Silicon carbide particle reinforced mullite composite foams

Silicon carbide particle reinforced mullite composite foams were produced by the polymer replica method using alumina and kaolin to form in situ mullite matrix. Up to 20 wt.% silicon carbide particles (SiCp) were added to aqueous ceramic slurry to explore its effect on the rheological behaviour of ceramic slurries and also properties of as sintered products. By means of solid loading optimisation and sintering enhancement by silicon carbide, mullite based ceramic composite foams of higher strength were obtained. The strength of the as sintered foams was found to depend greatly on the phase composition, relative density of the structures and the amount of SiCp addition. By studying the effect of the additive concentration, on the mechanical properties of the ceramic matrix, it is found that the optimal silicon carbide addition is 20 wt.%.     

Reaction sintering of alumina/mullite nanocomposites from nano-sized starting powders

Alumina/mullite ceramic nanocomposites were prepared by the mixtures of nano-sized starting powders of alumina with silica and alumina with silicon carbide. Silica from deliberate addition and as the product of silicon carbide oxidation reacted completely with alumina to form mullite. Silica from direct addition segregated at the grain boundary and intergranular mullite was formed whereas silica from oxidation was surrounded by alumina matrix and intragranular mullite was formed after reaction sintering. The most significant difference was fracture behaviour where intragranular mullite nanoparticles promoted transgranular fracture in alumina matrix due to thermal mismatch around nanoparticles and intergranular mullite nanoparticles gave rise to intergranular fracture similar to pure alumina. Wear resistance of the nanocomposites was better than that of alumina. Pull-out formation in the nanocomposites was less and pull-out size was also smaller. Fracture toughness of the nanocomposites was significantly higher than that of alumina.     

Effects of Current Confinement on the Spark Plasma Sintering of Silicon Carbide Ceramics

Despite numerous works have recently reported the densification of silicon carbide (SiC) ceramics using the Spark Plasma Sintering (SPS) technique, the effect of the localized electric current flow near the specimen over the liquid phase sintering process of SiC ceramics and on their microstructural features has not been completely addressed. In the present work, two different SPS setups affecting current flow are selected, one based on the ordinary die/punch setup configuration, and the other employing a BN electrically insulating coating on the inner wall of the die to force the electric current to locally flow through the inner graphite foil in contact with the ceramic compact. The effective electrical resistance and the energy consumed during the SPS runs for both setups, as well as the sintering behavior, microstructure, and mechanical properties of the SPSed materials are analyzed. The BN die coating considerably increases the effective resistance of the system, decreases the power consumption, and accelerates the SiC densification. Besides, ceramic specimens experience significantly higher real temperatures than the set values and, accordingly, coarser microstructures and tougher materials than those for the ordinary setting are produced. The thermoelectrical properties of SiC materials are proposed as fundamental in their SPS process, especially when electrical current is forced through the inner part of the SPS setting around the specimen.     

The thermal shock behaviour of ductile particle toughened alumina composites

Over recent years, it has been established that the incorporation of metallic particles into a ceramic matrix can lead to enhanced fracture properties. Relatively few attempts, however, have been made to establish whether or not the improved fracture toughness typically observed in such composite systems can offer improved performance in demanding environments. The current study is concerned with the thermal shock behaviour of a ceramic matrix composite consisting of an alumina matrix containing 20 vol% of discrete iron particles. The composite material has been produced by both hot pressing and conventional sintering techniques. The hot pressed composite shows a greater resistance to thermal shock than the monolithic matrix, both in terms of the critical temperature differential and retained strength, whereas the sintered material has been found to behave as a typical low strength refractory ceramic. The calculation of thermal shock resistance parameters for the composites and the monolith has indicated possible explanations for the differences in thermal shock behaviour.     

陶瓷-硬质合金复合刀片材料的力学性能与界面结构

采用热压烧结工艺制备了刀具用陶瓷－硬质合金复合片．复合片上层为具有一定厚度的Ａｌ２Ｏ３／ＴｉＢ２陶瓷材料,下层为ＷＣ／Ｃｏ硬质合金．结果表明：复合片综合了陶瓷材料的高硬度和硬质合金的高强度,其性能介于陶瓷材料和硬质合金之间．Ｘ射线衍射、电子探针和扫描电子显微镜分析表明：烧结过程中硬质合金和陶瓷材料中的元素在界面相互扩散,并有新相形成,增加了界面结合强度,其界面为化合物型和扩散型的混合．     

Intragranular nanocomposite powders as building blocks for ceramic nanocomposites

A powder-based bottom-up processing scheme is introduced for the production of ceramic nanocomposites. Internal displacement reactions between solid solution powders and metallic reactants proceeding via gaseous intermediates are utilized to generate nanostructured building blocks for the synthesis of ceramic nanocomposites. Subsequent rapid sintering results in ceramic nanocomposites, whose microstructures are inherited from the building blocks. This processing scheme is demonstrated for the production of titanium carbide nanocomposites featuring up to 28 wt.% intragranular tungsten inclusions derived from titanium-tungsten mixed carbide powders. Heat treatment of mixed carbide powders in evacuated ampoules containing titanium sponge and iodine at 1000°C for 24 h resulted in nanocomposite powders featuring tungsten precipitates within titanium carbide grains that were subsequently consolidated via spark plasma sintering at 1300°C for 10 min to produce titanium carbide/metallic tungsten nanocomposites. Transformation of mixed titanium–tungsten carbide powders to titanium carbide/metallic tungsten nanocomposite powders was analyzed via X-ray diffraction. Electron microscopy observations of microstructures pre- and post- sintering showed that the intragranular character of nanocomposite powders can be retained in sintered ceramic nanocomposites. The building block approach demonstrated in this work represents an improved method to make ceramic nanocomposites with majority intragranular character.     

Effect of Fe content on the mechanical properties of a regenerated cemented carbide

Impurity elements such as Fe, Ni, Zn, and Ca introduced in the recycling process of regenerated cemented carbide affect the structure and mechanical properties of the regenerated cemented carbide. Unlike other impurity elements, Fe and Co belong to group VIII elements and have similar chemical and physical properties. The nature of Fe is closely related to the size and type of waste cemented carbide during recycling and pollution observed during production. Waste cemented carbide is processed via zinc melting to obtain WC, and the regenerated YG10 cemented carbide was prepared via low-pressure sintering in this study. The effects of impurity element Fe on the microstructure and mechanical properties of a regenerated cemented carbide were studied via X-ray diffraction, scanning electron microscopy, magnetic property tests, and mechanical property tests. This study demonstrates that an increase in the Fe content leads to a decrease in the bending strength and hardness; further, the mechanism of grain growth caused by the increase in Fe content is revealed. In addition, the mechanism of decreasing wear resistance of the regenerated YG10 cemented carbide with increasing Fe content was discussed. The introduction of Fe as an impurity has evident adverse effects on the mechanical properties of the alloy. To satisfy the production requirements, the Fe content in the mixed powder must to be retained within 0.5%.     

Complex mullite structures fabricated via digital light processing of a preceramic polysiloxane with active alumina fillers

By taking advantage of the multi-functional properties of preceramic polymers, their transformation into ceramic material at low sintering temperatures and the processing capabilities of polymer manufacturing processes, mullite components were fabricated by additive manufacturing. A photocurable silicone preceramic polymer resin containing alumina particles was shaped into complex structures via Digital Light Processing. Dense and crack-free, highly complex porous mullite ceramics were produced by firing a mixture of a commercially available photosensitive polysiloxane as the silica source, containing alumina powder as active filler, in air at a low sintering temperature (1300 °C). In particular, the developed formulations, coupled with the additive manufacturing approach, allow for precise control of the architecture of the porous ceramic components, providing better properties compared to parts with stochastic porosity.     

Fabrication and characterisation of cellular alumina articles produced via radiation curable dispersions

A general and versatile method for the production of cellular materials from radiation curable solvent-free colloidal ceramic dispersions containing pore formers has been developed. By this technique cellular ceramic articles with a precisely controlled porosity, cell size and shape are obtained for compositions containing solid pore formers. Monolithic bulk samples are obtained by thermal curing, whereas thin films and multi-layered articles are advantageously produced by UV curing. In this work the influence of three different spherical pore former types, PE, PS and PMMA, on the processing and final properties of the porous materials using alumina as model material is studied. The effect of pore former type and concentration on rheology, curing behaviour, debinding and sintering steps as well as thermal conductivity and mechanical strength of the sintered cellular materials is presented. It is also shown that the choice of pore former type modifies the sintering behaviour and resulting properties.     

Pressureless sintering of multilayer graphene reinforced silicon carbide ceramics for mechanical seals

ABSTRACT

The silicon carbide (SiC) ceramics containing multilayer graphene derived from graphite exfoliation were successfully prepared by pressureless sintering, and the effect of graphene content on the sintering behaviours, microstructure, mechanical, tribological, electrical and thermal properties was investigated in detail. The bulk density, bending strength and hardness of the composite ceramics gradually decrease with the increase of graphene content, but the friction, conductance and thermal conductance properties are improved obviously. When the graphene content reaches 5?wt-%, the dry friction coefficient of 0.22, electrical conductivity of 2724.14 S?1?m?1 and thermal conductivity of 8.5?W?(m?1?K?1) can be obtained, indicating good comprehensive mechanical, tribological, electrical and thermal properties. This multilayer graphene reinforced silicon carbide ceramic is a promising seal material instead of SiC seal materials containing graphite to be applied in next-generation mechanical seals.     

红柱石基刚玉-莫来石复相陶瓷的显微结构及性能

以红柱石颗粒为主要骨料,辅以莫来石颗粒和刚玉颗粒,硅微粉、铝微粉为基质料,经混合、困料及成型后,经不同温度下烧成4h,制得莫来石基刚玉-莫来石复相陶瓷,分析了烧成温度对复相陶瓷的物相组成、显微结构、烧成性能、力学性能及热学性能。结果表明:红柱石在高温下转化成针状和柱状莫来石改善复相陶瓷的烧成性能和抗热震性能;在1480℃烧成时,红柱石刚玉-莫来石复相陶瓷具有优越性能,其抗折强度为15.4MPa,耐压强度为91.6MPa,热膨胀系数为5.5×10-6/K,1100℃下水冷的抗热震次数达到99次。     

Composites from WC powders sputter-deposited with iron rich binders

Composite powders of tungsten carbide (WC) and iron rich binder were prepared by an innovative approach, which consists in sputtering of a metallic binder on the tungsten carbide particles. The phase composition, microstructure and mechanical behaviour of WC coated powder composites with binder contents from 6 to 9 wt.% were characterized. η-Phase is early formed during sintering and its effect on the mechanical behaviour was investigated and related to the microstructure and atomic structure. The results show that the presence of η-phase has not a hazardous role in toughness as is previewed in conventional cemented carbide. Despite the presence of η-phase, a good compromise between toughness and hardness was attained in composites prepared from iron rich binders sputtered on WC powders.     

Feasibility of manufacturing of Al2O3–Mo HTCC by hybrid additive process

Additive manufacturing processes make it possible to produce increasingly complex 3D parts. In addition, these numerical processes can be usefully used to manufacture ceramic/metal parts of high dimensional resolution with thermal, electrical and electronic properties of interest for applications in the field of power electronics.In this context, a hybrid additive machine was developed to manufacture ceramic/metal parts. This machine consists in the combination of two additive manufacturing processes: stereolithography and robocasting.Using this hybrid process, the feasibility of HTCC components has been demonstrated by building dielectric alumina by stereolithography and molybdenum conductive network by robocasting. Molybdenum-based metallic formulation adapted to the process and allowing to obtain a high conductive metallic network has been developed. The co-debinding and co-sintering cycles have been optimized to minimize the content of residual carbon and to prevent the oxidation of molybdenum. The alumina/molybdenum interface has also been observed to conclude about a possible delamination between these two materials with different thermal expansion coefficients (CTE). Sintered HTCC parts have been characterized in the domain of hyperfrequency. The frequency responses deviate from the simulation due to a lack of dimensional accuracy of the metallic network.     

ρ-Al2O3添加量对原位反应结合碳化硅膜支撑体性能的影响

以黑色碳化硅为骨料,添加不同含量的ρ-Al2O3,在1430℃、无压条件下,用碳化硅表面氧化产生的SiO2与Al2O3反应制备多孔碳化硅膜支撑体,研究了ρ-Al2O3添加量对碳化硅支撑体粘结相组成及性能的影响。结果表明,ρ-Al2O3添加量由3wt%增加到15wt%时,试样烧结后粘结相中莫来石相增多,石英相减少但不能完全消除,支撑体的显气孔率降低,孔径减小,透气度由1127.8 m3?cm/(m2?h?kPa)下降到210.4 m3?cm/(m2?h?kPa);支撑体抗弯强度先增大后降低,ρ-Al2O3添加量为9wt%时,支撑体抗弯强度为25.1 MPa,透气度为372.7 m3?cm/(m2?h?kPa),支撑体的综合性能满足高温含尘气体过滤的要求。     

Sintering and Mechanical Properties of Mullite-Reinforced Boron Carbide Matrix Composite

In order to improve the mechanical properties of boron carbide (B₄C) ceramic, a mullite-reinforced B₄C matrix ceramic with complete densification was fabricated via hot pressing for the first time. The dense sintering mechanism of mullite-reinforced B₄C ceramic was discussed through the phase and element analysis. A new dense sintering mechanism was found in which the diffusion of Si in mullite through the B₄C matrix enhances the sintering of mullite-reinforced B₄C ceramics effectively. The mechanical properties and microstructure of the composite ceramics were investigated in contrast with monolithic B₄C and one kind of commercial B₄C ceramic. The flexural strength and fractural toughness of B₄C with 3 vol% mullite addition reached 560 MPa and 3.33 MPa·m^1/2, which is 154% and 96% higher than that of monolithic B₄C, respectively.     

Nondestructive Evaluation of Thermal Barrier Coatings Using Impedance Spectroscopy

This article reviews previous studies on nondestructive evaluation of thermal barrier coatings (TBCs) using impedance spectroscopy (IS). IS or electrochemical impedance spectroscopy has been widely used to measure the electrical properties of materials and electrochemical behavior at electrode/electrolyte interfaces. TBCs, which comprise metallic and ceramic multilayers, have been widely used in the hot section of aeroturbine engines to increase turbine efficiency and to extend the life of metallic components. Since 1999, IS has been developed to examine degradation of the TBCs as a nondestructive evaluation tool, which is critical for prediction of TBCs lifetime during service. IS has been used both at high temperature in dry environments and in aqueous solutions. Impedance spectra of TBCs reflect change in TBC thickness, porosity, cracks, sintering, and yttria-stabilized zirconia phase transformation. Meanwhile, impedance measurements indicate the thermally grown oxide growth and the failure in TBCs. In addition, the thermal conductivity of TBCs can be correlated to impedance measurement results.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.     

Effects of SiC addition on the structure and properties of reticulated porous mullite ceramics

Reticulated porous mullite ceramics (mullite RPCs) were produced by the polymer replica method using alpha alumina and kaolin to form in situ mullite. Up to 30% particulate silicon carbide (SiCp) was added to ceramic mixtures to explore its effect on the structure and properties of the sintered products. The ceramic slurry was coated onto polyurethane sponge using preset roller method, then recoating was applied by the spray coating technique. The strength of the mullite RPCs was found to depend greatly on the phase composition of the structures and on the amount of SiCp addition.     

In situ mullite foam fabrication using microwave energy

In situ mullite ceramic foams were fabricated using polymeric sponge replication method from ceramic slurry containing alpha alumina and kaolin mixtures. Ceramic preforms were processed using microwave energy and conventional heating. The sintered foam samples were characterized by SEM and XRD observations, density measurements and compression tests in order to observe the effect of two different sintering techniques on the structure and properties. It was found that the microwave processing was completed in a shorter burning out and sintering cycle and produced structures having higher mullite transformation ratio and fine grains.     

Al2O3／TiC陶瓷—WC／Co硬质合金复合刀片的研制

采用热压烧结工艺制备了陶瓷－硬质合金复合刀片，其上层为具有一定厚度的Ａｌ２Ｏ３／ＴｉＣ陶瓷材料下，下层为ＷＣ／Ｃｏ硬质合金。分析表明：复合片中残余应力值的大小与其上下两层厚度之比ｈ／Ｈ值有关。Ｘ射线衍射、电子探针和扫描电子显微镜分析表明：烧结过程硬质合金和陶瓷材料中的元素在界面存在相互扩散，增加了界面结合强度，其界面类型为扩散型。