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.     

Parameters for measuring the thermal shock of ceramic materials with an indentation-quench method

An indentation-quench method for measuring thermal shock has been evaluated. Experimental parameters such as sample thickness, initial crack length, and water bath temperature were surveyed. It was found that one single sample could be used throughout a whole test series of different quenching temperatures. The method can detect small differences in thermal shock resistance between materials, and was applicable to investigating thermal fatigue. The tested materials were two β-sialons with intergranular glass phase and with completely different thermal shock behaviour. The best resolution was obtained with a sample diameter Ø=12 mm, height h=4 mm, initial crack length=100 μm. and a water bath temperature=90 °C.     

炭/陶复合材料的抗热震性能研究

对含石墨的炭／陶复合材料优良的抗热震性能进行了讨论。这种性质与石墨的导热系数大、断裂功高、热膨胀和弹性模量小密切相关。     

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.     

Effect of surface heat transfer coefficient gradient on thermal shock failure of ceramic materials under rapid cooling condition

A rapid thermal processor (RTP) device as well as quenching technique is employed to systematically investigate the effect of surface heat transfer coefficient (h) gradient on thermal shock failure of a hot-pressed ZrB₂-based ceramic. Two typical kinds of quenchant with different surface h gradients during quenching tests, water and boiling water, are used for this study. When water as the cooling medium, two different cooling modes of indirect contact cooling by RTP device and direct contact cooling by quenching are also studied. The experimental results and related numerical simulations illustrate that surface h gradient plays an important role in thermal shock failure. This study confirms the previous presumption that the combination of body temperature gradient and surface h gradient leads to thermal stress damage and thermal shock failure. Under water quenching condition, water phase changes form bubbles randomly and produce great surface h gradient. Accordingly, critical body temperature gradient (V(max)_c) is small (~ 270?°C?s^?1). Under aqueous polymer quenching condition, the introduction of polymer chains into water lowers the random formation of steam bubble and mediates the surface h gradient. The corresponding V(max)_c hence become larger (~ 500?°C?s^?1). Under boiling water quenching condition, there is no surface h gradient and V(max)_c is even larger (> 600?°C?s^?1). This study provides useful complementary information for understanding the thermal shock behavior and gives suggests for predicting materials performance in actual service.     

The problem of thermal stability of ceramic materials

The thermal stability of ceramic materials is considered from the standpoint of fracture mechanics as a property determined by the capacity of the structure to resist the appearance and propagation of cracks on critical defects under the effect of thermal stresses. A method is suggested for comparative evaluation of the thermal stability of ceramics according to which specimens with a notch simulating a structural defect responsible for fracture are subjected to thermal shock. The absence of induced defects in the region adjoining the tip of the notch is a necessary condition for providing reproducible results. The resistance to thermal shock is determined from the relative decrease in the crack resistance after the thermal shock, the “insensitivity” of the structure to defects, and the degree of their accumulation in the region of the tip of the notch as a result of the thermal shock. The first and second criteria for evaluation of thermal stability involve the coefficient of relaxation of thermal stresses and the ultimate bending strength of the notched specimen. A ceramics of ZrO₂ partially stabilized by 3 and 12% Y₂O₃ and a cermet with a composition of ZrO₂-3 vol.% Y₂O₃-50 vol.% Cr are chosen for studying thermal stability by the method developed. Calculated results on the thermal stability of the cermet are compared with results obtained directly by thermocycling with variation of the metal content from 10 to 50%. The maximum mechanical properties are shown to correspond to a metal content of 40% due to formation of double-skeleton structure in the cermet. The method can be helpful for evaluating the initial stage of fracture caused by a thermal shock in structural ceramics. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp. 5–10, March, 1998.     

Synthesis and thermal shock evaluation of porous SiC ceramic foams for solar thermal applications

Reticulated porous ceramics with structural features spanning across multiple length scales are emerging as the primary media in a variety of demanding mass and heat transfer applications, most notably solar-assisted synthetic fuel processing. In this study, we focus on engineering of the open pore silicon carbide (SiC)-based foams in such catalytic applications. We evaluate the mechanical integrity and thermal stability of these porous structures. X-ray tomography analyses of the 3D structures reveal the presence of dual pore size distribution different by up to an order of magnitude in length scale. We further study the effect of thermal shock—induced via water quenching—on the SiC structures and we conclude that the mechanical properties of the ceramic foams are significantly reduced after thermal stress. Comparison of SEM micrographs—before and after thermal shock—reveals that needle-like features appear inside the foam matrix. These elongated defects may be responsible for structural and mechanical weakening.     

Fabrication of super flux and high thermal shock resistance ceramic membrane support

Ceramic membranes play an important role in high temperature gas-solid filtration. However, the thermal stability of the ceramic support at high temperatures has always been a problem. In this study, porous fused silica ceramic supports were fabricated with hexagonal boron nitride as a sintering aid. The results shown that hexagonal boron nitride could inhibit the crystallization of fused silica ceramic particles at high temperature and act as a sintering addictive to reduce firing temperature. The obtained supports have an average pore size of 72?µm, an open porosity of 42%, a bending strength of 16.5?MPa, a Weibull modulus of 8.67 and a gas permeability of 4.23?×?10⁵ m³/(m² h bar). The bending strength of the support remained 16?MPa after 30 cold-hot cycles, exhibiting high thermal shock resistance. After corrosion in 20?vol% H₂SO₄ solution for 8?h, the weight and the bending strength of the support were diminished by 0.6% and 24.32%, respectively. So, the ceramic support showed good acid corrosion resistance.     

High-Temperature tests of heat-insulating refractory materials for thermal conductivity

Specialists of D. I. Mendeleev Russian University of Chemical Engineering have created an installation for determining the thermal conductivity of heat-insulating refractory materials in the range of 20–1000°C using a measuring cross. The data obtained for materials tested by the steady-flow method and the hot-wire method (ISO 8894-1:1987 (E)) differ by no more than 12%.     

Erosion resistance of thermal insulating materials in streams of ionized gases

A study is summed up that was concerned with the erosion resistance in a stream of ionized gases of a composite made up of a corundum matrix and polycrystalline Al₂O₃ fibers in comparison with a ceramic composite using an oxygen-free compound. A process was developed for the manufacture of odd-shaped products of high erosion resistance. Translated from Steklo i Keramika, No. 3, pp. 16–18, March. 1997.     

The theory of thermal shock in heterogeneous refractory materials

Conclusions Increases in the thermal shock resistance with the addition of high-molecular organic liquids to the grog, proven experimentally, is confirmed by calculations of the factors for thermal-shock resistance of fireclay articles according to the theory of maximum stresses.The established reduction of thermal resistance with the addition of organic liquids in the bond is not reflected by the calculations. Hence, the theory of maximum stresses, although it can in certain cases be applied to bodies which are heterogeneous, it is not universal.On account of the growth in microcracks, the experimentally determined thermal-shock resistance for firebrick increases with chaotic orientation of the microcracks, and with their distribution over the grog grains, but diminishes when the microcracks are located in the bond parallel to each other.In this and other cases, the coefficient of homogeneity of the material diminishes, and the associated probability of destruction (calculated by Weibull) increases. Thus, the statistical theory of strength evolved by Weibull cannot be used to account for the effect of the orientation of microcracks in the structure on the distribution of cracks developed in the material during thermal shock.Calculation of the developed thermal stresses, as a function of the Biot and Fourier criteria, established that with the addition to the grog grains in the fireclay mass of organic liquids, these stresses diminish by a factor of 3–4; there is an increase in the time period and difference in temperature at which the cracks develop, and also the depth of the development of the latter. However, the distribution of the developing thermal cracks is limited when the microcracks are arranged chaotically and around the grog grains. This also increases the thermal resistance of the fireclay refractories when the high-molecular organic liquids are added to the grog grains.     

Influence of mullite additions on thermal shock resistance of dense alumina materials. Part 2: Thermal properties and thermal shock behaviour

Abstract

The thermal properties and thermal shock behaviour of homogeneous, fine grained (≈ 0·5–2 µm), dense (≥ 98% of theoretical density) alumina–mullite (5–15 vol.-%) composites have been studied and compared with those of a reference alumina ceramic with similar microstructure. Thermal expansion (25–600°C) thermal diffusivity, and thermal conductivity (25–400°C) were determined and a correlation between these results and microstructural characteristics was established. On the basis of the predicted theoretical relative performance under thermal shock, one of the composites (10 vol.-% mullite) was chosen and the thermal shock behaviour on quenching was compared with that of the reference alumina. A test that accounts for the detrimental effect of mullite on the heat transfer properties of alumina has been proposed. The results obtained agree with the theoretical predictions. In particular, the composite material had a critical temperature increment for failure larger (～ 12%) than that of alumina.     

Effect of MgO on thermal shock resistance of CaZrO3 ceramic

In order to improve the thermal shock resistance of CaZrO_3, CaZrO₃ was synthesized by a solid-state reaction method with analytical ZrO₂ and CaCO₃ as raw materials, and MgO as additive. The effects of MgO on Flexural strength at room temperature, thermal shock resistance, XRD and microstructure of CaZrO₃ were characterized. The results show that the grain growth of CaZrO₃ is inhibited and the thermal shock resistance of CaZrO₃ is improved by adding MgO. With the increasing of MgO, the flexural strength at room temperature of samples are improved due to the grain refinement. When the addition of MgO is 8%, the flexural strength at room temperature increases to 270.15?MPa. The thermal shock resistance of samples are improved by MgO deflecting and bridging cracks. When the addition of MgO is 4%, the residual flexural strength of samples is the maximum (26.94?MPa).     

Anorthite ceramic materials

Translated from Steklo i Keramika, No. 12, pp. 10–12, December, 1990.