Hot corrosion of TBC-coated components upon combustion of low-sulfur fuels

Gas turbine reliability is a crucial requirement for passenger safety in aviation and a secure energy supply. Hence, corrosive degradation of combustor parts, vanes, and blades in gas turbines must be prevented. One of the most severe forms of corrosion is alkali-sulfate-induced hot corrosion, which is associated with internal sulfidation of components and is usually anticipated to fade in importance in the absence of sulfur. However, the literature suggests that hot corrosion might still occur in low-sulfur combustion gases. In this study, established thermodynamic modeling methods are used to analyze the low-sulfur hot corrosion regime. Liquid sodium chromate is found to be stable in these conditions. A comparison of calculation results and engine findings suggests that high alkali levels can negatively impact thermal barrier coating life even if sulfur is absent in the fuel. Laboratory tests are carried out to validate the chromate formation on MCrAlY-coated specimens. It is shown that molten sodium chromate can alter the oxidation behavior of MCrAlY, promoting the formation of voluminous spinel. This represents a new and different form of hot corrosion compared to type I hot corrosion.     

Isothermal oxidation and TGO growth behaviors of YAG/YSZ double-ceramic-layer thermal barrier coatings

In this study, yttrium aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coatings (DCL TBC) and yttria-stabilized zirconia (YSZ) single-ceramic-layer thermal barrier coatings (SCL TBC) were deposited by atmosphere plasma spray (APS) on the Inconel 738 alloy substrate, and isothermal oxidation tests were performed to investigate the formation and growth behavior of thermally grown oxide (TGO). Results showed that the Al₂O₃ TGO thickness of both TBC groups increased by increasing the isothermal oxidation time，and then slowly decreased with the appearance and growth of the adverse multilayer structure comprising CoCr₂O₄, (Ni,Co)Al₂O₄, NiCr₂O₄, and NiO mixed oxides. However, since the significant inhibition effect of the YAG coating to oxygen ionic diffusion, the mixed oxides appearance time and TGO growth behaviors were delayed in the DCL TBC. As a result, the TGO thickness of the DCL TBC was always smaller than that of the SCL TBC in the entire oxidation process. And the Al₂O₃ layer thickness proportion in the total TGO of the DCL TBC was greater than or equal to that of the SCL TBC after oxidation for the same period. The results of weight gain showed that compared with the SCL TBC, the parabolic oxidation rate of the DCL TBC was decreased approximately 35%. Consequently, the DCL TBC has better high-temperature oxidation resistance than the SCL TBC.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Influence of Heat Treatment on the Bond Coat Cyclic Oxidation Behaviour in an Air-plasma-sprayed Thermal Barrier Coating System

THE METALLIC BOND COAT is an importantconstituent in a TBC system.It enhances the adhesionof the ceramic thermal barrier layer(the topcoat)to thesubstrate and also provides oxidation protection to thesubstrate metal.The composition of the bond coat,generalized as M-Cr-Al-Y,where M represents Ni,Coand/or Fe,generally allows a layer of alumina(A12O3)to form during high temperature exposure.If acontinuous scale of A12O3forms along the interfacebetween the bond coat and the ceramic to…     

双链式区块链交易监管研究

最近,作为去中心化的分布式帐本,区块链使人们能够在不可信赖的环境中进行安全可靠的交易,慢慢地被应用于各个领域。但是,逐渐出现了一些问题,用户隐私信息泄露,存在非法的交易且缺乏有效的监管,智能合约市场存在漏洞等。针对区块链非法交易,提出了监管链的概念,并建立了可以有效监管区块链交易的交易区块链（TBC）和监管区块链（RBC）双链结构。为了提高智能合约的安全性,进一步在监管链中建立智能合约市场,并设计了用于智能合约功能校验的零知识证明算法。理论和数据分析表明,双链结构具有高效率和可扩展性强。实验结果表明机器学习算法对于区块链交易数据的分类准确性达到95.0%,并证明了零知识证明校验智能合约功能的可行性。     

纳米ZnO催化合成无毒增塑剂柠檬酸三丁酯

以柠檬酸和正丁醇为原料，以纳米ZnO为催化剂催化合成无毒增塑剂柠檬酸三丁酯，探讨了催化剂用量、酸醇物质的量比、反应时间、反应温度对反应结果的影响，对合成的产品进行了红外光谱分析。实验结果表明，纳米ZnO催化合成柠檬酸三丁酯的最佳条件为n（柠檬酸）：n（正丁醇）：1：4．5，催化剂用量为柠檬酸质量的1．5％，反应时间为2．5h，反应温度为110—140％，酯化率可达97．23％，产品纯度〉99％。     

Effect of CNG flow rate on the performance and emissions of a Mullite-coated diesel engine under dual-fuel mode

On the diesel engines that are used to generate power in transportation and industries, many researchers have to deal with major problems of smoke emissions while extracting higher efficiency. There are many studies which reported the exhaust emission reduction strategies from diesel engines by applying new combustion methods that are capable of mitigating the formation of harmful emissions. One of the methods to reduce the exhaust emissions in diesel engines is to use the dual-fuel combustion mode. In this study, an attempt has been made to evaluate the effect of thermal barrier coating (TBC) on the dual-fuel engine and for this, experiments are carried out on a single-cylinder direct-injection diesel engine under dual-fuel and low heat rejection mode with compressed natural gas (CNG) as gaseous fuel. Engine components that are exposed to the combustion are coated with Mullite (3Al₂O₃-2SiO₂) TBC. Diesel at 200 bar injector opening pressure was used as pilot fuel and CNG at different flow rates (5, 10 and 15 litres per minute) was inducted into the combustion chamber through inlet manifold as main fuel. Experimental results show that the coating of TBC on the engine components has a positive effect on the performance emissions of the dual-fuel test engine. Brake specific fuel consumption (BSFC) was found improved significantly at all flow rates of CNG with coated engine. Emissions on the other hand were also noticed to be on the lower side with the coated engine except NO_x. Smoke emissions were significantly reduced with coated CNG operation of the test engine at all flow rates.     

不同方法制备的TBC涂层特性的研究

众所周知,热障涂层（TBC）主要包括粘接层和面层两部分。该文采用空气助燃超音速火焰喷涂（HVAF）和氧气助燃超音速火焰喷涂（HVOF）两种方法制备粘接层,大气等离子喷涂（APS）方法制备面层,来研究不同制备方法对TBC涂层微观结构和性能的影响。试验结果表明,HVAF和APS制备的TBC涂层,粘接层中氧化现象较少,热生长氧化物（TGO）生长相对致密均匀,以α-Al2O3为主,其它复合氧化物（NiO、CrNi、CoNi等）较少,表现出较好的高温性能。YSZ陶瓷面层隔热效果良好,是一种成本低廉的新型制备TBC涂层技术。     

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating system

Influence of asymmetric electrode geometry on an impedance spectrum of a plasma-sprayed thermal barrier coating (TBC) system was investigated. The impedance spectrum of the TBC system included impedance of the yttria stabilized zirconia (YSZ) grains, YSZ grain boundaries (negligible), the thermally grown oxide (TGO) and the electrode reaction. In the TBC system with a continuous TGO layer, the impedance of the YSZ grain was measured without influence of asymmetric electrode geometry above 100 kHz. In a frequency range below 100 kHz, asymmetric electrode geometry induced the spread of an electrical conduction region outside an electrode attached on the top coating surface. The impedance of the TGO and the electrode reaction was significantly affected by the asymmetric electrode geometry. The precise interpretation of an impedance spectrum of TBC systems measured under the condition of asymmetric electrode geometry requires further studies on the spread of an electrical conduction region due to decrease in frequency, increase in asymmetry of electrode geometry.     

Effect of superalloy substrate and bond coating on TBC lifetime

Several different single-crystal superalloys were coated with different bond coatings to study the effect of composition on the cyclic oxidation lifetime of an yttria-stabilized zirconia (YSZ) top coating deposited by electron beam physical vapor deposition from a commercial source. Three different superalloys were coated with a 7 μm Pt layer that was diffused into the surface prior to YSZ deposition. One of the superalloys, N5, was coated with a low activity, Pt-modified aluminide coating and Pt-diffusion coatings with 3 and 7 μm of Pt. Three coatings of each type were furnace cycled to failure in 1 h cycles at 1150 °C to assess average coating lifetime. The 7 μm Pt diffusion coating on N5 had an average YSZ coating lifetime > 50% higher than a Pt-modified aluminide coating on N5. Without a YSZ coating, the Pt-modified aluminide coating on N5 showed the typical surface deformation during cycling, however, the deformation was greatly reduced when constrained by the YSZ coating. The 3 μm Pt diffusion coating had a similar average lifetime as the Pt-modified aluminide coating but a much wider scatter. The Pt diffusion bond coating on superalloy X4 containing Ti exhibited the shortest YSZ coating lifetime, this alloy-coating combination also showed the worst alumina scale adhesion without a YSZ coating. The third generation superalloy N6 exhibited the longest coating lifetime with a 7 μm Pt diffusion coating.