Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

R-Curve Behavior and Thermal Shock Resistance of Ceramics

The influence of R -curve behavior of ceramics on the strength degradation associated with thermal shock is explored. Of particular significance for this interdependence is the observed nonlinear stress-strain behavior of materials that exhibit minimal strength degradation under severe thermal shock conditions. These two features, R -curve behavior and nonlinear behavior, are incorporated into a fracture mechanics analysis to provide a framework with which to understand severe thermal shock of ceramics. This analysis enables the estimation of the crack growth due to thermal shocking and also the anticipated strength degradation. The influence of specimen size is also addressed, and it is shown that greater strength degradation is anticipated with increasing specimen size. Experimental results for an alumina-zirconia composite material are presented to support the simple analysis.     

Acoustic Emission During Micro- and Macrocrack Growth in Mg-PSZ

Acoustic emission (AE) of a Mg-PSZ (TS grade) material has been studied during three- and four-point bending and stable macrocrack extension tests. During flexure, AE signals increased significantly upon the onset of nonlinear stress-strain behavior and were associated with phase transformation and microcracking on the tensile surface. Observations of crack extension in notched bars revealed relatively little AE during initial loading and onset of non-linear behavior. However, on subsequent loadings, the number of counts was in reasonable agreement with the extent of crack growth. These observations are discussed in terms of AE from phase transformation and crack growth at different grain size. They are compared with AE behavior of other zirconia-based materials.     

氧化铝-堇青石复合陶瓷抗热震性研究

为了提高氧化铝陶瓷的抗热震性,将具有低膨胀系数的堇青石加入到Al2O3中,通过无压烧结工艺,制备出了氧化铝-堇青石复合抗热震陶瓷.结果表明,堇青石加入量为w(堇青石)=10%,烧结温度 1520℃时,陶瓷样品能够承受1500℃温差(空冷)的热震破坏.采用SEM对陶瓷进行组织结构分析,发现在基体内部形成长柱状组织,并呈无规分布状态,这样的显微组织对提高陶瓷的抗热震性具有重要作用.     

Influence of a Piezoelectric Secondary Phase on Thermal Shock Resistance of Alumina-Matrix Ceramics

Using monolithic alumina ceramics as the reference, the thermal shock behavior of the hot-pressed alumina matrix ceramics with 3 mol% neodymium titanate was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three-point bending test to determine the flexural strength degradation. The hot-pressed composite exhibited a temperature differential of the thermal shock resistance 120°C higher than the monolith, mainly because of significantly improved fracture toughness.     

Cyclic Fatigue-Crack Propagation in Magnesia-Partially-Stabilized Zirconia Ceramics

The subcritical growth of fatigue cracks under (tension-tension) cyclic loading is demonstrated for ceramic materials, based on experiments using compact C(T) specimens of a MgO-partially-stabilized zirconia (PSZ), heat-treated to vary the fracture toughness K _c from ∼3 to 16 MPa·m^1/2 and tested in inert and moist environments. Analogous to behavior in metals, cyclic fatigue-crack rates (over the range 10⁻¹¹ to 10⁻⁵ m/cycle) are found to be a function of the stress-intensity range, environment, fracture toughness, and load ratio, and to show evidence of fatigue crack closure. Unlike toughness behavior, growth rates are not dependent on through0-thickness constraint. Under variable-amplitude cyclic loading, crack-growth rates show transient accelerations following low-high block overloads and transient retardations following high-low block overloads or single tensile overloads, again analogous to behavior commonly observed in ductile metals. Cyclic crack-growth rates are observed at stress intensities as low as 50% of K _c , and are typically some 7 orders of magnitude faster than corresponding stress-corrosion crack-growth rates under sustained-loading conditions. Possible mechanisms for cyclic crack advance in ceramic materials are examined, and the practical implications of such "ceramic fatigue" are briefly discussed.     

Acoustic Emission of Thermally Cycled Refractories

Acoustic emission (AE) technique was applied to continuously monitor property degradation during standard thermal shock fatique testing of three distinct types of refractories. AE results were correlated with respective strength losses of analyzed materials after a defined number of heating and cooling cycles. The complex nature of the AE signals was revealed by AE amplitude and duration analysis indicating that not all of the signals that were registered during thermal shock tests could be correlated with strength changes. On this basis, it was postulated that simple counting of a number of AE events should not be used as a measure of the degree of damage in refractories under thermal shock.     

Influence of the Compositional Profile of Functionally Graded Material on the Crack Path under Thermal Shock

Thermal cracking under a transient-temperature field in a ceramic/metal functionally graded plate is discussed. When the functionally graded plate is cooled from high-temperature, curved or straight crack paths often occur on the ceramic surface. It is shown that the crack paths are influenced by the compositional profile of the functionally graded plate. Transient-thermal stresses are treated as a linear quasi-static thermoelastic problem for a plane strain state. The crack paths are obtained using finite element method with Mode I and Mode II stress intensity factors.     

Thermal Shock Behavior of Iron-Particle-Toughened Alumina

The thermal shock behavior of an alumina monolith and two alumina–iron ceramic-matrix composites has been investigated by superimposing the measured K _R-curves of the materials onto the theoretically generated curves of the thermally induced stress intensity factor. Predictions of the critical-temperature differentials and retained strengths after quenching are in good agreement with the experimental data. The inclusion of metallic particles into an alumina matrix improves the thermal shock resistance, although the increase in toughness is not solely responsible for this improvement. There is a decrease in thermal stress-intensity factor that is generated for the composites; this decrease is due to a reduction in the Young's modulus and/or Biot modulus. However, the increased toughness for large crack lengths may offer increased damage resistance for severe thermal shock treatments.     

抗热震陶瓷材料的设计

随着高技术陶瓷的应用发展，迫切需要提高陶瓷材料的抗热震性，以适应各种恶劣的应用环境，本文讨论了各种抗热震性能优良的陶瓷，提出抗热震陶瓷的设计思想。     

Unique performance of thermal barrier coatings made of yttria-stabilized zirconia at extreme temperatures (>1500°C)

Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates.     

锆质定径水口热震稳定性的研究

本试验采用粉末成型工艺,以斜锆石和电熔部分稳定氧化锆(PMD、PCD)为原料,采用不同含量的添加剂,在300MPa压力下成型,1720℃烧制成锆质定径水口.结果表明:通过合理控制颗粒级配和优化工艺参数,氧化锆的稳定化率达到70%左右,烧成温度为1720℃保温6h后,可得到热震稳定性不小于5次的锆质定径水口.     

Mechanical Properties, Thermal Shock Resistance, and Thermal Stability of Zirconia-Toughened Alumina-10 vol% Silicon Carbide Whisker Ceramic Matrix Composite

Zirconia-toughened alumina (Al₂O₃–15 vol% Y-PSZ (3 mol% Y₂O₃)) reinforced with 10 vol% silicon carbide whiskers (ZTA-10SiC _w ) ceramic matrix composite has been characterized with respect to its room-temperature mechanical properties, thermal shock resistance, and thermal stability at temperatures above 1073 K. The current ceramic composite has a flexural strength of ∽550 to 610 MPa and a fracture toughness, K _IC , of ∽5.6 to 5.9 MPa·m^1/2 at room temperature. Increases in surface fracture toughness, ∽30%, of thermally shocked samples were observed because of thermal-stress-induced tetragonal-to-monoclinic phase transformation of tetragonal ZrO₂ grains dispersed in the matrix. The residual flexural strength of ZTA–10 SiC _w ceramic composite, after single thermal shock quenches from 1373–1573 to 373 K, was ∽10% higher than that of the unshocked material. The composite retained ∽80% of its original flexural strength after 10 thermal shock quenches from 1373–1573 to 373K. Surface degradation was observed after thermal shock and isothermal heat treatments as a result of SiC whisker oxidation and surface blistering and swelling due to the release of CO gas bubbles. The oxidation rate of SiC whiskers in ZTA-10SiC _w composite was found to increase with temperature, with calculated rates of ∽8.3×10⁻⁸ and ∽3.3×10⁻⁷ kg/(m²·s) at 1373 and 1573 K, respectively. It is concluded that this ZTA-10SiC _w composite is not suitable for high-temperature applications above 1300 K in oxidizing atmosphere because of severe surface degradation.     

Thermal Shock Behavior of Porous Silicon Carbide Ceramics

Using the water-quenching technique, the thermal shock behavior of porous silicon carbide (SiC) ceramics was evaluated as a function of quenching temperature, quenching cycles, and specimen thickness. It is shown that the residual strength of the quenched specimens decreases gradually with increases in the quenching temperature and specimen thickness. Moreover, it was found that the fracture strength of the quenched specimens was not affected by the increase of quenching cycles. This suggests a potential advantage of porous SiC ceramics for cyclic thermal-shock applications.     

Thermal Shock of Layered Ceramic Structures with Crack-Deflecting Interfaces

In this paper the influence of crack-deflecting interlayers on the thermal shock behavior of a ceramic body has been studied. It is observed that the presence of such interlayers inhibits the penetration of cracks into the body and that the magnitude of this effect is much greater than that of internal stresses or of possible increases in fracture energy of the layers, because cracking occurs in a manner different from that expected. A finite difference model has been used to estimate the temperature distribution in the body, from which the crack driving force and its variation with time and penetration into the body have been calculated. It is shown that these observations are consistent with quantitative predictions, if continued crack growth in the laminate requires that the stress in the outermost intact layer is equal to the failure strength of that layer, rather than the crack driving force for the overall penetrating crack being equal to the fracture energy of the material.     

Ultimate Strengths of Partially Stabilized Zirconia Ceramics

Previous results for ultimate tensile fracture strengths of partially stabilized zirconia (PSZ) containing preexisting finite cracks are reformulated in terms of a new transformation-strengthening parameter. This allows a simplified method for estimating the strengths of bulk PSZ containing arbitrarysized, preexisting critical cracks.     

TiC起始粉末粒径对TiC/Al2O3复合陶瓷热震行为的影响

通过使用不同粒径TiC起始粉末热压制备了AT1和AT2两种不同晶粒大小的TiC/Al_2O_3复合陶瓷。研究了TiC/Al_2O_3复合陶瓷的热震循环疲劳机制和TiC起始粉末粒径对热震循环疲劳抗力的影响。TiC起始粉末粒径的调整可以控制TiC/Al_2O_3复合陶瓷的晶粒大小。AT2材料晶粒细化,其力学性能的改善是其热震循环疲劳抗力改善的根本原因。热震循环过程中的微裂纹,裂纹桥接和偏转,热震裂纹间的“碎粒”等是引起TiC/Al_2O_3复合陶瓷热震疲劳的机制。热震循环疲劳机制和细晶强化的综合作用导致了AT2材料的热震抗力有较大改善。     

Thermal Expansion Anisotropy and Acoustic Emission of NaZr2P3O12 Family Ceramics

Most members of the NaZr₂P₃O₁₂ (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr₄P₆O₂₄ contracts on heating and that of SrZr₄P₆O₂₄ expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500°C, and Na and K compounds from 600°C. On the other hand, no acoustic emission signal is detected in Ca_0.5Sr_0.5Zr₄P₆O₂₄ ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500°C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques.