Oxidation of Micro-Sized Aluminium Particles: Hollow Alumina Spheres

Oxidized aluminium microparticles have recently been proposed for manufacturing new, environmentally-friendly, protective coatings on stainless-steels and Ni-base alloys. The oxidation mechanisms of spherical aluminium microparticles of an average particle size of 3.5 μm were studied. Accordingly, simultaneous differential thermal analysis–thermogravimetry tests were carried out in air at different temperatures, always above aluminium melting temperature. Scanning electron microscopy and XRD were also used for the interpretation of results. Weight gain and energy results were explained in terms of the different structural changes taking place in aluminium particles. Dehydroxylation process was identified. The transformation of amorphous alumina to γ-Al₂O₃ was numerically evaluated and the alumina phase transformation (γ-Al₂O₃→α-Al₂O₃) was also studied. The temperature ranges revealed the appearance of metastable phases (θ-Al₂O₃). Complete oxidation of particles can be obtained at 1,300 °C in <1 h, although this also takes place at lower temperatures if enough oxidation time is used. Activation energy of oxidation process at high temperature was also estimated, taking a value of 334 kJ/mol. High temperature oxidation causes the formation of hollow alumina spheres, without any aluminium left inside them.     

NiCoCrAlY粘结层喷涂工艺对8YSZ热障涂层抗氧化性能的影响

目的 提高热障涂层粘结层的抗高温氧化性能。方法 分别采用爆炸喷涂和等离子喷涂工艺制备了不同结构的NiCoCrAlY粘结层,之后通过等离子喷涂制备8YSZ陶瓷层,分析了两种粘结层结构的热障涂层的抗高温氧化性能。利用XRD、SEM和EDS对涂层物相、微观结构和成分进行分析,并对其与基体结合状态、抗高温氧化性能进行研究。结果 爆炸喷涂粘结层内部组织致密,缺陷较少,与基体结合处孔隙少;而等离子喷涂粘结层内部的层状特征明显,孔隙较多,表面粗糙度较低。爆炸喷涂粘结层氧化5 h后,表面生成了一层富Al2O3的致密氧化物膜;而等离子喷涂粘结层表面形成了富NiO、CoO、Cr2O3和Ni(Cr,Al)2O4的氧化物层,并出现了许多微裂纹和片层状氧化物。爆炸喷涂制备的热障涂层试样在前5 h氧化增重速率高于等离子喷涂试样,随后变平缓,而等离子喷涂试样氧化速率依然较高。爆炸喷涂热障涂层的热生长氧化物层（Thermally grown oxide, TGO）经50 h氧化后,仍呈连续状,厚度均匀,粘结层内氧化物缺陷较少。结论 爆炸喷涂粘结层组织均匀、致密,喷涂时涂层的氧化以及热处理的内氧化较少,使得足够的Al较快速地在粘结层表面形成致密的氧化铝,表面一定厚度的氧化铝层抑制了氧和其他金属原子的相向扩散反应,提高了涂层的抗高温氧化性能。     

TiAl合金表面EB-PVD制备热障涂层及高温抗氧化性能

利用EB-PVD技术在Ti Al合金表面制备了扩散铝/YSZ热障涂层。采用SEM、EDS和XRD分析了涂层原始及高温氧化后的微观组织及相组成,并测试了高温氧化性能。结果表明:涂层表面YSZ层为致密柱状晶结构,由非平衡四方相t′-ZrO_2组成。Ti Al合金沉积了扩散铝/YSZ热障涂层后高温氧化性能显著提高,氧化动力学曲线呈对数变化规律,900℃高温氧化时,氧化速率为2.2×10~(-5) mg/cm~2·h。1000℃高温氧化时,氧化速率为1.14×10~(-3) mg/cm~2·h。在高温氧化过程中,粘结层与基体之间发生元素扩散,膜基界面消失。在面层与中间粘结层之间形成了均匀连续的热生长氧化物层TGO。     

多层结构热障涂层抗热震性能的研究

本文采用电子束物理气相沉积技术(EB-PVD)在镍基单晶合金N5基体上制备了双层热障涂层(粘结层+陶瓷层)和三层热障涂层(粘结层+混合层+陶瓷层),并对两组涂层体系的热循环性能进行1100℃保温5min 水冷的热震实验,对其微观组织结构采用扫描电镜、能谱仪以及X射线衍射进行了分析。研究发现,在粘结层和陶瓷层之间添加的混合层(NiCrAlY+YSZ)能够延缓TGO层的生长,并具有缓解内应力的作用,两者的共同作用使得三层结构的热障涂层表现出更为优异的热匹配性。     

Thermal Shock Resistance of APS 8YSZ Thermal Barrier Coatings on Titanium Alloy

Thermal barrier coatings (TBCs) with a typical 8YSZ ceramic top coat and CoNiCrAlY bond coat were deposited on titanium alloy substrate (Ti-6Al-4V in wt.%) by air plasma spraying. Thermal insulation and thermal shock resistance of the TBCs at different temperatures as well as their failure behavior were investigated. The results showed that the test temperature had a significant effect on thermal shock life of the TBCs. Failure of the TBCs systems was caused by the formation of crack, bond coat oxidation and elemental diffusion. The vertical cracks induced by thermal shock cycles were probably responsible for the enhancement in thermal shock resistance of the TBCs. Furthermore, elemental diffusion had a great effect on the acceleration of the TBCs failure. The TBCs could provide a good thermal protection for the titanium alloy substrate.     

Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime

Oxidation of Metals - The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study,...     

Impedance Analysis of 7YSZ Thermal Barrier Coatings During High-Temperature Oxidation

ZrO₂-7 wt.%Y₂O₃ (7YSZ) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying. High-temperature oxidation of 7YSZ TBCs was accomplished at 950 °C and characterized by impedance spectroscopy and scanning electron microscopy with energy-dispersive spectrometry. The results indicated that the thermally grown oxide (TGO) mainly contained alumina. The increase of the thickness of the TGO layer appeared to follow a parabolic law. Impedance analysis demonstrated that the resistance of the TGO increased with increasing oxidation time, also following a parabolic law, and that characterization of the TGO thickness based on fitting an equivalent circuit to its measured resistance is feasible. The YSZ grain-boundary resistance increased due to increasing cracks within the coating for oxidation time less than 50 h. However, beyond 150 h, the YSZ grain-boundary resistance slightly decreased, mainly due to sintering of the coating during the oxidation process.     

Oxidation and Degradation of EB–PVD Thermal–Barrier Coatings

Thermal–barrier coatings (TBCs) consist of a magnetron-sputtered Ni–30Cr–12Al–0.3Y (wt.%) bond coat to protect the substrate superalloy from oxidation/hot corrosion and an electron-beam physical-vapor deposited (EB–PVD) 7 wt.% yttria partially stabilized zirconia (YPSZ) top coat. The thermal cyclic life of the TBC system was assessed by furnace cycling at 1050°C. The oxidation kinetics were evaluated by thermogravimetric analysis (TGA) at 900, 1000, and 1100°C for up to 100 hr. The results showed that the weight gain of the specimens at 1100°C was the smallest in the initial 20 hr, and the oxide scale formed on the sputtered Ni–Cr–Al–Y bond coat is only Al₂O₃ at the early stage of oxidation. With aluminum depletion in the bond coat, NiO, Ni(Cr,Al)₂O₄, and other spinel formed near the bond coat. During thermal cycling, microcracks were initiated preferentially in the YPSZ top coat along columnar grain boundaries and then extended through and along the top coat. The growth stress of TGO added to the thermal stress imposed by cycling, lead to the separation at the bond coat–TGO interface. The ceramic top coat spalled with the oxide scale still adhering to the YPSZ after specimens had been cycled at 1050°C for 300 cycles. The failure mode of the EB–PVD ZrO₂–7 wt.% Y₂O₃ sputtered Ni–Cr–Al–Y thermal-barrier coating was spallation at the bond coat–TGO interface.     

微弧氧化制备阻氚涂层的研究

通过微弧氧化技术在316L不锈钢表面制备了氧化铝阻氚涂层。研究了不同微弧氧化工艺参数对涂层相结构、表面形貌、膜厚的影响,并分别通过XRD、SEM、涡流法进行了分析。通过划痕实验、抗热震实验、气相氘渗透试验,经过分析得到了制备最佳涂层的工艺参数。结果表明：涂层相结构、表面形貌、膜厚受到微弧氧化电流密度、电压及反应时间的影响。电流密度、电压能过影响涂层相结构,电流密度增大时更多铝相变成氧化铝相,电压增大时将促进α相氧化铝的形成;增大电压及反应时间,膜层表面不均匀质量变差,电流密度为9A/dm2时,膜层表面形貌最佳;膜层厚度受到电流密度、电压及反应时间的影响。经过一系列的测试（划痕实验、抗热震实验、气相氘渗透试验）,发现参数为6A/dm2, 300V, 30min时,得到了膜基结合力为 86.0N、能抗280次热震、阻氚性能至少达达3数量级的涂层。     

Comparative Structure,Oxidation Resistance and Thermal Stability of CoNiCrAlY Overlay Coatings With and Without Pt and Their Performance in Thermal Barrier Coatings on a Ni-Based Superalloy

It is demonstrated that the addition of Pt to CoNiCrAlY overlay coating can significantly improve its oxidation resistance and thermal stability as well as its performance in thermal barrier coatings. The addition of Pt is found to stabilize a surface layer with composition based upon NiAlPt₂ with L1_o superlattice in addition to enhancing a more favorable distribution of Y and restricting the outward diffusion of detrimental substrate elements particularly Ta and Ti. Due to these beneficial effects, utilizing the Pt-modified bond coating in a TBC system with top coating of zirconia stabilized by yttria is found to extend its lifetime from 410 ± 42 h to 956 ± 48 h as determined from cycling oxidation tests at 1150 °C. However, spallation of the top coatings in the two systems has been correlated with loss of oxide adherence to the bond coatings.     

热循环下梯度热障涂层热氧化物生长的应力分析

建立了一种预测多层复合梯度热障涂层热应力的理论模型,并通过有限元方法分析了梯度涂层分布指数n、热循环过程中热氧化物的生长对涂层热应力大小及分布的影响。结果表明,通过控制梯度涂层的成分分布指数可以显著降低热应力和改善应力分布。当n=1时,涂层热应力较小且变化平缓,结合性能优异。与双层非梯度涂层的热应力对比可知,功能梯度涂层能显著地缓和涂层系统的热应力和消除应力集中。另外,热循环过程中梯度热障涂层与基体界面附近生长的热氧化物急剧地提升了界面附近的热应力,复杂而又集中的热应力对梯度涂层有很大的破坏。同时采用了一种方法来抑制热氧化物的生长,结果显示该方法能较好地优化涂层的热应力和改善涂层质量。     

Influence of Manufacturing Route on the Oxidation Resistance of Platinum-Modified Aluminide Bond Coatings and Their Performance in Thermal Barrier Coatings Deposited on a Ni-Based Superalloy

The importance of manufacturing route of Pt-aluminide bond coatings in determining the life of thermal barrier coatings on Ni-based superalloys is demonstrated. It is shown that bond coatings aluminized by chemical vapor deposition exhibit higher resistance to oxidation in comparison with the pack cementation route, which results in about twofold increase in the life of the thermal barrier coating as determined under cycling oxidation conditions. This difference in behavior is correlated with the initial microstructures of the bond coatings and their effect on thermal stability and oxidation resistance. However, thermal barrier coatings utilizing the two types of bond coatings are found to fail by the same mechanism involving spallation of the top coating due to loss of adhesion between the thermally grown oxide and underlying bond coating. It is concluded that manufacturing by the CVD route with low Al activity decelerates the kinetics of the processes leading to degradation of the coating system in comparison with the pack cementation route with high Al activity.     

Oxidation Behavior of a High-Nb-Containing TiAl Alloy with Multilayered Thermal Barrier Coatings

Oxidation of TiAl alloys has been recognized as an obstacle for high-temperature applications such as aero engine and gas turbine. Substantial efforts have been made to improve oxidation resistance of TiAl alloys at elevated temperatures. In this study, multilayered thermal barrier coatings are prepared to protect a high-Nb-containing TiAl alloy from oxidation by air plasma spraying. The combination of Al₂O₃-13wt.%TiO₂ ceramic coatings and NiCoCrAlY metallic coatings can improve thermal stability and increase the service lifetime of coatings. The fully melted TiO₂ particles distribute in ceramic coatings uniformly and act as sealing pores and microcracks, which decrease porosity of the ceramic coatings and reduce diffusion channels of oxygen atoms. The porosity of surface and cross-section morphology are 5.5?±?0.8 and 5.1?±?0.8%, respectively. The results of oxidation experiment carried out at 800 and 900 °C for 100 h indicate that the coatings can effectively protect a high-Nb-containing TiAl alloy from oxidation. The mass gain of the high-Nb-containing TiAl alloys with coatings is lower than that of the one without coatings. The ceramic coatings retard diffusion of large amount of oxygen atoms, and bond coatings avoid to be excessively oxidized. Thus, the multilayered thermal barrier coatings exhibit an excellent long-term stability.     

热喷涂NiCoCrAlYTa+7YSZ热障涂层颗粒沉积行为

分别采用低压等离子喷涂和大气等离子喷涂在K4169基体上收集了NiCoCrAlYTa颗粒沉积物及涂层,并对颗粒沉积物的形貌及涂层性能进行了观察分析。结果表明:低压等离子喷涂收集到的单个NiCoCrAlYTa扁平颗粒主要呈圆盘状,涂层致密且氧含量低。而大气等离子喷涂收集到的扁平颗粒主要呈溅射状,涂层孔隙率和氧含量均较高。又在经镜面抛光的NiCoCrAlYTa涂层和K4169基体上分别收集了7YSZ颗粒沉积物,并对其沉积形貌进行了观察分析,结果表明:在K4169基体上收集到的7YSZ颗粒沉积物主要呈圆盘状,表面存在大量的网状微裂纹及宏观环状贯通裂纹。在镜面抛光的NiCoCrAlYTa涂层表面收集的7YSZ颗粒沉积物,周围有少量的指状溅射物,中心部存在一定数量的网状微裂纹,但宏观环状裂纹消失。     

基于太赫兹技术的热障涂层平行裂纹监测研究

目的监测热障涂层内部的平行裂纹。方法针对采用大气等离子喷涂方法制备的热障涂层,在内部预制不同规格的两种平行裂纹,利用反射式太赫兹时域光谱系统进行测试,获取时域谱图,联合Hanning窗函数-小波降噪滤波方法进行反卷积处理,提取不同反射峰间的时间间隔,建立数学模型进行裂纹规格参数计算。结果太赫兹时域光谱可以有效测量服役前未出现平行裂纹的热障涂层的陶瓷顶层厚度D。针对不同类型和规格的平行裂纹,通过提取时域光谱反射峰1和反射峰2的时间间隔Δt1,用于计算裂纹上端距涂层表面预制值d1和判断裂纹的位置类型。当d1相似文献   

热喷涂纳米结构热障涂层的最新研究

分析列举了近些年来热喷涂纳米结构热障涂层（TBCs）的最新研究进展和成果。重点比较了热喷涂纳米结构TBCs与传统TBCs之间的性能差异。最后对热喷涂TBCs的研究前景作了展望。     

等离子喷涂氧化铝涂层的热震性能及微观组织特征

以团聚氧化铝复合颗粒为原料,采用等离子喷涂法,变化特征喷涂参数(CPSP),在Q235钢表面制备陶瓷涂层。对制备的涂层进行热震性能测试,并用扫描电镜和X射线衍射仪分析粉末和热震性能较优涂层的微观结构和相组成特征。结果表明:随特征喷涂参数(CPSP)值的上升,热震循环次数先增大后降低;以适中的特征喷涂参数(CPSP=750 A.V/L.min-1)制备的陶瓷涂层,热震循环次数最大;其微观结构为典型的双模结构,由部分熔化区和完全熔化区组成;涂层中主要物相为γ-Al2O3、α-Al2O3和Al2TiO5。     

YAG/8YSZ双陶瓷热障涂层等温氧化性能研究

本文对比研究了YAG/8YSZ双陶瓷和8YSZ单陶瓷热障涂层体系的抗高温氧化性能。采用爆炸喷涂(D-GUN)在310S耐热不锈钢基体上制备粘结层(NiCoCrAlY),用大气等离子喷涂(APS)分别在粘结层试样上制备YAG/8YSZ双陶瓷和8YSZ单陶瓷热障涂层,利用SEM和XRD分析涂层氧化前后截面与表面特征,对比研究2种热障涂层体系在1100 ℃等温氧化不同时间后的氧化增重动力学、YAG陶瓷层微观结构与物相及TGO生长过程和生长动力学。结果表明,YAG陶瓷层在1100 ℃等温氧化200 h后无明显相结构转变,孔隙率稍有降低;YAG/8YSZ双陶瓷层体系较8YSZ单陶瓷层体系氧化增重速率降低1.7倍,TGO生长速率降低1.4倍,粘结层β-NiAl相消耗速度及岛状氧化物生长速度更低,表现出更好的抗高温氧化性能。     

硬质纳米复合薄膜的热稳定性、抗氧化性与韧性

综述了纳米复合薄膜的硬度增强特性和热稳定性、在高温条件下(≥1 000℃)的抗氧化性和热稳定性、在X射线下呈非晶态的薄膜在高温条件下的热稳定性、以及具有高韧性新型硬质纳米复合薄膜的相关性能。这些新型高韧性硬质纳米复合薄膜具有较低的等效弹性模量E*(H/E*0.1),高弹性恢复系数(We≥60%),优异的摩擦学性能,以及良好的抗断裂能力。等效弹性模量E*=E(1-ν2),E为弹性模量,ν为泊松比。