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 and fatigue of Zircon-Mullite composite materials

Zircon-Mullite (ZrSiO₄-3Al₂O₃·2SiO₂) composites are refractory materials widely employed in many industries. Thermal shock behaviour of these materials must be considered because it is sometimes its failure mechanism.In this work both thermal shock resistance (TSR) and fatigue (TFR) of Zircon-Mullite composites with different compositions and microstructure configurations were experimentally evaluated by a non destructive measurement of the elastic modulus (E) and compared with the prediction made from the theoretical parameters (R, R? and R_ST).A typical solid brittle material behaviour was found; a simple mathematical expression facilitated the TFR analysis. Although the microstructural configurations studied differed, the experimental behaviour of this group of materials was almost equal. This fact was satisfactorily predicted by the theoretical parameters (R and R?) showing the importance and potential of the evaluation mechanical evaluation of the ceramic material that define these two parameters. On the other hand the slight difference evaluated in the TFR was correlated with the only parameter that takes into account the fracture toughness (R_ST) showing that the significance of this property in a more deep characterization of the ceramic materials.     

Microstructure and performances of corundum−mullite composite ceramics for heat transmission pipelines: Effects of Ho2O3 additive content

Ho₂O₃ was employed to improve the microstructural densification and performances of pressureless sintered corundum–mullite ceramic composites. This study investigated the influences of Ho₂O₃ addition on the microstructure, physical properties and thermal shock resistances of the composites. The results indicated that sample AH5 (80 wt% Al₂O₃, 20 wt% coal series kaolin, and 5 wt% additional Ho₂O₃), which was sintered at 1550 °C, showed the best comprehensive properties. In this Al₂O₃-rich and SiO₂-poor system, a reaction between the Ho₂O₃ and Al₂O₃–SiO₂ system produced an Ho₂O₃–Al₂O₃–SiO₂ liquid phase. This liquid phase increased the microstructural densification and resulted in a lower sintering temperature. The generation of mullite and holmium disilicate during thermal shocks improved the thermal shock resistance. The high bending strength and satisfactory thermal shock resistance of the as-prepared corundum–mullite ceramic composites showed their potential for use in heat transmission pipelines.     

Thermal shock resistance of Al2O3/SiO2 composites by sol-gel

In this paper, the SiO₂ ceramic matrix composites were reinforced by the two-dimensional (2D) braided Al₂O₃ fibers by sol-gel. To develop the high performance aeroengine with excellent resistance to thermal shock for advanced aerospace application, two different thermal shock temperatures (1100?°C and 1300?°C) and three different thermal shock cycles (10, 20 and 30 cycles) were tested and compared in this paper; besides, the thermal shock resistance of Al₂O₃/SiO₂ composites was investigated in air. Our results suggested that, the flexural strength of the untreated composites was 78.157?MPa, while the residual strength of Al₂O₃/SiO₂ composites under diverse thermal shock cycles and temperatures had accounted for about 95% and 50% of the untreated composites, respectively. Meanwhile, the density and porosity of the composites were gradually increased with the increase in test temperature. Moreover, the changes in fracture morphology and micro-structural evolution of the composites were also observed. Our observations indicated that, the fracture morphology of the composites mainly exhibited ductile fracture at the thermal shock temperature of 1100?°C, whereas brittle fracture at the thermal shock temperature of 1300?°C. Additionally, Al₂O₃/SiO₂ composites belonged to the Oxide/Oxide CMCs, so no new phase was formed after thermal shock tests. Above all, findings of this paper showed that Al₂O₃/SiO₂ composites had displayed outstanding thermal shock resistance.     

Role of ZrO2 in sintering and mechanical properties of CaO containing magnesia from cryptocrystalline magnesite

As main components of magnesia-based refractories, magnesia exhibits excellent properties such as high refractoriness and good basic slag corrosion resistance. However, magnesia produced from CaO containing cryptocrystalline magnesite has limited application owing to the low hydration resistance and poor thermal shock resistance (TSR). This work aimed to investigate the reinforcing effects of microscale monoclinic ZrO₂ on free CaO containing magnesia for optimizing mechanical properties, TSR and hydration resistance. The results showed that adding ZrO₂ could promote the removal of the open pores, strengthen the interface bonding between various grains and produce crack deflection, which improved flexural strength and fracture toughness. As a result, the TSR of the specimens was enhanced effectively due to increased strength and toughness and reduction in the thermal expansion coefficient. Besides, as the ZrO₂ was introduced, hydration resistance of the specimens improved significantly, mainly attributing to the decrease in apparent porosity and elimination of the free CaO by forming CaZrO₃ and cubic ZrO₂ phases.     

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.     

Preparation and thermal shock resistance of anorthite solar thermal energy storage ceramics from magnesium slag

Anorthite solar thermal energy storage ceramics were fabricated from magnesium slag solid waste by pressureless sintering. The effects of CaO/SiO₂ ratio and sintering temperature on the physical, chemical, and thermophysical properties of ceramics were explored. X-ray diffraction results demonstrated that thermal shock process contributed to the formation of anorthite, and increasing CaO/SiO₂ ratio promoted the transformation of anorthite (CAS₂) into melilite (C₂AS). Some micro-cracks were found according to SEM analysis, forming by the mismatch of thermal expansion coefficients among phases. The combined effects of the low thermal expansion coefficient of anorthite and micro-crack toughing endowed the ceramic with good thermal shock resistance. Optimum comprehensive performances were observed in the sample with a CaO/SiO₂ ratio of 0.58 sintered at 1160°C, of which the specific thermal storage capacity was 0.63 J·g^-1·°C^-1(room temperature). The bending strength increased by 0.22% after 30 thermal shock times (room temperature-800°C, wind cooling). Therefore, the anorthite ceramics exhibited great potential for solar thermal energy storage.     

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.     

Role of Mg(OH)2 in pore evolution and properties of lightweight brine magnesia aggregates

Lightweight magnesia aggregates were fabricated using high-purity MgO agglomerates with the addition of Mg(OH)₂ as a pore former. The pore evolution and its relationship to the resulting properties were investigated. Mg(OH)₂ decomposition increased the number of inter-agglomerate pores, which subsequently affected the porosity and pore structure. When Mg(OH)₂ was 0–20 wt%, the inter-agglomerate pores were converted to both open and closed small pores, which effectively reduced the thermal conductivity and improved the thermal shock resistance (TSR) by accommodating thermal stress and inducing crack deflection. Small pores also favored the formation of a dense (Mg, Fe)O corrosion layer, preventing further slag penetration. However, large open pores occurred with further increasing Mg(OH)₂ content, which dramatically deteriorated the TSR and slag resistance. The specimen with 20 wt% Mg(OH)₂ exhibited the best overall performance, with a thermal conductivity of 16.6 W/(m·K) at 500 °C, and a residual flexural strength ratio of 32.3%; its slag resistance was comparable with that of dense magnesia.     

Effects of zinc oxide on thermal shock behavior of zinc sulfide– silicon dioxide ceramics

Thermal shock resistances of ZnO and non-ZnO containing ZnS–SiO₂ composite ceramics are observed using water quenching method. The residual strengths are measured as function of quenching temperature differences. The thermal shock damage parameters R and R are evaluated to compare with experimental results. Specimens with low thermal shock damage parameters show acute strength degradation up to 76% at a lower quenching temperature difference of 250 °C. The 1% ZnO containing specimen with medium density and higher thermal shock damage parameter values demonstrates a minimal strength drop of 36% at a higher quenching temperature difference of 300 °C. The evaluated R and R values correspond well with the residual strength at elevated temperature difference. It implies that the good thermal shock resistance of ZnS–SiO₂ system can be achieved by improving fracture toughness with moderate ZnO addition and pores.     

Effects of SDBS and ZrO2 additives on the microstructure and properties of silicon-bonded SiC porous ceramics

Porous silicon-bonded silicon carbide (SBSC) ceramics were prepared under argon atmosphere, with silicon as pore former and bonding material, simultaneously, sodium dodecyl benzene sulfonate (SDBS) and ZrO₂ as sintering additives, the effects of SDBS and ZrO₂ on the porosity, pore size, mechanical, physical and thermal properties and microstructures were investigated. The results suggested that suitable content of SDBS and ZrO₂ could not only effectively lower the sintering temperature to 1450 °C due to the sticky flow of molten silicon, but also increase the pore structure and improve the bending strength. The reason for this is that SDBS decomposed into Na₂O which reacted with ZrO₂ and impurity SiO₂, which was the native oxide film on the surface of SiC particles, to form a bonding phase between SiC particles to improve the bending strength; meanwhile, the disappearances of impurity SiO₂ would benefit the bond of molten silicon and silicon carbide particles, and silicon melt leaving pores in its original position to increase the pore structure. The optimal apparent porosity, bending strength, average pore size, gas permeance and residual bending strength after thermal shock cycles of SBSC porous ceramic sintered at 1450 °C with 5 wt% SDBS and 6 wt% ZrO₂ were 38.33%, 55.4 MPa, 11.3 μm, 106.4 m³/m²·h·kPa and 28.2 MPa, respectively.     

Effect of SiC nanowires on the thermal shock resistance of joint between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics

In order to improve the bonding property of joint between SiC modified carbon/carbon (C/C) composites and Li₂O–Al₂O₃–SiO₂ (LAS) glass ceramics, SiC nanowires were attempted as the reinforcement materials in the interface region of SiC transition layer and Li₂O–MgO–Al₂O₃–SiO₂ (LMAS) gradient joining interlayer. The C/C–LAS joint with SiC nanowire-reinforced interface layer was prepared by a three-step technique of pack cementation, in situ reaction and hot-pressing. The microstructure and thermal shock resistance of the as-prepared joints were examined. The average shear strength of the joined samples with SiC nanowires increased from 24.9 MPa to 31.6 MPa after 40 thermal cycles between 1000 °C and room temperature, while that of the joined samples without SiC nanowires dropped from 21.4 MPa to 8.3 MPa. The increase of thermal shock resistance of the C/C–LAS joints was mainly attributed to the toughening mechanism of SiC nanowires by pullout, bridging and crack deflection.     

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.     

Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al₂O₃. Intergranular MgAl₂O₄ phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al₂O₃ presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m^?1 k^?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.     

某些碳化硅材料的内耗和热震损伤

研究了某些碳化硅材料的力学行为，考察了抗折强度与弹性模量和内耗的关系。根据损伤力学，引进了热震损伤变量，并考察了碳化硅材料热震损伤与弹性模量和内耗的关系。由于弹性模量和内耗均可用非破坏实验方法测得，因此，在确定了强度和热震损伤与弹性模量和内耗的关系之后，可用非破坏实验来估计材料的强度和热震损伤。此外，还研究了常温强度和高温强度均高、抗热震性均优的碳化硅材料。该材料适用于制造陶瓷窑具。     

Ablation behavior and mechanism of SiCf/Cf/SiBCN ceramic composites with improved thermal shock resistance under oxyacetylene combustion flow

The ablation properties and mechanisms (under oxyacetylene combustion) together with thermal shock behavior of SiC_f/C_f/SiBCN ceramic composites were investigated. The solid ablation products are primarily amorphous SiO₂ and cristobalite. The primary ablation mechanisms include fiber and ceramic matrix oxidation, evaporation of B₂O₃ (l) and SiO₂ (l), and mechanical exfoliation. SiC_f/C_f/SiBCN has a significantly low mass ablation rate and a desirable linear ablation rate. The combination of crack deflection caused by SiC and carbon fibers, fiber pull-out and debonding improves thermal shock resistance and thus leads to the absence of surface macrocracks.     

Thermal shock behavior of co-continuous TiCx-Cu cermets in air and anaerobic environment

The present work investigated the microstructure and mechanical properties of TiC_x-Cu cermets before and after thermal shock tests. Thermal shock temperature was from 800 °C to 1200 °C and number of cycles was from 1 to 20. The results indicated that TiC_x-Cu cermets with co-continuous structure exhibited a stable and excellent thermal shock resistance. When quenched at 1000 °C for 1 cycle, the residual flexural strength of the cermet increased to 882 MPa, which was 10.1% higher than that without thermal shock. After 20 cycles of thermal shock, the residual flexural strength still maintain 760 MPa. When quenched at 800 °C and 1200 °C, the strengths of cermet decreased correspondingly, which were caused by the thermal mismatch between metal and ceramics and effusion of Cu or collapses of oxide layer, respectively. Herein, the recrystallization and refinement of metal phase grains caused during the thermal shock process, resulting in the superior thermal shock performance of cermet.     

Preparation and thermal shock resistance of solar thermal absorbing corundum ceramics for concentrated solar power

The absorptivity of solar thermal absorber materials affects the heliothermal conversion efficiency of concentrated solar power systems. The solar absorbing ceramics were prepared by the fixed mixture of bauxit, Fe₂O₃, and TiO₂ with adding CuO in different percentages. The absorptivity and thermal shock resistance with the effect of adding CuO in different percentages were studied. Fe₂O₃ and TiO₂ have excellent optical properties, and CuO decreases the material's band gap to boost the electronic transition and increase the material absorptivity. The results showed that the material is sintered at 1380°C with an excellent absorptivity of 94.00% in the spectrum range of 0.3–2.5 μm, and the bending strength is 132.94 MPa. The bending strength was increased by 21.07% after 30 thermal shock cycles (1000°C-room temperature, air cooling). The liquid phase facilitates the synthesis of hercynite with excellent high temperature properties. The hercynite improves the thermal shock resistance of the material.     

Thermal-shock stable Al2O3 crucible for superalloy smelting through slip casting with particle gradation

Crucible is a key vessel in superalloy smelting, and high-performance crucible is quite important to the development of superalloy. Pointed at the easy cracking and poor thermal-shock stability of Al₂O₃ crucibles for superalloy smelting, particle gradation was managed in slip casting of Al₂O₃ crucibles. The ceramic slurry in good dispersion was prepared by optimizing particle gradation, with solid content of 86 wt% and viscosity of lower than 0.5 Pa s. For the crucible with optimized particle gradation, the density was 2.37 g/cm³, the linear shrinkage was less than 2%, the porosity was 18.72%, the bending strength was close to 22 MPa and the thermal expansivity was 2.77 × 10^?6/°C. Strength retention rate was raised to evaluate the thermal shock stability of crucibles, which was up to 73.5% after five-time thermal shock, well meeting the requirements of crucibles for superalloy smelting, and the superior thermal-shock stability was attributed to the uniform network microstructure and low thermal expansivity. The optimal particle gradation in experiments was explained in terms of Dinger-Funk model. This work provides a reference for preparing high-performance Al₂O₃ crucibles for superalloy smelting with low thermal expansivity as well as excellent thermal-shock stability.