Key Engineering Materials Vols. 280-283

Abstract: The rapid development of magnetic materials has witnessed a sustained consumption increase in corundum-mullite kiln furniture application, yet a comparatively short duration of them. In this view, the thermal shock resistance of sagger plate in floppy magnets sintering turns out a factor of critical importance. This paper makes a study concerning the influence of factors including the size of critical electro-melted mullite particles, the electro-melted corundum or mullite as medium particles and the addition of upon the thermal shock resistance. The result shows that, with critical particles of 2 mm, the sheet of about 10mm thickness is characterized by better performance in modulus of rapture and thermal shock resistance. The latter proves to be more outstanding in samples rather than mullite. When zircon powder is added in a two-hour sintering process at the temperature of 1550°C, a remarkable improvement is made by the sample in terms of its thermal shock resistance. SEM analysis shows the existence of micropores in the section of samples with corundum as medium particles, whereas the glasses phase in that of mullite sample, resulting in a dense structure. Finally, zircon powder added makes for the improvement of thermal shock resistance due to the formation of baddeleyite on the particle surface of the corundum.

Abstract: Heterogeneous-nucleation formation of Al2O3 coatings on graphite powders via a controlled hydrolysis process of aluminum chloride was investigated, and surface morphology and properties of the coated graphite were examined. The formed Al2O3 coatings were uniform and essentially amorphous. They were bonded well with graphite via chemical interaction between their O atoms and C atoms on graphite surface. Graphite particle size increased from 7.61 µm (as-received graphite) to 9.53 µm (the coated graphite), revealing that average thickness of the coating was ~1µm. Compared with those of as-received graphite, specific surface area and fractal dimension of the coated graphite also increased, which was caused by the porosity present in the coating. The isolectric point (IEP) of the coated graphite (~9) was close to that of pure Al2O3, indicating that the electro-kinetic behavior of the coated graphite was similar to that of Al2O3.

Abstract: Carbon-containing refractories are widely used in metallurgical industry due to their excellent thermal property and resistance to corrosion by molten iron. In the present work, slip casting colloidal process, which is used to fabricate high performance structural ceramics, has been introduced into the preparation for magnesia-carbon refractory with selection of suitable dispersant. The results indicated that the curve of zeta-potential of magnesia was similar with that of carbon just containing dispersant and the homogenized mixtures were attained under a controlling of low pH value. Three kinds of preparing routes for green bodies were designed, and all of the final densities and the resultant hardness of sintered bodies using colloidal process were superior to these of the specimen using conventional dry pressing, whereas its high green density can be plausibly observed in dry pressing body because of high compaction pressure used.

Abstract: Hydration resistance of three commercially available high-purity MgO-CaO clinkers were investigated and compared in the present paper. The results show that, as well as CaO content, microstructural morphology of MgO–CaO clinker affects its hydration resistance. The microstructure in which CaO is present at the grain boundaries of MgO leads to poorer hydration resistance, whereas the one in which CaO and MgO are distributed alternately confers much improved hydration resistance on the clinker, especially when MgO level is high enough to cover most of the CaO crystals.

Abstract: β-sialon bonded corundum-based sliding gate plate was developed from raw materials of Al, metallic silicon and alumina through nitriding-reaction process. It was found that the effects of β-sialon content on the physical properties are significant and the mechanical properties of developed sliding gate plate are strongly related to the Z-value. The materials with Z value of 2 and 20wt% β-sialon, exhibit the highest hot modulus of rupture and cold crushing strength. It was also found that sialon bonded corundum-based material has good oxidation resistance because a protective layer is formed during oxidation process.

Abstract: The corrosion behavior of BF slag to sialon bonded corundum refractories using reaction nitriding technology at 1500°C was investigated by crucible test method. The results show that the reaction products of slag corrosion are anorthite, magnalium-spinel and glass phases. The corundum grains can react with MgO in the slag to produce magnalium-spinel, accompanying by expansion and liquating stepwise by slag. Erosion mechanism is oxidation-erosion-dissolving. This study is very necessary and useful to improve the property of BF slag resistance and to yield better overall properties of sialon bonded corundum refractories.

Abstract: Engineering ceramics have a high application potential for wear-protection of different equipment used for electric power generating, mining and mineral industries. The ZTA (zirconia toughened alumina) and alumina (92%, 95%) have been prepared. Effects of varying the impact velocity and volume content of erodent particle and erosive time are investigated. Scanning electron microscopic examination of the eroded ceramic surfaces provides insights into the erosive wear mechanisms. The material removal processes include chipping out of lateral cracks caused by impact of the erodent particles, grain boundary cracking and grain pull out, as well as plastic deformation caused by the repeated sliding and impact of the particles.

Abstract: A SiO2-SiC coating on 3-D carbon fiber perform (raw perform) was successfully prepared through several steps. The appearance and composition of the coating/3-D carbon fiber preform (coated perform=CP) was studied through IR, XRD and SEM. The isothermal weight-loss and non-isothermal thermogravimetric (TGA) were used to study the oxidation behavior of CP. The results showed that a uniform coating on every fiber of preform was achieved and the coating adhered well with fiber. The oxidation process of CP in isothermal condition was reaction-controlled in the first step, gas diffusion and reaction-controlled in the second step. The characteristic of the oxidation process in non-isothermal condition was self-catalytic.

Abstract: Magnetic ceramic particles (NiFe2O4) and aluminum borate whiskers reinforced aluminum matrix composite was fabricated by squeeze casting technique, which had lower coefficients of thermal expansion (CTEs). The components and microstructure were characterized by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that the CTE of the composite was lower than that of the composite only reinforced by aluminum borate whisker, and with increasing the number of thermal cycling, the CTE of the composite became low. The results indicated that the lower CTEs of composite resulted from ferromagnetic-paramagnetic transformation of the magnetic ceramic particles.

Abstract: A new 3D network ceramics (skeleton) reinforced aluminum alloy composites was designed and fabricated. Dry friction and wear behaviors of the new composites were investigated. The friction coefficients and wear rates of the matrix and the composites were measured at various temperatures and loads. The worn surfaces of the samples were examined with a scanning electron microscope, and the effect of the 3D network ceramics on the wear mechanisms was discussed accordingly. The composites had much better wear resistance than the matrix. The improvement in the wear resistance of the composites became more prominent at higher ceramics volume fraction, higher temperature, and larger normal load. Moreover, the friction coefficients of the composites remained stable with increasing normal load, especially at elevated temperature. It is suggested that the 3D network ceramics support the load applied onto the sliding surface, which not only restricts the plastic deformation and high temperature softening of the alloy, but also protects the oxide film(Al2O3) on the surface.