Influence of high-temperature creep stress on growth of thermally grown oxide in thermal barrier coatings

Recently, a large local stress has been found, caused by the change of both the diffusion rate of oxygen through an existing oxide and the rate of chemical reaction at the oxide/oxidized material interface. Since high thermal stress occurs in the thermal barrier coating (TBC) system, the volume expansion of the newly grown oxide, and centrifugal force, the growth rate of the thermally grown oxide (TGO) may change depending on the temperature, the exposure time, and the stress. The aim of this study is to make clear the influence of stress on the growth rate of the TGO thickness under static oxidation. The results show that TGO thickening was affected by the increase of not only the exposure temperature but also the applied stress. The tensile stress in a longitudinal direction in a TBC system due to the applied load makes the oxidant transport and oxide flow more easy, i.e., the tensile stress promotes a volume expansion of newly formed oxide. The increase rate of the TGO thickness was approximately 34% when the applied stress increased from 0 to 205 MPa at 900 °C for 325 h, and approximately 25% when the stress increased from 0 to 150 MPa at 950 °C for 125 h.     

NiCoCrAIY粘结底层的高温氧化性能研究

采用增重法研究了NiCoCrAlY粘结底层的高温氧化性能,通过SEM、EDS等分析手段对高温氧化膜层TGO的显微形貌和组成进行了检测分析,初步探讨了TGO膜层的生长过程、生长机制以及热障涂层失效的影响因素。     

氧化铝封孔对大气等离子喷涂热障涂层抗高温氧化性能的影响

为了减少氧气在大气等离子喷涂热障涂层（TBC）中的渗透通道,进而提高涂层抗高温氧化性能。采用溶胶凝-胶法制备Al2O3封孔剂,通过1100 ℃高温氧化试验,研究封孔作用对陶瓷层中氧气传输、TBC微观形貌以及热生长氧化物（TGO）生长行为的影响规律,阐明封孔对提高TBC抗高温氧化性能的作用机理。结果表明:Al2O3封孔有效阻碍了陶瓷层氧气的渗透,降低了TBC的氧化速率。直至氧化100 h,通过封孔的TBC始终具有更小的TGO厚度与氧化增重,而且抛物线氧化速率（Kp）相比传统TBC下降了14.99 %。     

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO₂-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO₂ reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al₂O₃ layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO₂ doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.     

Model construction and effect of thermally grown oxide dynamic growth on distribution of thermal barrier coatings

A physical geometric model of the dynamic growth of thermally grown oxide (TGO) was established based on an analysis of the TGO growth of 8YSZ thermal barrier coatings during thermal cycling. Finite-element simulation was used to simulate the evolution law between the coating residual stress and thermal cycling, and the linear elasticity, creep effect, and stress accumulation in each thermal cycle were studied. The interface between the top coat (TC) and the bond coat (BC) was covered with a TGO layer that grew vertically and slowly in a layer-like manner. The stress in the TGO was distributed with a “layer” zonal gradient, and the TGO/BC boundaries were distributed uniformly with a large compressive stress, which decreased the TGO layer thickening. With the longitudinal rapid random TGO growth, the boundaries were subjected to a tensile stress, and a high tensile stress concentration area developed at the boundaries. The internal stress consisted of an alternating and mixed distribution of concentrated compressive and tensile stresses. The concentration area of the maximum equivalent stress was distributed in the one-layer TGO near the TC/TGO interface. When a microcrack formed at the TGO/BC boundaries, the crack was subjected to a tensile stress of different size, with a higher tensile stress at both ends, which facilitated crack expansion. Thus, the 8YSZ thermal barrier coating was prone to crack formation and expansion at the TGO/BC boundaries and in the TGO layer near the TC/TGO boundaries.     

Characterization of metallic interconnects extracted from Solid Oxide Fuel Cell stacks operated up to 20,000 h in real life conditions: The fuel side

The performance of an SOFC stack is related to the evolution over time of the materials during operation. Metallic interconnects (MICs) are exposed to high temperature (ca. 850 °C) and dual atmosphere (fuel and air streams), and hence react to form an oxide scale on top. This paper focuses on the fuel side of the MIC and investigates the nature of the oxides formed in SOFC stacks field operated by the SUNFIRE GmbH Company for up to 20,000 h in the frame of the EU-H2020 ADASTRA project. This opportunity supports the understanding on how the metal (i.e., CROFER®22APU) reacts once exposed to the fuel stream by performing investigations on the surface and on the cross-section of the samples by SEM-EDS and micro-Raman spectroscopy. The results show the presence of a layered thermally grown oxide (TGO) rich in Cr at the interface with the metal. On top of the scale, a predominant presence of Mn_xO_y is found supporting the hypothesis of a further reaction of the TGO with selective Cr depletion. The manganese oxides may represent a severe issue due to their very low conductivity.     

基于TGO系统的地方坐标系约束平差解决方案

在GPS控制测量中,若未知地方独立坐标系的相关定义参数时,便不能进行约束平差而只能用坐标转换的方法来求取未知点的地方坐标,在TGO系统中用点校正来解决问题。基于此,对点校正问题进行了理论和实例分析,结合笔者应用TGO的经验,得出了一些在工程测量中有益的结论和建议。     

Determination of mechanical properties of thermally grown oxide on Fecralloy by nano-indentation

Nano-indentation was used to measure the mechanical properties of the thermally grown oxide (TGO) on a Fecralloy substrate. Due to the influence both of the substrate and the indenter size effect (ISE), the measured hardness and Young's modulus of the TGO system decreased with increasing indentation depth. Models were proposed to determine the mechanical properties of the TGO with consideration of both the substrate effect and the ISE. In addition, the ratio of hardness to Young's modulus (H/E) can be related to the ratio of irreversible work to total work (W_ir/W_t) during the indentation process.     

混合氧化物对等离子喷涂热障涂层热循环寿命的影响

采用梯度热循环试验与理论模型相结合的方式研究了混合氧化物对热障涂层寿命的影响。喷涂态热障涂层系统由Inconel 738基体、冷喷涂沉积的NiCoCrAlTaY粘结层(BC)及大气等离子喷涂制备的ZrO2+Y2O3(8%)陶瓷层组成。在1 150℃空气气氛条件下,经过保温20h制备了含有混合氧化物的涂层。每个热循环包括70s的加热,50s的保温及120s的压缩空气冷却。陶瓷层表面最高温度为1 150℃,此时250μm厚的陶瓷层隔热150℃。结果表明:涂层中存在混合氧化物时,其寿命显著降低,且涂层寿命降低程度与混合氧化物和α-Al2O3生长诱发的YSZ应变量,以及混合氧化物的覆盖率有关。与α-Al2O3相比,较高生长速率的混合氧化物易诱发YSZ产生较高的应变,过早地诱发涂层未结合界面扩展、合并,进而导致涂层快速失效。     

等离子喷涂热障涂层高温 TGO 的形成与生长研究

目的研究热障涂层(TBC)和纯粘结层(BC)在1100℃下的氧化动力学,探讨热障涂层中热生长氧化物(TGO)组织结构的演化规律。方法运用大气等离子喷涂技术(APS)制备涂层,对比分析热障涂层和纯粘结层涂层在1100℃下等温氧化2,5,10,20,50,100,200,350 h后TGO的厚度变化,并对粘结层表面和热障涂层截面分别进行XRD和SEM分析。结果热障涂层和纯粘结层在1100℃下的氧化动力学均遵循抛物线规律,其氧化速率常数分别为0.344,0.354μm/h0.5。等温氧化5 h后,TGO的主要成分为α-Al2O3;随氧化时间的增加,生成Cr2O3、尖晶石、Co O和Ni O的混合氧化物;等温氧化100 h后,Co O消失,Ni O的含量减少,Cr2O3和尖晶石氧化物的含量增加;等温氧化350 h后,TGO中出现了裂纹,但涂层仍未剥落,TGO最终由顶层多孔的混合氧化物层和底层具有柱状晶结构的α-Al2O3层组成。结论顶层陶瓷层(TC)对热障涂层氧化速率常数的影响很小。TGO中α-Al2O3首先形成并以柱状结晶的方式生长,混合氧化物在α-Al2O3上形成,TGO生长速度逐渐变缓。