Experiments and transient finite element simulation of γ-Y2Si2O7/B2O3-Al2O3-SiO2 glass coating on porous Si3N4 substrate under thermal shock

A dense γ-Y₂Si₂O₇/B₂O₃-Al₂O₃-SiO₂ glass coating was fabricated by slurry spraying method on porous Si₃N₄ ceramic for water resistance. Thermal shock failure was recognized as one of the key failure modes for porous Si₃N₄ radome materials. In this paper, thermal shock resistance of the coated porous Si₃N₄ ceramics were investigated through rapid quenching thermal shock experiments and transient finite element analysis. Thermal shock resistance of the coating was tested at 700 °C, 800 °C, 900 °C and 1000 °C. Results showed that the cracks initiated within the coating after thermal shock from 800 °C to room temperature, thus leading to the reduction of the water resistance. Based on the finite element simulation results, thermal shock failure tended to occur in the coating layer with increasing temperature gradient, and the critical thermal shock failure temperature was measured as 872.24 °C. The results obtained from finite element analysis agree well with that from the thermal shock tests, indicating accuracy and feasibility of this numerical simulation method. Effects of thermo-physical properties for the coating material on its thermal shock resistance were also discussed. Thermal expansion coefficient of the coating material played a more decisive role in decreasing the tangent tensile stress.     

Thermal shock resistance of porous ceramic foams with temperature-dependent material properties

The impact of temperature dependence of material properties on thermal shock resistance of porous ceramic foams is studied in this paper. Two cases of thermal shock are carried out: sudden heating and sudden cooling. Finite difference method and weight function method are employed to get the thermal stress field at crack tip. The effects of time dependence and temperature dependence of material properties on thermal shock behavior are analyzed. The thermal shock resistance is acquired based on two different criteria: fracture mechanics criterion and stress criterion. By comparison analysis, results show that taking temperature dependence of the material properties into account is crucial in the assessment of thermal shock resistance of ceramic foams. Cold shock fracture experiments of Al₂O₃ foams with different relative densities are also made, and the obtained results are in coincidence with theoretical results very well.     

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.     

Thermal Shock Behavior of Open-Cell Ceramic Foams

Specimens of heated, open-cell ceramics were thermally shocked by immersion in water or oil. It was found the strength retained after thermal shock underwent a gradual decrease with increasing quench temperature, indicative of a cumulative damage mechanism which manifests itself with increasing thermal stress. This damage could also be monitored using measurements of the elastic constants before and after quenching. The thermal shock resistance of the open-cell materials was found to be strongly dependent on cell size (increased with increasing cell size) and weakly dependent on density (increased with increasing density). Two possible sources of thermal stress were considered; one was associated with the temperature gradient across the microscopic struts and the other with the heating of the quenching medium as it infiltrates the cellular structure. Such heating was confirmed and it was concluded that this was the dominant source of thermal stress in this particular study, controlling the thermal stress in this particular study, controlling the thermal shock resistance of the open-cell ceramics.     

Experiment and simulation analysis on thermal shock resistance of laminated ceramics with graphite and boron nitride interfaces

Laminated ZrB₂–SiC ceramics with graphite and BN as the interface layers were fabricated by tape casting and consequent hot pressing. The influences of interface materials on the thermal shock resistance of the laminated ceramics were studied by both experimental and theoretical analyses. Experimental results show that the critical temperature differences of G/LZS (laminated ZrB₂–SiC/G) and BN/LZS (laminated ZrB₂–SiC/BN) were 237 and 99 °C, respectively. The stress gradients of G/LZS and BN/LZS calculated via finite element analysis were 709 and 809 MPa/mm, respectively. Thus, the thermal shock resistance of G/LZS was higher than that of BN/LZS. Such merits of G/LZS can be explained by the following two reasons: one is the thermal conductivity of the graphite interface layer was higher than that of the BN interface layer; the other is the difference of the thermal expansion coefficient for the graphite interface layer and the matrix layer was lower than that for the BN interface layer and the matrix layer.     

Thermal shock resistance of rare-earth doped in-situ SiAlON reinforced h-BN matrix ceramics under vacuum thermal cycling

In-situ SiAlON reinforced BN-matrix ceramics were prepared by hot pressing sintering, and the effects of different rare earth oxides on the thermal shock resistance of the materials were investigated. The effects of rare earth oxides on the phase composition, microstructure, bending strength, thermal properties and thermal shock resistance of the composites were studied. The results show that the phase composition and bending strength of ceramics with different rare earth oxides had no obvious change. However, the influence on the thermal expansion coefficient of the material was notable. The thermal expansion coefficient of the ceramics with CeO₂ increased by 24.6% compared with Sm₂O₃ in the test temperature range. After 50 cycles of thermal shock at Δt = 1150 °C, the residual strength of ceramics with CeO₂ was down to 157.1 MPa, decreased by 40.6% compared with the one tested in room temperature. And the Sm₂O₃-added ceramics reduced by 34.7%–167.1 MPa after thermal shock. The decrease of the residual strength of ceramics is mainly caused by the internal stress generated by the mismatch between the growth of quartz and SiAlON phase in the matrix and the thermal expansion coefficient of the matrix. However, no macro cracks were observed on the surface of the samples after thermal shock.     

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.     

Modeling the thermal shock induced cracking in ceramics

Prediction of surface cracking in ceramics due to quenching is performed numerically using either the coupled criterion or a cohesive zone model. Under such a thermal shock, a network of short cracks with minimal spacing between them initiate and propagate until some of them stop while the others continue propagating. The numerical implementation consists of a periodic array of cracks modeled by a representative volume element. It allows crack initiation, simultaneous propagation and period doubling to be predicted. The investigation of the crack period doubling allows a precise determination of the optimal crack spacing, which decreases with an increasing thermal shock amplitude. The predicted crack spacing results are in agreement with experimental measurements.     

Thermal shock resistance behavior of auxetic ceramic honeycombs with a central crack or an edge crack

In this paper, the thermal shock resistance of an auxetic ceramic honeycomb plate is studied based on the fracture mechanics concept for the cases of a central crack or an edge crack. The transient temperature field and transient thermal stress field are obtained for both auxetic and non-auxetic structures. The relationship between the thermal stress intensity factor (TSIF) and the internal cell angle, crack length and time is determined and the critical temperature for the initiation of crack propagation is predicted. Results show that compared with the non-auxetic ceramic honeycombs which are at an internal cell angle of 30°, the critical temperature of the auxetic ceramic honeycombs whose cell are orientated at an angle of −30° increases by 78.5% and the TSIF at the crack tip decreases by 40%, respectively. Hence, the auxetic structures have better thermal shock resistance. This study indicates that auxetic ceramic honeycombs have significant potential applications in harsh temperature environments.     

Alumina/NiCu Laminate under Thermal Shock up to 1000°C: II, Prediction of the Laminate Behavior

In this investigation, a multilayered, multimaterial system with strong interface subjected to thermal shock loading was analyzed. The analysis was based on a one-dimensional spatio-temporal finite difference scheme of the temperature field, and the thermal residual stresses and zero misfit stress temperature were considered. Using a failure criterion based on crack initiation, the number of broken layers due to thermal shock and residual mechanical strength at room temperature could be predicted. Furthermore, the room temperature residual strength of the laminate as a function of thermal shock temperature was constructed, demonstrating steplike behavior. Using this model, the mechanical behavior of the alumina/NiCu laminate system subjected to thermal shock loading of up to 1000°C was predicted. The model revealed the superiority of this material system over monolithic ceramics under thermal shock conditions.     

Comparative evaluation of two different methods for thermal shock resistance of laminated ZrB2-SiCw/BN ceramics

The thermal shock resistance (TSR) of laminated ZrB₂–SiC_w/BN ceramic was evaluated through indentation-quench and quenching-strengthening methods. It was correspondingly compared to monolithic ZrB₂–SiC_w ceramic. In the indentation-quench method with consideration to crack propagation on the surface layer, the critical thermal shock temperature of laminated ZrB₂–SiC_w/BN ceramic with surface residual tensile stress was 550?°C, which was lower than monolithic ZrB₂–SiC_w ceramic (600?°C). Unlike the microscopic method of crack growth measurement through indentation-quench testing, the quenching-strengthening method, which was based on the macroscopic properties of the material, mainly characterizing the residual strength subsequently to thermal shock, the critical thermal shock temperatures of the laminates and monolithic were 609?°C and 452?°C, respectively. Compared to the brittle fracture of ZrB₂–SiC_w ceramics, the deflection, bifurcation and delamination of the cracks as the main TSR mechanisms of the laminated ceramics, were revealed through quenching-strengthening method, which was more suitable for the TSR characterization of laminated ceramics.     

Thermal shock and thermal fatigue resistance of Al2O3/(W,Ti)C/TiN/Mo/Ni multidimensional graded ceramics

Al₂O₃/(W, Ti)C/TiN/Mo/Ni multidimensional graded ceramics and homogeneous reference ceramic were prepared by two step hot press sintering. The thermal shock and thermal fatigue resistance of the multidimensional graded ceramics were tested using the water quenching method. Scanning electron microscopy (SEM) and optical microscope were used to investigate microscopic failure mechanism of ceramics. The results showed that the retained flexural strength of two-dimensional and one-dimensional graded ceramics was almost same, but higher than that of the homogeneous ceramic. The crack growth (∆c) of homogeneous ceramic increased rapidly, while that of two-dimensional graded ceramics is the lowest. Hence, thermal fatigue resistance of the two-dimensional graded ceramics was highest. The residual compressive stress in the first layer induced by the optimal graded structure played an important role. In addition, the increasing toughness on the crack propagation path by adding different amounts of metals was also a contributing factor.     

Measurement of ceramics cracking during water quenching by digital image correlation

Measuring the thermal shock crack growth process is crucial for revealing ceramic materials and structures’ thermal shock failure mechanisms and evaluating their reliability. We used a self-made water quenching system to conduct thermal shock tests on alumina and zirconia ceramics. The thermal shock process was recorded by high-speed digital image correlation (DIC) during the test. The process of thermal shock crack initiation and propagation in two kinds of ceramics was determined by analyzing the speckle image change on the sample’s surface. It is found that the crack growth rate of alumina is faster than that of zirconia, which is caused by different material parameters. This paper presents an in-situ measurement method for the initiation and propagation of thermal shock cracking in ceramic materials. It can provide a measurement method to identify and predict the thermal shock damage of ceramic components.     

Thermal shock resistance of Cr2O3–Al2O3–ZrO2 bricks with the microstructure of Cr2O3 aggregates coated by ZrO2

Zirconia-coated Cr₂O₃ aggregates synthesized by mixing ZrO₂ powders and Cr₂O₃ aggregates with a phenolic resin binder followed by thermosetting treatment were employed as modified Cr₂O₃ aggregates to obtain Cr₂O₃–Al₂O₃–ZrO₂ bricks (high-chromia bricks). The elastic modulus (E) and cold modulus of rupture (CMOR) of these high-chromia bricks before and after thermal shock cycles were systematically investigated, and the residual ratios of CMOR and E were calculated. The thermal shock resistance of the high-chromia bricks was significantly improved by the factor of modification of Cr₂O₃ aggregates. The mechanism of the improved thermal shock resistance of these high-chromia bricks was studied via microstructure analysis, and the crack propagation behavior was analyzed by scanning electron microscopy (SEM). The fracture work (γ_WOF), thermal shock damage factor (R′′′′), and thermal stress crack stability parameter (R_st) were measured and calculated using the wedge splitting test (WST). The results indicate that the porous ZrO₂ coating layer wrapped the Cr₂O₃ aggregates, forming modified Cr₂O₃ aggregates that can increase crack deflection, free path of crack propagation, and fracture work, thus improving the thermal shock resistance of high-chromia bricks. The thermal shock resistance of the fabricated high-chromia bricks was highly correlated with the thickness of the ZrO₂ coating layer surrounding the Cr₂O₃ aggregates. The variation trend of R_st is well consistent with the experimental results, which is suitable to evaluate the thermal shock resistance of high-chromia bricks.     

多孔碳化硅陶瓷的抗热震性研究

本文考察了多了孔碳化硅陶瓷的抗热震性,并探讨了不同制造工艺对多孔碳化硅陶瓷抗热震性的影响。同时研究了ＳｉＣ陶瓷在热处理过程中ＳｉＣ颗粒表面氧化形成的ＳｉＯ２在不同热处理温度的状态变化及其对试样抗热震性的影响。     

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.     

内压作用下斜接弯管纵向半穿透裂纹应力强度因子研究

采用三维有限元技术,应用ANSYS对内压作用下含纵向半穿透裂纹斜接弯管的应力强度因子进行了系统的分析。建立了三维有限元裂纹模型,对裂纹前沿应力强度因子的影响因素进行了研究,分析了不同内压下无量纲参数裂纹相对深度a/t、裂纹相对形状b/a、管壁相对厚度Do/t对应力强度因子的影响。     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Effect of Sm2O3 on microstructure,thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm₂O₃. The effects of Sm₂O₃ on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm³⁺ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3 wt%) of Sm₂O₃ could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100 °C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm₂O₃ addition (3 wt%) had a lower thermal conductivity than that of composite ceramics without Sm₂O₃ addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.     

High thermal-conductivity rGO/ZrB2-SiC ceramics consolidated from ZrB2-SiC particles decorated GO hybrid foam with enhanced thermal shock resistance

Graphene derivative materials exhibit excellent mechanical and thermal properties, which have been extensively used to toughen ceramics and improve thermal shock resistance. To overcome the thermal agglomeration of graphene oxide (GO) during heating and drying process, ZrB₂-SiC particles decorated GO hybrid foam with uniformly anchored ceramic particles was synthesized by electrostatic self-assembly and liquid nitrogen-assisted freeze-drying process. Densified rGO/ZrB₂-SiC ceramics with varying microstructure, thermal physical and mechanical properties were obtained by adjusting the content of decorated ceramic particles. Although the flexural strength of rGO/ZrB₂-SiC ceramics have an attenuation compared with that of ZrB₂-SiC ceramic, the thermal conductivity, work of fracture and thermal shock resistance are greatly improved. rGO/ZrB₂-SiC ceramics exhibit delayed fracture and increasing R-curve behavior during the crack propagation. The novel preparation technology allows for the well dispersion of rGO in ZrB₂-SiC ceramics and can be easily extended to other ceramic or metal materials systems.