Search Results (50)

Facing challenges of low efficiency and severe wear in deep hard formations with conventional drilling bits, this study investigates the synergistic rock-breaking technology combining a pulsed CO₂ laser with mechanical bits. The background highlights the need for novel methods to enhance drilling speed in high-strength, abrasive strata where traditional bits struggle. The theoretical analysis explores the thermo-mechanical coupling mechanism, where pulsed laser irradiation rapidly heats the rock surface, inducing thermal stress cracks, micro-spallation, and strength reduction through mechanisms like mineral thermal expansion mismatch and pore fluid vaporization. This pre-damage layer facilitates subsequent mechanical fragmentation. The research employs finite element numerical simulations (using COMSOL Multiphysics with an HJC constitutive model and damage evolution criteria) to model the coupled laser–mechanical–rock interaction, capturing temperature fields, stress distribution, crack propagation, and assessing efficiency. The results demonstrate that laser pre-conditioning significantly achieves 90–120% higher penetration rates compared to mechanical-only drilling. The dominant spallation mechanism proves energy-efficient. Conclusions affirm the feasibility and significant potential of CO₂ laser-assisted drilling for deep formations, contingent on optimized laser parameters, composite bit design (incorporating laser transmission, multi-head layout, and environmental protection), and addressing challenges, like high in-situ stress and drilling fluid interference through techniques like gas drilling. Future work should focus on high-power laser downhole transmission, adaptive control, and rigorous field validation. Full article

The chemical composition of FeCrAl alloy significantly influences its thermal-mechanical as well as anti-corrosive properties. This study investigates the impact of silicon and yttrium additions on the thermal-mechanical properties and high-temperature oxidation resistance of FeCrAl alloy. The results indicate that thermal conductivity gradually decreases with the incorporation of Y or Si into the lattice, whereas the mechanical strength of the alloy can be enhanced through the addition of Y. A trace amount of Y can improve the alloy’s high-temperature oxidation resistance by mitigating the spallation of the surface oxidation film and promoting the growth of the film, characterized by heterogeneous chemical composition and microstructure. It is observed that Y possesses a higher charge density than FeCrAl, suggesting that Y can lose electrons more readily than other elements, which implies a reduction in oxygen diffusion. Full article

Neutron beams, being electrically neutral and highly penetrating, offer unique advantages for the irradiation of biological species such as plants, seeds, and microorganisms. We comprehensively investigated the potential of neutron irradiation for inducing genetic mutations by using simulations of spallation, reactor, and compact neutron sources based on J-PARC BL10, the JRR-3 TNRF, and KUANS. We analyzed neutron flux, energy deposition rates, and Linear Energy Transfer (LET) distributions. The KUANS simulation demonstrated the highest dose rate of 17 Gy/h, significantly surpassing that obtained at BL10, due to the large solid angle achieved with optimal sample placement. The findings highlight KUANS’s suitability for efficiently inducing specific genetic mutations and neutron breeding, particularly for inducing targeted mutations in biological samples, also on account of its LET range of 20–70 keV/μm. Our results emphasize the importance of choosing neutron sources based on LET requirements to maximize mutation induction efficiency. This research study shows the potential of compact neutron sources such as KUANS for effective biological irradiation and neutron breeding, offering a viable alternative to larger facilities. The neutron filters used at BL10 and the TNRF effectively exclude low-energy neutrons while keeping the high-LET component. The neutron capture reaction, ¹⁴N(n,p)¹⁴C, was found to be the main dose contributor under thermal neutron-dominated conditions. Full article

Thermal shock testing has long been employed to assess thermal barrier coatings (TBCs), with crack formation and propagation on TBC surfaces serving as important indicators of fracture toughness. In this study, the influence of preexisting cracks within TBC coatings was investigated. These cracks can help alleviate stress concentrations at the interface and within the thermally grown oxide (TGO) layers of the TBC model. In other words, surface crack propagation may eventually intersect the interface, leading to delamination and spallation. This research focused on modifying the volume fraction of functionally graded materials (FGMs) and optimizing preexisting surface cracks in TBCs to extend their lifespan before delamination occurs. The accuracy of the J-integral and displacement correlation technique (DCT) methods was evaluated for use in thermal shock testing simulations. The results showed that both the stress intensity factor (SIF) and interface tensile stress of preexisting cracks were significantly reduced when the volume fraction was set at N = 3. Furthermore, the SIF values demonstrated strong agreement with calculations using the J-integral and DCT methods. The SIF for preexisting cracks dropped to below 62.42% of the fracture toughness when the crack length was approximately 50% of the TBC coating thickness in FGM structures, with a crack density of 10 cracks per inch (CPI). Full article

A novel concept of cold neutron source employing chessboard or staircase assemblies of high-aspect-ratio rectangular para-hydrogen moderators with well-developed and practically fully illuminated surfaces of the individual moderators is proposed. An analytic approach for calculating the brightness of para-hydrogen moderators is introduced. Because the brightness gain originates from a near-surface effect resulting from the prevailing single-collision process during thermal-to-cold neutron conversion, high-aspect-ratio rectangular cold moderators offer a significant increase, up to a factor of 10, in cold neutron brightness compared to a voluminous moderator. The obtained results are in excellent agreement with MCNP calculations. The chessboard or staircase assemblies of such moderators facilitate the generation of wide neutron beams with simultaneously higher brightness and intensity compared to a para-hydrogen-based cold neutron source made of a single moderator (either flat or voluminous) of the same cross-section. Analytic model calculations indicate that gains of up to approximately 2.5 in both brightness and intensity can be achieved compared to a source made of a single moderator of the same width. However, these gains are affected by details of the moderator–reflector assembly and should be estimated through dedicated Monte Carlo simulations, which can only be conducted for a particular neutron source and are beyond the scope of this general study. The gain reduction in our study, from a higher value to 2.5, is mostly caused by these two factors: the limited volume of the high-density thermal neutron region surrounding the reactor core or spallation target, which restricts the total length of the moderator assembly, and the finite width of moderator walls. The relatively large length of moderator assemblies results in a significant increase in pulse duration at short pulse neutron sources, making their straightforward use very problematic, though some applications are not excluded. The concept of “low-dimensionality” in moderators is explored, demonstrating that achieving a substantial increase in brightness necessitates moderators to be low-dimensional both geometrically, implying a high aspect ratio, and physically, requiring the moderator’s smallest dimension to be smaller than the characteristic scale of moderator medium (about the mean free path for thermal neutrons). This explains why additional compression of the moderator along the longest direction, effectively giving it a tube-like shape, does not result in a significant brightness increase comparable to the flattening of the moderator. Full article

Microbubbles have been applied in various fields. In the mercury targets of spallation neutron sources, where cavitation damage is a crucial issue for life estimation, microbubbles are injected into the mercury to absorb the thermal expansion of the mercury caused by the pulsed proton beam injection and reduce the macroscopic pressure waves, which results in reducing the damage. Recently, when the proton beam power was increased and the number of injected gas bubbles was increased, unique damage morphologies were observed on the solid–liquid interface. Detailed observation and numerical analyses revealed that the microscopic pressure emitted from the gas bubbles contracting is sufficient to form pit damage, i.e., the directions of streak-like defects which are formed by connecting the pit damage coincides with the direction of the gas bubble trajectories, and the distances between the pits was understandable when taking the natural period of gas bubble vibration into account. This indicates that gas microbubbles, used to reduce macroscopic pressure waves, have the potential to be inceptions of cavitation damage due to the microscopic pressure emitted from these gas bubbles. To completely mitigate the damage, we have to consider the two effects of injecting gas bubbles: reducing macroscopic pressure waves and reducing the microscopic pressure due to bubble dynamics. Full article

Thermo-mechanically stabilized nanocrystalline (NC) alloys are increasingly valued for their enhanced mechanical strength and high-temperature stability, achieved through thermodynamic and kinetic stabilization methods. However, their fine-grained structure also increases susceptibility to internal oxidation due to higher atomic diffusivity associated with a greater volume fraction of grain boundaries (GBs). By incorporating solutes that form protective oxides, or the so-called thermally growing oxides (TGO), this vulnerability can be mitigated. The TGO scale acts as a diffusion barrier for oxygen that slows down the oxidation kinetics and prevents internal oxidation that impairs the structural integrity of the metal. This review examines advancements in oxidation-resistant NC alloys, focusing on the interplay between grain size and alloy chemistry. We explore how grain refinement influences diffusion coefficients, particularly the enhanced GB diffusion of Ni and Cr in Ni-Cr-based alloys, which improves oxidation resistance in NC variants like Ni-Cr and Cu-Cr compared to coarse-grained counterparts. We also analyze the role of third elements as oxygen scavengers and the impact of reactive elements such as Hf, Zr, and Y in NiAl alloys, which can slow down diffusion through early establishment of protective TGO layers and enhance oxidation resistance. The concomitant effect of grain size refinement, modifications in alloy stoichiometry, and enhanced atomic diffusion is shown to manifest via drastic reductions in oxidative mass gain, and visualization of the stable, protective oxide scales is delivered through characterization techniques such as TEM, SEM, and EDS. A brief overview is provided regarding stress effects and the role of induced stress in driving oxide scale spallation, which can negatively impact oxidation kinetics. Lastly, we propose future research directions aimed at developing micro-structurally stable NC alloys through multi-solute strategies and surface modification techniques, targeting robust materials for high-stress applications with improved oxidation resistance. Full article

In this study, Gd₂O₃ and Yb₂O₃ co-doped YSZ (GYYSZ) ceramic coatings were prepared via atmospheric plasma spraying (APS). The GYYSZ ceramic coatings were subjected to heat treatment at different temperatures for 5 h to analyze their high-temperature phase stability and sintering resistance. The thermophysical properties of GYYSZ, YSZ, and composite coatings were compared. Three types of thermal barrier coatings (TBCs) were designed: GYYSZ (TBC-1), YSZ/GYYSZ (TBC-2), and YSZ/Composite/GYYSZ (TBC-3). The failure mechanisms of these three TBCs were investigated. The results indicate that both the powder and the sprayed GYYSZ primarily maintain a homogeneous cubic phase c-ZrO₂, remaining stable at 1500 °C after annealing. The sintering and densification of the coatings are influenced by the annealing temperature; higher temperatures lead to faster sintering rates. At 1500 °C, the grain size and porosity of GYYSZ are 4.66 μm and 9.9%, respectively. At 1000 °C, the thermal conductivity of GYYSZ is 1.35 W·m⁻¹ K⁻¹, which is 44% lower than that of YSZ. The thermal conductivity of the composite material remains between 1.79 W·m⁻¹ K⁻¹ and 1.99 W·m⁻¹ K⁻¹ from room temperature to 1000 °C, positioned between GYYSZ and YSZ. In the TBC thermal shock water quenching experiment, TBC-3 demonstrated an exceptionally long thermal shock lifetime of 246.3 cycles, which is 5.8 times that of TBC-1 and 1.8 times that of TBC-2. The gradient coating structure effectively reduces the thermal mismatch stress between layers, while the dense surface microcracks provide a certain toughening effect. Failure analysis of the TBC reveals that TBC-3 exhibits a mixed failure mode characterized by both spallation and localized peeling. The ultimate failure was attributed to the propagation of transverse cracks during the final stage of water quenching, which led to the eventual spallation of the ceramic blocks. Full article

Laser nanostructuring of thin films with ultrashort laser pulses is widely used for nanofabrication across various fields. A crucial parameter for optimizing and understanding the processes underlying laser processing is the absorbed laser fluence, which is essential for all damage phenomena such as melting, ablation, spallation, and delamination. While threshold fluences have been extensively studied for single compound thin films, advancements in ultrafast acoustics, magneto-acoustics, and acousto-magneto-plasmonics necessitate understanding the laser nanofabrication processes for functional multilayer films. In this work, we investigated the thickness dependence of ablation and delamination thresholds in Ni/Au bilayers by varying the thickness of the Ni layer. The results were compared with experimental data on Ni thin films. Additionally, we performed femtosecond time-resolved pump-probe measurements of transient reflectivity in Ni to determine the heat penetration depth and evaluate the melting threshold. Delamination thresholds for Ni films were found to exceed the surface melting threshold suggesting the thermal mechanism in a liquid phase. Damage thresholds for Ni/Au bilayers were found to be significantly lower than those for Ni and fingerprint the non-thermal mechanism without Ni melting, which we attribute to the much weaker mechanical adhesion at the Au/glass interface. This finding suggests the potential of femtosecond laser delamination for nondestructive, energy-efficient nanostructuring, enabling the creation of high-quality acoustic resonators and other functional nanostructures for applications in nanosciences. Full article

Zirconium hydride is commonly used for next-generation reactor designs due to its excellent hydrogen retention capacity at temperatures below 1000 K. These types of reactors operate at thermal neutron energies and require accurate representation of thermal scattering laws (TSLs) to optimize moderator performance and evaluate the safety indicators for reactor design. In this work, we present an atomic-scale representation of sub-stoichiometric ZrH_2−x$(0.3\le x\le 0.6)$, which relies on ab initio molecular dynamics (AIMD) in tandem with velocity auto-correlation (VAC) analysis to generate phonon density of states (DOS) for TSL development. The novel NJOY+NCrystal tool, developed by the European Spallation Source community, was utilized to generate the TSL formulations in the A Compact ENDF (ACE) format for its utility in neutron transport software. First, stoichiometric zirconium hydride cross sections were benchmarked with experiments. Then sub-stoichiometric zirconium hydride TSLs were developed. Significant deviations were observed between the new $\delta$-phase ZrH_2−x TSLs and the TSLs in the current ENDF release. It was also observed that varying the hydrogen vacancy defect concentration and sites did not cause as significant a change in the TSLs (e.g., ZrH_1.4 vs. ZrH_1.7) as was caused by the lattice transformation from $ϵ$- to $\delta$-phase. Full article

In this study, a NiCr/YSZ coating was applied to a γ-TiAl surface using multi-arc ion plating technology to enhance its high-temperature performance and explore the mechanisms of high-temperature oxidation and thermal corrosion. The thermal corrosion properties of the γ-TiAl matrix and NiCr/YSZ coating were investigated at 850 °C and 950 °C using a constant-temperature corrosion test in a 75% Na₂SO₄ + 25% NaCl mixture. The results indicate that after 100 h, the thermal corrosion weight gain of the coating samples was 70.1 mg/cm² at 850 °C and 118.2 mg/cm² at 950 °C. At these temperatures, sulfide formation on the surface increases, leading to a loose and porous surface. After 100 h of high-temperature corrosion at 850 °C, the primary oxidation product on the surface of the coating was tetragonal-ZrO₂. At 950 °C, Y₂O₃, which mainly acts as a stabilizer in YSZ, reacted with Na₂SO₄, resulting in the continuous consumption of Y₂O₃. This reaction caused a substantial amount of tetragonal-ZrO₂ to transform into monoclinic-ZrO₂, altering the volume of the ceramic layer, which induced internal stress, crack propagation, and minor spallation. A continuous and dense internal thermally grown oxide (TGO) layer effectively impeded the diffusion of molten salt substances and oxygen, thereby significantly improving the thermal corrosion resistance of the thermal barrier coating. Full article

The concentration of interfacial normal stress at the free edges of thermal barrier coatings (TBCs) can result in coating spallation. Fatigue cracking is one of the main reasons for creating free edges under complex loads. It is crucial to investigate the fatigue cracking of coatings under cyclic loads to assess potential coating failure. To address this issue, a novel model was proposed to predict the fatigue life of the YSZ topcoat under stress parallel to the interface. Firstly, this study conducted uniaxial and tensile-torsional fatigue tests at elevated temperatures on specimens with atmospheric plasma-sprayed TBCs. The test results revealed that fatigue cracks appeared in the topcoat under cyclic loads, but these cracks did not propagate into the bondcoat or substrate immediately. The number of cycles before the topcoat cracked was found to be associated with the magnitude of the cyclic load. Secondly, this study analyzed the test conditions using the finite element method. Simulations indicated that the crack direction in the topcoat under complex loading conditions was aligned with the first principal stress direction. Finally, the fatigue life prediction model of the topcoat was established based on experiments and simulations. The predicted results fell within a fourfold scatter band. Full article

The failure mechanism of a thermal barrier coatings (TBCs) system is investigated using cyclic thermo-mechanical loading with a thermal gradient. Hollow circular cylindrical specimens are employed, consisting of a nickel-based single-crystal alloy DD6 coated with a NiCoCrAlYHf bond coat via arc-ion plating and a surface electron beam physical vapor deposited (EB-PVD) yttria-stabilized zirconia topcoat. The experimental setup allows for a surface temperature of 1130 °C and a substrate temperature of 1070 °C, while a tensile mechanical load of 200 MPa is employed to simulate the centrifugal stress in the middle of the high-pressure turbine blade. The comparison between TBCs with and without mechanical loading implies that the coupled thermo-mechanical load significantly promotes coating spallation since the superposition of mechanical strain enhances the local tensile stress at the peak region of the topcoat/thermally grown oxides (TGOs) interface. A subsequent interfacial morphology analysis demonstrates that the topcoat/TGO interface exhibited a degradation in the direction parallel to the mechanical loading axis. For all the specimens, TGO comprises a duplex structure, consisting of outer spinel and inner α-Al₂O₃. Full article

To fully realize the potential application of spalled thermal barrier coating systems (TBCs) in gas turbine blades, it is essential to evaluate the service behavior of TBCs and the critical spallation size for safety servicing. For this purpose, the evaluation of the localized spallation of TBCs under high-temperature gas was investigated experimentally and numerically. Thermal insulation experiments and a conjugate heat transfer numerical algorithm were used to clarify the over-temperature phenomenon, temperature distributions, the relevant flow characteristics of the high-temperature gas in the localized spallation region of TBCs, and the influencing mechanisms that consider the spallation width were identified. The results suggested that when the spallation width was less than 10 μm, the temperature in the TBCs did not change due to the weak impression of gas. When the spallation width exceeded the security coefficient of about 3 mm, the TBCs were difficult to service safely due to the impact of high-temperature gas. Furthermore, the concept of an over-temperature coefficient was proposed to describe the over-temperature damage and a nonlinear fitting equation was obtained to reveal and predict the evolution of the over-temperature coefficient. The over-temperature coefficient may serve as a valuable metric in determining the performance degradation of TBCs. Full article

This review offers a comprehensive analysis of thermal barrier coatings (TBCs) applied to metallic materials. By reviewing the recent literature, this paper reports on a collection of technical information, involving the structure and role of TBCs, various materials and coating processes, as well as the mechanisms involved in the durability and failure of TBCs. Although TBCs have been successfully utilized in advanced applications for nearly five decades, they continue to be a subject of keen interest and ongoing study in the world of materials science, with overviews of the field’s evolution remaining ever relevant. Thus, this paper outlines the current requirements of the main application areas of TBCs (aerospace, power generation and the automotive and naval industries) and the properties and resistance to thermal, mechanical and chemical stress of the different types of materials used, such as zirconates, niobates, tantalates or mullite. Additionally, recent approaches in the literature, such as high-entropy coatings and multilayer coatings, are presented and discussed. By analyzing the failure processes of TBCs, issues related to delamination, spallation, erosion and oxidation are revealed. Integrating TBCs with the latest generations of superalloys, as well as examining heat transfer mechanisms, could represent key areas for in-depth study. Full article