Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal René N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.     

Thermal radiation and cycling properties of (Ca,Fe) or (Sr,Mn) co-doped La2Ce2O7 coatings

La₂Ce₂O₇ with low thermal conductivity as a potential candidate of thermal barrier coatings (TBCs) was co-doped with (Ca, Fe) or (Sr, Mn) in order to further improve its thermal radiation at high temperatures. The microstructure, chemical composition, infrared emission properties (reflection and absorption properties) and thermal cycling lifetime of the coatings were respectively investigated. The results revealed that La_2-xCa_xCe_2-xFe_xO₇₊δ and La_2-xSr_xCe_2-xMn_xO₇₊δ coatings had defected fluorite structure and their infrared emittances were much higher than that of the parent La₂Ce₂O₇. The superior infrared emission could be ascribed to the enhancement of the intrinsic absorption (electron transition absorption), free-carrier absorption and impurity absorption as well as lattice vibration absorption. However, the thermal cycling lifetime of La₂Ce₂O₇ coatings presented a reduction after the (Ca, Fe) or (Sr, Mn) substitution, primarily due to the decrease in the fracture toughness and the increase in the thermal conductivity.     

Effect of material and fabrication technique on marginal fit and fracture resistance of adhesively luted inlays made of CAD/CAM ceramics and hybrid materials

This study evaluated the fracture resistance and marginal fit of CAD/CAM ceramic and composite inlays. Molars (N = 80) were prepared to receive Mesio-occlusal-distal (MOD) inlays and randomly divided into four groups to be restored depending on the materials: (a) HLD: heat-pressed lithium disilicate ceramic (IPS e.max Press), (b) CLD: CAD/CAM-fabricated lithium disilicate ceramic (IPS e.max CAD), (c) NC: CAD/CAM nano-ceramic resin (Lava Ultimate), (d) RC: Indirect resin composite (Filtek P60). Each group was randomly divided into two subgroups regarding the resin cement: (a) High-viscosity resin cement (Syntac, Variolink II), (b) Self-adhesive low-viscosity cement (RelyX Ultimate). After marginal gap and cement thickness measurements, specimens were loaded to fracture in a Universal Testing Machine (1 mm/min). Intact molars acted as the control group (n = 10). Data were analyzed using one-way and two-way ANOVA, Tukey’s tests (α = 0.05). Before cementation, CLD group showed significantly lower mean marginal gap (65 ± 22.4 μm) and after cementation, cement thickness was again the lowest with CLD (82.6 ± 24.6 μm) and the highest with HLD (108.4 ± 21.3 μm) (p < 0.001). The mean marginal gaps of inlays at the gingival margin were significantly higher than at the occlusal and the axial margins (p < 0.05). While material type significantly affected the mean fracture resistance (p < 0.001), the cement type had no effect on the results (p = 0.083). NC group (2486 ± 40 N) showed significantly higher mean fracture load compared to those of other three groups (1997.5 ± 60–2007 ± 30) (p < 0.05). The mean fracture resistance of control group with the intact teeth was significantly higher than those of all groups (p < 0.05) except for NC (p > 0.05).     

热喷涂复合结构MCrAlY/8YSZ热障涂层的抗剥落能力

在陶瓷涂层与金属粘接层之间制备一层NiCoCrAlTaY/YSZ复合过渡层和通过半熔化团聚YSZ粉末制备层状/多孔团状复合结构YSZ隔热层,用SEM表征了涂层的显微组织;依照ASTM C633标准测试了涂层的结合强度;用压痕法测试了陶瓷层的弹性模量和断裂韧性.用激光脉冲法测试了陶瓷层的热导率.用高温水淬快速冷却实验验证...     

纳米陶瓷与纳米陶瓷粉末

八十年代中期发展起来的纳米陶瓷,使材料的超塑性、强度大为提高,对材料的电学、热学、磁学、光学性质产生重要影响,为材料的利用开拓了一个崭新的领域,已成为材料科学研究的热点之一．本文对纳米陶瓷的烧结与性能、纳米陶瓷粉体的制备及性能表征作了简要的综述．     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Experimental investigation of Al2O3/8YSZ and CeO2/8YSZ plasma sprayed thermal barrier coating on diesel engine

In this research work, aluminium oxide/yttria stabilized zirconia (20%Al₂O₃/80%8YSZ) and ceria/yttria stabilized zirconia (20%CeO₂/80%8YSZ) were coated through atmospheric plasma spray technique (APS) as thermal barrier coating (TBC) over CoNiCrAlY bond coat on aluminium alloy (Al-13%Si) substrate piston crown material and their thermal cycling behavior were studied experimentally. Thermal cycle test of both samples were conducted at 800?°C. Microstructural, phase and elemental analysis of the TBC coatings were experimentally investigated. The performance, combustion and emission characteristics of Al₂O₃/8YSZ, CeO₂/8YSZ TBC coated and uncoated standard diesel engine were experimentally investigated. The test results revealed that CeO₂/8YSZ based TBC has an excellent thermal cycling behavior in comparison to the Al₂O₃/8YSZ based TBC. The spallation of the Al₂O₃/8YSZ TBC occurred mainly due to the formation of thermally grown oxide (TGO), and growth of residual stresses at top coating and bond coating interface. The experimental results also revealed that the increase of brake thermal efficiency and reduction of specific fuel consumption for both TBC coated engine. Further reduction of HC, CO and smoke and increase of NOx emission were recorded for both TBC coated engine compared to the standard diesel engine.     

Hot corrosion behavior of Y4Al2O9 ceramics for thermal barrier coatings exposed to calcium-magnesium-alumina-silicate at 1250 °C

The degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) attack has become increasingly dramatic. Y₄Al₂O₉ ceramic, a new potential TBC candidate, has received an increasing attention. In this study, porous Y₄Al₂O₉ ceramic pellets, instead of actual TBCs, are used to investigate the CMAS corrosion resistance at 1250 °C. Results indicate that Y₄Al₂O₉ reacts with CMAS melt to form an impervious sealing layer mainly containing Ca-Y-Si apatite, which could mitigate CMAS further penetration. Once the sealing layer formed, further reaction would occur above the layer accompanying by the recession of sealing layer. This process is probably related to a solid state diffusion.     

Hot corrosion behavior of YSZ/ZrW2O8 composites exposed to V2O5 at 700 °C and 850 °C in air

The hot corrosion behavior of YSZ/ZrW₂O₈ composites as a promising thermal barrier coating system exposed to V₂O₅ at 700 °C and 850 °C was investigated in order to better understand the influence of the incorporated ZrW₂O₈ with isotropic negative thermal expansion performance on the corrosion resistance. Results indicate that the ZrW₂O₈ incorporation could retard the degradation of YSZ from V₂O₅ attack and the corrosion process is significantly related to the inclusion content and the temperature. The corrosion resistance could be determined by the incorporation content, while the reaction products are only temperature dependent. At 700 °C, ZrV₂O₇, YVO₄ and m-ZrO₂ were the main corrosion products, while ZrW₂O₈ recrystallized under the acidic environment provided by V₂O₅. At 850 °C, ZrW₂O₈ decomposed and only WO₃, YVO₄ and m-ZrO₂ could be detected as final corrosion products. The corrosion mechanisms of YSZ/ZrW₂O₈ composites at 700 °C and 850 °C were discussed based on the phase diagrams and Lewis acid-base rule as well as the volume compensation of the positive and negative expansion ceramics.     

CMAS penetration-induced cracking behavior in the ceramic top coat of APS TBCs

The penetration of calcium-magnesium-alumino-silicate (CMAS) is one of the most vital factors inducing the failure of air plasma sprayed thermal barrier coatings (APS TBCs). In present study, a two-dimensional periodical model considering the microstructures in ceramic top coat (TC) is built to study the cracking behavior in the TC of APS TBCs penetrated by CMAS during the cooling process. The CMAS penetration process is considered by filling the microstructures with the same shape of CMAS. The results show that CMAS penetration into the microstructures of the TC changed the stress distribution around the microstructures and induced a mixed crack type here. A microstructure with a relatively sharper geometry will experience a more severe stress state when penetrated by CMAS. The material discontinuity due to CMAS penetration also causes a slightly higher stress level around the microstructure at the CMAS deposit/TC interface, the CMAS penetrated layer and TC/BC interface. Thus, the horizontal cracks are easier to initiate from the microstructures with sharper geometry in these three regions.