

Heat conduction of composites and its dependence on the microstructure of Al2O3-Cr3 C2 composite

Heat conduction behavior of Al₂O₃-Cr₃C₂ composite with Cr₃C₂ content varying from 0 to 40% in volume were investigated using a laser-flash method. Thermal diffusivity of the composite decreased sharply as first incorporating 10 vol.% of Cr₃C₂, then increased with further increase of Cr₃C₂ content up to 40 vol.%. Similar trends were observed for thermal conductivity behaviour. The significantly decreased thermal diffusivity as well as thermal conductivity at lower content of Cr₃C₂ was assumed to be ascribed mainly to the formation of “composite grain” wherein the fine Cr₃C₂ particle being trapped into Al₂O₃ grain. Furthermore, the formation of an amorphous layer with thickness approx. 10 Å at inclusion/matrix interface may also act as a barrier to phonon conduction.     

Anodic behaviour of the Pb–Co3O4 composite coating in copper electrowinning

The electrochemical and corrosion properties of Pb–Co₃O₄ (about 3% Co) composite anode for copper electrowinning without additives as well as in the presence of two combinations of organic additives in the electrolyte have been investigated and compared with those of Pb–Sb 5.85% anode. The formation of PbO₂ layer on the surface of these electrodes was traced by cyclic voltametric measurements using rotating disc electrode (RDE) method. The Pb–Co₃O₄ composite anode shows a depolarizing effect on the process of oxygen evolution as compared to Pb–Sb anode. The corrosion rate of Pb–Co₃O₄ anode during prolonged polarization is approximately 6.7 times lower than that of Pb–Sb anode. The influence of the tested organic additives on the anodic behaviour of both anodes is negligible.