Shrinkage behavior and mechanical properties of alkali activated mortar incorporating nanomaterials and polypropylene fiber

Blended ground granulated blast slag (GGBS), low-calcium fly ash (FA), nano silica (NS), and nano alumina (NA) with/without polypropylene fiber (PPF) had a significant effect on the development length of alkali activated mortar (AAM) cured at various humidity levels and curing ages. This paper presents the behavior of alkali activated mortar with 50% FA and 50% GGBFS binder materials and the alkali ratio of sodium silicate to sodium hydroxide (SS/SH) is 2.5. The molar concentration of sodium hydroxide (NaOH) used was 12 M. Feasibility Comparisons between the different humidity 60%–98% of at 23 ± 3 C° were examined. The strength behavior of alkali-activated mortar with different curing ages was also evaluated. Scanning electron microscopy (SEM) analysis and X-ray diffractions (XRD) were also conducted to clarify the effect of nanomaterials and PPF on the microstructures of AAM. It was found that the shrinkage values of AAM were decreased with the addition of polypropylene fiber and nanomaterials. The combined use of both nanomaterials had better performance than the use of nano SiO₂ and/or nano Al₂O₃ alone. The combined use of PPF with nanomaterials had a superior reduction in shrinkage and expansion and increment on strength values; the minimum shrinkage, expansion values, and maximum strength were found at AAM mix incorporating 2%NS-1%NA-0.5%PPF. The SEM analysis and XRD evaluation indicates the significant effect of nanomaterials on the microstructures and bond strength of AAM. The microstructure of the mixes incorporating both nanomaterials and PPF was denser than other mixes without nanomaterials and/or PPF and showed lower micro cracks.     

Analysis of methyl methacrylate adhesive (MMA) relaxation with non-linear stress–strain dependence

One of the problems regarding high strength and stiffness adhesive connections is the concentration of stresses on the edges of the bond. The use of adhesives characterized by high strength but also greater flexibility would avoid this effect and prevent brittle failure. Therefore, the results of a series of laboratory tests are presented in the paper for a semi-rigid methyl methacrylate adhesive (MMA). The research included quasi-static tensile and relaxation tests of the adhesive at a temperature of 23 °C and 50% humidity. Based on the test results, the complex characteristics of the viscoelastic material parameters of the adhesive for different stress levels were determined. The Maxwell model was used to describe the material model of the adhesive during relaxation, for which Prony series were calculated. The work also indicates the approach to numerical modelling using the Finite Element Method (FEM) for selected adhesive, ensuring good convergence of results in the elastic work of the material. Numerical analyses were performed in the ABAQUS software, using material models such as hyperfoam and viscoelasticity. Finally, the relaxation function values obtained during the laboratory tests and numerical analyses were compared and a forecast for longer term function values was presented. The presented results can be used for numerical analysis of adhesive joints with the use of a selected methacrylate adhesive, in the scope of the elastic range subjected to short- or long-term loads.     

A comprehensive exploration on the development of nano Y2O3 dispersed in AA 7017 by mechanical alloying and hot-pressing technique

The main objective of the present study is to develop AA 7017 alloy matrix reinforced with yttrium oxide (Y₂O₃, rare earth element) nanocomposites by mechanical alloying (MA) and hot pressing (HP) techniques for armor applications. AA 7017+10 vol % Y₂O₃ nanocomposites were synthesized in a high-energy ball mill with different milling times (0, 5, 10, and 20 h) to explore the structural refinement effect. The phase analysis and homogeneous dispersion of Y₂O₃ in AA 7017 nanocrystallite matrix were investigated by X-ray diffraction (XRD), various electron microscopes (HRSEM, and HRTEM), Particle Size Analyzer (PSA), and Differential Thermal Analysis (DTA). The nanostructured powders were hot-pressed at 500 MPa pressure with a temperature of 673k for 1hr. The consolidated sample results revealed significant grain refinement and the enhanced mechanical properties with the function of milling time in which the 20h sample exhibited improvement in the hardness (142 VHN - 260 VHN) and ultimate compressive strength (514 MPa–906.45 MPa) due to effective dispersion of Y₂O₃. The various strengthening mechanisms namely, grain boundary (27.02–32.69 MPa), solid solution (57.21 MPa), precipitate (189.79–374.62 MPa), Orowan (135.68–206.92 MPa), and dislocation strengthening (84.99–149.82 MPa) were determined and correlated to the total strength.     

Pressureless-sintered boron nitride nanosheets/glass composite ceramics for excellent mechanical,dielectric and thermo-conductive performances

Because they have a high application potential in the thermal management of insulation environments, high-quality hexagonal boron nitride (h-BN)-based multiphase ceramics have been highly desired. However, so far, their synthesis is still full of challenges. Here, a kind of boron nitride nanosheets (BNNSs)/glass (GS) composite ceramics was prepared by a pressureless sintering method at a lower temperature of 900 °C. Due to a tightly bonded interaction between BNNSs and GS, the formed BNNSs/GS ceramics exhibit excellent multifunction performance. They have an outstanding compressive strength in the range of 19 ∼ 64 MPa and Vickers hardness ranging from 50 to 179 HV. For the BNNSs/GS ceramics with BNNS’s filling fraction of 90 wt%, their maximum side-surface TC values are 12.01 ± 0.18 W m⁻¹ K⁻¹ at 25 °C and 13.64 ± 0.37 W m⁻¹ K⁻¹ at 300 °C, respectively. In the ultra-high frequency range of 26.5 ∼ 40 GHz, the dielectric constant values of the BNNSs/GS ceramics are primarily between 2 and 3, and the corresponding loss tangent values are < 0.3. In addition, based on the remarkable integrity of their structure, these BNNSs/GS ceramics exhibit outstanding thermal-shock stability and prominent thermal management capacity during lots of heating/cooling-testing cycles. Therefore, we believe this kind of BNNSs/GS ceramic system will have great application potential in the new-generation thermal management and/or insulation packaging fields.     

Fabrication of sustainable magnesium phosphate cement micromortar using design of experiments statistical modelling: Valorization of ceramic-stone-porcelain containing waste as filler

Magnesium phosphate cement (MPC) is a potential sustainable alternative to Portland cement. It is possible to lower the total CO₂ emissions related to MPC manufacturing by using by-products and wastes as raw materials. When by-products are used to develop MPC, the resultant binder can be referred to as sustainable magnesium phosphate cement (sust-MPC). This research incorporates ceramic, stone, and porcelain waste (CSP) as a filler in sust-MPC to obtain a micromortar. Sust-MPC is formulated with KH₂PO₄ and low-grade MgO (LG-MgO), a by-product composed of 40–60 wt% MgO. CSP is the non-recyclable glass fraction generated by the glass recycling industry. The effect of water and CSP addition on the mechanical properties of sust-MPC was analyzed using design of experiments (DoE). A statistical model was obtained and validated by testing ideally formulated samples achieved through optimization of the DoE. The optimal formulation (15 wt% of CSP and a water to cement ratio of 0.34) was realized by maximizing the compressive strength at 7 and 28 days of curing, resulting in values of 18 and 25 MPa respectively. After one year of curing, the micromortar was physico-chemically characterized in-depth using backscattered scanning electron microscopy (BSEM-EDS) and Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR). The optimal formulation showed good integration of CSP particles in the ceramic matrix. Thus, a potential reaction between silica and the K-struvite matrix may have occurred after one year of curing.     

Microstructural and main mechanical properties of novalac based carbon–carbon composites as the pyrolysis heating rate

In this study, the effects of pyrolysis heating rate on microstructural and main mechanical properties of Novalac-based carbon/carbon composites were investigated by CHNS, optical microscope, FE-SEM, BET N₂ adsorption, XRD, Raman, FT-IR, wear analyzing, three-point bending test, tensile and Vickers micro-hardness tests. Firstly, PAN-derived carbon nanofibers (reinforcing agent) was synthesized using electrospinning followed by the functionalizing via the wet chemical oxidation to improve the strength of nanofiber bonding to the matrix of composites. Firstly, novalac resin (acting as a matrix), hexamethylenetetramine (hardener agent) and carbon nanofibers (reinforcing agent) were mixed and hot-pressed at 180 °C under the compression load of 40 kN to produce compressed CNFs-Novolac composites. Carbon/Carbon composites were obtained from the pyrolysis of CNFs-Novolac composites up to 1000 °C by the various heating rates under the compression press of 400 bar, finally. Structural and mechanical studies confirmed that the heating rates below or equal to 10 °C.min⁻¹ resulted in the production of low porosity (≤17%) carbon composite with high carbon content (>90 wt%), high fracture strength (≥270 MPa), high toughness (≥9 MPa m^1/2), high hardness (≥156 Hv), and low friction coefficient (<0.6).     

Pressureless consolidation of boron nitride fiber ceramics via a chemical bonding approach

Hexagonal boron nitride (h-BN) materials with excellent physicochemical stability and processibility have been considered as promising engineering materials. However, their practical applications are rather limited by the difficulty in constructing the well-bonded h-BN ceramic. Herein, we present a reaction welding strategy for facile preparation of well-bonded h-BN fibers (BNFs) network by using ethanolamine (ETA) containing alcoholic hydroxyl and primary amine groups as crosslinking agent. The formed >B-O-H₂C-H₂C-H₂N|→B< bond bridges through the esterification and intramolecular coordination (N|→B) reactions between ETA molecules and BNFs containing surface hydroxyls, play an essential role in the construction of the thermodynamically stable >B-N-B< covalent bond between BNFs. Based on this synthetic strategy and a room-temperature molding process followed by a calcination step, a series of resilient BNFs ceramics with high processibility, typical diamagnetism, low dielectric constant (∼ 2.1) and loss, good resistance to temperature different, corrosion, oxidation and abrasion are produced. Besides, they also exhibit excellent strain and damage tolerance, the maximum compression strain and strength can reach up to 38% and ∼ 33 MPa, respectively. Significantly, the deliberately designed BNFs vessel can be successfully used for the alloying of high melting point metals, showing both structural/chemical robustness and good reusability. This ingenious design for well-bonded BNFs network not only is a good model for exploring the welding way for h-BN powders, but also opens up numerous opportunities for practical applications.     

Migration,transformation and solidification/stabilization mechanisms of heavy metals in glass-ceramics made from MSWI fly ash and pickling sludge

In consideration of recycling solid waste to achieve high value-added products, glass-ceramics have been fabricated from municipal solid waste incineration (MSWI) fly ash, pickling sludge (PS), and waste glass (WG) by melting at 1450 °C firstly to achieve parent glass and then crystallizing at 850 °C. Results demonstrated that heavy metals have been well solidified in the prepared glass-ceramics, and relatively/extremely low leaching concentrations of heavy metals have been detected. The synthetic toxicity index of heavy metals has been greatly reduced from 7-18 to <3.2 after crystallization treatment, and the leaching concentrations of Cr, Ni, Zn, Cu, and Pb are 0.15, 0.05, 0.26, 0.12, 0.19 mg L^-1 respectively. Chemical morphology analysis, principal component analysis, TEM and EPMA were utilized to clarify the migration, transformation, and solidification mechanism of heavy metals from the as-received solid wastes. The major heavy metals, Cr and Ni which is responsible for the most toxicity, mainly exist in form of the oxidation state and residual state in parent glass, while the residual state in the glass-ceramics. The solidification performance was mostly positively correlated with the form of residue state, which the stability of heavy metals in glass-ceramics is improved. The solidification mechanism of heavy metals in glass-ceramics could be explained by the combination of chemical solidification/stabilization and physical coating. The TEM and EPMA confirmed that Cr and Ni mainly exist in the spinel crystalline (NiCr₂O₄, Fe_0.99Ni_0.01Fe_1.97Cr_0.03O₄) by solid solution or chemical substitution, and a small amount of Cr in the diopside phase. Pb, Cu, and Zn are homogenously dispersed in the glass-ceramics, which is considered as physical coating solidification.     

Study on the microstructure and mechanical properties of microwave sintered NbC–Ni cermets reinforced by multilayer graphene

In recent years, NbC–Ni cermets has been proposed as a potential substitute for WC-Co cemented carbide in machining and other fields because of its economy and good performance, which has attracted extensive attention of scholars. Research on improving its mechanical properties will help to explore its application potential. Graphene-reinforced NbC–Ni cermets were prepared using a microwave sintering technique, and the effects of multilayer graphene (MLG) on its mechanical properties and microstructure were investigated. The experimental results show that the addition of a certain content of graphene contributes to the densification of the material and inhibits the grain growth. The Vickers hardness, toughness, and bending strength increased and then decreased with an increase in the MLG content. When 0.75 wt% MLG was added, the comprehensive mechanical properties of NbC–Ni cermets were optimal, with a Vickers hardness, fracture toughness, and bending strength of 1297.5 kg/mm², 18.23 MPa m^1/2, and 1464.5 MPa, respectively, which were 12.01%, 38.95%, and 18.97% higher than those without MLG. At low MLG content, the graphene sheet layers were well dispersed in the matrix grain boundaries, whereas graphene agglomerates and pores appeared in cermets with 1 wt% MLG, which degraded their mechanical properties. The strengthening and toughening mechanisms of MLG include grain refinement, large-angle deflection of cracks, crack bridging, and pullout of graphene sheet layers.     

Phase-gradient atomic layer deposition of TiO2 thin films by plasma-induced local crystallization

Atomic layer deposition (ALD) is a thin-film fabrication method that can be used to deposit films with precise thickness controllability and uniformity. The low deposition temperature of ALD, however, often interrupts the facile crystallization of films, resulting in inferior optical and electrical properties. In this study, the extremely localized crystallization of TiO₂ thin films was demonstrated by per-cycle plasma treatment during the plasma-enhanced ALD process. By layering crystalline and amorphous films, a phase-gradient TiO₂ film with precisely modulated optical and electrical properties was fabricated. Moreover, the ratio between the amorphous and crystalline layer thicknesses for a high dielectric constant and low leakage current density was optimized.     

Cracking behaviors of EB-PVD thermal barrier coating under temperature gradient

In this paper, we study the cracking behaviors of single-crystal nickel-based superalloy samples coated with electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) under a thermal gradient experimentally and via the finite element method (FEM). Our results indicate that the stress distribution and failure mode of the TBC samples under the thermal gradient are different from those of samples under a uniform temperature field. The failure of the TBCs under uniform temperature is initiated by interfacial and horizontal cracks, which can result in the separation and buckling of the top coat (TC) layer. However, for the TBCs under a thermal gradient, failure is mainly caused by both vertical TC cracks and interfacial cracks because of the increased transversal stress in the TC layer. Moreover, the initiation and propagation of vertical and horizontal cracks change the failure mode to local spallation of the TC layer. We believe that our findings can contribute to further developments in TBC technology.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

Ultra-high-temperature tantalum-hafnium carbonitride ceramics fabricated by combustion synthesis and spark plasma sintering

We report the fabrication of dense single-phase (Ta,Hf)CN carbonitride ceramics using a combination of combustion synthesis (CS) and spark plasma sintering (SPS). The ceramic powder was produced by high-energy ball milling of the reactants (Ta, Hf, C) in different atomic ratios followed by CS of the obtained nanostructured composites in a nitrogen atmosphere. X-ray diffraction analysis of the combustion products revealed the formation of (Ta,Hf)CN with cubic B1 structures as the dominant phases for all investigated compositions. The SPS of the as-synthesized powders allowed both homogenization of the composition and consolidation of the bulk single-phase carbonitride ceramics with a relative density of 98 ± 1 %. Ta₂₅Hf₇₅CN showed the highest hardness (19.4 ± 0.2 GPa) and fracture toughness (5.4 ± 0.4 MPa m^1/2) among the investigated composites and excellent oxidation resistance in air.     

Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu³⁺) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu³⁺ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu³⁺ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu³⁺ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu³⁺ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu³⁺ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.     

A study on retention of MWCNT in robocasted MWCNT-HAP scaffold structures using vacuum sintering technique and their characteristics

Bioceramic scaffolds are being widely employed in bone tissue engineering applications for their ability to interact with host tissues without inducing any toxicity. Additionally, bioceramics possess good biocompatibility, osteointegration, osteoinduction, and biodegradation characteristics. Hydroxyapatite (HAP) is one such bioceramic known to exhibit closeness to natural bone in terms of chemical composition. The present reports additive manufacturing of HAP and Multiwalled carbon nanotubes (MWCNTs) reinforced HAP scaffold structures for bone tissue engineering applications using the Robocasting technique. Carboxymethyl Cellulose (CMC) was employed as the polymeric binder in this study to prepare the highly viscous HAP and CNT-HAP slurry ideal for robocasting of the scaffold structures. Different percentages of MWCNT (0.5, 1 and 2 wt%) incorporated into the developed CNT-HAP scaffold structures and were vacuum sintered at 1000 °C for 15 min. Vacuum sintering was found to effectively prevent oxidation of MWCNT which is subjected to decomposition at temperatures above 400 or 500 °C in Oxygen atmosphere as per literature. Further, the retention of MWCNTs in the developed CNT-HAP structures post sintering was confirmed using FESEM imaging. The mechanical characterizations revealed that 0.5CNT-HAP structures exhibited highest compression strength (3.36 ± 0.67 MPa) in comparison to 1CNT-HAP and 2CNT-HAP structures. Also, the in vitro biological characterizations demonstrated that the developed CNT-HAP scaffold structures were cytocompatible and remained stable for about 35 days at 37 °C.     

Mechanical and dielectric properties of porous magnesium aluminate (MgAl2O4) spinel ceramics fabricated by direct foaming-gelcasting

Porous ceramic materials have been broadly applied in various fields due to their multifunctional properties. Optimization of their microstructural characteristics, such as pore morphology, total porosity, and pore size distribution, which determine various properties of the final products, is crucial to improve their performances and thus extend their applications. In this study, single-phase porous MgAl₂O₄ materials were fabricated by direct foaming–gelcasting. With an increase in the foam volume from 260 to 350 mL, the total porosity and pore size of the porous ceramic increased, and its microstructure varied from mostly closed cells to open cells containing interconnected large pores (40–155 μm) and small circular windows (10–40 μm) in the ceramic skeleton. The total porosity could be tailored from 84.91% to 76.08% by modulating the sintering temperature and foam volume and the corresponding compressive strengths were in the range of 2.8–15.0 MPa. The compressive strength exhibited a power-law relationship with the relative density with indices of approximately 3.409 and 3.439, respectively. Porous MgAl₂O₄ ceramics exhibited low dielectric constants in the range of 1.618–1.910 at room temperature, which are well matched with theoretical calculations on account of a modified Bruggeman model. The porous MgAl₂O₄ ceramics with good mechanical and dielectric properties controlled easily by various sintering temperatures and foam volumes are promising for practical applications.     

Effect of the calcined andalusite aggregates on the micro-crack formation and thermal shock resistance of mullite castables

The effect of calcined andalusite aggregates on the micro-crack formation and thermal shock resistance of mullite castables was investigated and analyzed. The mullite castables were prepared from andalusite aggregates calcined at high temperatures. The results show that the micro-cracks from the transformation of andalusite to mullite can effectively relieve thermal stress, improving the thermal shock resistance of mullite castables. The micro-cracks generated decreased with increasing calcine temperatures of the andalusite aggregates. When the calcine temperature was increased from 1300 °C to 1500 °C, the thermal shock resistance of mullite castables was found to continuously increase. However, the thermal shock resistance of mullite castables with the andalusite aggregates calcined at 1600 °C is lower than those with the andalusite aggregates calcined at 1500 °C.     

Thermal cycling behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coating with La0.8Ba0.2TiO3−δ top layer

In this study, a La_0.8Ba_0.2TiO_3?δ (LBT) upper layer was deposited on an yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) through atmospheric plasma spraying. The thermal cycling behaviors of the YSZ single-ceramic-layer and LBT–YSZ double-ceramic-layer coatings at 1000 °C were investigated through a water quenching method. Moreover, phases, microstructural evolution, and elemental distributions were studied through by X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy. The results showed that the thermal cycling lifetime of the LBT–YSZ coating was 27% higher than that of conventional YSZ coating. The conventional YSZ coating failed after 251 cycles because of the joining of the continuous horizontal and vertical cracks caused by the formation of thermal growth oxides and the bending effect of the single-ceramic-layer structure. The thermal cycling behavior of the LBT–YSZ coating was different from that of the YSZ coating at the edge and center. Although the former was similar to the failure behavior of the YSZ coating, the cracks in the vertical direction were deflected as a result of the bending effect of the double-ceramic-layer structure during quenching. This deflection led to the formation of slope cracks with longer propagation paths and slope spallation zones. The latter showed small-debris spallation on top of the LBT upper layer due to the lower fracture toughness of the LBT, which protected the central coating from the structural damage of the ceramic coating. These two behaviors would either release the thermal stress or increase the crack-propagation energy requirement in the ceramic coating, consequently improving the thermal cycling lifetime of the LBT–YSZ coating. In summary, depositing an LBT upper layer could potentially improve the thermal cycling lifetimes of TBCs.