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.     

Structural design and energy absorption mechanism of laminated SiC/BN ceramics

The laminated silicon carbide/boron nitride (SiC/BN) ceramics with different structural designs were fabricated by pressureless sintering at 1900?°C for 1?h in argon flow. The alumina (Al₂O₃)-and yttrium(III) oxide (Y₂O₃)-doped SiC ceramic exhibited a significant intergranular fracture behavior, which could be attributed to the yttrium aluminum garnet (YAG) phase located at the grains boundaries. The bending strength and fracture toughness were used to characterize the crack propagation including the delamination cracking, crack kinking, and crack deflection. The energy absorption in the process of crack propagation was characterized by the work of fracture (WOF) and damping capacity. The mode of crack propagation changed with the change in the structure and variation of BN content in the BN layer. The delamination cracks occurred inside the BN layer or at the interface between SiC and BN layers. The sample with a gradient structure exhibited the combination of delamination cracks occurring at the interface and inside the BN layer, which showed the maximum WOF of 2.43?KJ?m^?2, bending strength of 300?MPa, and fracture toughness of 8.5?MPa?m^1/2. The damping capacity varied with the change of the structure and the amplitude. The sample with a gradient structure exhibited the damping capacity of 0.088 and the maximum loss modulus of 9.758?GPa.     

Electrochemically assisted room-temperature crack healing of ceramic-based composites

We demonstrated for the first time the room-temperature crack healing in ceramic-based composites. For this purpose, 20 and 30 vol% of fine titanium metal (Ti) were homogeneously dispersed in electrically insulating alumina (Al₂O₃) ceramic to obtain composites that exhibited excellent electrical conductivity. Electrochemical anodization at room temperature was used to successfully heal cracks induced in the Al₂O₃/Ti composites and recover their fracture strength and reliability. The bending strength of as-prepared, crack-induced, and electrochemically healed composites was measured to evaluate the crack-healing ability. Moreover, the effects of the anodization current density, crack size (including length and crack open distance), and the conductivity of the composites on their crack-healing behavior were investigated and discussed. The results indicate that the bending strength of crack-induced composites, which was approximately 61% of the crack-free composite strength, was completely recovered after the crack-healing procedure at room temperature under appropriate anodization conditions. The titanium oxides obtained after anodization formed bridges that healed the crack; this was considered to be the main strength recovery mechanism. By anodizing Al₂O₃/Ti composites, we developed a new and convenient approach to heal cracks and recover the fracture strength of cracked ceramics at room temperature.     

In-situ fabrication of laminated SiC/TiSi2 and SiC/Ti3SiC2 ceramics by liquid silicon infiltration

The laminated silicon carbide/titanium silicon (SiC/TiSi₂) and silicon carbide/titanium silicon carbide (SiC/Ti₃SiC₂) ceramics were successfully designed and fabricated by liquid silicon (Si) infiltration. When the thickness of TiC layer was 150 and 450?µm, the TiSi₂ and Ti₃SiC₂ phases were the main products in the TiC layer, respectively. The as-fabricated structural unit of laminated SiC/Ti₃SiC₂ ceramics consisted of five layers of functionally graded materials, which has multiscale layered structure containing macro-layered structure and nano layered structure. The generation of hierarchical structure was attributed to the diffusion of Ti elements and in-situ formation of TiSi₂ and Ti₃SiC₂. The growth direction of Ti₃SiC₂ was anisotropic, thus providing more paths for the crack propagation via deflection, branching, and delamination during fracture process. However, the crack propagation inside the Ti₃SiC₂ phase included the pull out, bridging, lamination, deflection, and fracture of the single layer, which are the energy absorption and damage tolerance mechanisms of the Ti₃SiC₂ phase.     

The effects of microstructure on mechanical and electrical properties of W dispersed Al2O3 ceramics

This work aims to enhance the fracture toughness of brittle Al₂O₃ ceramics and apply insulated Al₂O₃ ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al₂O₃–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot-press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al₂O₃ matrix grains and W particles. The existence of three phases of Al₂O₃, W, and AlWO₄ was confirmed by X-ray diffraction patterns. All Al₂O₃–W composites showed higher fracture toughness than monolithic Al₂O₃. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 10¹² Ω cm of monolithic Al₂O₃ to 4.1 × 10² Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al₂O₃ grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased.     

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.     

Fabrication and mechanical properties of Al2O3-Si3N4/ZrO2-Al2O3 laminated composites

In this paper, Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were fabricated by tape casting and hot press sintering, and the relationships between the process, microstructure, and mechanical properties of Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were determined. The SiAlON phase was found in the Al₂O₃-Si₃N₄ layer, and liquid-phase sintering was proposed. Nano-scratch tests were carried out to investigate the interface bonding strength of the laminates. The distribution of residual stresses, generated due to the different coefficients of thermal expansion between the different layers, was estimated according to lamination theory and confirmed using Vickers indentation. When the sintering temperature was 1550 °C, the sintered laminated ceramics had good mechanical properties, with a maximum strength and toughness of 413 MPa and 6.2 MPa m^1/2, respectively. The main toughness mechanics of laminated composites was residual stress.     

Molecular dynamics study on nano‐particles reinforced oxide glass

Energy release rate and fracture toughness of amorphous aluminum nanoparticles reinforced soda‐lime silica glass (SLSG) were measured by performing fracture simulations of a single‐notched specimen via molecular dynamics simulations. The simulation procedure was first applied to conventional oxide glasses and the accuracy was verified with comparing to experimental data. According to the fracture simulations on three models of SLSG/‐Al₂O₃ composite, it was found that the crack propagation in the composites is prevented through following remarkable phenomena; one is that a‐Al₂O₃ nanoparticles increase fracture surface area by disturbing crack propagation. The other is that the deformation of a‐Al₂O₃ nanoparticle dissipates energy through cracking. Moreover, one of the models shows us that the crack cannot propagate if the initial notch is generated inside a‐Al₂O₃ nanoparticle. Such strengthening is partly due to the fact that the strength of the interface between nanoparticle and SLSG matrix is comparable to that of SLSG matrix, implying that their interface does not reduce crack resistance of the oxide glass.     

Effects of nano-ZrO2 content on microstructure and mechanical properties of GNPs/nano-ZrO2 reinforced Al2O3/Ti(C,N) composite ceramics

Dense Al₂O₃/Ti(C,N) composite ceramics reinforced with GNPs/nano-ZrO₂ were fabricated by hot-press sintering. The effects of nano-ZrO₂ content on the microstructure and mechanical properties of the prepared Al₂O₃/Ti(C,N)/GNPs/ZrO₂ composites were investigated. Results showed that nano-ZrO₂ inclusions refined the matrix grains significantly and resulted in the formation of intra-granular structure. Excellent comprehensive mechanical properties were achieved via addition of combined GNPs and nano-ZrO₂. In particular, the fracture toughness of composites incorporating GNPs (0.4 wt%)/ZrO₂ (1 wt%) exceeded 11 MPa m^1/2, which was increased by more than 86 % compared with that of Al₂O₃/Ti(C,N) ceramic composites without GNPs/ZrO₂. The main toughening mechanisms have been identified as stress-induced phase transformation, crack bridging, deflection and pull-out of GNPs. The toughening effects originated from GNPs were enhanced with the introduction of nano-ZrO₂ because of not only the residual stress resulted from phase transformation but also the formation of intra-granular structure with uneven surface around GNPs.     

Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker

In this study, SiC whisker (SiC_w) was introduced to ZrB₂ matrix layer of laminated ZrB₂/BN ceramics to improve fracture toughness. Laminated ZrB₂-SiC_w/BN ceramics were prepared by tape casting and spark plasma sintering. For comparison, monolithic ZrB₂-SiC_w and laminated ZrB₂-SiC_p/BN ceramics were also prepared using the same method. The introduction of SiC whiskers increased fracture toughness of laminated ZrB₂-SiC_w/BN ceramics to 13.31?±?0.33?MPa?m^1/2 for all samples. This was related to the multi-scale toughening mechanism, including delamination and crack deflection issued from the laminate structure at the macroscopic level, as well as whiskers bridging and pullout at the microscopic view. The R-curve behaviors of all samples revealed improved resistance to crack propagation of laminated ZrB₂-SiC_w/BN when compared to ZrB₂-SiC_p/BN and ZrB₂-SiC_w issued from multi-scale toughening design.     

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.     

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.     

CNT-induced TiC toughened Al2O3/Ti composites: Mechanical,electrical, and room-temperature crack-healing behaviors

This study addressed novel multiphase composite of Al₂O₃/Ti/TiC that exhibited enhanced fracture toughness and room-temperature crack-healing function. Al₂O₃/Ti/TiC composites were fabricated through hot-press sintering of CNT, TiH_2, and Al₂O₃ mixed powders, where the TiC was in-situ formed by reaction of CNT and Ti. The effects of CNT (TiC) content on mechanical and electrical properties were studied. Electrochemical anodization process at room temperature was attempted to these composites to heal cracks introduced in the surface of composites. Results indicated that added CNT was invisible while metal Ti and reaction product TiC coexisted in all samples. The reaction between CNT and Ti[O] representing dissolved active oxygen into Ti was considered as the main formation route of TiC. The toughening mechanism was demonstrated as crack deflection and bridging due to the presence of TiC. In spite of the increase in electrical resistivity because of the higher resistivity of TiC than Ti, the present Al₂O₃/Ti/TiC composites still remain high enough electrical conductivity (8.0 × 10⁻³ Ωcm ~1.8 × 10⁻² Ωcm for 0-2 vol% CNT addition) which could be regarded as conductors; it allowed to heal cracks in the composites by electrochemical anodization that formed titanium dioxide phase at room temperature. It was found that crack-healing ability in 1 vol% CNT added composite exhibited higher strength recovery ratio of 95.6% to the crack-free sample than that of Al₂O₃/Ti composite (the recovery ratio of 89.6%). After crack-healing process, mechanical strength of samples increased by 52.3% compared to cracked composites. It was concluded that the formed TiC could contribute to the appropriate electrical conduction as well as interface strengthening in the Al₂O₃/Ti composites. Furthermore, it was firstly speculated that the TiC could be electrochemically anodized to form an oxide like Ti metal. These characteristics enable Al₂O₃/Ti/TiC composites as the crack-healing materials at room temperature.     

Role of CeAl11O18 in reinforcing Al2O3/Ti composites by adding CeO2

This work discusses the reinforcement effect of elongated CeAl₁₁O₁₈ phase on multifunctional Al₂O₃/Ti composites by adding CeO₂ to inhibit interfacial reaction and strengthen interface for inducing optimized performances. For this purpose, Al₂O₃/Ti composite with different contents of CeO₂ was fabricated and the microstructure, mechanical and electrical properties were studied. Results indicated that after CeO₂ was added, elongated CeAl₁₁O₁₈ phase was formed within these composites. Owing to inhibited interfacial reaction between Al₂O₃ and Ti, Ti content was increased and compositions of composites were calculated using Rietveld method based on X-ray diffraction patterns. Attributed to the strengthening and toughening effects of CeAl₁₁O₁₈ phase, 2 mol% CeO₂ added composite showed the highest flexural strength and fracture toughness of 576 MPa and 5.15 MPa·m^1/2, which increased by 21% and 20% compared to Al₂O₃/Ti composite without CeO₂ addition. In this case, crack bridging by both Ti and CeAl₁₁O₁₈ particles was the major toughening mechanism and the additional fracture toughness caused by CeAl₁₁O₁₈ toughening effect reached a maximum. The role (crack bridging or particle pull-out mechanism) of CeAl₁₁O₁₈ in toughening Al₂O₃/Ti composites depended on the aspect ratio of these elongated particles, which was directly related to CeO₂ content. Because of the inhibition of interfacial reaction and the increase in Ti content, excellent electrical resistivity of composites after CeO₂ addition was maintained in spite of the formation of insulated CeAl₁₁O₁₈ phase. All samples showed relatively low electrical resistivity of ~10⁻³ Ωcm.     

Fine Ti‐dispersed Al2O3 composites and their mechanical and electrical properties

Al₂O₃/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al₂O₃ and TiH₂ powders. During sintering, TiH₂ decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al₂O₃ matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al₂O₃. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm^1/2, which was 37% greater than that of monolithic Al₂O₃. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10^?1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al₂O₃ increased the fracture toughness and improved the conductivity of Al₂O₃.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

In situ TEM observation of crack propagation in LiTaO3 particle toughened Al2O3 ceramics

Direct observation of crack propagation in LiTaO₃/Al₂O₃ composite ceramics was carried out using in situ transmission electron microscopy (TEM). Domain switching induced by crack propagation, crack deflection and branching at domain boundaries and ripples similar to the contrasts of 180° domains at the microcrack tip inside LiTaO₃ grains were detected evidently. Domain switching, crack deflection, branching and energy dissipation resulting from the formation of contrasts similar to the 180° domains at the microcrack tip, were proposed as the toughening mechanisms in LiTaO₃/Al₂O₃ ceramics.     

Bioinspired alumina/reduced graphene oxide fibrous monolithic ceramic and its fracture responses

Natural composites have very simple compositions and complex hierarchical architectures consisting of several different levels. These features simultaneously endow them with strength, toughness, functional adaptation, and damage-healing characteristics. Inspired by the microstructural features of natural materials, this work successfully fabricated Al₂O₃/reduced graphene oxide (rGO) fibrous monolithic ceramics with bamboo-like structures by introducing a thin graphene oxide around Al₂O₃ fiber cells to form the rGO boundary phase. The detailed evolutions of the crack extension and fracture responses were investigated by a J-integral method, and these bamboo-like composites demonstrated high structural reliability with excellent damage tolerance and progressive plastic failure behavior. With the fiber cell diameter of 0.6 mm, such composites exhibited fracture toughness (29.46 ± 3.04 MPa m^1/2) and work of fracture (799 ± 127 J m⁻²) that were 475% and 1075% higher than those of the monolithic Al₂O₃ ceramic, respectively. Their excellent fracture-resistant behavior was attributed to the hierarchical architectures that provide crack deflection, delamination, and load redistribution. The results also established the structure-activity relationships to guide the design and fabrication of these bamboo-like composites.     

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.     

Fabrication of in-situ Ti(C,N) phase toughened Al2O3 based ceramics from natural bauxite

Al₂O₃–Ti(C,N) ceramics were fabricated via carbothermal reduction nitridation method with high-titania special-grade bauxite as the raw material. The formation mechanism of in-situ Ti(C,N) phase and its effect on the properties of materials are discussed. After nitrided at 1700 °C, Ti(C,N) phase could be formed in-situ with appropriate C/TiO₂ molar ratio. Due to the residual stress field formed by Ti(C,N) particles, the path of crack propagation is changed, leading to the crack deflection and pinning. Therefore, the mechanical properties of the materials are improved by forming in-situ Ti(C,N) phase. With a C/TiO₂ molar ratio of 2.2 and nitridation temperature of 1700 °C, Al₂O₃–Ti(C,N) ceramic with a hardness of 13.9 GPa, a fracture toughness of 8.28 MPa m^1/2 and a flexural strength of 387 MPa could be fabricated.