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.     

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.     

Biomimetic micro-textures,mechanical behaviors and intermittent turning performance of textured Al2O3/TiC micro-composite and micro-nano-composite ceramics

This work was conducted to investigate biomimetic micro-textures, mechanical behaviors and intermittent turning performance of textured Al₂O₃/TiC micro-composite and micro-nano-composite ceramics. Chip characteristics and the geometry features of the structures on the cuticle of Procambarus clarkia were considered in the preparation of the laser-induced biomimetic micro-textures on the composite ceramic surfaces. Characteristics of the biomimetic micro-textures were revealed in terms of geometry, morphology and chemical composition. The connection between the thermal stress resulting from laser pulse and the fractal dimension of the micro-crack on the micro-textures was analyzed and identified. The correlation between the mechanical behaviors (damage and fracture toughness) of the textured composite ceramics and the fractal dimension of the micro-crack was fitted and revealed quantitatively. The performance of the textured composite ceramic tools was pre-evaluated by means of a proposed indicator. Damage, fracture toughness and tool stress were incorporated in the indicator. The indicator and the experimental tool wear were compared for validating the effectiveness of the proposed indicator. It was found that the micro-composite ceramic exhibited greater sensitivity to laser than the micro-nano-composite ceramic did. The damage of the textured micro-composite ceramic was larger than that of the textured micro-nano-composite ceramic when the same laser parameters were utilized in the micro-texture preparation. On the contrary, the fracture toughness of the textured micro-composite ceramic was found to be smaller. There was a negative correlation between the damage of the textured composite ceramic and the fractal dimension of the micro-crack. Conversely, there was a positive correlation between the fracture toughness and the fractal dimension. The performance of the textured micro-nano-composite ceramic tool was better than that of the textured micro-composite ceramic tool. The proposed indicator can be used to predict the combination of laser parameters that resulted in the optimum performance of the textured composite ceramic tool.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

Effects of SiC content on thermal shock behavior and elastic modulus of cordierite–mullite composites

Cordierite (Mg₂Al₄Si₅O₁₈) is a commercially available ceramic with low fracture toughness that hampers its broad industrial applications. Although several studies have reported the mechanical improvement of cordierite using various reinforcements, modulating its mechanical and thermal shock characteristics is not explored precisely. In the present research, we investigated the manufacturing of cordierite–mullite ceramics and the role of SiC on their thermomechanical properties. The in-situ formed mullite particles were obtained by mixing andalusite-talc-alumina and addition of SiC. It was found that thermal shock behavior and elastic moduli are dependent on SiC content and retained porosity. Furthermore, the addition of SiC to cordierite-based ceramics could enhance the thermal shock resistance via proper activation of the crack bridging mechanism in the matrix of the prepared composite.     

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.     

Grain growth kinetics in microwave sintered graphene platelets reinforced ZrO2/Al2O3 composites

The grain growth kinetics and mechanical properties of graphene platelets(GPLs) reinforced ZrO₂/Al₂O₃(ZTA) composites prepared by microwave sintering were investigated. The calculated grain growth kinetics exponent n indicated that the GPLs could accelerate the process of the Al₂O₃ columnar crystal growth. And the grain growth activation energy of the Al₂O₃ columnar crystal indicated that the grain growth activation energy of the GPLs doped ZTA composites is much higher than those of pure Al₂O₃ and ZTA in microwave sintering. The optimal mechanical properties were achieved with 0.4?vol% GPLs, whose relative density, Vickers hardness and fracture toughness were 98.76%, 18.10?GPa and 8.86?MPa?m^1/2, respectively. The toughening mechanisms were crack deflection, bridging, branching and pull-out of GPLs. The results suggested that GPLs-doped are good for the Al₂O₃ columnar crystal growth in the ZTA ceramic and have a potentially improvement for the fracture toughness of the ceramics.     

Environment-oriented low-cost Al2O3 ceramics with hierarchical pore structure fabricated from SiC solid waste

Environment-oriented low-cost Al₂O₃ reticulated porous ceramics with hierarchical pore structure were fabricated by the polymer sponge replica method combined with vacuum infiltration methods, using Al₂O₃ powders and SiC solid waste (SCSW) as raw material and a pore-forming agent. The effects of SCSW addition amount on mechanical properties, microstructure and pore size of Ceramics were investigated. The results showed that the thermal shock resistance of specimens increased gradually with addition of SCSW, however, the median pore diameter increased firstly and then decreased, due to the generation of mullite and liquid phase. After calcination, the residual stress was generated within the coating layer because of the difference in the thermal expansion coefficients of ceramic matrix and coating layer, which could improve the properties of Ceramics by deflecting and bifurcating crack growth path. The results showed that the best dosage of SCSW was 30 wt%.     

Strong and tough laminated ceramics prepared from ceramic foams

Here, we present a novel strategy to prepare laminated ceramics by combining the ceramic foams and hot-pressing sintering. Al₂O₃ and ZrO₂ ceramic foams prepared by the particle-stabilized foaming method was cut into thin slices and then directly laminated and hot-pressing sintered. Al₂O₃/ZrO₂ laminated ceramics with various structures were prepared. Compared with the slices prepared by conventional process, ceramic foams can easily regulate the thickness of laminate to resemble the nacre-like structure. In addition, the grain in the ceramic foams have lower activity and shrinkage rate, thereby weakening the residual tensile internal stress caused by grain coarsening and differences in coefficient of thermal expansion. The effects of layer number and thickness ratio on residual stress and the structure-activity relationship between mechanical properties and microstructure were investigated. The fracture toughness, flexural strength, and work of fracture of the optimal Al₂O₃/ZrO₂ laminated ceramics are 8.2 ± 1.3 MPa·m^1/2, 356 ± 59 MPa, and 216 J·m^?2, respectively.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

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.     

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.     

Thermal stress durability and optical characteristics of a promising solar air receiver based black alumina ceramics

In solar thermal technology, solar receiver materials with high solar absorptivity and photocatalytic efficiency have recently become a mandatory requirement for promoting and maximizing the released thermal and electrical energy. This paper presents a detailed study of the optical performance and thermal stress durability of the promising solar receiver material, black Al₂O₃/CuO ceramics. Different Al₂O₃/CuO ceramics with different CuO content (10–40 wt%) were obtained by the pressureless sintering method. Optical properties such as solar absorbance and reflectance, band gap energy and photoluminescence are inclusively investigated. The thermal stress resistance of the obtained ceramic receivers is simulated using the finite element modeling (FEM) method at different temperatures. Results indicated that adding and increasing the content of CuO to Al₂O₃ has a significant role in transforming alumina from a non-solar light-absorbed material to a solar absorber material with high absorptivity in the Ultraviolet–Visible-Near Infrared (UV-VIS-NIR) spectrum. Composite with 40 wt% CuO has recorded the maximum absorbance of 75% in the visible light region. Moreover, Al₂O₃/CuO ceramics gave multiple graded band-gaps in the range of (1.6–5 eV). Composites with high wt% of CuO have the most increased photocatalytic activity and light emissivity efficiency. Thermal stress analysis of the different ceramics showed outstanding stress durability with uniform heat distribution. Hence, black Al₂O₃/CuO ceramics can be considered ideal solar absorber materials with high sustainability and durability at high temperatures.     

Evaluation of the Thermal Resistance of Structural Ceramics in Testing Notched Prismatic Samples

The considered approach to determining thermal resistance in structural ceramics involves thermal cycling of samples with a special notch. The proposed thermal resistance characteristics include the loss of crack resistance after a thermal shock, the insensitivity of the material structure to defects formed at the notch apex as the result of a thermal shock, and the degree of defect accumulation at the notch apex. The testing results of ceramics made of Al₂O₃, ZrO₂, partly stabilized Y₂O₃ (molar part 3%), and cermet ZrO₂ – Y₂O₃ (molar part 3%) with metallic chromium additive (50 vol.%) are indicated.     

AC Impedance Spectroscopy of CaF2‐doped AlN Ceramics

The electrical conductivity of CaF₂‐doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF₂‐doped AlN ceramics were sintered with a second additive, Al₂O₃, added to control the electrical conductivity. The effects of calcium fluoride (CaF₂) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF₂ additive have no secondary phases at grain boundaries. Addition of Al₂O₃ caused the formation of amorphous phases at grain boundaries. Addition of Al₂O₃ to CaF₂‐doped AlN ceramics at temperatures 200°C–500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al₂O_3. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity.     

Thermal‐Shock Fracture and Damage Resistance Improved by Whisker Reinforcement in Alumina Matrix Composite

Fracture resistance of SiC‐whiskers‐reinforced Al₂O₃‐matrix composite under thermal shock was examined. Equibiaxial tensile thermal stress in the composite was significantly reduced before fracture, because the whiskers made percolation paths that increase heat flux and thereby reduced the temperature gradient. The thermal‐shock fracture resistance (R′) of the composite is thus much higher than that of monolithic Al₂O₃. Thermal‐shock damage resistance (R″″) was estimated from the thermal‐shock stress when a surface crack propagates. R″″ of the composite is also much higher than that of monolithic Al₂O₃ owing to an increment of work‐of‐fracture due to crack‐face bridging of the whiskers.     

The properties of Al2O3 coated fine‐grain temperature stable BaTiO3‐based ceramics sintered in reducing atmosphere

Chemical coating, an effective doping modification method, was employed to fabricate fine‐grain BaTiO₃‐based ceramics. Based on the consideration of subsequently using base metal as inner electrodes in multilayer ceramic devices, green bodies are generally sintered in reducing atmosphere, which generates more charged point defects and thus affects the electric properties. According to the elements distribution analysis, Al element is greatly enriched in the grain boundary and shell region. Coating Al₂O₃ achieves not only a smaller grain size and narrower distribution but also a higher breakdown strength, discharge energy density and energy efficiency at ambient temperature. In addition, temperature dependences of dielectric and energy storage properties under a same field were also investigated. Over the whole measuring temperature range, the sample with Al₂O₃ remains higher discharge energy density and energy efficiency.     

Fibrous monolithic ceramic with a single alumina phase: Fracture and high-temperature tribological behaviors

Fabrication of fibrous monolithic ceramic with bamboo-like structures is a promising method to improve the mechanical properties of ceramics through extrinsic reinforcement. Nevertheless, heterogeneous boundaries are easily oxidized at high temperatures, which seriously limits the long-term use of these materials when employed in high-temperature and high/low-temperature alternating environments. In this study, a “plain” ceramic—a single-component and complex-structure Al₂O₃ ceramic—was successfully designed and prepared using nano-sized Al₂O₃ as polycrystalline fibers and micro-sized Al₂O₃ as boundaries to obtain a structure with a fibrous monolithic architecture. Self-toughening of Al₂O₃ ceramics can be achieved by introducing hierarchical architectures derived from the difference between grain sizes of fibers and boundaries, which gives the ceramics high fracture toughness and reliability. Moreover, the material demonstrated a low friction coefficient and high wear-resistance properties when coupled with C/C composites at room temperature, 800°C, and in the alternating temperature enviroment between room temperature and 800°C.