Analysis of the literature data [1][2][3][4][5][6][7] shows that the destruction of refractories can ten ~. tatively be divided into two stages: initiation and development.In the first stage the growth of cracks occurs and the second is characterized by their development.The potential of the refractory, operating in conditions of periodically acting thermal stress, is limited to the duration of the second stage. In order to increase the time of this stage various researchers have attempted a rational "design" of the structure of the refractories.A granular structure [i] has been created, a structure from spherical granules, consisting of an internal nucleus, coated with a layer of dispersed powder [2], a structure made of tetrahedra [3], and microspheres [4]. Also known are structures with diffusion porosity [5] and microcracked structures.*In addition laminated or layer--granular structures are known [6].These structures produce a complex character for the development of the cracks, and as a consequence a high resistance to the development of the destruction.The present authors obtained corundum laminated-~ranular material by pressing batches from cubic layered granules and subsequently sintering the green brick.The form of the cubic layered material is shown schematically in Fig. i. This ~ranule is formed by interconnecting dense and porous layers.The dense layers are obtained by sintering finely dispersed powdered corundum with particle sizes of 1-5 pm (layer A), the porous layer --by sintering technical alumina powder with particle sizes of 80-100 ~m (layer B) o The thickness of the layers is 150-200 ~m.The particles of technical alumina have a spherolytic structure.The structure of the sintered material is shown in Fig. 2. The properties of the material (relative linear shrinkage A~, open porosity P, apparent density p, thermal-shock resistance T~ dynamic modulus of elasticity E, and strength) sintered at 1700~ were determined with the wellknown methods [7]. In addition we evaluated the tensile strength by the diametral compression method [4], the relative residual strength under compression after 200 heat cycles 850~ --water.The compressive strength Ocomp was determined on cylindrical specimens of diameter 12 and height 24 mm, and the tensile strength on cylindrical specimens, diameter and height 12 mm, and the bending strength ~. on specimens measuring 7 x 7 x 70 mm using the three-point loading 1 scheme.We also determined the parameters of crack resistance [4, 5] --the critical coefficient of stress intensity KIc and the specific work of initiating destruction YI, typifying the stage of destruction inception; the specific effective work of destruction y~, characterizing the process of integral failure, taking into account the various stages (initiation of the breakdown and its development); the magnitude of the ratio yF~. --the kinetic characteristic of destruction [9]. The parameters were determined on specimens measuring 8 • i0 x 50 mm notched to half of their height, using three-point loading and a ...

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The known methods for forming stress concentrators (cracks and notches) in ceramic specimens in order m determine their crack resistance are described. A method for forming a notch with a tip curvature radius of at most 10 btm is suggested. The notch is first formed in the process of pressing the specimen in a specially designed mold and then the green specimen is cut additionally from the tip of the notch by a steel blade with a thickness of 0.1 mm and a grinding angle of 14L After sintering, this specimen does not contain induced defects that are possible when sintered specimens are notched by a diamond disk by the conventional method. It is shown for a ZrO2 ceramics partially stabilized by 12 mol.% CeO2 and an AIzO3 ceramics with 0.5 wt.% MgO possessing a layered granular structure that an incorrect choice of the tip curvature radius can result in an erroneous evaluation of the optimality of the structure of the material and an incorrect choice of the technological parameters for its production. Notching by the suggested method made it possible to establish the discrete nature of fracture of the layered granular structure of the ceramics from strain diagrams and the mechanisms of crack propagation causing this kind of fracture.

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