陶瓷圆柱体的抗热震参数

通过求解第三类边界条件下无限长圆柱体的瞬态温度场及瞬态热应力场，研究了陶瓷圆柱体的抗热冲击行为，建立了引起无限长陶瓷圆柱体表面临界热应力的临界温差表达式，并以此作为陶瓷圆柱体的抗热震参数。计算结果表明，无限长陶瓷圆柱体的临界温差大于具有相同Biot模数的无限大陶瓷平板的临界温差，但其表面达到临界热应力的时间小于无限大平板的数值。     

梯度功能陶瓷圆球的抗热震性

推导和提出了第三类边界条件下梯度功能材料(functionally gradient materials,FGM)圆球的瞬态温度场及瞬态热应力场表达式.基于陶瓷材料的临界应力断裂判据,通过求解梯度功能陶瓷圆球表面达到其局部断裂强度的时间,建立了引起其表面临界热应力的临界温差△Tc的表达式,并以此作为梯度功能陶瓷圆球的抗热震参数.通过计算实例并与均质陶瓷圆球对比,分析了材料的热-物理性能分布规律对其抗热震性的影响,并提出了高抗热震性FGM陶瓷圆球的设计原则:线膨胀系数和热扩散率应由表及里增大,而弹性模量应由表及里减小.     

Thermal shock behavior of nano-sized ZrN particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications owing to its excellent stability and mechanical properties such as high rigidity and good chemical stability. Thermal shock resistance is a major concern and an important performance index of refractories and high-temperature ceramics. While zirconium nitride (ZrN) particles have been proven to improve mechanical properties of AlON ceramic, the thermal shock behavior has not been evaluated yet. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of hot-pressed 2.7% ZrN–AlON composites by a water-quenching technique over a temperature range between 225 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing temperature range and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. The presence of nano-sized ZrN particles exhibited a positive effect on the improvement of both residual strength and critical temperature difference of AlON ceramic due to the toughening effects, the higher thermal conductivity of ZrN, the refined grain size and the reduction of porosity. Different toughening mechanisms including crack deflection, crack bridging and crack branching were observed during thermal shock experiments, thus effectively enhancing the crack initiation and propagation resistance and leading to a considerable improvement in thermal shock resistance in the ZrN–AlON composites.     

陶瓷涂层三明治板的抗热震性

采用解析法研究了第3类边界条件下双面陶瓷涂层三明治板的瞬态温度场及瞬态热应力场.对不同Biot模数的热冲击过程中,Al2O3涂层/硬质合金(WC-8%Co,质量分数)基体/Al2O3涂层三明治板的瞬态热应力进行了数值计算.分析了涂层/基体厚度比、涂层与基体热-物理性能匹配对陶瓷涂层三明治板表面热应力峰值的影响.结果表明:陶瓷涂层三明治板的基体的热导率、线膨胀系数和弹性模量应高于涂层,这样可以降低其表面热应力,获得高抗热震性陶瓷涂层三明治板.此外,涂层厚度应尽可能小,以利于改善涂层的抗热震性.     

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.     

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 Resistance of Laminated ZrB2–SiC Ceramic Evaluated by Indentation Technique

In this work, the thermal shock behavior of laminated ZrB₂–SiC ceramic has been evaluated using indentation‐quench method based on propagation of Vickers cracks and compared with the monolithic ZrB₂–SiC ceramic. The results showed that the laminated ZrB₂–SiC ceramic exhibited better resistance to crack propagation and thermal shock under water quenching condition, and the critical temperature difference (ΔT_c) of laminated ZrB₂–SiC ceramic (ΔT_c ≈ 590°C) was much higher than that of monolithic ceramic (ΔT_c ≈ 290°C). The significant improvement in thermal shock resistance was attributed to residual stresses enhancing the resistance to crack growth during thermal shock loading.     

The Thermal Shock Resistance Analysis of Ceramic Foams

This article studies the thermal shock resistance behavior of ceramic foams under sudden thermal load induced by a sudden temperature variation. Two types of thermal shock loading conditions are considered: cold shock and hot shock. Variations of the stress and stress intensity factor with thermal shock time, location, crack size, medium thickness, and relative density of the ceramic foam are given. Crack growth behavior is studied and crack growth velocity is explained from energy equilibrium consideration. The thermal shock resistances of ceramic foams are established from the view points of energy criterion and fracture mechanics concept.     

Assessment of viscoelastic crack bridging toughening in refractory materials

Viscoelastic bridges can be formed in refractory ceramics while cooling from high temperatures. Such bridges can shield crack tips, thus reducing the effective crack tip stress intensity factors leading to higher resistance to creep and thermal shock. The extent to which the crack tip stress intensity is reduced can be estimated from fracture mechanics models that include experimental measurement of crack bridging and microstructural parameters. In this paper a novel approach is proposed for the assessment of the effective crack bridging toughening from combining destructive and non-destructive test methods. Fracture toughness values were determined applying chevron notched specimen technique and surface damage of the specimen was monitored by image analysis. Different cordierite–mullite compositions characterized by different microstructure morphologies and crack propagation behaviour were investigated. A brief discussion about the correlation between thermo-mechanical properties, microstructure, crack propagation behaviour and thermal shock resistance is presented. Moreover, an empirical model able to determine the presence and effectiveness of the viscoelastic crack bridging ligaments acting in the microstructure under thermal shock conditions and their degradation with increasing thermal shock cycles from parameters measured at room temperature is presented.     

A new arrangement of SiC particles in reaction bonded Si3N4-SiC ceramic refractory and crack behaviors under thermal shock

High temperature protection brick lining is important for super-charged boilers. In practice, Si₃N₄ bonded SiC ceramics are usually chosen as the raw material of refractory bricks due to their excellent performance under high temperature. In the field of the ceramic refractory material, a main goal is to improve the resistance of ceramics under thermal shock because their inherent brittleness may cause failure under sudden change in temperature. In this paper, we fulfilled this goal by introducing a new particle arrangement called “double dispersion” for the SiC particle-reinforced ceramic refractory material. And we established the micro-structure models for both the original and the modified ceramic refractory material. To study the influence of the particle arrangement on the fracture toughness, we performed simulations of the crack initiation and propagation under the same thermal load for the original and the modified material. The results showed that the “double dispersion” method can improve the thermal shock resistance of the reaction-bonded Si₃N₄-SiC ceramic refractory.     

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.     

蜂窝陶瓷蓄热体的抗热震性研究进展

本文针对高风温燃烧技术和煤矿乏风瓦斯氧化技术的蜂窝陶瓷蓄热体,系统地综述了影响蓄热体抗热震性的因素,数值模拟分析了蓄热体在热冲击下的温度场和热应力场的分布特点,总结了蜂窝陶瓷蓄热体热震损伤机理等的研究进展情况,并提出今后的研究及发展方向。     

Thermal shock resistance of additively manufactured gas-permeable SiO2 ceramic structures for HIP-quench applications

The increasing demand for hot isostatic pressing (HIP) means that a reliable and efficient operation of modern HIP units with fast cool capability is indispensable. A key factor for efficient operation is the ceramic crucible used as the load basket. Its task is to keep as much of the HIPed parts as possible effectively within the hot zone and to prevent them touching the furnace wall. This work focuses on designing a gas-permeable ceramic structure with a high thermal shock resistance that can be scaled up to a load basket for future HIP applications. Stereolithography (SL) 3D printing of a ceramic resin is employed to build various scalable framework structures inspired by nature and by existing engineering applications. Thermal shock tests with water quenching reveal that framework structures with evenly distributed triangular bracings offer the highest flexural strength, whereas auxetic structures are best at retaining their flexural strength after thermal shock.     

Crack propagation speed in ceramic during quenching

The effects of water quenching temperature and specimen size on the propagation speed of thermal shock crack are investigated in real time by water quenching of translucent ceramic and high-speed imaging. The results show that the crack growth rate increases with the increase of quenching temperature difference or specimen size. Within 100?ms, average crack speed is 20.3?mm/s at a temperature difference of 400?°C in 20?mm wide ceramic and is 11.9?mm/s at a temperature difference of 220?°C in 5?mm wide ceramic, respectively. Compare with specimen size, the influence of quenching temperature difference on the crack propagation speed is larger. The calculations based on meso-damage mechanics have similar results to those of experiments. This paper quantitatively studies the thermal-shock crack growth of ceramic in real time and expands the scientific understanding of thermal shock cracking phenomenon of ceramic.     

Thermal Fracture Resistance of Ceramic Coatings Applied to Metal: I, Elastic Deformation

A study was made of the resistance to thermal fracture of four ceramic coatings of the cobalt-bearing ground-coat type applied to enameling-grade iron specimens. The study was made of coated-metal systems in the unsteady state, symmetrically cooled, and in the absence of viscous or plastic flow. Determinations were made of the elastic characteristics of the coating-metal composites, the effective coefficient of linear expansion, the temperature at which the coating and base metal were at dimensional equilibrium, and the temperature differential sufficient to induce coating fracture when water quenched. Coating-metal thickness ratios were correlated with the maximum specimen temperature withstood in water quenching without coating fracture. Studies indicated that ceramic coatings, after receiving a given thermal treatment, fracture when subjected to a thermal shock by a critical temperature differential. When no residual coating stress is present, thermal shock resistance is inversely related to the thermal expansion characteristics of the coating. The critical stress at which coating fracture occurs may be expressed as the sum of thermal and residual stresses developed in annealed systems in which viscous or plastic flow does not occur. Residual compressive stress in a coating is a major factor in improved thermal shock resistance. Increased thermal shock resistance is gained by decreased coating thickness.     

Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

原位生成堇青石结合红柱石太阳能热发电储热陶瓷的抗热震性能

为提高储热陶瓷材料的抗热震性能,采用原位生成堇青石增强技术,以红柱石为主要原料,通过半干压成型,无压烧结研制了用于太阳能高温热发电红柱石储热陶瓷材料样品。研究了配方组成、烧成温度、相组成、微观结构对样品抗热震性能的影响。结果表明：红柱石添加量为 70%,经1 400 ℃烧成的样品抗热震性能最佳：30 次热震实验（热震条件：1 100 ℃～室温,风冷）的强度不仅没有损失,反而增加了 26.20%。相组成和微观结构分析表明样品的晶相为堇青石、莫来石、硅线石、α-方石英、α-石英等,原位生成的堇青石晶体均匀分布在由红柱石转化的莫来石晶体之间,赋予样品较好的抗热震性能     

Theoretical prediction of temperature-dependent fracture strength for ultra-high temperature ceramic composites considering the evolution of damage and thermal residual stress

Based on the maximum storage energy density criterion of material fracture, a model of temperature-dependent fracture strength for ultra-high temperature ceramic composites is established. The combined impacts of the evolution of damage and thermal residual stress with temperature are considered. The model predictions are highly consistent with available experimental values. Besides, the critical crack sizes of ZrB₂–30 vol%SiC in air from 1400 to 1600 °C are predicted using the proposed model, which agree well with the total oxidation thickness of the reported literature at 1400 and 1500 °C, and a more reasonable definition of critical crack size at 1600 °C are given. Moreover, the quantitative effect of crack size on the fracture strength is analyzed under different environment temperature, and a useful conclusion is obtained that decreasing crack size is more effective to improve the fracture strength of the composites at low temperatures. This study not only provides a feasible and convenient method to predict the fracture strengths at different temperatures, but also offers a theoretical support for the design of ultra-high temperature ceramic composites.     

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 ceramic materials in melt immersion tests

A melt immersion test is applied to determine the relative resistance of ceramic materials to thermal shock failure under high heat flux conditions. The testing method is demonstrated mainly for Al₂O₃ pellets, while AlN is included to represent elevated thermal shock resistance. In order to quantify the resistance to crack formation, the critical temperature difference ΔT_c between sample and metal melt is determined from the failure probability distribution of a set of pellets.

In quenching tests correspondence of ΔT_c with the thermal shock parameter R = σ(1 − μ)/E was found, if the initial surface temperature of the sample was correctly estimated. This assessment was the main concern of the evaluation work.

ΔT_c resulting from heating tests was correlated with the maximum tensile stress in the sample by modeling calculations. The stress limits determined show that the ultimate bending strength could serve as a rough approximation for the materials tested.