Rapid fabrication of Si3N4 ceramics with gradient appearance and microstructure

Si₃N₄ ceramics with tailored gradient in color and microstructure were prepared by a rapid cost-effective one-step approach. The gradient microstructure was obtained by the manipulation of the dissolution-reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si₃N₄ ceramics, the β-phase content gradually changed from 84% to 11%. The Si₃N₄ ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m^1/2 and may be suitable for bio-implantation applications.     

Preceramic Polymer‐Derived SiAlON as Sintering Aid for Silicon Nitride

Two kinds of sintering additives based on the polysiloxanes or polysilazanes filled with nano‐sized powders as SiAlON precursors were tested for the densification of Si₃N₄‐based ceramics. The results showed that both systems can be successfully used as additives for the preparation of Si₃N₄ ceramics with favorable mechanical characteristics. The ceramics were sintered with 18 wt% of preceramic polymer‐based mixture, and good fracture resistance and high hardness values were obtained after sintering in optimized conditions (temperature, dwell time, nitrogen pressure). Higher densification temperatures and longer holding times were required for sintering of samples with polysilazane‐based precursors. The best toughness values were approximately 5 MPa·m^0.5, while the highest hardness was about 19 GPa. The differences in mechanical properties of the prepared composites can be related to the phase composition, microstructure and different chemical bonds present in the ceramic residue generated upon pyrolysis and final densification.     

Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.     

Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si₃N₄/SiC_w ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si₃N₄/SiC_w ceramics was 13.5 MPa m^1/2, which was almost 1.6 times that of Si₃N₄/SiC_w composite ceramics, namely 8.5 MPa m^1/2. Moreover, the indentation strength of laminated Si₃N₄/SiC_w ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si₃N₄/SiC_w ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si₃N₄/SiC_w ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Si3N4/graphene nanocomposites for tribological application in aqueous environments prepared by attritor milling and hot pressing

Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si₃N₄ structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si₃N₄/MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si₃N₄ ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.     

Toughening and ablation mechanism of Si3N4 short fiber toughened ZrB2-based ceramics

ZrB₂-based ceramics with Si₃N₄ short fiber (ZSN) were prepared by wet-spinning extrusion and hot pressing. The toughness of ZSN was 5.6 MPa·m^1/2, which was 20% higher than that of monolithic ceramic (4.7 MPa·m^1/2). The ablation performance of ZSN was evaluated by air discharge plasma ablation platform with a heat flux of 8.04 MW/m² for 120 s. The mass and linear ablation rates of ZSN were − 0.19 mg/s and − 0.25 µm/s, respectively. The specimens of ZSN remained intact while monolithic ceramics exhibited destructive fracture. The better ablation performance of ZSN is attributed to the addition of Si₃N₄ short fiber which increased the fracture toughness, reduced the elastic modulus, and improved the thermal conductivity at high temperature.     

Microstructure and mechanical properties of Ti (C,N) based cermets reinforced with different ceramic particles processed by spark plasma sintering

The addition effect of different ceramic particles such as TiB₂, TiN and nano-Si₃N₄ on the microstructure and mechanical properties of TiCN-WC-Co-Cr₃C₂ based cermets, which are prepared by spark plasma sintering, was studied. Microstructural characterization of the cermets was done by scanning electron microscope. X-ray diffraction was performed to study the crystal structures. Mechanical properties such as hardness and fracture toughness were measured for the different developed cermets. The hardness and fracture toughness of the TiCN-WC-Co-Cr₃C₂ cermets without TiN, TiB₂, and nano-Si₃N₄ were 8.4 GPa and 3.4 MPa m^1/2, respectively. It was found that 5 wt% TiB₂ addition alone improved the corresponding hardness and fracture toughness to 19.2 GPa and 6.9 MPa m^1/2, respectively. The addition of 5 wt% TiN, improved the hardness and fracture toughness to 16.7 GPa and 6.9 MPa m^1/2, respectively. With the combination of 5 wt% TiN and 5 wt% TiB₂, the hardness and fracture toughness were improved to 15.5 GPa and 6.6 MPa m^1/2, respectively. But, the addition of 5 wt% Si₃N₄ showed a balanced improvement in both hardness (17.6 GPa) and toughness (6.9 MPa m^1/2). Fracture toughness did not change much for all the above cermets with different ceramic inclusions.     

Preparation,microstructure, and properties of GPS silicon nitride ceramics with β-Si3N4 seeds and nanophase additives

Si₃N₄ ceramics with high thermal conductivity and outstanding mechanical properties were prepared by adding β-Si₃N₄ seeds and nanophase α-Si₃N₄ powders as modifiers. The introduction of β-Si₃N₄ seeds enhanced the growth of β-Si₃N₄ grains. Owing to the interlocked structure induced by the β-Si₃N₄ grains, the fracture toughness of Si₃N₄ ceramics reached a high value of 7.6 MPa·m^1/2; also, the large-sized grains increased the contact possibility of Si₃N₄ grains, improving the thermal conductivity of Si₃N₄ ceramics (64 W/(m·K)). Because of the introduction of nanophase α-Si₃N₄, the flexural strength, fracture toughness, and thermal conductivity of the Si₃N₄ ceramics increased to 754 MPa, 7.2 MPa·m^1/2, and 54 W/(m·K), respectively. According to the analysis of the growth kinetics of Si₃N₄ grains, the rapid growth of Si₃N₄ grains was ascribed to the reduction in the activation energy resulting from the introduction of β-Si₃N₄ seeds and nanophase α-Si₃N₄.     

Si3N4-ZrB2 ceramics prepared at low temperature with improved mechanical properties

Si₃N₄-ZrB₂ ceramics were hot-pressed at 1500 °C using self-synthesized fine ZrB₂ powders containing 2.0 wt% B₂O₃ together with MgO-Re₂O₃ (Re = Y, Yb) additives. Both Si₃N₄ and ZrB₂ grains in the hot-pressed ceramics were featured with elongated and equiaxed morphology. The presence of elongated Si₃N₄ and ZrB₂ grains led to the partial texture of the ceramics under the applied pressure. Vickers hardness and fracture toughness of Si₃N₄-ZrB₂ ceramics with MgO-Re₂O₃ additives prepared at low temperature were about 19–20 GPa and 9–11 MPa m^1/2, respectively, higher than the reported values of Si₃N₄-based ceramics prepared at high temperature (1800 °C or above) under the same test method.     

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.     

Toughening of silicon nitride ceramics by addition of multilayer graphene

Silicon nitride (Si₃N₄) ceramics have superior mechanical properties allowing their broad application in many technical fields. In this work, Si₃N₄-based composites with 1–5?wt% multilayer graphene (MLG) content were fabricated by spark plasma sintering at different temperatures and holding time in order to improve the fracture resistance of the Si₃N₄ ceramic. Our investigation focused on understanding the relationships between the microstructure and mechanical properties with special attention to the intergranular phases between Si₃N₄ matrix and MLG reinforcement.We have found that nanopores developed at the Si₃N₄-MLG interface due to a reaction between carbon and the oxygen available in the topmost layer of the Si₃N₄ particles. Interface porosity has an optimum for the toughening effect. In 1?wt% MLG/Si₃N₄ composites nanopores are local, but separated at the Si₃N₄-MLG interface, which promote the MLG pull-out mechanism imparting a significant toughening effect on the composite. Beyond the optimal 1?wt% MLG content, MLG platelets agglomerate and excessive porosity are developed at the Si₃N₄-MLG interfaces, leading to weaker matrix- graphene adhesion and thus lower fracture toughness.     

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.     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

Temperature-dependent mechanical and oxidation behavior of in situ formed ZrN/ZrO2-containing Si3N4-based composite

In this work, Si₃N₄ and Zr(NO₃)₄ were used as raw materials to prepare ZrN/ZrO₂-containing Si₃N₄-based ceramic composite. The processing, phase composition, and microstructure of the composite were investigated. Hardness and fracture toughness of the ceramics were evaluated via Vickers indentation in Ar at 25°C, 300°C, 600°C, and 900°C. During spark plasma sintering, Zr(NO₃)₄ was transformed into tetragonal ZrO₂, which further reacted with Si₃N₄, resulting in the formation of ZrN. The introduction of ZrN enhanced the high-temperature mechanical properties of the composite, and its hardness and fracture toughness reached 13.4 GPa and 6.1 MPa·m^1/2 at 900°C, respectively. The oxidation experiment was carried out in air at 1000°C, 1300°C, and 1500°C for 5 h. It was shown that high-temperature oxidation promoted the formation and growth of porous oxide layers. The microstructure and phase composition of the formed oxide layers were investigated in detail. Finally, it was identified that the obtained composite exhibited a higher thermal diffusivity than that of monolithic Si₃N₄ in the temperature range of 100°C–1000°C.     

High hardness and high fracture toughness B4C-diamond ceramics obtained by high-pressure sintering

The B₄C-diamond composite with high hardness and toughness was first prepared by high-pressure sintering of B₄C and diamond powders at 5 GPa and 1600 °C. The effect of the diamond fraction on the densification, microstructure and mechanical properties of B₄C-diamond composite were investigated. The results indicated that the hardness of the as-prepared composite ceramics increased gradually with the increase in diamond content. The composite having 40 vol% diamond exhibited excellent comprehensive mechanical properties with a relative density of 98.3%, a density of 2.86 g/cm³, Vickers hardness of 39.8 GPa and fracture toughness of 8.1 MPa·m^1/2. The use of superhard diamond enhanced the fracture toughness of the B₄C while maintaining its lightweight and high hardness. The main toughening mechanisms were crack bridging, crack deflection and pull-out of homogeneously dispersed diamond grains. Superhard second phase dispersion high-pressure sintering provides a new technical route to improve the properties of advanced composites.     

Graded Si3N4 ceramics with hard surface and tough core by two-step hot pressing

Graded Si₃N₄ ceramics with hard surface and tough core were prepared by two-step hot pressing with the homogenous starting composition. The inner Si₃N₄ layer was firstly hot-pressed at 1800 °C, subsequently covered with Si₃N₄ powders on both sides, and finally hot-pressed at 1600 °C. After two-step hot pressing, the resulting ceramics exhibited a zoned microstructure, differentiated by the phase assemblage of Si₃N₄ and grain size. The outer layers were well bonded to the inner layer. The outer layer exhibited bimodal and fine-grained microstructure, whereas the inner layer exhibited bimodal and coarse-grained microstructure. Vickers hardness of outer and inner layers were 18.1±0.2 GPa and 16.0±0.2 GPa, respectively, and fracture toughness were 4.2±0.1 MPa m^1/2 and 5.5±0.2 MPa m^1/2, respectively.     

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.     

Effect of ZrB2 content on phase assemblage and mechanical properties of Si3N4–ZrB2 ceramics prepared at low temperature

Based on the previous work on Si₃N₄–ZrB₂ [Wu et al. J Eur Ceram Soc;2017,37:4217], the influence of ZrB₂ addition on the phase and microstructure evolution of Si₃N₄–ZrB₂ composites was emphatically investigated, and the mechanical properties were compared with pure Si₃N₄ ceramics. It was revealed that the ratio of β‐ to (α+β)‐Si₃N₄ significantly increased from 14.3% in pure Si₃N₄ ceramics to 39.8% in Si₃N₄ with 15 vol% ZrB₂ addition, indicating that the introduction of ZrB₂ promoted α‐ to β‐Si₃N₄ phase transformation. As a consequence, the microstructure of the composite showed the bimodal distribution, containing both elongated and equiaxed Si₃N₄ grains. For the pure Si₃N₄, Vickers hardness, fracture toughness and flexural strength was 22.8 GPa, 7.6 MPa m^1/2, and 334.5 MPa, respectively. In contrast, the composite of Si₃N₄–30 vol% ZrB₂ simultaneously possessed an excellent combination of mechanical properties: 19.5 GPa in hardness, 9.8 MPa m^1/2 in toughness and 702.0 MPa in strength. Present study suggested that Si₃N₄‐based ceramics with high hardness, high toughness, and high strength could be obtained by the combination of appropriate ZrB₂ content and low hot‐pressing temperature.     

The effect of graphene nanoplatelet thickness on the fracture toughness of Si3N4 composites

Si₃N₄ composites with 3 and 5?wt% of graphene nanoplatelet (GNP) additions were prepared by spark plasma sintering. We used both commercially available GNPs and thinner few-layer graphene nanoplatelets (FL-GNPs) prepared by further exfoliation through ball milling with melamine addition. We found that by employing thinner FL-GNPs as filler material a 100% increase in the fracture toughness of Si₃N₄/3?wt% FL-GNP composites (10.5?±?0.2?MPa?m^1/2) can be achieved as compared to the monolithic Si₃N₄ samples (5.1?±?0.3?MPa?m^1/2), and 60% increase compared to conventional Si₃N₄/3?wt% GNP composites (6.6?±?0.4?MPa?m^1/2). For 5?wt% filler content the increase of the fracture toughness was near 50% for both GNP and FL-GNP fillers. The hardness of the composites decreased with increasing GNP content. However, composites reinforced with 5?wt% of FL-GNPs displayed 30% higher Vickers hardness (12.8?±?0.2?GPa) than their counterparts comprising conventional GNP fillers (9.8?±?0.2?GPa). We attribute the enhanced mechanical properties obtained with thinner FL-GNPs to their higher aspect ratio leading to a more homogeneous dispersion, higher interface area, as well as smaller pores in the ceramic matrix.