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The gradual transition of hydrogen as a fuel for land-based gas turbines has resulted in direct changes to the combustion environment. The inadequate combustion of hydrogen fuel can lead to a transition from an oxidizing environment to a partially reducing environment, and further introduces a new potential failure mode for existing thermal barrier coating materials. In this study, tests were conducted on thermal barrier coating samples at 1000 °C in Ar + 5 % O 2 and Ar + 5 % H 2 environments, in addition to samples subjected to heat-treatment in pure argon and air environments to provide a comparison against low oxygen partial pressure and conventional failure modes. The results demonstrated that the degree of sintering of the top coat decreased gradually with a decreasing oxygen partial pressure, and was significantly inhibited in a reducing environment. Faster cooling rates led to the expansion of vertical cracks in the top coat toward the interface, which was accompanied by the generation of numerous transverse cracks in the reducing environment. In contrast, the structure of the top coat remained intact in the other three environments. Furthermore, effective methods for improving the coating durability in reducing environments are discussed. This study therefore contributes to a comprehensive understanding of the failure behavior of thermal barrier coatings in reducing environments, providing new insights into enhancing the stability under such conditions. [ABSTRACT FROM AUTHOR]

Suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) are advanced thermal spraying techniques that enable creation of the coatings with desirable properties. In both techniques, it is necessary to entrain the liquid feedstock into the thermal jet so that it can be effectively transformed into oxide particles. In this study, the suspension containing 3wt.% solid load of 40 nm ZrO2-7%Y2O3 (7YSZ) powder with a medium of zirconium acetate-yttrium nitrate solution (instead of an aqueous or non-aqueous solvent) was used as titled solution medium suspension plasma spray (SMSPS) to produce the YSZ thermal barrier coating (TBC). The microstructure of SMSPS coating exhibited a columnar structure, with an inter-columnar spacing of approximately 5 microns. This columnar structure was attributed to the trajectory of nano-sized particles being affected by the plasma jet and their deposition at shallow angles on surface asperities, resulting in shadowing effect. The presence of vertical cracks within some of the columns in the microstructure of SMSPS coating was similar to the structure observed in SPPS coatings, highlighting the unique nature of this TBC structure. The oxidation testing at 1000°C for 12, 50, 120, and 250 hours revealed the formation of a thermally grown oxide (TGO) layer consisting of aluminum oxide and mixed oxides. The TGO layer growth rate was initially high, but then significantly decreased during the diffusion-controlled and steady-state stages of the test. It was found that the inter-columnar spacing in the coating facilitated oxygen diffusion, resulting in an accelerated oxidation of the bond coat during the initial stages. In addition, the SMSPS coating exhibited an average life of 753 cycles in 1-hours, 1000 ˚C thermal cycling test. The failure mechanism observed involved insular collapsing of adjacent clusters due to crack propagation which was attributed to the presence of vertical cracks and the columnar structure. [ABSTRACT FROM AUTHOR]

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Pipe piles have been widely used because of their high bearing capacity and strong stability, but vertical cracks were appeared in the service process of various fields. In order to fully clarify the causes of vertical cracks happened in pipe piles, provide effective prevention and control measures based on the mechanism of cracks, improve the service life of pipe piles in different environments, and solve various engineering problems caused by pipe pile cracking. By summarizing and analyzing the existing research results, the generation mechanism, crack hazards, crack prevention measures and crack detection technology of vertical cracks in pipe piles in different fields of use are reviewed. The results can provide reference for crack treatment of pipe pile engineering. [ABSTRACT FROM AUTHOR]

The development of vertical cracks in loess slopes often affects slope stability. Compared with the plane strain mechanism, the slope stability analysis under the three-dimensional (3D) failure mechanism is closer to the actual slope instability. Based on the upper bound method of plastic limit analysis, different failure mechanisms (face failure, toe failure and base failure) of 3D loess slope with pre-existing cracks are considered, the energy balance equation and its dimensionless critical height expression γH/c are established, and the upper bound solution of critical height is obtained by random search method. The effects of constraint width, slope angle, internal friction angle and crack depth on the critical height of 3D vertical cracked loess slopes are analyzed. The results indicate that for the toe failure mechanism, the critical height decreases with the increase of crack depth, and the increase in crack depth no longer affects the critical height after reducing to the critical crack depth (δ /H)min. The critical crack depth increases with the increase of slope angle β and decreases with the increase of internal friction angle φ. When the constraint width B/H

Microbially induced calcium carbonate deposition (MICP) restoration technology has good restoration effects and low environmental pollution compared with traditional restoration methods, and this technology has been applied in actual projects. However, grouting components such as bacterial solutions are easily lost in vertical cracks due to their low viscosity as well as their self-weight. In this paper, cracks were repaired using microbial mineralization by sealing the bottom of vertical cracks and pressure grouting. Controlled tests were designed based on three variables: crack width, presence or absence of media in the crack, and grouting method. The results show that the smaller the crack width, the easier it is to achieve a seal. The addition of media in the cracks can improve the repair efficiency. As the amount of grouting increases, the amount of calcium carbonate production in the cracks gradually increases, and the height and thickness of calcium carbonate deposition also gradually increases. The bottom closure pressure grouting method can not only reduce the amount of grouting fluid, but also shorten the test period. This method overcomes the difficulty that grouting poses as it should not be left in vertical cracks and plays an important role in reducing costs and shortening repair time. [ABSTRACT FROM AUTHOR]

The solution precursor plasma spray (SPPS) process involves injecting atomized droplets of a precursor solution into the plasma plume. In this study, two different solution precursors were used in the SPPS process including zirconium oxynitrate and zirconium acetate to make thermal barrier coatings (TBC). The effect of these solutions on microstructure and high-temperature properties of thermal barrier coating made by SPPS has been studied. The results showed that the use of zirconium acetate as a precursor compared to zirconium oxynitrate led to the formation of more vertical cracks with an average spacing of 150-200 μm in the microstructure of coatings. This was due to the presence of more un-pyrolysis or semi-pyrolysis materials in the coating structure and their shrinkage stresses during the deposition that was dependent on the solution chemistry in the plasma plume. The oxidation test at 1000 °C for various times of 12, 50, 120, and 250 h showed similar behavior for both coatings. In addition, the coating obtained from zirconium acetate precursor exhibited an average life of 648 cycles in a 1-h 1000 °C thermal cycling test, which was more than about 1.5 times that of SPPS-TBC coating made of zirconium oxynitrate precursor (431 cycles) due to the presence of much more vertical cracks in its structure. [ABSTRACT FROM AUTHOR]

Nondestructive detection and sizing of internal cracks initiated in reinforced concrete have been critical problems. Geometric characterization of internal cracks and delaminations contributes to predicting their propagation path and failure pattern followed by structural prognostics. There have been limited studies on quantitative detection and characterization of vertical cracks in concrete using wave‐based NDE techniques. This study focuses on solving the problem of imaging deep internal crack planes in the concrete medium by leveraging the ultrasonic shear horizontal (SH) waves from a transducer array. Post‐processing the array full matrix capture (FMC) data with both total focusing method (TFM) and plane wave imaging (PWI) reveals their efficiency in mapping planar defects inclined between 0° and 60°. However, their performance on vertical/near‐vertical defects is found to be inferior with imaging limited to the tip of the defect planes. Further, half‐skip modes of wave dispersion have been adopted in addition to the directly scattered pulses for imaging the vertical and near‐vertical cracks. We propose an imaging framework based on the outcomes of our investigation on the best suitable methodology to detect and map planar defects like cracks inclined in the range of 0–90°. Statistical quantification of defect inclinations from the reconstructed images is compared to the ground‐truth orientations, and they are found to be 94%–99% accurate. Besides accuracy, the computational efficiency of the proposed techniques makes them desirable and reliable for quick on‐site inspections on built infrastructure. [ABSTRACT FROM AUTHOR]

The propagation characteristics of F‐(flexural) wave and the reflection from vertical cracks in pipe‐piles subjected to an eccentric vertical excitation were numerically investigated using elastodynamic finite integration technique (EFIT). The accuracy of the self‐written code was validated by comparing computed results against the measured data. The snapshots of circumferential, radial and vertical velocities in intact and cracked pipe‐piles, as well as the velocity histories of the pile‐top were presented. It is found that not only L‐(longitudinal) wave (corresponding to axisymmetric mode) but also F‐wave (corresponding to non‐axisymmetric mode) can be excited in pipe‐piles subjected to an eccentric vertical excitation. F‐wave causes motion in all directions, while L‐wave only causes vertical motion. The three‐dimensional (3‐D) interference is weak at 90° away from the excitation position for circumferential velocity response, while at 45° or 135° for radial velocity response. The reflected F‐wave (RFW) from the pipe‐toe has multiple peaks due to the dispersion effect of F‐wave. Reflection occurs when F‐wave encounters vertical cracks, while L‐wave is not sensitive to vertical cracks. The intensity of RFW from vertical cracks varies with excitation‐and‐sensor modes. For shallow vertical cracks, the RFW strength varies with the sensor positions; while the intensity of the RFW caused by deep vertical cracks varies little with the sensor positions. With the increase of crack length, the strength of the first arrival RFW due to the vertical crack has no obvious change; however, the strength of RFW from the pile‐toe decreases. [ABSTRACT FROM AUTHOR]

Prefabricated post‐tensioned pre‐stress concrete (PC) T‐girder is one of the common structural forms for bridge superstructure to achieve better quality control, higher construction speed and lower downtime to the transportation system. However, vertical cracks are frequently observed at the web of girder ends after prestress is applied, which could impose an adverse effect on the durability of girders when exposed to aggressive environment. In this paper, the mechanism of this cracking pattern and corresponding countermeasures were investigated by a numerical study as well as practical engineering applications. To begin with, a nonlinear three‐dimensional (3D) finite element model was developed to simulate a typical PC T‐girder in the platform of ABAQUS. The model was loaded based on the sequence of post‐tensioning. It was found that the vertical crack pattern is caused by the large friction resistance and bond strength at the contact surface between the girder and supporting steel pedestal as well as the stress concentration at the girder ends due to the cambering effect of girder during the application of prestress. In addition, a practical method was proposed to control this crack pattern by introducing an isolation cushion made of a thin wood piece to the bottom of girder end. The reliability of this method was verified by numerical analysis and engineering practice. Therefore, it can be used to control vertical cracks at the ends of prefabricated post‐tensioned PC bridge T‐girders during construction. [ABSTRACT FROM AUTHOR]

In this study, La2(Zr0.7Ce0.3)2O7 nanostructured coatings were applied by plasma spraying of the nanostructured agglomerates. Initial nanostructured precursor, precursors calcined at 400°C, 600°C and 900°C with La2(Zr0.7Ce0.3)2O7 compound were used to apply top coat layer of the coatings. XRD results and microstructural observations indicated that a uniform distribution of LC phase in LZ matrix was obtained in the coatings applied by the precursors calcined at 400°C, 600°C and 900°C. The coatings were composed of vertical cracks. The isothermal oxidation results showed that TGO layer in the coating applied by the precursor calcined at 600°C (coating 600D60) was composed of Al2O3 protective compound. While, TGO layer in the conventional YSZ coating contained both compounds of protective Al2O3 and Ni(Cr, Al)2O4 detrimental spinel. The coating 600D60 had better isothermal oxidation resistance compared to the conventional YSZ. But, its thermal cycling resistance was approximately equal to the conventional YSZ. [ABSTRACT FROM AUTHOR]

This study examines sintering resistance of a thermal barrier coating (TBC), composed of a 7YSZ suspension plasma sprayed (SPS) top coat (TC), an air plasma sprayed (APS) NiCoCrAl bond coat (BC), and an INCONEL 625 substrate, under isothermal and cyclic conditions with a peak temperature of 1080 °C for 400, 800, and 1300 h/cycles. Microstructure, phase composition and microstrain were examined using SEM and XRD. Mechanical properties of fracture toughness, hardness and elastic modulus were obtained using nano-indentation. Samples under cyclic conditions presented faster sintering rate than under isothermal condition due to larger compressive strain and frequent heating and cooling cycles. Faster degradation of mechanical properties due to sintering leads to shorter lifetime of SPS coating under cyclic conditions. Moreover, vertical cracks within SPS coatings reduces compressive stress leading to a greater lifetime as compared to APS coatings exposed to similar conditions. [ABSTRACT FROM AUTHOR]

A novel direct current non-transferred arc plasma torch that can generate long, silent and stable laminar plasma jet in air is applied to thermal spraying process in this article. Such long laminar plasma jet, making a low level of noise (< 80 dB) in air, features a jet length over 350 mm, and presents regular variation in length with the increase of output power and gas flow rate. Microstructures and properties of YSZ coatings obtained under spraying distances ranging from 130 mm to 300 mm are discussed in the following. The results show different intervals of vertical cracks across the transverse sections of the coatings and orderly distributed island-protrusions on the top surface of the coatings. A mass of vapor deposited structures can be found in the samples, that means the refractory 8YSZ powders can be melted sufficiently when flowing in long laminar plasma jet, although the stable output power of the laminar plasma torch is only 25–26 kW during the spraying process. This may provide extensive options for different plasma spraying applications and improve the controllability of plasma spraying technology. [ABSTRACT FROM AUTHOR]

This study aims to elaborate the effect of microstructural defects in plasma sprayed yttria partially stabilized zirconia (YPSZ) coating on its corrosion resistance to CaO-MgO-Al 2 O 3 -SiO 2 (CMAS). The microstructural evolution of YPSZ coating under corrosion condition was comparatively studied. The results suggested that the defects that existed in the as-sprayed coating significantly decreased its corrosion resistance to CMAS. The penetration of CMAS could be effectively suppressed through depositing low-defected YPSZ coating by controlling the velocity and temperature of in-flight particles before they impacted onto the substrate. The dissolution of Y 2 O 3 into CMAS caused the phase transformation from the metastable tetragonal phase (t′-ZrO 2 ) to the monoclinic phase (m-ZrO 2 ) and some components in CMAS can act as sintering additives to accelerate the densification of coatings. The stress caused by the thermal gradient and sintering resulted in the initiation and propagation of coarse cracks, which were basically vertical to the coating surface. These vertical cracks can greatly promote the penetration of CMAS, and vice versa. [ABSTRACT FROM AUTHOR]

The paper aims at demonstrating various microstructures which can be obtained using the suspension spraying technique and their respective significance in enhancing the thermal insulation property of a thermal barrier coating. Three different types of coating microstructures are discussed which were produced by the Axial Suspension Plasma Spraying. Detailed characterization of coatings was then performed. Optical and scanning electron microscopy were utilized for microstructure evaluations; x-ray diffraction for phase analysis; water impregnation, image analysis, and mercury intrusion porosimetry for porosity analysis, and laser flash analysis for thermal diffusivity measurements were used. The results showed that Axial Suspension Plasma Spraying can generate vertically cracked, porous, and feathery columnar-type microstructures. Pore size distribution was found in micron, submicron, and nanometer range. Higher overall porosity, the lower density of vertical cracks or inter-column spacing, and higher inter-pass porosity favored thermal insulation property of the coating. Significant increase in thermal diffusivity and conductivity was found at higher temperature, which is believed to be due to the pore rearrangement (sintering and pore coarsening). Thermal conductivity values for these coatings were also compared with electron beam physical vapor deposition (EBPVD) thermal barrier coatings from the literature and found to be much lower. [ABSTRACT FROM AUTHOR]

Abstract: High resolution ground-coupled GPR is useful for determining thickness, deterioration, jointing and cracks in pavements. Although only millimetres in width, vertical cracks can present a significant target in reflection profiles. On composite pavements, consisting of asphalt over concrete, we have observed responses at locations above joints in the underlying concrete that we attribute to cracks within the asphalt layer. On some asphalt pavement, we observed significantly stronger diffraction and waveguide effects responses at a 250MHz centre frequency rather than at 1000MHz, despite the small crack apertures. Using numerical modelling we show the importance of crack filling material, crack aperture, crack height, asphalt conductivity and the GPR centre frequency on the observed response. Our observations and models clearly show the potential of GPR to detect and characterise vertical cracks. We recommend a multi-frequency approach to GPR surveys of pavement: high frequency for crack characterisation and lower frequency for crack detection. [Copyright &y& Elsevier]

Abstract: In this study the authors present and discuss the results of the investigation on the fatigue fracture behaviors in a series of as-soldered and thermal-aged copper/lead-free solder joints deformed under both monotonic and cyclic loadings. The observation results showed that fatigue cracks generally initiate around the IMC/solder interface when the loading axis is vertical to the interface. The intrinsic deformation behaviors are little different for different solder joints resulting from strain localization induced by the stain mismatch. Fracture surface observations revealed the crack propagation path and fatigue resistance of the solder joints to be affected by the yield strength and mechanical property of the solder. When the copper/solder interface is parallel to the loading axis, the interfacial IMC layer failed approximately perpendicular to the interface when the cumulative strain exceeded the fracture strain, then the cracks propagated to the IMC/solder interface, leading to the fracture along the interface. The failure mechanisms and factors influencing interfacial fatigue are discussed. [Copyright &y& Elsevier]

Abstract: Oxide coatings used for various components in the hot section of aero-turbine engines experience temperature gradients at various stages during their flight cycle. One gradient exists during steady-state, due to the combination of the combustion environment next to the free surface and internal cooling of the underlying superalloy substrate. Other gradients develop during cooling of the surface when engine power is reduced. It will be argued that delaminations, when observed within the oxide layer, can only be explained by the presence of a significant stress gradient in the coating, governed by these thermal circumstances. Two extreme cool-down scenarios are envisaged. In one, the surface is cooled suddenly to a lower temperature, followed by slow uniform cooling. In the other, the entire system reduces its temperature uniformly before the temperature gradient in the TBC is eliminated. Criteria for guarding against delaminations within the oxide layer and along the interface with the substrate are provided and the outcome visualized in the form of delamination maps. A comparison with engine experience provides a preliminary assessment of the relevant thermal scenarios, as well as pathways for continuing research. [Copyright &y& Elsevier]

Abstract: When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spray (SPPS) process produces a microstructure containing uniformly vertical cracks. These cracks provide a high degree of strain tolerance to the ceramic top coat. In order to understand the formation of vertical crack in SPPS process, coatings of various thicknesses were deposited on a variety of substrates with vastly different thermal properties. These coatings were characterized in the as-sprayed state and after heat treatment. It has been determined that the tensile stress derived from the pyrolysis of precursor occurring during coating deposition or post heat-treatment is the major driving force for the formation of vertical cracks in SPPS TBCs. [Copyright &y& Elsevier]

In this study, we designed a thermal barrier coating (TBC) system—Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2 (YGYZ) coating—with a Lewis neutral B–SiO 2 layer applied on its surface via a sol–gel-based glassification process and investigated its long-term corrosion resistance under corrosive environments. The corrosion behavior of the B–SiO 2 -covered YGYZ coating was assessed after 20, 40, 80, 160, and 320 h exposure to a hot corrosive environment of Na 2 SO 4 + V 2 O 5 at 1100 °C. Results indicated that the B–SiO 2 layer effectively prevented the penetration of corrosive salts and formation of m -ZrO 2 phase in the coating layer as well as generated vertical cracks that could release the residual stress. Therefore, it is concluded that depositing a Lewis neutral B-SiO 2 layer on YGYZ coatings is highly beneficial for extending the lifetime of the TBCs. • Lewis neutral B–SiO 2 layer was coated on Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2. • B–SiO 2 -covered YGYZ coatings effectively blocked the penetration of corrosive salts. • With increasing exposure time, B–SiO 2 -covered YGYZ coating generate vertical cracks. • B–SiO 2 -covered YGYZ coatings have better thermal durability than existing systems. [ABSTRACT FROM AUTHOR]

The segmented thermal barrier coatings (TBCs) with vertical cracks have similar thermal insulation properties to layered coatings prepared by atmosphere plasma spraying (APS), and have similar mechanical properties to columnar coatings prepared by electron beam physical deposition (EB-PVD), which are very suitable for gas turbines with frequent start-up and variable load. Vertical cracks in the segmented APS-TBCs change the surface morphologies and internal microstructures, so it is necessary to study the internal heat transfer performance of the coating and the conjugate heat transfer characteristics between the TBCs and the cooling film. In this work, the numerical reconstruction models of TBCs with different crack lengths and openings were obtained based on the optimized quartet structure generation set (QSGS) method. By using the double distribution function lattice Boltzmann method (DDF-LBM), the temperature distribution of the coating, the conjugate heat transfer between TBCs and the cooling film and the flow characteristics of the cooling film were studied. The results showed that the vertical cracks with diverse sizes have significant effects on the coating-film cooling system, on which the crack opening has a greater effect. The temperature non-uniformity coefficient shows bimodal distribution in TBCs and the highest temperature is located at the crack center on the coating surface, which can easily lead to local hot spots near crack tips and further result in an acceleration of coating structure's fatigue failure. Meanwhile, the presence of cracks strengthens the interfacial convection heat transfer characteristics. The convection heat transfer coefficient change rate goes up 10% with the crack opening widens. Correspondingly, the greater deformation of the cooling film streamline and value of the vertical velocity are generated under the pre-existing larger size cracks. The results can provide us a powerful guide for the design of advanced coating structures and a strong foundation for a better understanding of the failure behavior of segmented TBCs. • An optimized reconstruction model of TBCs with vertical cracks is proposed. • Microstructure changes of TBCs caused by vertical cracks have significant effects on the coating-film cooling system. • The temperature non-uniformity coefficient shows bimodal distribution in segmented TBCs. • The convection heat transfer coefficient goes up 10% with the crack opening widens. • Stronger vortex and larger negative velocity produced in wider crack passage reduce the stability of the cooling film. [ABSTRACT FROM AUTHOR]

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is a promising cathode material for solid oxide fuel cells (SOFCs). Hence, structured LSCF coatings were deposited in this study by atmospheric laminar plasma spraying (ALPS) at the spraying distances from 150 mm to 250 mm. Particles with high temperature were obtained at low plasma arc power of only 15 kW. The microstructural characterization of the coatings showed cluster-like morphologies on the top surfaces and columnar features with large-scale vertical cracks at the cross section. An evolution mechanism of the coatings was proposed based on particle size. The movement of particles during laminar plasma jet depended on particle size, which was studied experimentally and by simulation to explain the selective deposition. The coatings showed lower polarization resistance when compared to LSCF coatings deposited by atmospheric plasma spraying (APS) due to large vertical crack densities. The high output performance of the SOFCs with the LSCF cathode also proved this phenomenon. The results suggest that ALPS provides extensive options for SOFCs manufacturing. • LSCF coatings were deposited at the long spraying distances, although at low plasma arc power of only 15 kW. • The selective deposition depended on particle size was proposed to explain various structures of coatings. • Structured coatings with large-scale vertical cracks revealed lower polarization resistances compared to APS coatings. [ABSTRACT FROM AUTHOR]