Biomorphic silicon/silicon carbide ceramics from birch powder

A novel process has been developed for the fabrication of biomorphic silicon/silicon carbide (Si/SiC) ceramics from birch powder. Fine birch powder was hot-pressed to obtain pre-templates, which were subsequently carbonized to acquire carbon templates, and these were then converted into biomorphic Si/SiC ceramics by liquid silicon infiltration at 1550 °C. The prepared ceramics are characterized by homogeneous microstructure, high density, and superior mechanical properties compared to biomorphic Si/SiC ceramics from birch blocks. Their maximum density has been measured as 3.01 g/cm³. The microstructure is similar to that of conventional reaction-bonded silicon carbide. The Vicker's hardness, flexural strength, elastic modulus, and fracture toughness of the biomorphic Si/SiC were 19.6 ± 2.2 GPa, 388 ± 36 MPa, 364 ± 22 GPa, and 3.5 ± 0.3 MPa m^1/2, respectively. The outstanding mechanical properties of the biomorphic Si/SiC ceramics are assessed to derive from the improved uniform microstructure of the pre-templates made from birch powder.     

The Role of Infiltration Temperature in the Reaction Bonding of Boron Carbide by Silicon Infiltration

The present paper is concerned on the effect of infiltration temperature on the components, microstructure, and mechanical properties of reaction‐bonded boron carbide (RBBC) ceramics. RBBC ceramics were fabricated by reactive infiltration of molten silicon (Si) into porous preforms containing boron carbide (B₄C) and free carbon. It has been found that infiltration temperatures have significant influence on the infiltration reactions involved and therefore the evolution of different phases formed in the RBBC ceramics. An increase in grain size of boron carbide particles through the coalescence of neighboring grains was observed at certain infiltration temperatures. The morphology of silicon carbide (SiC) phases developed from discontinuous and cloud‐like SiC to continuous and integrated SiC zones with the increase of infiltration temperatures. With increasing temperatures up to 1600°C, the hardness, flexural strength, and fracture toughness all increased. When the temperatures exceeded 1600°C, while the hardness and flexural strength decreased, the fracture toughness continued to increase.     

Study of Fracture Behavior of Very Fine-Grained Silicon Carbide Ceramics

Dense, fine-grained silicon carbide (SiC) ceramics were fabricated by a hot-pressing technique using pyrolyzed polycarbosilane powders. Hot-isostatic pressing treatments were also applied to some of these hot-pressed samples. The grainsize range of the obtained sintered bodies was from 0.2 to 1.4 μm, which was much finer than that of ordinary sintered SiC ceramics. Relationships among sintering conditions, microstructures, and fracture toughness of the obtained ceramics were investigated. A clear grain-size dependence of fracture toughness was observed in this very fine-grain region (0.2 to 1.4 μm). Fracture toughness showed its maximum (5.1 MPa.m^1/2) at the average grain size of ∼0.7 μm. Also, the fracture toughness of the samples having similar grain sizes increased with increasing relative density.     

High-toughness mullite ceramics fabricated by hot-press sintering of pyrophyllite and AlOOH at low temperatures

High-toughness mullite ceramics were fabricated through hot-press sintering (HPS) of pyrophyllite and AlOOH, which were wet-milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high-toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200°C for 120 min. Increasing the sintering temperature from 1000 to 1300°C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m^1/2 were obtained for samples sintered at 1300°C. Further increase of temperature to 1400°C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high-toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three-dimensional network structure obtained through HPS at a low temperature of 1300°C.     

Microstructures and mechanical properties of high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C ceramics with the addition of SiC secondary phase

The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m^1/2 and 5.24 ± 0.41 MPa·m^1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles.     

SiC纳米线增强反应烧结碳化硅陶瓷的性能研究

以单晶SiC纳米线作为增强体,碳化硅-碳为陶瓷基体,在1550℃下,采用反应烧结制备碳化硅基陶瓷复合材料(SiCnf/SiC).结合X射线衍射、万能试验机和扫描电镜等检测和分析,研究SiC纳米线对复合材料的微结构和力学性能的影响.研究表明:与未加入SiC纳米线的反应烧结碳化硅陶瓷相比,添加SiC纳米线的复合陶瓷的抗弯强度和断裂韧性都得到显著的提高,抗弯强度提高了52％,达到320 MPa(SiC纳米线含量为12wt％),断裂韧性提高了40.6％,达到4.5 MPa· m1/2(SiC纳米线含量为15wt％);反应后的SiC纳米线仍然可以保持原有的竹节状结构,且随着SiC纳米线的加入,复合陶瓷的断口可以观察到SiC纳米线拔出现象.但由于SiC纳米线“架桥”的现象,添加过量的纳米线会降低复合陶瓷的密度和限制复合陶瓷力学性能的提高.同时还讨论了SiCnf/SiC的增强机理.     

Microstructural Control for Strengthening of Silicon Carbide Ceramics

Fine-grained (<1 μm) silicon carbide ceramics with high strength were obtained by using ultrafine (∼90 nm) β-SiC starting powders and a seeding technique for microstructural control. The microstructures of the as-hot-pressed and annealed ceramics without α-SiC seeds consisted of fine, uniform, and equiaxed grains. In contrast, the annealed material with seeds had a uniform, anisotropic microstructure consisting of elongated grains, owing to the overgrowth of β-phase on α-seeds. The strength, the Weibull modulus, and the fracture toughness of fine-grained SiC ceramics increased with increasing grain size up to ∼1 μm. Such results suggested that a small amount of grain growth in the fine grained region (<1 μm) was beneficial for mechanical properties. The flexural strength and the fracture toughness of the annealed seeded materials were 835 MPa and 4.3 MPa·m^1/2, respectively.     

Interpenetrating Al2O3-TiAl3 alloys produced by reactive infiltration

Dense alumina-TiAl₃ composites with interpenetrating networks have been fabricated by reactive gas-pressure infiltration and squeeze casting of Al into sintered porous preforms containing 30 vol% TiO₂ and 70 vol% Al₂O₃. Strength of up to 543±21 MPa with corresponding fracture toughness of 8·6±0·4 MPa√m and hardness of H_V10=565±27 have been obtained. The present paper discusses processing parameters such as particle size of oxide precursor and preform porosity which control microstructural development and mechanical properties of the composites.     

Reactive Melt Infiltration Processing of Biomorphic Si–Mo–C Ceramics from Wood

Different kinds of natural wood specimens (beech, pine, and rattan) were converted to biomorphic, microcellular Si–Mo–C ceramics by reactive melt infiltration processing. During processing at 1500°C under vacuum the carbon of the pyrolyzed native preforms reacted with Si of the infiltrating Si–Mo eutectic melt (90.4 wt% Si). The resulting materials have fractional densities of >90%, and contained silicon carbide (SiC), MoSi₂, in addition to unreacted Si and carbon. The residual carbon remains covered by SiC. The conversion process retains the microcellular morphology of the parent wood tissue. Depending on the initial morphology of the different kinds of wood, reactive melt infiltration processing of Si–Mo–C ceramics can be used to manufacture light weight materials with low density but adequate strength for wide ranging applications where anisotropic behavior is required.     

黄杨木制备SiC木质陶瓷的性能与表征

SiC木质陶瓷是近年来应用前景广阔的新型陶瓷材料,以绿色可再生的木材为原材料,通过反应烧结工艺制备出的陶瓷具有优良的高温力学性能。为探究影响生物质陶瓷性能的因素,将黄杨木在800 ℃氮气保护下热解形成生物质炭坯,然后在1 650 ℃和1 900 ℃两种高温下进行熔融渗硅制备SiC木质陶瓷。利用X射线衍射仪(XRD)和扫描电子显微镜(SEM)研究SiC木质陶瓷的物相组成和微观结构,采用阿基米德排水法和三点弯曲法测定陶瓷的开孔率、密度和弯曲强度,再使用维氏硬度计测定SiC木质陶瓷的显微硬度。研究结果表明:在1 650 ℃下通过熔融渗硅可以得到微观结构均匀的SiC木质陶瓷,在1 650 ℃比在1 900 ℃下熔融渗硅制备陶瓷的力学性能更优异,陶瓷的密度更大,为2.27 g/cm³,此时弯曲强度为192.45 MPa;游离Si会提高SiC木质陶瓷的密度,增强陶瓷的弯曲强度。     

Pressureless Sintering of ZrB2–SiC Ceramics

A pressureless sintering process was developed for the densification of zirconium diboride ceramics containing 10–30 vol% silicon carbide particles. Initially, boron carbide was evaluated as a sintering aid. However, the formation of a borosilicate glass led to significant coarsening, which inhibited densification. Based on thermodynamic calculations, a combination of carbon and boron carbide was added, which enabled densification (relative density >98%) by solid-state sintering at temperatures as low as 1950°C. Varying the size of the starting silicon carbide particles allowed the final silicon carbide particle morphology to be controlled from equiaxed to whisker-like. The mechanical properties of sintered ceramics were comparable with hot-pressed materials with Vickers hardness of 22 GPa, elastic modulus of 460 GPa, and fracture toughness of ∼4 MPa·m^1/2. Flexure strength was ∼460 MPa, which is at the low end of the range reported for similar materials, due to the relatively large size (∼13 μm long) of the silicon carbide inclusions.     

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65 μm, 33.70 μm and 63.35 μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74 kg/mm² to 1674.90±100.00 kg/mm² and fracture toughness drops from 5.76±0.26 MPa m^1/2 to 3.4±0.37 MPa m^1/2. However, flexural strength decreases from 403.41±5.70 MPa to 256.15±25.05 MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.     

Fabrication and Fracture Behavior of Novel SiC Ceramics Having Rodlike Grains

A new type of silicon carbide having rodlike grains has been fabricated by hot-pressing or hot-pressing followed by hot isostatic pressing using SiC whiskers as the raw powder. Preliminary relationships between sintering conditions and microstructure and fracture toughness of the processed ceramics were established. Ceramics with relative density greater than 98% were prepared at temperatures of 2050°C or greater. The fracture toughness of these ceramics was better than that of ordinary SiC, and its maximum value was 7.3 MPa · m^1/2. Grain pullout, grain bridging, and crack deflection were considered to be the main operative mechanisms which led to improved fractrue toughness.     

Fabrication and characterization of a new-style structure capillary channel in reaction bonded silicon carbide composites

A new-style structure capillary channel was fabricated by using boron carbide powder mixtures with an appropriate multimodal particle size distribution to promote the liquid silicon infiltration in reaction bonded silicon carbide composites. Two types of core–rim structure were observed and the secondary SiC produced in the siliconisation reaction existed in two forms: nucleating on the original SiC and occupying the original positions of the residual silicon. The size of the secondary SiC in the latter form was in a range of tens to hundreds nanometers. These nano-sized SiC grains and the additive of fine boron carbide particles refined the crystalline structure and broke up the residual silicon phase into small pieces. Using this method, the microstructure was refined and the mechanical properties improved significantly. The lowest residual silicon volume fraction was 4.0% and the flexural strength and fracture toughness reached peak values of 526 ± 21 MPa and 6.2 ± 0.4MPa m^1/2, respectively.     

Three-Dimensional Carbon/Silicon Carbide Composites Prepared by Chemical Vapor Infiltration

Continuous-carbon-fiber-reinforced silicon carbide composites (C/SiC) were prepared by chemical vapor infiltration in which the preforms were fabricated with the three-dimensional braid method. The mechanical properties and microstructures were investigated. For the composites with no interfacial layer, flexural strength and fracture toughness increased with density of the composites, and the maximum values were 520 MPa and 16.5 MPa·m^1/2, respectively. The fracture behavior was dependent on the interfacial bonding between fiber/matrix and fiber bundle/bundle which was determined by the density of the composites. Heat treatment had a significant influence on the mechanical properties and fracture behavior. The composites with pyrolysis interfacial layers exhibited characteristic fracture and relatively low strength (300 MPa).     

Fabrication and mechanical properties of nacre-like alumina with addition of silicon nitride

How to deal with the brittleness of ceramic materials is always one of the hot topics in the field of materials science. Design of layered ceramics with textured structure is one of the effective methods to improve their fracture toughness. The introduction of additives as interlayer phases can balance fracture toughness and flexural strength. However, the research about addition of interlayer phases and their mechanisms in the layered ceramics is still limited. In this work, nacre-like alumina ceramics were successfully fabricated by freeze casting followed by hot pressing. Silicon nitride was incorporated as the interlayer phase, and the effect on the mechanical properties of the nacre-like alumina was investigated. The addition of silicon nitride was beneficial to improvement of interlayer bonding between the alumina layers due to formation of sialon phase, leading to increase of flexural strength but decease of fracture toughness. When the content of silicon nitride was 3.3 wt%, flexural strength and fracture toughness of the sample was 468 MPa and 6.2 MPa m^1/2, respectively. Compared with the sample without silicon nitride, the flexural strength was enhanced significantly. Additionally, both flexural strength and fracture toughness were improved as compared the sample without any additives. This work can provide available references for design and fabrication of high-strength and high-toughness ceramics by properly tuning the layer structure and interlayer phase composition.     

Fabrication of biomorphic SiC composites using wood preforms with different structures

Biomorphic SiC composites were fabricated from wood, including high-density compressed cedar, high-density fiberboard (HDF) and low-density paulownia followed by the fabrication of a preform and liquid silicon infiltration (LSI) process. The degree of molten silicon infiltration was strongly dependent on the cell wall thickness and pore size of the carbon preform. The mechanical properties of the biomorphic SiC composites were characterized by compressive tests at room temperature, 1000 °C and 1200 °C, and the relationship between the mechanical properties and the microstructural characteristics was analyzed. The compressive strength of the biomorphic composites was found to be strongly dependent on their bulk density and decreased as the test temperature increased to 1200 °C. Strength reduction in the biomorphic SiC composites occurred due to the deformation of the remaining Si at elevated temperatures under ambient atmospheric conditions.     

Effects of heat treatment on mechanical and dielectric properties of 3D Si3N4f/BN/Si3N4 composites by CVI

In this study, three-dimensional silicon nitride fiber-reinforced silicon nitride matrix (3D Si₃N_4f/BN/Si₃N₄) composites with a boron nitride (BN) interphase were fabricated through chemical vapor infiltration. Through comparing the changes of microstructure, thermal residual stress, interface bonding state, and interface microstructure evolution of composites before and after heat treatment, the evolution of mechanical and dielectric properties of Si₃N_4f/BN/Si₃N₄ composites was analyzed. Flexural strength and fracture toughness of composites acquired the maximum values of 96 ± 5 MPa and 3.8 ± 0.1 MPa·m^1/2, respectively, after heat treatment at 800 °C; however, these values were maintained at 83 ± 6 MPa and 3.1 ± 0.2 MPa·m^1/2 after heat treatment at 1200 °C, respectively. The relatively low mechanical properties are mainly attributed to the strong interface bonding caused by interfacial diffusion of oxygen and subsequent interfacial reaction and generation of turbostratic BN interphase with relatively high fracture energy. Moreover, the Si₃N_4f/BN/Si₃N₄ composites also displayed moderate dielectric constant and dielectric loss fluctuating irregularly around 5.0 and 0.04 before and after heat treatment, respectively. They were mainly determined based on the intrinsic properties of materials system and complex microstructure of composites.     

Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

Reduced-graphene-oxide-reinforced boron carbide ceramics fabricated by spark plasma sintering from powder mixtures obtained by heterogeneous co-precipitation

Reduced graphene oxide (rGO) sheets were uniformly dispersed in boron carbide ceramics by a heterogeneous co-precipitation method. This approach was used to improve the fracture toughness of boron carbide ceramics and to address the problem of agglomeration of graphene in the boron carbide matrix. Cetyltrimethyl ammonium bromide was used as a heterogeneous co-precipitation reaction initiator to prepare a homogeneously dispersed graphene oxide/boron carbide (GO/B₄C) mixture. Reduced graphene oxide/boron carbide (rGO/B₄C) powder mixtures with good dispersion were obtained by high temperature heat treatment. Dense rGO/B₄C composite ceramics were fabricated by spark plasma sintering at 1800 °C and 50 MPa. The fracture toughness and flexural strength of the rGO/B₄C with an rGO content of 2 vol% composite increased by 42% (from 3.43 to 4.88 MPa·m^1/2) and 28% (from 372 to 476 MPa) compared with those of pure B₄C, respectively. The markedly improved fracture toughness and flexural strength of the boron carbide ceramics were attributed to the effect of crack bridging and crack deflection by graphene sheets, graphene interface sliding, and pulling out of graphene.