The scope is considered for increasing the high-temperature stability to deformation in ceramic items as regards comprehensive improvement in characteristics corresponding to current economic trends.It is becoming increasingly important to examine deformation during firing for molded semifinished products made of materials sintered at high temperatures and subsequently acquiring a vitrocrystalline nature, particularly on account of the tendency to reduce the firing times and temperatures by the use of suitable additives, and also in connection with tightening specifications for the size accuracy. Large ceramic products such as drainage pipes may be reduced in mass by reducing the wall thickness, which reduces the amount of raw material used and the energy consumption, but then there is an increase in the tendency of the semifinished product to pyroplastic deformation.Researches have been done on a large scale on the creep in ceramics at high surface temperatures, but they have not been focused on the deformation in multicomponent systems such as porcelain during firing [1]. The variety of compositions and firing modes must determine the substantial differences in deformation [2]. It is not always possible to extend the phenomenology of deformation for materials whose elementary rheological properties differ substantially to other materials characterized by more complicated behavior. One approach is to examine porous vitrocrystalline material such as porcelain during firing, whose behavior is represented by a very complicated set of properties.The task of giving a proper description of deformation splits up into several very complicated subtasks: determination of key parameters of the structure and composition, the changes in them during firing under load, the effects on the deformation, determination of the effects of firing conditions and shape of a component on processes, and this combines with the choice or creation of rheological models and software for calculating the deformation from the significant parameters.The chemical and mineral compositions had the main effects on the deformation at high temperatures. Although there are many forms of porcelain, one can distinguish in their compositions the quartz component, whose grains are rarely greatly altered during firing, and which only partially dissolve in the liquid, together with the clay-mineral component and the fluxes, which form a liquid at a high temperature. One can control the properties of the porcelain, including the deformation behavior on firing, by adjusting the nature, grain size, and method of preparation for those components.It is simplest to regulate the properties of a porcelain by varying the contents of low-activity crystalline phases: the oxides of silicon, aluminum, zirconium [3], and so on. One needs to consider not only the grain size but also the firing conditions, since the amounts of the crystalline phases may be reduced substantially by prolonged hold. There has been a study [4] of the reactions between porcelain components with m...
The deformation behavior of porcelain and half-porcelain samples during calcination with different type of loads -bending, tension, and torsion -is investigated. The temperature dependences of the effective viscosity of the materials are calculated. The simplest method of determining the deformation stability is bending cantilevered samples, and the most informative and accurate method is torsion of thin-wall tubes.The drive to decrease calcination time and temperature by using active fluxes and the more stringent requirements imposed on the dimensional exactitude of ceramic articles are making investigations of the deformation of ceramic articles increasingly more urgent.Three basic groups of factors have a large effect on the deformation of ceramic articles: the characteristics of the raw materials (chemical, phase, and size dispersion composition) and of the intermediate product (uniformity), parameters of drying and calcination (temperature uniformity, gas-dynamic regimes, charging, and so forth).The creation of a stable framework from relatively large grains, the use of fluxes with high melt viscosity, and accurate prediction of phase-formation make it possible to decrease deformation by adjusting the composition of the raw materials. The use of formation methods that give an intermediate product with high uniformity (slip casting, isostatic pressing) makes it possible to avoid nonuniform shrinkage of articles which occurs because of density nonuniformity and texturing of the intermediate product. Improvements of the equipment used for heat treatment and optimization of its parameters for a concrete material and shape of articles as well as the use of special charging techniques all help to decrease deformation at these stages. The possibilities of all of these methods of decreasing deformation are always limited by the material and the specific product, but one other important tool is remains -adjustment of the shape of an article.In the conventional technology working forms are fabricated in steps -first a rough and then the final model. If an article undergoes unacceptable deformation during calcination, then dimensions of the model must be adjusted and all steps must be repeated. The time expended on adjustment and fabrication of forms can be greatly reduced by means of computer models, which are used together with CNC machine tools to remove burrs. CAD and CAE programs are used to develop such programs; this requires knowledge of the basic characteristics of the article being calcined.The volume-stress state of an article during calcination is determined by the temperature field, shrinkage, phase transformations of the material, and other factors. The rheological behavior of the material during calcination can be described by one of the known models taking account of the factors indicated.The procedure for calculating the inelastic deformation of ceramic articles is given in the monograph [1], where relations for the elastic deformations of the structural elements are modified in the equations of so-cal...
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