Tough and damage-tolerant monolithic zirconia ceramics with transformation-induced plasticity by grain-boundary segregation

Conventional ceria-stabilized tetragonal zirconia (Ce-TZP) with modest flexural strength has rarely been used as compared to yttria-stabilized zirconia, even though it has excellent hydrothermal stability and high toughness. Ce-TZP-based composites were recently developed, being tough and remarkably in combination with transformation-induced plasticity. However, distinct from the widely applied composite approach to improve the strength of Ce-TZP, in this study, a simpler and easily tailorable method was proposed by doping aliovalent oxides that are able to segregate at the zirconia-grain boundaries. 0.2–1 mol% divalent oxides with different cation size (Mg²⁺, Ca²⁺, Ba²⁺ and Sr²⁺) were selected to dope 10 mol% ceria-stabilized zirconia. CaO and MgO dopants were able to enter the tetragonal Ce-TZP lattice and showed a grain-boundary segregation effect, thereby tailoring the microstructure and transformation behavior. At a higher dopant concentration of 1 mol% MgO or CaO, the ceramics were strong but brittle with a typical elastic linear fracture behavior, whereas at a low dopant concentration of 0.1–0.2 mol% CaO or 0.2–0.4 mol% MgO doping, the ceramics deformed in-elastically to a certain degree without changing the Young’s modulus. At the transition between both fracture behaviors, the best combination of toughness (>10 MPa m^1/2), biaxial strength (≥1200 MPa), reliability (Weibull modulus up to 30) and damage-tolerance was obtained. In-situ fracture testing revealed that transformation in such tough and deformable zirconia ceramics took place well before crack initiation with a large transformation zone of ～100 µm ahead of the crack tip having been formed before failure.     

Spark plasma sintering of boron nitride micron tubes reinforced boron carbide ceramics with excellent mechanical property

In this paper, the novel boron nitride micron tubes (BNMTs) were used to reinforce commercial boron carbide (B₄C) ceramics prepared via spark plasma sintering technology. The effects of the sintering parameters, sintering temperature, the holding time, and the BNMTs content on the microstructure and mechanical properties of B₄C/BNMTs composite ceramics were studied. The results indicated that adding a proper amount of BNMTs could inhibit the grain growth of B₄C and improve the fracture toughness of the B₄C/BNMTs composite ceramics. The prepared composite ceramic sample with 5 wt% BNMTs at 1850°C, 8 min and 30 MPa displayed the best mechanical properties. The relative density, hardness, fracture toughness, and bending strength of the samples were 99.7% ± .1%, 35.62 ± .43 GPa, 6.23 ± .2 MPa m^1/2, and 517 ± 7.8 MPa, respectively. Therein, the corresponding value of hardness, fracture toughness, and bending strength was increased by 10.3%, 43.59%, and 61.5%, respectively, than that of the B₄C/BNMTs composite ceramic without BNMTs. It was proved that the high interface binding energy and bridging effect between boron carbide and BNMTs were the toughening principle of BNMTs.     

Novel strong and tough Ta/TaHfC2 composites with multi-scale laminated structure

Ta-Hf-C ternary ceramics are good candidates for structural components used in extreme thermal environment. However, intrinsic brittleness greatly restricts their applications in reality. Here, we report a novel TaHfC₂-based composite with superior strength and toughness by employing Ta foil as interlayers to generate a multi-scale laminated structure together with the in situ formed Ta₂C MXene during sintering. Owing to the unique composition and microstructure, the tensile strength, shear strength, fracture toughness (K_IC), and work of fracture of Ta/TaHfC₂ have reached 233 ± 12 MPa, 64 ± 6 MPa, 13.16 ± 0.03 MPa·m^1/2, and 1726 ± 448 J/m² simultaneously, and all showed substantial increments when compared with TaHfC₂ monolithic ceramic and related Ta-Hf-C materials as showed in literatures. This study significantly improved the toughness of TaHfC₂, and it provides a new high-performance thermal structural material for the aerospace industry as well as a new idea for toughening ultra-high temperature ceramics (UHTCs) of transition metal carbides.     

Mechanical properties and pre-oxidation behavior of spark plasma sintered B4C ceramics using (Ti3SiC2+CeO2/La2O3) as sintering aid

B₄C ceramic with the addition of 5 wt % (Ti₃SiC₂+ CeO₂/La₂O₃) as sintering aids was fabricated by spark plasma sintering at a relatively low temperature of 1650 °C for 5 min at 80 MPa. The phase composition, microstructures, and comprehensive mechanical properties of the ceramics were studied in detail. The existence of reinforced second phase particles, the refinement of the matrix grains, the formation of residual stress along the grain boundaries and the appearance of the mixed fracture mode had a synergetic strengthening effect on the mechanical properties. The flexural strength, fracture toughness and Vickers hardness of B₄C ceramics reached 565.2 ± 21.8/551.0 ± 25.2 MPa, 6.28 ± 0.01/6.41 ± 0.12 MPa·m^0.5, and 28.51 ± 0.86/27.23 ± 1.08 GPa, respectively. In addition, to reduce the crack sensitivity of the ceramic, the ceramics were pre-oxidized at 800 °C for different durations. The flexural strength was increased by approximately 13.4% after the ceramic was oxidized at 800 °C for 45 min due to the crack-healing effect induced by the oxide glass B₂O₃ on the ceramic surface.     

Micro-scale fracture toughness of textured alumina ceramics

Enhanced fracture resistance of textured alumina is ascribed to crack deflection along grain boundaries. In this work, we quantify and compare the micro-scale fracture toughness of textured alumina grains and grain boundaries by micro-bending tests. Notched micro-cantilevers were milled from single alumina textured grains (perpendicular to the [0001] direction) and across several textured grains (along the [0001] direction), using a focused ion beam technique. Bending tests were performed with a nanoindenter. A shape function for notched pentagonal-shaped cantilevers was developed using finite element analysis. The critical stress intensity factor at the notch tip was determined based on the measured fracture loads. The micro-scale fracture toughness of the textured alumina grain boundaries (2.3 ± 0.2 MPa m^1/2) was about 30% lower than that of the grains (3.3 ± 0.2 MPa m^1/2). These findings at the micro-scale are paramount for understanding the macroscopic fracture behaviour of textured alumina ceramics.     

In situ toughened two-phase B12(C,Si, B)3–SiC ceramics fabricated via liquid silicon infiltration

In situ toughened B₁₂(C, Si, B)₃–SiC ceramics were successfully fabricated via the liquid silicon infiltration process. Two types of B₁₂(C, Si, B)₃ phases, with high and low Si contents, respectively, and plate-like SiC particles were formed by the reaction between B₄C and Si. The in situ toughening mechanism involved two effects: the multiple crack deflections caused by the increased grain boundaries, and the pullout and rupture of a significant amount of plate-like SiC particles. Block ceramics with a high fracture toughness of 6.5 ± 0.5 MPa·m^1/2 were fabricated via the in situ toughening mechanism. A strong interface bond was present between the high- and low-B₄C-content layers in the laminated ceramics, which led to residual compressive stress inside the materials. As a result, the laminated structural design enhanced the fracture toughness to 7.5 ± 0.5 MPa·m^1/2.     

Effects of SmF3 addition on aluminum nitride ceramics via pressureless sintering

Aluminum nitride (AlN) ceramics with dense structure, high thermal conductivity, and exceptional mechanical properties were fabricated by pressureless sintering with a novel non-oxide sintering additive, samarium fluoride (SmF₃). The results showed that the use of a moderate amount of SmF₃ promoted significant densification of AlN and removed the oxygen impurity. This led to the formation of fine and isolated secondary phase that cleaned the grain boundaries and increased the contact between AlN grains, remarkably enhancing thermal conductivity. Furthermore, SmF₃ also exhibited grain refinement and grain boundary strengthening effects similar to traditional sintering additive, samarium oxide (Sm₂O₃), leading to high mechanical properties in SmF₃-doped AlN samples. The most optimal characteristics (thermal conductivity of 190.67 W·m⁻¹·K⁻¹, flexural strength of 403.86 ± 18.27 MPa, and fracture toughness of 3.71 ± 0.19 MPa·m^1/2) were achieved in the AlN ceramic with 5 wt% SmF₃.     

Cost-effective fabrication of α-SiAlON ceramics with CeO2 addition for cutting tool applications

SiAlON ceramic tools exhibit unique performance superiority in the machining of nickel-based heat-resistant superalloys (HRSA). Production cost is a critical obstacle for the wide application of SiAlON inserts. In this work, the cost-effective preparation of α-SiAlON ceramics was implemented using aqueous slurry, and the effects of CeO₂ content on the densification, phase composition, microstructure, mechanical properties, and toughening mechanism of the materials were investigated. The addition of CeO₂ enhanced the anisotropic growth of α-SiAlON grains, which enabled the ceramics to obtain higher flexural strength and fracture toughness. Although a decrease in Vickers hardness would occur when CeO₂ content was 3 or 5 wt.%, ascribed to the amount increase of the intergranular phase in the samples. The α-SiAlON ceramics doped with 1-wt.% CeO₂ achieved the extremely high density and mechanical properties with the relative density, flexural strength, Vickers hardness, and fracture toughness of 99.6% ± .1%, 931 ± 62 MPa, 19.7 ± .2 GPa, and 7.2 ± .2 MPa m^1/2, respectively, which could be a candidate cutting tool material for use in the efficient machining of nickel-based HRSA.     

Synergistic effect of binary fluoride sintering additives on the properties of silicon nitride ceramics

A variety of combinations of YF₃ and MgF₂ were used as sintering aids in the fabrication of Si₃N₄ ceramics via gas pressure sintering (GPS). The synergistic effects of YF₃ and MgF₂ on the liquid viscosity, mechanical properties, thermal conductivities, and grain growth kinetics of the Si₃N₄ ceramics were investigated. The results showed that appropriately adjusting the YF₃/MgF₂ ratio could decrease liquid viscosity, reducing the diffusion energy barrier of the solute atom and promoting mass transfer. Meanwhile, the chemical bonding strength in the grain boundary complexions formed by the metal cations also influenced grain boundary migration. Samples doped with 4 mol% YF₃ and 2 mol% MgF₂ achieved the lowest grain growth exponent (n = 2.9) and growth activation energy (Q = 616.7 ± 16.5 kJ mol^?1) as well as the highest thermal conductivity (83 W m^?1 K^?1) and fracture toughness (8.82 ± 0.13 MPa m^1/2).     

Pressureless Sintering of Hafnium Carbide–Silicon Carbide Ceramics

HfC‐30 vol%SiC ceramics with a relative density of 99.7% was obtained by pressureless sintering at 2300°C for 0.5 h. The resultant ceramics showed fine microstructure with HfC grain size around 1 μm. The hardness (20.5 ± 0.2 GPa), bending strength (396 ± 56 MPa), and fracture toughness (2.81 ± 0.18 MPa·m^1/2) of HfC‐30 vol%SiC ceramics were at least 20% higher than those of monolithic HfC ceramics. The influences of SiC particle size, volume fraction, and the oxide impurity on the microstructure evolution of HfC‐based ceramics were examined. The results indicate that SiC addition and the oxygen impurity introduced by ball milling play opposite roles in the HfC grain growth during sintering. The oxide impurity introduced by ball milling caused the HfC grain coarsening, whereas SiC particles inhibited the grain growth of HfC significantly.     

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.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Preparation and mechanical properties of laminated B4C–TiB2/BN ceramics

Laminated B₄C–TiB₂ ceramics with h-BN interface layers were successfully prepared by roll forming and tape casting, and samples with different numbers of stacked layers were obtained. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and interlayer crystal phases of the composites, and the bending strength, fracture toughness, and work of fracture were measured. As the number of h-BN layers increased, the fracture toughness increased from 7.38 ± 0.5 MPa m^1/2 to 9.01 ± 0.61 MPa m^1/2, which is 2–3 times higher than that of monolithic B₄C ceramics. As the fracture toughness increased, the hardness remained at a high level (31.67 GPa). Bending tests showed that cracks deflected when they encountered the h-BN interfacial layers. The toughening mechanisms included the deflection and branching of cracks and generation of new microcracks, which increased the length of the propagation path and work of fracture.     

Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, α/β-Si₃N₄ composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN–AlN–Al₂O₃/Y₂O₃, and spark plasma sintering at 1550 °C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered α/β-Si₃N₄ composite ceramics with ZrN–AlN–Al₂O₃ contained the most α phase, which resulted in a maximum Vickers hardness of 18.41 ± 0.31 GPa. In the α/β-Si₃N₄ composite ceramics with ZrN–AlN–Y₂O₃ additives, Zr₃AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated β-Si₃N₄ grains and denser microstructure when 20 wt% TiC particles were mixed into Si₃N₄ matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 ± 0.24 μm. As a result, the compact α/β-Si₃N₄ composite ceramics containing ZrN–AlN–Y₂O₃ additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 ± 28.75 MPa and 8.53 ± 0.21 MPa?m^1/2, respectively. Moreover, the synergistic toughening of rod-like β-Si₃N₄ grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si₃N₄ ceramic matrix.     

Influence of whisker-aspect-ratio on densification,microstructure and mechanical properties of Al2O3 whiskers-reinforced CeO2-stabilized ZrO2 composites

A novel composite of 12 mol% CeO₂-stablized tetragonal ZrO₂ reinforced with Al₂O₃ whiskers (designated as Ce-TZP/A_w) has been prepared and studied in this work. The objective of this investigation was to systematically study the influence of whisker-aspect-ratio on the densification behaviors, microstructure evolution, and mechanical properties of Ce-TZP/A_w composite. Results showed that the sintered density of composite increased and the grain growth tended to diminish with the decrease in whisker aspect radio. Both the fracture toughness and flexural strength reached maximum values of 475 ± 12 MPa and 11.4 ± 0.2 MPa m^1/2, respectively at a whisker aspect ratio of about 12. It was also observed that the fracture toughness, flexural strength and tetragonal to monoclinic ZrO₂ transformation of the dual-phase composite exhibited similar variation trend as a function of the whisker-aspect-ratio, which suggested that the stress-induced phase transformation should be the main toughening and strengthening mechanism in the Ce-TZP/A_w composite.     

Pressureless sintered Al4SiC4 ceramics with Y2O3 addition

Highly densified Al₄SiC₄ ceramics with a relative density of 96.1% were prepared by pressureless sintering using 2 wt% Y₂O₃ as additives. The densification mechanism, phase composition, microstructures and mechanical properties of Al₄SiC₄ ceramics were investigated. Y₂O₃ in-situ reacted with the oxygen impurities in Al₄SiC₄ powder to form a yttrium aluminate liquid phase during sintering, which promoted the densification and anisotropic grain growth. The final Al₄SiC₄ ceramics were composed of equiaxed grains and columnar grains, and presented a bimodal grain distribution. The mechanical properties of the pressureless sintered Al₄SiC₄ ceramics were better than those reported for hot pressed Al₄SiC₄, including a flexural strength of 369 ± 24 MPa, fracture toughness of 4.8 ± 0.1 MPa m^1/2 and Vickers hardness of 11.3 ± 0.2 GPa. Pressureless sintering of Al₄SiC₄ ceramics is of great significance for the development and practical application of Al₄SiC₄ ceramic parts, especially with big size and complex shape.     

Degradation evaluation of Si3N4 ceramic surface layer in contact with molten Al using microcantilever beam specimens

Although Si₃N₄ ceramics are often utilized as structural components in the Al casting industry due to their excellent properties, they occasionally suffer breakage after long-term use. In this study, the bending strength, fracture toughness, and Young’s modulus in the vicinity of the Si₃N₄ ceramic surfaces after contact with molten Al were evaluated using microcantilever beam specimens, which were fabricated using a focused ion beam technique. Fracture testing of the specimens was carried out by nanoindentation. The bending strength of the ceramic surface before and after contact with molten Al was 5.89 ± 1.33 and 3.03 ± 0.28 GPa, respectively. The fracture toughness of the corroded layer in Si₃N₄ ceramics also decreased compared to that of the polished surface. Using fractography by observation with scanning electron microscopy, it was shown that changes in the grain boundary glassy phase resulted in the degradation of strength and fracture toughness.     

Pressureless sintering of titanium carbide doped with boron or boron carbide

Titanium carbide ceramics with different contents of boron or B₄C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB₂ platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B₄C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m^1/2) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B₄C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB₂ platelets.     

Mechanical properties of LaFeO3 ceramics

The fracture strength, fracture toughness and apparent Young’s modulus of LaFeO₃ ceramics in the temperature region 25–800 °C are reported. The fracture strength of the material was observed to increase from 202 ± 18 MPa at room temperature to 235 ± 38 MPa at 800 °C. The room temperature fracture toughness was 2.5 ± 0.1 MPa m^1/2. The fracture toughness decreased to 2.1 ± 0.1 MPa m^1/2 at 600 °C, followed by an increase to 3.1 ± 0.3 MPa m^1/2 at 800 °C. The temperature dependence of the fracture toughness correlates well with the crystallographic strain, |(a−c)|/(a+c), and ferroelastic toughening of LaFeO₃ materials is inferred. Non-elastic stress–strain behaviour of the LaFeO₃ materials due to ferroelasticity was confirmed by cyclic compression experiments, and residual strain was observed in the material after unloading.     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.