Microstructure and fracture-mechanical properties of carbon derived Si3N4 + SiC nanomaterials

The microstructure and basic mechanical properties, as hardness, fracture toughness, fracture strength and subcritical crack growth at room temperature were investigated and creep behavior at high temperatures was established. The presence of SiC particles refined the microstructure of Si₃N₄ grains in the Si₃N₄ + SiC nanocomposite. Higher hardness values resulted from introducing SiC nanoparticles into the material. A lower fracture toughness of the nanocomposite is associated with its finer microstructure; crack bridging mechanisms are not so effective as in the case of monolithic Si₃N₄. The strength value of the monolithic Si₃N₄ is higher than the characteristic strength of nanocomposites. Fractographic analysis of the fracture surface revealed that a failure started principally from an internal flaw in the form of cluster of free carbon, and on large SiC grains which degraded strength of the nanocomposite. The creep resistance of nanocomposite is significantly higher when compared to the creep resistance of the monolithic material. Nanocomposite exhibited no creep deformation, creep cracks have not been detected even at a test at 1400 °C and a long loading time, therefore the creep is probably controlled mainly by diffusion. The intergranular SiC nanoparticles hinder the Si₃N₄ grain growth, interlock the neighboring Si₃N₄ grains and change the volume fraction, geometry and chemical composition of the grain boundary phase.     

Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

Preparation of Si3N4-SiC composites by microwave route

Si₃N₄-SiC composites have been microwave sintered using β-Si₃N₄ and β-SiC as starting materials. Si₃N₄ rich compositions (95 and 90 vol.% Si₃N₄) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si₃N₄ limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m^1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si₃N₄ matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Mechanical properties and microstructure in sintered and HIPed SiC particle/Si3N4 composites

Pressureless sintered (PLS) and gas-pressure sintered (GPS) Si₃N₄, PLS and GPS SiC particle/Si₃N₄ composites, and PLS + HIP and GPS + HIP SiC particle/Si₃N₄ composites were produced. Investigation of their mechanical properties showed that PLS + HIP and GPS + HIP composites, containing SiC particles in the beta-silicon nitride grains, yield higher bending strength, although its fracture toughness remains at the same level. This is attributed to the fact that the added SiC particles inhibit excessive growth of beta-Si₃N₄ grains without changing the fracture behaviour. However, this investigation also found precipitation during the reaction between SiC and nitrogen in gas pressure sintering, resulting in a low Young's modulus and low density in the GPS composite.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Formation Mechanism of Microstructure of Fe2(MoO4)3/Si3N4 Composite Powder by Hydrogen Reduction

观测Fe₂(MoO₄)₃和Fe₂(MoO₄)₃/Si₃N₄粉末H₂还原后的微结构特征, 研究了其微观组织结构的演变。 结果表明: Fe₂(MoO₄)₃还原后转变为20 nm厚的Fe薄层包覆Mo颗粒的微结构; Fe₂(MoO₄)₃/Si₃N₄粉末被还原后转变为两种结构形式颗粒粉末, 一种为3--5 nm的薄层Fe包覆在Mo颗粒表面粉末, 一种为粘附有纳米Fe--Mo氮化物、Si、Mo等颗粒的Si₃N₄粉末。Fe₂(MoO₄)₃/Si₃N₄粉末还原后形成这种微结构的原因是, 在还原过程中同时发生了两种反应: 一种是Fe₂(MoO₄)₃自身发生分解还原反应, 另一种是Fe₂(MoO₄)₃与Si₃N₄颗粒表面发生反应。     

Influence of SiC, Si3N4 and Al2O3 particles on the structure and properties of electrodeposited Ni

The structure and properties of electrodeposited nickel composites reinforced with inert particles like SiC, Si₃N₄ and Al₂O₃ were compared. A comparison was made with respect to structure, morphology, microhardness and tribological behaviour. The coatings were characterized with optical microscopy, Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) technique. The cross-sectional microscopy studies revealed that the particles were uniformly distributed in all the composites. However, a difference in the surface morphology was revealed from SEM studies. The microhardness studies revealed that Si₃N₄ reinforced composite showed higher hardness compared to SiC and Al₂O₃ composite. This was attributed to the reduced crystallite size of Ni — 12 nm compared to 16 nm (SiC) and 23 nm (Al₂O₃) in the composite coating. The tribological performance of these coatings studied using a Pin-on-disk wear tester, revealed that Si₃N₄ reinforced composite exhibited better wear resistance compared to SiC and Al₂O₃ composites. However, no significant variation in the coefficient of friction was observed for all the three composites.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si₃N₄) with lithium yttrium oxide (LiYO₂) and zirconia (ZrO₂) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si₃N₄ grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si₃N₄ grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.     

Microstructure and mechanical properties of Si3N4–TiC nanocomposites

Abstract

Si₃N₄–TiC nanocomposites are fabricated by hot press sintering from silicon nitride nanopowders and ultrafine TiC powders. The microstructure and mechanical properties are analysed and discussed. Scanning electron microscopy images show that the microstructure consists of equiaxed grains and grain boundary phase. The TiC added as a dispersed phase reacts with the nitrogen from Si₃N₄ during the liquid phase sintering, with the formation of TiC_0.7 N_0.3 , trace of SiC and N₂. The adding of a proper amount of TiC powders increases the flexural strength and has little influence on fracture toughness. The hardness increases with increasing TiC content.     

Crack-healing behaviour and resultant high-temperature fatigue strength of machined Si3N4/SiC composite ceramic

The crack‐healing behaviour of machining cracks in Si₃N₄/20 wt% SiC composite was investigated. The machining cracks were introduced by a heavy machining process, during the creation of a semicircular groove. The machined specimens were healed at various temperatures and times in air. The optimized crack‐healing condition of the machined specimens was found to be a temperature of 1673 K and a time of 10 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens healed. Moreover, the bending strengths and the fatigue limits of the machined specimens healed were systematically investigated at temperatures from room temperature to 1673 K. The machined specimens healed at the optimized condition exhibited an almost constant bending strength (～700 MPa) up to 1673 K. Also, the specimens exhibited considerably high cyclic and static fatigue limits at temperatures from 1073 to 1573 K. These results demonstrated that the crack‐healing could be an effective method for improving the structural integrity and reducing machining costs of the Si₃N₄/SiC composite ceramic.     

Processing and mechanical properties of short fibers/Si3N4p reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Dense short silicon carbide (SiC_sf) and carbon fibers (C_sf) reinforced BaAl₂Si₂O₈ (BAS) glass-ceramic composites with silicon nitride were fabricated by hot-pressing technique. The phase characterization, microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and three-point bending tests. The results showed that short silicon carbide and carbon fibers disperse homogeneously in BAS matrix, and had good chemical compatibility with the glass-ceramic matrix. The addition of Si₃N₄ could successfully eliminate the microcracks in the BAS matrix induced by the thermal mismatch between the fiber and matrix. Both the added short fibers could effectively reinforce the BAS glass-ceramic by the associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects.     

R-Curve Behavior of Si3N4/BNNT Composites

The crack propagation rsistance behavior of Si₃N₄ ceramics reinforced by boron nitride nanotubes (BNNT) has been discussed in the work. And, bending strength and fracture toughness of Si₃N₄/BNNT composites were tested by three point bending method. It is shown that crack propagation resistance of BNNT/Si₃N₄ composites is increased distinctly owing to addition of BNNT. It is attributed to the pinning and bridging roles of BNNT. One kind of mathematical model was constructed for calculating crack propagation resistance of Si₃N₄ ceramics and BNNT/Si₃N₄ composites. Crack resistance curve (R-Curve) of Si₃N₄ ceramics and BNNT/Si₃N₄ composites was also calculated. Crack propagation of them was simulated using finite element methods. The results show that strong shielding area is formed in crack tip owing to existence of BNNT and crack propagation is prevented by strong stress shielding roles.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

纳米SiC及Si3N4/SiC的高温等静压研究

采用高温等静压工艺，制备了纳米结构的单相ＳｉＣ及Ｓｉ３Ｎ４／ＳｉＣ复相陶瓷，并通过Ｘ射线衍射分析透射有高分辨电镜对其相组成及结构进行了表征。实验表明，在温度１８５０℃，压力２００ＭＰａ条件下保温１ｈ，要获得晶粒尺寸〈１００ｎｍ，结构均匀，致密的单相ＳｉＣ纳米结构陶瓷。     

Mechanical and electromagnetic wave absorption properties of C_f-Si_3N_4 ceramics with PyC/SiC interphases

Aiming to obtain microwave absorbing materials with excellent mechanical and microwave absorption properties, carbon fiber reinforced Si_3N_4 ceramics(Cf-Si_3N_4) with pyrolytic carbon(PyC)/SiC interphases were fabricated by gel casting. The influences of carbon fibers content on mechanical and microwave absorption properties of as-prepared Si_3N_4 based ceramics were investigated. Results show that chemical compatibility between carbon fibers and Si_3N_4 matrix in high temperature environment can be significantly improved after introduction of Py C/SiC interphases. As carbon fibers content increases from 0 to 4 wt%, flexural strength of Si_3N_4 based ceramics decreases slightly while fracture toughness obviously increases. Moreover, both the real and imaginary parts of complex permittivity increase with the rising of carbon fibers content within the frequency range of 8.2–12.4 GHz. Investigation of microwave absorption shows that the microwave attenuation ability of Cf-Si_3N_4 ceramics with Py C/SiC interphases is remarkably enhanced compared with pure Si_3N_4 ceramics. Effective absorption bandwidth(-10 d B) of10.17–12.4 GHz and the minimum reflection less of-19.6 d B are obtained for Si_3N_4 ceramics with 4 wt%carbon fibers in 2.0 mm thickness. Cf-Si_3N_4 ceramics with Py C/SiC interphases are promising candidates for microwave absorbing materials with favorable mechanical property.     

Mechanical Properties and Thermal Shock Resistance Analysis of BNNT/Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Composites

BNNT/Si₃N₄ ceramic composites with different weight amount of BNNT fabricated by hot isostatic pressing were introduced. The mechanical properties and thermal shock resistance of the composites were investigated. The results showed that BNNT-added ceramic composites have a finer and more uniform microstructure than that of BNNT-free Si₃N₄ ceramic because of the retarding effect of BNNT on Si₃N₄ grain growth. The addition of 1.5 wt.% BNNT results in simultaneous increase in flexural strength, fracture toughness, and thermal shock resistance. The analysis of the results indicates that BNNT brings many thermal transport channels in the microstructure, increasing the efficiency of thermal transport, therefore results in increase of thermal shock resistance. In addition, BNNT improves the residual flexural strength of composites by crack deflection, bridging, branching and pinning, which increase the crack propagation resistance.     

Fabrication and mechanical properties of Al2O3/Ti(C0.7N0.3) nanocomposites

Two kinds of Al₂O₃/Ti(C_0.7N_0.3) nanocomposites were fabricated with traditional hot pressed sintering and repetitious-hot-pressing technology, one is added with nano-scale SiC, and the other is without SiC. The results showed that the mechanical properties of the former are higher than that of the latter, especially the fracture toughness can reach up to 8.3 MPa m^1/2. Although the fracture toughness remains high, repetitious-hot-pressing results in the reduction of flexural strength. The improvement of the mechanical properties is interpreted from the different microstructure and fracture mode. The microstructure shows that the addition of nano-scale Ti(C_0.7N_0.3) and nano-scale SiC lead to the refinement of matrix grain, and the inter/intragranular microstructure can be formed instead of the intergranular microstructure in monolithic alumina. The higher fracture toughness resulted mainly from the transgranular fracture mode.