Investigation On Near Net-Shape Forming Technique For Ceramic

High performance ceramic has been widely used in some significant and advanced fields, such as aeronautics and astronautics, national defence and military, and chemical industry with a series of excellent properties, such as high strength, high hardness, high temperature and corrosion resistance, etc. However, there are two factors limiting its applications caused by its high strength and toughness, one is the difficulty for processing after sintering, the other is low reliability of property, resulting high cost for production. To solve this problem and obtain the ceramic parts with precise size, the machining process is arranged before sintering, decreasing the machining redundancy after sintering.Gel-casting, is one of the method to realize the precise molding for ceramic. Monomers in slurry polymerize into gelatin with three-dimensional network, in which ceramic powders are locked and in-situ mold into green body with relative high strength for machining and uniform density. This technique provides a prospective routine to produce high performance ceramic parts with complex shape, uniform structure.Traditional polymerize system for gel-casting is acryl amide system, which is harmful to our nervous system seriously. In this paper, NMAM was used as monomer in stead of AM. Factors affecting idle time and properties of gelatin were studied. Bone china powders were used as raw materials to fabricate green body by gel-casting. Factors related to properties(strength, shrinking ratio, density) of green body were studied. We analysis the drying process to obtain a reasonable system for dehydration,Conclusions are as follows:(1) For NMAM gelatin system, according to the production practice, reasonable gelatinization time is during60min to90min, not only to avoid settlement, but also remove bubbles in the slurry. To achieve this goal, usually, monomer concentration is during20wt%to25wt%, the usage of initiator and catalyst are respectively2.4ml/100m1H20,1.6m1/100m1H20.(2) The cross-linking degree is very important for the strength and safety of green body while drying. Results show that, when the monomer concentration is20wt%, the proportion of cross-linking to monomer is during1/15to1/10, the cross-linking degree is reasonable.(3) Ethanol, volatile easily, can improve the remolding effects, however, too much ethanol would prolong idle time. The proper proportion of ethanol to water is1/10to1/5.(4) The time for adding initiator is important for the structure of green body. It is very useful to obtain uniform structure to add initiator before milling.(5) It is crucial to enhance the strength of green body for machining precisely. Results show that, the green body with density above1.25g/cm3and strength above16MPa enough for machining is obtained, under the condition of solid content70-75wt%, proportion of cross-linker to monomer1/10, dispersant content0.2%, initiator content0.4-0.6ml per100g ceramic powders.(6) As the gelatin is very sensitive to humidity and temperature, dehydration for green body is crucial. Drying with high humidity or in PEG solution can decrease the rate of water losing. Once the water losing reaches70%, drying rate can be increased, realizing the consistent of safety and high effectiveness.(7) Process for removing organics is determined by TG analysis. Green body should be heated at300℃for2hours to remove free water and bound water thoroughly, then heated at520℃for3hours to provide enough time for organics releasing.