Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia

Yttria-stabilized zirconia powders, containing different levels of SiO₂ and Al₂O₃, have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<∼50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Sintering Kinetics of Plasma-Sprayed Zirconia TBCs

The sintering of free-standing plasma sprayed TBCs has been modeled, based on variational principles of free energy minimization and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with the experimental data obtained by dilatometry and BET analysis, respectively. The sensitivity of the predictions to initial pore architecture and material properties is assessed. The model can be used to predict the evolution of the contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously developed analytical model (I.O. Golosnoy, et al. J. Therm. Spray Technol., 2005, 14(2), p 205-214). This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

ZrO2纳米粉等离子喷涂层制备研究

经二次造粒的纳米ZrO2为喂料制备了等离子喷涂层。X衍射物相分析、SEM形貌观察表明．涂层主要以四方相和立方相构成，含有少量的单斜相；在喷涂过程中粉未熔化状况良好，涂层含有网状微裂纹。用金相法测定了涂层孔隙率，与普通ZrO2等离子喷涂层相比，涂层敛密度显著提高。在模拟熔炼条件下，对涂层进行了真空热震试验，涂层均能经受一次真空热震而涂层完好、单斜相含量较低、具有一定孔隙率和均匀网状微裂纹的涂层，经二次真空热震后，涂层仍处于完好状态。     

The Influence of Heat Treatments on the Porosity of Suspension Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed coatings are produced using fine-grained feedstock. This allows to control the porosity and to achieve low thermal conductivity which makes the coatings attractive as topcoats in thermal barrier coatings (TBCs). Used in gas turbine applications, TBCs are exposed to high temperature exhaust gases which lead to microstructure alterations. In order to obtain coatings with optimized thermomechanical properties, microstructure alterations like closing of pores and opening of cracks have to be taken into account. Hence, in this study, TBC topcoats consisting of 4 mol.% yttria-stabilized zirconia were heat-treated in air at 1150 °C and thereafter the coating porosity was investigated using image analysis (IA) and nuclear magnetic resonance (NMR) cryoporometry. Both IA and NMR cryoporometry showed that the porosity changed as a result of the heat treatment for all investigated coatings. In fact, both techniques showed that the fine porosity decreased as a result of the heat treatment, while IA also showed an increase in the coarse porosity. When studying the coatings using scanning electron microscopy, it was noticed that finer pores and cracks disappeared and larger pores grew slightly and achieved a more distinct shape as the material seemed to become more compact.     

Characterization of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings by Cathodoluminescence

The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs.     

Effect of Deposition Rate on the Stress Evolution of Plasma-Sprayed Yttria-Stabilized Zirconia

The deposition rate plays an important role in determining the thickness, stress state, and physical properties of plasma-sprayed coatings. In this article, the effect of the deposition rate on the stress evolution during the deposition (named evolving stress) of yttria-stabilized zirconia coatings was systematically studied by varying the powder feed rate and the robot-scanning speed. The evolving stress during the deposition tends to increase with the increased deposition rate, and this tendency was less significant at a longer spray distance. In some cases, the powder feed rate had more significant influence on the evolving stress than the robot speed. This tendency can be associated with a deviation of a local deposition temperature at a place where sprayed particles are deposited from an average substrate temperature. At a further higher deposition rate, the evolving stress was relieved by introduction of macroscopic vertical cracks as well as horizontal branching cracks.     

Effect of Powder Injection on the Interfacial Fracture Toughness of Plasma-Sprayed Zirconia

Adhesive strength of the plasma-sprayed thermal barrier coating is one of the most important parameters which influence their durability and reliability during service. While many methods exist to measure the adhesive strength, in general, they require cumbersome and time-consuming specimen preparation. Furthermore, considerations of the adhesion strength from the point-of-view of fracture toughness or for that matter, their systematic correlation to both processing variances are limited. Consequently, there is an opportunity to both simplify the measurement procedure and establish correlations among methods and linkages between processing parameters and interfacial fracture toughness. In this paper, we report results on adhesion strength of plasma-sprayed yttria-stabilized zirconia (YSZ) coating on aluminum substrates based on both interfacial indentation test (to measure interfacial fracture toughness) and the modified tensile adhesive test. Carrier gas flow for powder injection into the plasma torch was systematically varied to introduce variances in particle melting with concomitant impact on the measured adhesive strength. The results indicate the correlation between the particle melting index and the measured interfacial fracture toughness.     

Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings

Substrate temperature variation was measured during plasma spraying of ZrO₂ 7% Y₂O₃ powder using fast-response thermocouples embedded in the stainless steel surface. Coatings were deposited with both stationary and moving torches. The substrate was either kept at room temperature at the start of coating deposition or pre-heated to 270-300 °C. Peak temperature during spraying reached 450 °C for a surface initially at room temperature, and 680 °C for a surface preheated to 300 °C before coating deposition. Preheating the substrate reduced coating porosity by approximately 40%. The porosity at the center of the deposit was significantly lower than that at its periphery since particle temperature and velocity were lower at the edges of the plasma plume than along its axis. When a coating was applied with a moving torch the substrate temperature did not increase above 450 °C, at which temperature heat losses to the ambient equalled the heat supplied by the plasma plume and particles. Coating porosity decreased with distance from the substrate. As sequential layers of coating are applied surface temperature increases and roughness decreases. Both of these factors suppress break-up of particles landing on the substrate and thereby reduce coating porosity.     

Influence of Deposition Temperature on the Splat Bonding and Mechanical Properties of Plasma-Sprayed Tungsten Coatings

The mechanical properties of thermally sprayed metallic coatings are limited by the bonding between splats.In this study,tungsten coatings were deposited at different deposition temperatures by controlling the substrate temperature through shrouded plasma spraying.The dependence of the splat bonding and mechanical properties of W coatings on deposition temperature was investigated.The results showed that the apparent porosity of the coatings decreased from 3.2%to 0.3%with the increase of the deposition temperature.The Young’s modulus of W coating was significantly increased from 128 to 307 GPa as the deposition temperature increased from room temperature to 800°C.The microhardness of the coatings was less influenced by the deposition temperature.It was found that splat bonding across lamellae was formed when the deposition temperature was higher than 600°C compared to the obvious lamellae interface in the coatings deposited at temperatures lower than 600°C.The results evidently revealed that the mechanical properties of plasma-sprayed W coatings could be controlled through the splat bonding by altering deposition temperature.     

Microstructure and Creep Behavior of Plasma-Sprayed Yttria Stabilized Zirconia Thermal Barrier Coatings

The purpose of this study was to determine the creep/sintering characteristics of thermally sprayed zirconia coatings and attempt to understand the influence of microstructure on the creep resistance of deposits. The major modification, compared with more typical practice, was employment of a new powder feedstock with agglomerated sub-micron size particles (Nanox), which is compared to one of the best commercially available powders (HOSP). Thick plasma-sprayed coatings were prepared and their physical and mechanical properties were characterized. Creep/sintering experiments were then conducted to investigate the response of the materials when exposed to high temperatures under load. The results showed that it could be possible to correlate the splat thickness to the creep behavior of the coatings.     

氮对Fe-Mn合金阻尼性能和力学性能的影响

通过内耗检测技术研究了氮对Fe-14．1Mn和Fe-17．6Mn合金阻尼性能的影响,以及氮对Fe,Mn系合金相组成和力学性能的影响。结果表明,Fe-14．1Mn比Fe-17．6Mn合金具有更高的阻尼性能;合金中分别加入质量分数为0．2％的氮,两种合金的阻尼性能略有改变,同时Fe-17．6Mn合金的σb从465MPa增加到695MPa,δ从3．7％增加到12．6％,而Fe-14．1Mn合金的σh从470MPa增加到690MPa,δ从5．4％降到4．0％。     

Influence of Deposition Temperature on the Microstructures and Properties of Plasma-Sprayed Al2O3 Coatings

Al₂O₃ coatings were deposited on 1Cr13 substrates by atmospheric plasma spraying at different deposition temperatures of 140, 275, 375, 480, 530, and 660 °C to investigate the effect of coating surface temperature on the lamellar bonding formation. The fractured cross section morphology was characterized by scanning electron microscopy to reveal the lamellar interface bonding. X-ray diffraction was used to characterize the phase contents in the coating. Micro-hardness, Young??s modulus, and thermal conductivity of the deposits were measured for examining the dependency of coating properties on its microstructure. The results show that the interface area bonded through columnar grain growth across splat-splat interfaces was increased with increasing deposition temperature. Moreover, micro-hardness, Young??s modulus and thermal conductivity were increased with the increase of deposition temperature. However, the phase structure of the coating changed little with deposition temperature. The results evidently indicate that the apparent bonding ratio and properties of deposits can be significantly changed in a wider range through controlling the deposition temperature.     

Simulated and Experimental Damping Properties of a SMA/Fiber Glass Laminated Composite

In this article, an advanced laminated composite is developed, combining the high damping properties of shape memory alloy (SMA) with mechanical properties and light weight of a glass-fiber reinforced polymer. The composite is formed by stacking a glass-fiber reinforced epoxy core between two thin patterned strips of SMA alloy, and two further layers of fiber-glass reinforced epoxy. The bars of the laminated composite were assembled and cured in autoclave. The patterning was designed to enhance the interface adhesion between matrix and SMA inserts and optimally exploit the damping capacity of the SMA thin ribbons. The patterned ribbons of the SMA alloy were cut by means of a pulsed fiber laser source. Damping properties at different amplitudes on full scale samples were investigated at room temperature with a universal testing machine through dynamic tension tests, while temperature dependence was investigated by dynamic mechanical analyses (DMA) on smaller samples. Experimental results were used in conjunction with FEM analysis to optimize the geometry of the inserts. Experimental decay tests on the laminated composite have been carried out to identify the adimensional damping value related to their first flexural mode.     

玻纤含量对玻纤增强聚丙烯拉伸性能的影响

采用不同玻纤含量的标准拉伸试样,应用试验方法,通过玻纤含量对玻纤增强PP复合材料的拉仲性能进行比较,找出了实现最佳抗拉强度的玻纤含量。     

氧化钇含量对氧化锆陶瓷微波介电性能的影响

研究了Y2O3含量对钇稳定氧化锆(YSZ)陶瓷微波介电性能的影响.通过热压烧结方法制备了YSZ陶瓷,对材料进行了X射线衍射分析和复介电常数测量.结果表明,当Y2O3含量从2%(摩尔分数)增加到12%时,复介电常数实部在19.49到23.39之间变化.当Y2O3含量为6%时,微波损耗达到最大值0.0789.对YSZ陶瓷的微波损耗机理进行了详细探讨,由于氧离子空位随交变电场的震动和移动而产生的漏导电流是电磁波损耗的主要原因.     

Systematic Investigation on the Influence of Spray Parameters on the Mechanical Properties of Atmospheric Plasma-Sprayed YSZ Coatings

In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence.     

几种铝合金的力学性能及阻尼特性

对Al-Mg-Si、Al-Mg和Al-Li-Cu-Mg-Zr合金进行了不同的时效工艺处理,测试了合金在不同时效状态下的力学性能和阻尼特性。结果表明,合金在不同时效工艺状态,力学性能有明显差异,阻尼特性也有较大差别。可以通过调整合金的时效工艺来改变合金的阻尼特性。