Papers by Keyword: Ceramic Composite

Abstract: The Si3N4 ceramics are usually known as strongly refractory and enduring materials and they have typical insulating material properties on room temperature. If reinforcing phase of the Si3N4 matrix composite is a good electrical conductor, in that case it is worth to investigate the composite in electrical aspect. In our case carbon nanotubes, black-carbon and graphite with good electrical conductivity were mixed in the base ceramic. During our electrical investigations DC resistivity measurements were used to determine the percolation threshold and the conductivity of the composites. In case of high resistance samples AC impedance spectroscopy was applied. As result of the impedance spectroscopy capacitive properties were found. In some cases of conductor samples combined mechanical – electrical measurements were done to study the integrity of the additions in the matrix.

Abstract: For enhanced mechanical properties of ceramics for structural application, a great deal of attention has concentrated on preparation of layered composites. In this study, numerical simulation technique, which is applicable for the evaluation of mechanical performance for layered composite material, was developed. A generalized material constitutive equation coupled with material damage model based on the continuum damage mechanics approach was proposed and has been implemented to an in-house type finite element analysis code. The material behaviors for each component of layered composites can be simulated by the pre-defined material model on the simulation. A series of finite element analyses was carried out in order to elucidate the effect of fabrication related residual stress on the structural capacity of the layered composites.

Abstract: Industrial wood cutting is a highly demanding application for cutting tool materials as a wedge angle of ~50°, a tip radius in the 1 m range, and high wear, temperature and corrosion resistance are needed. For this application Si3N4 based ceramic matrix composites (CMC) were developed. Cutting tests showed that reinforcing Si3N4 with 30 wt.% SiC gives a good balance between fracture toughness and wear resistance. The use of an yttria / lanthana sintering aid system resulted in a fine-grained microstructure without degrading the fracture toughness. Post heat treatment was essential for the integrity of the cutting edge. Finally, cutting tests proved that the CMC cutting tips had a 3-fold lifetime compared to tungsten carbide.

Abstract: The dynamic compressive behavior of Al2O3 (10% vol.) / TiB2 ceramic composite had been tested by using a split Hopkinson pressure bar in this paper. The results show that the main failure modes of the ceramic composite include crushed failure and split fracture along the loading direction. The former is the typical compressive failure of brittle materials. The later is tensile failure along the flaws produced during the composite manufacturing. The numerical simulation was also used to study the effect of the diameter/length ratio of the samples on the experimental results. The effect of the deformation in the bars’ ends, which contacted with the samples, was also studied in the numerical models.

Abstract: Porous SiC/Si3N4 composites were fabricated by reaction between Si3N4 and C powder in 0.5 MPa nitrogen atmosphere at 1700oC, 1800oC and 1900oC for 0.5h, 1h and 2h, respectively. The characteristics of in situ porous Si3N4/SiC composites were further investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Results show that the SiC particles are very fine. And the transformation from α-Si3N4 to β-Si3N4 is obvious at 1800oC for 2h and at 1900oC. Otherwise, at 1700oC, the formation of rod-like β-Si3N4 is inhibited. The compressive test was conducted at a cross-head speed of 0.5 mm/min to get the basic mechanical performance of the porous ceramics. The highest value of strength of these composites was obtained when sintered at 1800oC for 1h.

Abstract: Effects of the body centered cubic distribution of the dispersed SiC and (W,Ti)C particles on the residual thermal stresses of (W,Ti)C/SiC/Al2O3 ceramic composite were analyzed with the finite element method, especially with the comparison with that of the face centered cubic distribution. Results show that the residual thermal stress is affected not only by each material compositions but also by the distribution forms of the dispersed SiC and (W,Ti)C particulates. Both distribution and magnitude of residual thermal stress differs with the difference in the particle distribution forms. The calculation results of different particle distribution forms, however, are just the reflection of different cases of the residual thermal stresses inside the ceramic composite from different points of view. Both reflections can research agreement in trends.

Abstract: An optimum model for the compositional design of advanced ceramic composites is built based on the impact resistance. The relative impact modulus IM0 is defined as an index for the characterization of impact resistance of brittle ceramics. Computer aided optimum technique is used to get the optimum compositions of the material. Results show that the material can be expected to have the highest impact resistance which is nearly 86% higher than that of the pure alumina when the volume fraction of Al2O3, SiC and Ti(C,N) is 72.3%, 14.8% and 12.9%, respectively. An advanced SiC/Ti(C,N)/Al2O3 ceramic composite is then fabricated according to the optimum results. When used as cutting tools, its impact fracture resistance is approximately 71%-76% higher than that of the pure alumina ceramic in the machining of hardened carbon steel. The increment coincides well with that predicted directly from the optimum model. It suggests that the method is feasible in the design and fabrication of ceramic composites especially for machining application.

Abstract: One of the authors proposed a non-equilibrium powder metallurgy process, which enables the fabrication of a near net-shape product using TiC and TiN/Silicide ceramic composites. The PM process in combination with mechanical alloying (MA) and Spark Plasma Sintering (SPS) are applied to produce nano-grain composite, TiC/Ti5Si3. Powders of elements Ti and SiC whose composition is Ti-20 mass%SiC are blended for MA. After the alloying, the MA powder whose average particle size is 20~30 μm, has amorphous-like structures, and then the MA powder is compacted by SPS. Results of compression-tests indicate the occurrence of unusual hightemperature deformation behaviors such as low flow stress at the lower deformation temperature or at the high initial strain rates were observed in the SPS compacts. TEM observations of the deformed compacts after the compression-tests indicate the microstructure has no-strain equiaxial - grains and clear boundaries. This serves as proof of a superplasticity deformation. In addtion, the results of the XRD analysis of the compressed-compacts show that new phases are formed during the compression-test. Therefore, the above deformation is attributed to a "pseudo" superplasticity in which the phase transition of metastable microstructure occurs during the deformation.

Abstract: Most structural materials existing in nature take the form of composite. After centuries’ evolution, these materials gain highly optimized microstructures and performances. In this work, a kind of natural biomaterial, shinbone, is observed with a scanning electron microscope (SEM). The observation result shows that the bone is a bioceramic composite consisting of hydroxyapatite layers and collagen matrix. The observation also shows that the hydroxyapatite layers are composed of hydroxyapatite sheets. The hydroxyapatite sheets are of thin and long shape and parallel distribution along the orientation of the maximum main stress of the bone. The shape and distribution of the hydroxyapatite sheets may endow the bone with favorable fracture toughness, which is analyzed and illuminated based a representational model of the hydroxyapatite sheets and the idea of maximum pullout force.

Abstract: In this paper a method to produce laminated ceramic composites containing residual stresses is described. The method consist in superimposing thin layers obtained by tape casting, their worm-pressing and sintering. Detailed information on the process and on the slurry compositions are reported. The reasons why laminated structure can exhibit improved performances are also illustrated. The model on which a multilayer composite, containing residual stresses, can be designed is briefly illustrated. The relationship among the physical, chemical and microstructural properties of the different layers, necessary to stimulate the residual stresses outlined.