Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.     

Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock

The physical properties which affect the propagation of cracks in specimens fractured by thermal shock are discussed. The driving force for crack propagation is provided by the elastic energy stored at fracture. The mechanism of energy dissipation which will tend to arrest the propagating cracks is provided by the "effective surface energy" required to produce the newly formed crack surfaces. An expression is derived applicable to a body of spherical shape for the mean area traversed by cracks nucleated by thermal shock. Three numerical examples are given for materials with widely different physical properties, and their fracture behavior is predicted. Good agreement with experiment was obtained. Thermal shock damage resistance parameters suitable for the relative comparison of the "degree of damage" to be expected in materials fractured by thermal shock are proposed. The criteria for a low degree of damage are high values of Young's modulus of elasticity, Poisson's ratio, and effective surface energy and low values of strength. Recommendations are made for the selection of materials for severe thermal shock, where the best materials available are known to fail.     

Crack Engineering in Thick Coatings Prepared by Spray Pyrolysis Deposition

Cracks should normally be avoided in the deposition of coatings, but vertical cracks in thermal barrier coatings are engineered to absorb thermo‐mechanical stress. Thick lanthanum zirconate coatings were deposited by spray pyrolysis deposition from aqueous nitrate‐based precursor solutions, and cracks were formed during decomposition of the nitrate species due to the associated volume change. The crack spacing and crack opening in the deposited coatings were analyzed in terms of thickness, pH of the precursor solution, and deposition and decomposition temperatures and kinetics by thermogravimetry, scanning calorimetry, mass spectroscopy, and electron microscopy. The thickness of the coatings demonstrated the most important effect on the crack pattern. The crack opening and the crack spacing varied linearly with increasing thickness, leading to small delamination at the interface. The cracks were stable after the crystallization of the films by further heat treatment. Knowing the influence of the different parameters, coatings with a designed crack pattern can be deposited.     

Investigation of Thermal Shock in a High-Temperature Refractory Ceramic: A Fracture Mechanics Approach

The results of a study on the thermal shock behavior of a high-temperature refractory ceramic that is used as a furnace liner in the melting of steels are presented in this paper. The experimental studies show that thermal shock damage initiates by edge cracking after the first shock cycle. Subsequent subcritical crack growth occurs by the incremental extension of dominant cracks until catastrophic failure occurs. The observations of the crack profiles also reveal the formation of viscoelastic bridges that promote crack-tip shielding/toughening via crack bridging. Following a brief discussion of the respective mechanisms of fracture and thermal shock damage at different temperatures and temperature ranges, the implications of the results are discussed for refractory ceramics that are toughened by viscoelastic crack bridging.     

纳米改性Ti(C,N)基金属陶瓷的力学性能及抗热震性能

真空烧结制备了Ti(C,N)基金属陶瓷,测试了不同金属黏结相成分的纳米TiN改性Ti(C,N)基金属陶瓷的力学性能及抗热震性能.力学实验结果表明:金属相含量越多,材料的强度和断裂韧性越高,而硬度则越低.金属相含量相同时,Ni能提供更高的强度与韧性,而Co能带来更高的硬度.热震试验结果表明:热循环温度较低时,40%TiC-10%TiN-15%WC-14%Mo-20%Ni-1%C.50%TiC-10%TiN-15%WC-4%Mo-10%Ni-10%Co-1%C和50%TiC-10%TiN-15%WC-4%Mo-20%Co-1%C(质量分数,下同)3组试样缺口处裂纹的形成均存在一定的孕育期,随着循环温度的升高,孕育期明显缩短,裂纹的扩展速率加快;与金属相为4%Mo-20%Co的金属陶瓷抗热震性能相比,4%Mo-10%Ni-10%Co的较好,14%Mo-20%Ni的最好.扫描电镜观察表明:微孔洞的连通形成裂纹,裂纹主要沿陶瓷相晶界及金属相扩展.     

Thermal and mechanical properties of crack-designed thick lanthanum zirconate coatings

Vertical cracks are beneficial in thermal barrier coatings due to enhanced thermo-mechanical compliance. Accordingly, an aqueous nitrate based precursor solution was atomized on stainless steel substrates by spray pyrolysis to deposit thick crack-designed lanthanum zirconate coatings. Coatings with designed crack patterns were deposited and characterized by electron microscopy, tribology, Vickers indentation, and thermal diffusivity. The crystallization of the coatings was investigated by in situ high temperature X-ray diffraction. The green coatings crystallized from 600 °C and the pyrochlore structure was formed after heat treatment at 1000 °C. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, lower thermal diffusivities, and higher thermal conductivities compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings, ∼28 mm²/s at room temperature, was similar to the values reported for yttria-stabilized zirconia plasma sprayed coatings.     

Isothermal Crack Healing and Strength Recovery in UO2 Subjected to Varying Degrees of Thermal Shock

Isothermal crack healing and strength recovery were investigated in UO₂ pellets subjected to varying degrees of thermal shock by water quenching. It is shown, as was speculated in an earlier paper, that crack density and/or crack width in the specimen strongly influence the subsequent healing behavior. Experimental data were used to demonstrate that, under the present thermal shock conditions, the time required for complete healing increased exponentially with the number of cracks/unit length of the specimen.     

Preparation and structure control of a scalelike MoSi2-borosilicate glass coating with improved contact damage and thermal shock resistance

The scalelike coatings with MoSi₂ as emittance agent, flake fused quartz as coarse fillers, silica sol as dispersive medium of coating slurry, were prepared on rigid mullite fibrous ceramics to avoid fatal radial cracks and improve thermal shock resistance via combining slurry method with sol-gel method. And the particle size distribution of fused silica was changed to get different crack structure in scalelike coatings. Microstructure and phase composition of the coatings with different crack structure were investigated comprehensively. Contact damage resistance, thermal shock resistance and infrared radiating property were also studied. The results showed that only the coatings with mixing large and small particles of fused silica as coarse fillers could form crack network. The scalelike coatings with crack network went through 25 thermal cycles between 1500 °C and room temperature without peeling and spalling, and the bonding strength did not decrease after test. The scalelike structure with crack network also avoided radial cracks and exhibited some softness and less stiffness in Hertzian indentation test. The emissivity of coatings with different crack structure were all higher than 0.85; and the coatings with lower proportion of crack areas had higher emissivity.     

A lifetime prediction method based on Cumulative Flaw Length Theory

Ultrasonic pulse velocity testing (UPVT) was carried out to perform non-destructive quality control of refractory plates. Used in conjunction with fracture mechanics, ultrasonic velocity measurements have proved a powerful technique for detecting, positioning and sizing internal voids and cracks in the samples, originated from the manufacturing process. Two cordierite-mullite refractory compositions exhibiting different microstructure and crack propagation behaviour were characterized through their lifetime during which they were subjected to thermal shock loading. In this paper, a new statistical method is proposed which allows to estimate the lifetime when the stress state that will be applied in service (loading) and the scattering of the ultrasonic velocity data in the as-received state are known. Since this lifetime prediction method is based on a non-destructive technique, it could be implemented into a code in an automatic quality control device for continuous lifetime estimation. The correlation between crack propagation behaviour and thermal shock resistance is discussed and semi-empirical models were developed to predict the service life of refractory plates from the measured values of ultrasonic velocities on as-received samples.     

Residual Strength of a Low‐Strength Ceramic with a Precrack Induced by Thermal Shock

A method of introducing a single sharp crack with controllable length and position in brittle materials by thermal shock is proposed. This method is simple to conduct and suitable for the precise testing of critical fracture parameters, with accurate values of fracture toughness of brittle ceramics, such as ZrB₂–SiC–graphite (ZSG), able to be obtained. Moreover, this provides an experiment foundation for the study of the relationship between mechanical properties and cracks: The effects of crack length and specimen thickness on the residual strength of ZSG were investigated here. Further comparison between the experimental data and the results of the extended finite‐element calculation was made. Through proper control over the thermal shock, a desired number of uniformly distributed and roughly parallel cracks can be obtained.     

Direct numerical simulations on crack formation in ceramic materials under thermal shock by using a non-local fracture model

In this work, a non-local failure model was proposed and implemented into a finite element code. It was then used to simulate the crack evolution in ceramic materials subjected to thermal shock. By using this numerical model, the initiation and propagation of cracks in water quenched ceramic specimens were simulated. The numerical simulations reproduced faithfully the crack patterns in ceramic specimens underwent quenching tests. The periodical and hierarchical characteristics of the crack patterns were accurately predicted. The numerical simulations allow a direct observation on whole the process of crack initiation and growth, which is quite a difficult task in experimental studies. The failure mechanisms and the fracture procedure are discussed according to the numerical results obtained from the simulations. It is shown that the numerical model is simple, robust, accurate and efficient in simulating crack evolution in real structures under thermal shock.     

Extension of Hasselman's thermal shock theory for crack/microstructure interactions in refractories

Thermal shock damage resistance in advanced refractories depends on the crack interactions with the microstructure. These energy dissipation mechanisms during crack propagation are not directly considered in the original classical thermal shock model of Hasselman. They are imbedded within the N and γ terms of his derivations. In this extension of Hasselman's work, an expression is presented, which estimates the final crack size (?_f) as the fracture surface energy ratio between γ_NBT and γ_WOF. That expression directly considers the crack interaction mechanisms with the refractory microstructure as it includes the R-curve behavior effects. In addition, the equation presented allows a quantitative evaluation of the volumetric density of cracks in refractories.     

Bimaterial Composites via Colloidal Rolling Techniques: II, Sintering Behavior and Thermal Stresses

Shrinkage behavior and crack formation during firing have been investigated for Al₂O₃/Ce-TZP composites that have been fabricated by colloidal rolling and folding. These composites show improved sinterability and sinter isotropically after repeated rolling. Interface instability in rolling creates corrugated interfaces with large layer waviness; therefore, rolling can substantially alleviate the in-plane sintering constraints, which leads to improved sinterability. A loss of sintering anisotropy also is observed and is directly correlated to the microstructure instability, which is coincident with the laminate-cellular transition. Sintering cracks during heating and thermal cracks during cooling both are limited to the thick Ce-TZP layers in the composites. The critical layer thickness and the normalized crack spacing of the thermal cracks follow the predicted behavior of elasticity theory. Thus, crack-free, high-density Al₂O₃/Ce-TZP composites with either a laminate or cellular microstructure can be obtained, with a layer thickness of 4-60 µm, via pressureless sintering.     

Thermal shock resistance study of stereolithographic additive-manufactured Al2O3 ceramics by in situ digital radiography

Thermal shock resistance is critical to ensure the service safety of ceramic hot-end components. The thermal shock performance of stereolithographic additive-manufactured ceramics has not yet been studied. In this study, a series of thermal shock experiments with various temperature differences was conducted on stereolithographic additive-manufactured Al₂O₃ ceramics. The surface cracks were analysed based on photographs captured before and after the thermal shock experiments. Three-point bending tests with in situ X-ray digital radiography were conducted to determine the thermal shock resistance. Crack initiation, propagation, and coalescence were observed under flexural loads. The critical temperature difference of the stereolithographic additive-manufactured Al₂O₃ ceramics was determined to be 267.22 °C. The crack length increased and residual strength decreased with increasing temperature differences. The layered structure of the stereolithographic additive-manufactured ceramics slowed crack propagation. We expect that this study will serve as a reference for the performance of stereolithographic additive-manufactured Al₂O₃ ceramics in extreme environments.     

Thermal Shock Resistance of Silicon Nitrides Using an Indentation–Quench Test

Thermal shock resistance of silicon nitrides is investigated using an indentation–quench method. Four commercially available silicon nitrides with different microstructures are investigated. The extension of Vickers radial cracks is measured as a function of quenching temperature for each material, up to the critical temperature for failure. An indentation fracture mechanics analysis is used to account for the crack responses, with due allowance for R -curve behavior. The analysis confirms the important role of microstructure in thermal shock resistance.     

Microstructure,oxidation and thermal shock resistance of graphene reinforced SiBCN ceramics

SiBCN ceramics were prepared using various volumes of graphene platelets (GPLs) as nanofiller. The effects of the nanofiller on microstructure, and oxidation and thermal shock resistance of as-sintered ceramics were investigated. The phase composition and microstructures were very similar for all investigated ceramics consisting primarily of β-SiC, BNC and small amounts of α-SiC with relatively homogeneously distributed 5–10 nm thick GPLs in the matrix. For SiBCN ceramics incorporating graphene as nanofiller, a porous oxide layer forms at 1500 °C and the oxidation behavior shows a linear kinetics by thickness measurement method. Gas evolution during heating lead to a passive oxidation behavior and weight loss. Graphene reinforced SiBCN ceramics exhibit thermal shock resistance superior to monoliths of the same material. The graphene distributed in SiBCN matrix can dissipate the energy of crack growth and acts as a stopper to cracks. The toughening mechanisms offered by graphene, including pull-out and bridging appear to aid in ameliorating thermal shock effects. Furthermore, the existence of a dense oxide surface layer retards oxygen diffusion into the inner matrix and heals surface pores and cracks, which also contributes to thermal shock resistance.     

Distribution of Matrix Cracks in a Uniaxial Ceramic Composite

Conventional shear-lag analyses of matrix cracking and debonding in uniaxial composites loaded in tension predict that the matrix stress varies only very slowly with position except near existing cracks. It therefore follows that the location of subsequent cracks is very sensitive to minor local variations in matrix strength, leading to significant statistical variation in crack spacing. This question is investigated using a discrete random process model of a composite and by direct experimental measurements of crack spacing. In the limit of a completely homogeneous composite, it is shown that the crack spacing distribution tends to an inverse square distribution between the theoretical maximum spacing and half that value. The random process model recovers this behavior in the limit and exhibits an approximately Weibull distribution of crack spacings when the matrix strength has significant variance. The theoretical predictions are compared with experimental results obtained for a unidirectional ceramic-matrix composite (SiC fibers in a calcium aluminosilicate matrix). The experimental results exhibit features similar to those predicted by the model and are compatible with a matrix strength whose standard deviation is of the order of 40% of the mean strength. An important point is that, with this magnitude of strength variation, the material exhibits a significant size effect and it is essential to take this into account in estimating the mean crack spacing from the corresponding mean matrix properties.     

Mechanism for abnormal thermal shock behavior of Cr2AlC

Some MAX phases exhibit abnormal thermal shock behavior, i.e. the residual strength of as-quenched MAX phases decreases gradually with increasing quenching temperatures and then unbelievably increases after quenching from even higher temperatures. To date, the abnormal thermal shock behavior has been found in some MAX ceramics for about 17 years and has resisted interpretation. Herein Cr₂AlC as a representative member of the MAX phases is chosen to investigate such unique behavior. Thermal shock tests show that the abnormal thermal shock phenomenon for Cr₂AlC occurs at above 1200 °C. The main mechanism for this phenomenon is that the thermal shock induced cracks are instantly healed by the formation of reactants well adhering to the crack faces during quenching. The mechanism demonstrated here can be applied to other materials with the abnormal thermal shock behavior. Such materials may find applications in environments where temperature changes abruptly.     

Cracking behavior in reinforced concrete members with steel fibers: A comprehensive experimental study

The addition of fibers in concrete determines a cracking phenomenon characterized by narrower and more closely spaced cracks, with respect to similar members without fibers. Fiber Reinforced Concrete (FRC) may significantly improve the tension stiffening into the undamaged portions of concrete among cracks, and, in addition, may provide noticeable residual stresses at a crack due to the bridging effect provided by its enhanced toughness.This paper aims at further investigating the ability of fibers in controlling cracks by discussing more than ninety tension tests on Reinforced Concrete (RC) prisms, carried out at the University of Brescia, having different sizes, reinforcement ratios, amount of fibers and concrete strengths. In particular the influence of FRC in reducing the crack spacing and the crack width is evaluated as a function of the FRC toughness.Finally, the most recent available models for predicting the crack spacing of FRC composites are evaluated and critically discussed.     

Constrained sintering and cracking of air plasma sprayed thermal barrier coatings: Experimental observation and modeling

The development of vertical cracks in air plasma sprayed (APS) thermal barrier coatings (TBCs) during thermal cycling and constrained sintering under a temperature gradient is investigated. Microstructural analysis shows that the development of the vertical cracks is associated with multiple processes, including sintering during the hold period and cleavage during cooldown. Inspired by the experimental observations, an image-based sintering model is used to simulate the development of vertical cracks as the coating sinters while constrained by a substrate. The computational results show that microstructural imperfections can develop into vertical cracks, which then propagate toward the interface. A simple analytical model is presented for the threshold level of in-plane stress for the onset of propagation of a vertical crack during constrained sintering. By combining the results of these different modeling approaches, the cross-coupling of the material and geometric parameters, and how this determines the sintering response (microstructure evolution) and vertical crack formation is evaluated. In addition, the growth of vertical cracks by a cleavage mechanism during cooldown is examined and the coupling between sintering, cleavage crack growth, and TBC lifetime is explored.