Advanced Materials Research Vols. 105-106

Abstract: Nano-SiCp/MoSi2 composites with different SiC volume fraction have been synthesized by in situ reactive hot press sintering with the milled Mo, Si and C elements powders. The X-ray diffraction (XRD) detection shows the composites are composed of α-MoSi2 and β-SiC, and transmission electron microscopy (TEM) shows that the particle sizes of β-SiC formed by in situ reaction are in range of 20-100nm, and most of the them locate in grain boundary and some particles embed in MoSi2 grain. The isothermal oxidation and thermal cycling test of nano-SiCp/MoSi2 composites were carried out in air at temperatures of 500°C. The pest phenomenon was not found during oxidation and cyclic oxidation at 500°C for 500 hours and 50 cycles, respectively. All the composites have both excellent oxidation resistant and good cyclic oxidation resistant. The mechanisms of oxidation at low temperature were discussed also.

Abstract: Artificial neural network (ANN) back-propagation model was developed to predict the thermal expansion behavior and internal residual strains in reinforced ceramic matrix composites (CMCS).The ANN training model has been used to predict the thermal expansion behavior and internal residual strains, exhibiting excellent comparison with the experimental results. It was concluded that predicted thermal expansion behavior and internal residual strains by the trained neural network model seem more reasonable compared to approximate methods. It is possible to claim that, ANN is fairly promising prediction technique if properly used. Training ANN model was introduced at first. And then the neural network architecture is designed. The performance of system is summarized at last. In order to facilitate the comparisons of predicted values, the error evaluation and mean relative error are obtained. The result shows that the training model has good performance, and the experimental data and predicted data from ANN are in good coherence.

Abstract: The thermal fatigue behavior of alumina-magnesia based and alumina-chromia based purging plug materials are comparatively studied. By comparing thermal shock parameters, the changes of elastic modulus and hot modulus of rupture after thermal shock cycles, we come to a conclusion that microcracks emerge in the alumina-magnesia based material, which hinder the crack growth during thermal shock cycles. The fine-grained and network structure of alumina-magnesia based material are also helpful to improve thermal shock resistance. However, cracks are difficult to form in the alumina-chromia based material but it tends to fracture damage quickly once the cracks nucleation due to coarse-grained structure of alumina-chromia based material.

Abstract: High temperature oxidation testing was carried out on hot-pressed ZrB2-SiC-graphite composite by using high electric current heating. The composites oxidation behavior was investigated, the temperature of oxidized specimens was above 2000°C. The results found that the UHTC composite was ruptured at oxidized temperature 2055°C. The microstructure of surface and cross-section of posttest samples were investigated by scanning electron microscopy along with energy and X-ray diffraction. The failure mechanism of rupture was also discussed.

Abstract: Dense ZrB2-20vol.%SiC-10vol.%BN (ZSB) ceramic composites by introducing BN as the third phase are fabricated through hot pressing sintering under inert gas protected. The static oxidation behavior of the ZrB2-SiC-BN ceramic composite at 1200°C in air is analyzed using differential thermal analysis technique, and the surface morphology of the composites after oxidation at 1200°C is examined using scanning electron microscopy along with energy dispersive spectroscopy. The microstructure change and oxidation behaviors of the ZrB2-SiC-BN ceramic composite are investigated. The effect of BN grain size is analyzed and the oxidation mechanism in ceramic composites is discussed correspondingly.

Abstract: In this work, silicone/alumina composites with 33 wt.% silicone content are prepared by compression mould technique. TGA, XRD and SEM were used to characterize the ceramifying behavior when the samples were heated from 600 to 1600°C. XRD results suggest that the ceramic residue of SR could restrain the phase transformation of γ-Al2O3 below 1100°C, indicating that the chemical bonds preserve the mechanism strength of SR/Al2O3 composites under high temperature. The flexural strength increases from 18.19 MPa to 75.31 MPa with the increasing temperature from 600 to 1600°C, owing to the sintering reaction between SiO2 and Al2O3 forming mullite phase under higher temperature. It has low liner shrinkage after firing and nearly no macrocracks or deformation under any temperature, illustrating that the SR/Al2O3 composites can be used as organic fire-retardancy materials.

Abstract: In this paper, rare-earth fluorides were used as sintering additives instead of rare-earth oxides, the influence of the type and the amount of the fluoride on the thermal conductivity, hardness and strength of Si3N4 ceramics was studied. Results show that the thermal conductivity of a sample sintered with CeF3 is nearly 10% higher than that of the sample sintered with Ce2O3, while the samples sintered with LaF3 increases further 15% as comparing with the samples sintered with CeF3. The strength of the samples changes reversely. These results express that the thermal conductivity of Si3N4 ceramics can be improved by using rare-earth fluoride additives instead of the oxides, and the type of rare-earth fluorides has a significant impact on the properties of Si3N4 ceramics.

Abstract: By using the ring-block friction and wear machine, the friction and wear behavior of nano-ZrO2 ceramic test block against GCr15 steel test ring has been studied. At the same time, friction and wear test of 45# steel block against GCr15 steel ring has also been conducted for comparison. By using self-made test machine for the wear ability of the hip joint, the friction test of nano-ZrO2 ceramic hip joint head against ultra-high molecular weight polyethylene (UHMWPE) acetabulum has been done, the friction test of Co-Cr-Mo alloy hip joint head against UHMWPE acetabulum has also been made for comparison. The results show that the friction factor of nano-ZrO2 ceramic test block against GCr15 steel test ring is less 37.3% than that of 45# steel block against GCr15 steel ring, the wear weight of nano-ZrO2 ceramic test block against GCr15 steel test ring is only 0.76% of that of 45# steel block against GCr15 steel ring. The wear weight of UHMWPE acetabulum against nano-ZrO2 ceramic hip joint head is 61.5% of UHMWPE acetabulum against Co-Cr-Mo alloy hip joint head. This indicates nano-ZrO2 ceramic has good wear resistance property and is the ideal materials for artificial Hip joint head Prostheses.

Abstract: Fully dense samples of TiB2-TiCX and TiB2-TiCX/15SiC ceramic composites were fabricated by in-situ synthesis under hot isostatic pressing from TiH2, B4C and SiC powders. Their oxidized behaviors at different temperatures were tested. Optical micrograph studies and thermo-gravimetric analyses show that the highest effective temperature of oxidation resistance is 700°C for TiB2-TiCX, and 1100°C for TiB2-TiCX/15SiC. The weight gain of TiB2-TiCX/15SiC below 1100°C is quite low, and it rises up suddenly when the temperature reaches 1200°C. Thus, the highest effective temperature of oxidation resistance is 1100°C for TiB2-TiCX/15SiC. The oxidation dynamic curves of TiB2-TiCX/15SiC ceramics accord with the parabola’s law. The activation energy of TiB2-TiCx/15SiC (189.87kJ.mol-1) is higher than that of TiB2-TiCx (96.44kJ.mol-1). In the oxidation process of TiB2-TiCx/15SiC, TiB2 reacts with oxygen and generates TiO2 and B2O3 at first. A layer of whole homogeneous oxide film cannot be formed, in the mean time, the oxidation of TiC begins. When temperature goes up to 1000°C, TiC phase is totally oxidized. SiC is oxidized to SiO2 at about 900°C, Meanwhile, TiO2 forms denser film than B2O3, which grows and covers the surface of the material, and gives better property of oxidation resistance.

Abstract: AlON/SiC composites were synthesized by hot-pressing sintering technology using SiC and prepared AlON powder as raw materials. The influence of SiC content on relative density, hardness, bend strength and microstructure of multiphase material was studied by several testing methods, such as XRD and SEM. The strengthening mechanism of SiC was also explored in this paper. The experimental results indicated that with the addition of SiC, the bend strength of the multiphase material was obviously enhanced, comparing with AlON single phase material, and ultimately it reached 310 MPa. The best adding mass content of SiC was 8% and its strengthening mechanism was mainly thermal expansion mismatch and fine grain strengthening. As the adding mass content of SiC increased, the hardness of AlON/SiC ceramic matrix composite was increased gradually, the bend strength firstly increased and then decreased, and the main fracture way of composite was changed from intergranular fracture to transgranular fracture and finally both of the two modes of fracture occurred. This research had positive significance on deep popularization and application of AlON ceramic material.