Design of crack deflection induced high toughness laminated Si3N4 ceramics based on hollow oriented one-dimensional microstructure

A laminated silicon nitride (Si₃N₄) ceramic material with a hollow, oriented, one-dimensional microstructure was successfully prepared based on the tape casting and sacrificial template method. The results show that hollow, oriented, one-dimensional microstructures can effectively induce crack deflection. Different arrangements of the structural design layer and dense layer will have different effects on the material. In particular, bulks with a single-layer orthogonal arrangement of the structural design layer possess high toughness and obvious crack deflection during the fracture process. A kind of multiscale crack deflection mode was realized. Compared with the fracture toughness of the monolithic Si₃N₄ ceramic bulk (5.55 MPa m^1/2), the fracture toughness can reach 8.73 MPa m^1/2, and the flexural strength can still reach 391.47 MPa with only a slight decrease.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Microstructure and fracture toughness of SiAlCN ceramics toughened by SiCw or GNPs

Mechanical alloying and spark plasma sintering (SPS) were used to prepare dense SiAlCN ceramic and SiAlCN ceramic toughened by SiC whiskers (SiC_w) or graphene nanoplatelets (GNPs). The influences of different reinforcements on the microstructure and fracture toughness were investigated. The SiAlCN ceramic exhibited a fracture toughness of 4.4 MPa m^1/2 and the fracture characteristics of grain bridging, alternative intergranular and transgranular fracture. The fracture toughness of SiC_w/SiAlCN ceramic increased to 5.8 MPa m^1/2 and toughening mechanisms were crack deflection, SiC_w bridging and pull-out. The fracture toughness of GNP/SiAlCN ceramic increased significantly, which was up to 6.6 MPa m^1/2. GNPs played an important role in grain refinement, which resulted in the smallest grain size. Multiple toughening mechanisms, including crack deflection, crack branch, GNP bridging and pull-out could be found. The better toughening effect could be attributed to the larger specific surface area of GNPs and the appropriate interface bonding between GNPs and matrix.     

A high-temperature structural and wave-absorbing SiC fiber reinforced Si3N4 matrix composites

The SiC_f/Si₃N₄ composite with low–high–low permittivity sandwich structure was designed for high-temperature electromagnetic (EM) wave absorption and mechanical stability. The SiC_f/Si₃N₄ possessed the remarkable mechanical properties at room temperature (the flexural strength is 357 ± 16 MPa and the fracture toughness is 10.8 ± 1.7 MPa m^1/2) for the strong fiber strength, moderate interface bonding strength and uniform matrix. Furthermore, the retention rate is as high as 80% at 800 °C. The A/B/C nanostructure and the sandwich meta-structure endowed the SiC_f/Si₃N₄ with an excellent EM absorbing property at room temperature. The SiC_f/Si₃N₄ still absorbed 75% of the incident EM waves energy in X and Ku bands when the temperature increases up to 600 °C, which is only 6% lower than that at room temperature, for the partial compensation of the decreased interfacial polarization loss for the increased conductivity loss and dipole polarization loss.     

A novel method of strong and tough Si3N4 ceramic from the particle-stabilized foam

The brittleness of Si₃N₄ ceramics has always limited its wide application. In this paper, Si₃N₄ ceramics were prepared based on foam. Combining the unique honeycomb structure of the ceramic foams and the self-toughening mechanism of Si₃N₄, the strengthening and toughening of Si₃N₄ ceramics can be further achieved by adjusting the microstructure of Si₃N₄ ceramic foams. The powder particles are self-assembled by particle-stabilized foaming to form a foam body with a honeycomb structure. It was pretreated at different temperatures (1450–1750°C). The microstructure evolution of foamed ceramics at different pretreatment temperatures and the conversion rate of α-Si₃N₄ to β-Si₃N₄ at different pretreatment temperatures were explored. Then the foamed ceramics with different microstructures are hot-press sintered to prepare Si₃N₄ dense ceramics. The effects of different microstructures of foamed ceramics on the strength and toughness of Si₃N₄ ceramics were analyzed. The experimental results show that the relative density of Si₃N₄ ceramics prepared at a particle pretreatment temperature of 1500°C is 97.8%, and its flexural strength and fracture toughness are relatively the highest, which are 1089 ± 60 MPa and 12.9 ± 1.3 MPa m^1/2, respectively. Compared with the traditional powder hot-pressing sintering, the improvement is 21% and 33%, respectively. It is shown that this method of preparing Si₃N₄ ceramics based on foam has the potential to strengthen and toughen Si₃N₄ ceramics.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

Effects of SiC amount and morphology on the properties of TiB2-based composites sintered by hot-pressing

This investigation intended to assess the influence of SiC morphology on the sinterability and physical-mechanical features of TiB₂-SiC composites. For this aim, different volume percentages of SiC particles and SiC whiskers were introduced to TiB₂ samples hot-pressed at 1950 °C for 2 h under an external pressure of 25 MPa. The characterization of as-sintered specimens was carried out using X-ray diffraction, optical microscopy, and scanning electron microscopy. The relative density studies revealed that SiC_w had a more significant impact on the sinterability of TiB₂-based composites. The XRD investigation confirmed the production of an in-situ TiC phase during the hot-pressing; however, some peaks related to the graphitized carbon also appeared in the patterns of SiC_w-doped ceramics. The addition of 25 vol% SiC_p halved the average grain size of TiB₂ while introducing the same content of SiC_w decreased this value by just around 20%. Finally, the highest Vickers hardness and fracture toughness were obtained for the sample reinforced with 25 vol% SiC_w, standing at 29.3 GPa and 6.1 MPa m^1/2, respectively.     

Efficient multiscale strategy for toughening HfB2 ceramics: A heterogeneous ceramic–metal layered architecture

A multiscale structural design was innovatively adopted herein to increase the toughness of monolithic HfB₂ ceramics. SiC whiskers (SiC_w) and graphene oxide (GO) were used as fillers for the HfB₂ matrix, whereas a ductile W foil was introduced as an interlayer to synthesize laminated HfB₂-SiC_w-rGO/W ceramics. Monolithic HfB₂-SiC_p (particulate) and laminated HfB₂-SiC_p/W ceramics were prepared using the same routes and used as controls. Following tape casting and spark plasma sintering at 1800°C, the toughness of the prepared laminated HfB₂-SiC_w-rGO/W samples was increased to 14.2 ± 0.6 MPa·m^1/2, with minimal sacrifice in flexural strength (421 ± 16 MPa). Morphological analysis of the fracture surface revealed the synergistic effects of micro-toughening (including bridging and pullout of whiskers and rGO) and macro-toughening (including crack deflection, bifurcation, and delamination) mechanisms responsible for improving the fracture toughness of the laminated HfB₂-SiC_w-rGO/W composites.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

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.     

Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense β-Si₃N₄ coating toughened by β-Si₃N₄ nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si₃N₄ ceramics in this work. A porous β-Si₃N₄ nanowires/nanobelts layer was synthesized in situ on porous Si₃N₄ ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si₃N₄ nanowires/nanobelts, fine β-Si₃N₄ particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m^1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si₃N₄ ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.     

On the fabrication and mechanical properties of Si3N4 ceramics with low content sintering additives

Si₃N₄ ceramics were prepared by hot pressing (HP) and spark plasma sintering (SPS) methods using low content (5 mol%) Al₂O₃–RE₂O₃(RE = Y, Yb, and La)–SiO₂/TiN as sintering additives/secondary additives. The effects of sintering additives and sintering methods on the composition, microstructures, and mechanical properties (hardness and fracture toughness) were investigated. The results show that fully density Si₃N₄ ceramics could be fabricated by rational tailoring of sintering additives and sintering method, and TiN secondary additive could promote the density during HP and SPS. Besides, SN-AYS-SPS possesses the most competitive mechanical properties among all the as-prepared ceramics with the Vickers hardness as 17.31 ± .43 GPa and fracture toughness as 11.07 ± .48 MPa m^1/2.     

Crack tolerant silicon carbide ceramics prepared by liquid-phase assisted oscillatory pressure sintering

Silicon carbide ceramics were prepared by liquid-phase assisted oscillatory pressure sintering (OPS) with graphene and in-situ synthesized SiC whisker as the reinforcements. The effects of sintering temperature on the densification, morphology and mechanical performances of the SiC_p-SiC_w-graphene ceramics were investigated. In the temperature range from 1700 to 1800 °C, the densification rate of SiC_p-SiC_w-graphene ceramics was accelerated, ascribing to the reduction in viscosity of the glassy phase. At 1800 °C, the flexural strength and fracture toughness of the OPS ceramics corresponded to 697 MPa and 5.8 MPa m^1/2, respectively, which were higher than that of the hot-pressed ceramics under the same temperature conditions. Multiphase toughening mechanisms, such as whisker bridging and pullout, graphene bridging and delamination, were considered as the primary mechanisms. This work demonstrates an effective strategy to prepare silicon carbide ceramics at low sintering temperature.     

Effect of ZrB2 on promoting the phase transformation mechanism of Si3N4-based ceramics at low temperature

In this study, the mechanism of the effect of ZrB₂ on phase transformation of Si₃N₄ at a low temperature and the influence of its content on Si₃N₄-based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B₂O₃ and ZrO₂ on ZrB₂ particles, alone cannot contribute to phase transformation of Si₃N₄ at a low temperature. But, the introduction of 0.5 vol% ZrB₂ into Si₃N₄ ceramics can promote the α-β phase transformation of Si₃N₄, which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB₂ content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si₃N₄-based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB₂ increases to 5 vol%, the Si₃N₄-based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si₃N₄-based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m^1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB₂.     

Effects of whisker-like β-Si3N4 seeds on phase transformation and mechanical properties of α/β Si3N4 composites using MgSiN2 as additives

α/β Si₃N₄ composites with β-Si₃N₄ content ranging from 26% to 100% were hot-pressed with or without β-Si₃N₄ seeds, using MgSiN₂ as additives, and their mechanical properties were investigated. When the α-Si₃N₄ content was over 58%, the microhardness of α/β Si₃N₄ composites was in the range of 23–24 GPa, and then the indentation hardness decreases with decreasing the content of α-Si₃N₄, whether with β-Si₃N₄ seeds or not. The toughness increased with increasing elongated β-Si₃N₄ grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0 MPa m^1/2 with 1 wt% β-seeds. In comparison with α/β Si₃N₄ composite with a similar phase composition, the fracture strength was improved by adding β-Si₃N₄ seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si₃N₄ composite with 5 wt% β-seeds showed a high strength of 1253 MPa, a high hardness of 20.9 GPa and a toughness of 6.9 MPa m^1/2.     

Preparation,mechanical, dielectric and microwave absorption properties of hierarchical porous SiCnw-Si3N4 composite ceramics

Herein, hierarchical porous SiC_nw-Si₃N₄ composite ceramics with good electromagnetic absorption properties were prepared. A porous Si₃N₄ matrix with different pore structures was first prepared by gelcasting-pressureless sintering (G-PLS) and gelcasting combined with particle stabilized foam-pressureless sintering (G-PSF-PLS). SiC_nw was then introduced by catalytic chemical vapor deposition (CCVD). An increase in solid loading (25–40 vol%) decreased apparent porosity (47.7–41.3%) and improved flexural strength (142.19–240.36 MPa) and fracture toughness (2.25–3.68 MPa·m^1/2). The addition of foam stabilizer propyl gallate (PG, 0.5–1.0 wt%) significantly increased apparent porosity (73.2–86.4%) and realized large-sized spherical pores, reducing flexural strength (58.23–38.56 MPa) and fracture toughness (0.75–0.41 MPa·m^1/2). High apparent porosity and large-sized pores facilitated the introduction of SiC_nw. The 25 vol% sample exhibited a reflection loss of ? 14.67 dB with an effective absorption bandwidth of 3.47 GHz, suggesting a development potential in the electromagnetic wave absorption field.     

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.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

Refined measurements of indentation fracture resistance of alumina using powerful optical microscopy

A round-robin of the indentation fracture (IF) method using two alumina ceramics was performed in 12 laboratories to confirm the significantly improved reproducibility of indentation fracture resistance K_IFR, using powerful optical microscopy. Powerful optical microscopy with both an objective lens of 40× or 50× and a traveling stage was employed to reduce the error in reading crack length. Indentations at 98 N for the two samples had moderate between-laboratory standard deviations of 0.3 and 0.2 MPa m^1/2 for K_IFR of 4.3 and 3.6 MPa m^1/2, respectively, which indicates the effectiveness of this measurement technique to improve the reliability of the IF method. The deviations of the grand average K_IFR reported by the laboratories from those re-measured by the authors using the returned samples were only ca. 0.4 MPa m^1/2, which was attributed to the slight misreading of the crack length by the participant laboratories. Thus, the reliability of the IF method seems reasonable by this advanced approach because our recent round-robins, together with this study, have confirmed that the precision for the three major structural ceramics, SiC, Si₃N₄ and alumina, could meet the necessary condition of reproducibility.