Silicon Ceramic Core. Sintering Process And Component Design,

Sillica based ceramic cores is the main core materials in domestic presently when used to manufacture hollow turbine blades, for the reasons of its series advantage, such as low coefficient of thermal expansion, high mechanical strength, excellent thermal and chemical stability, ease to leach from castings, and so on. It is very improtant and meaningful to investigate this kind of cores since it always generate deformation even rapture because of its poor high temperature mechanical properties, when casting directional solidification and single crystal hollow blades, which is often held up the production of blades.Bending at room temperature and high temperature deformation tests are performed to characterize the mechanical properties of the experimental cores. The influence of sintering process, composition of cores and impurity content of fused silica to the properties of the cores and the microstructural evolution are systematically established by means of particle size and chemistry composition analysis of raw materials, X-ray Diffraction(XRD) and Scanning Electron Microscopy(SEM). The main results are concluded below:1. Much fault, such as tympanites and sand fusion appeared at the surface of ceramic cores if the heating-up rate was too quick in the period of deprivation plasticizer at low temperature. The optimal heating-up treament is as follows:room temperature～300℃/3h,300℃/2h,300℃～500℃/2h and 500℃/2h.2. When sintering at 1250℃within the experimental temperature, the cores obtained the best properties. The strength at room temperature improved with temperature increasing when below 1250℃, because the increment of strength by temperature rise was greater than the damage to strength by volume conversion of cristoballite. On the contrary, the damage of cristroballite was dominant when temperature exceed 1250℃, which results to the reduction of strength. Meanwhile, a mass of cristoballite in the cores after sintering was able to accelerate the remain fused sillica transform to cristoballite before the molten metal casting, which improved the thermal deformation resistance of the cores. 3. With addition of cristoballite from 10% to 20%, flexure strength at room temperature of experimental cores declined since the damage to strength by cristoballite was strengthen as the cristoballite content increasing at the same sintering temperature. The high temperature deflection decreased from 16.1mm sudden to 0.5mm, resulted from that the gross of cristoballite in the cores after sintering raised as the addition of cristoballite to cores increase. The optimal addition content of cristoballite to cores is 20%.4. With impurity content in fused sillica from 0.4% to 0.69%, flexure strength of experimental cores increased from 5.21MPa steeply to 9.86MPa, while the high temperature deflection reduced slightly to 0.2mm. Meanwhile, with impurity content in zirconia from 0.04% to 4.82%, flexure strength increased steeply to 8.5MPa. By contrast, the high temperature deflection raised substantially to 4mm. The reason why strength increase was that liquid phase formed at high temperature by low-melting compound which produced by chemical reaction between impurity and SiO2 in the process of sintering passivated the tip of crack generated by cristoballite conversion, thereby to alleviate the concentration of stress in cores. The optimal impurity content in fused sillica is less than 0.69% and the impurity content in zirconia should be controled rigidly.