Formation of??-sialon in the system Si3N4-Al2O3-AlN

Abstract

The study of the phase formation in the system Si-AI-O-N is interesting in connection with the production of new ceramic materials (sialons) which are products of the reaction between nitrides and oxides.Sialons combine to a certain degree the advantages of nitrides and oxides, and are promising for use in various regions of technology.The sialon phases (while preserving the normal valence of the elements constituting the quaternary system Si-AI-O-N) are situated on the plane Si3N4-SiOz-AI203-AIN, formed by the binary compounds in the system. This plane may be divided into two equilateral triangles, consisting of the subsystems Si3N4-AI203-SiO 2 and Si3N4-AI203-AIN [i].In a study of the partial system Si3N4-AI203-AIN [1-3], the formation of solid solutions was detected on the basis of ~-Si3N 4. The crystallochemical prerequisites for the formation of such phases consist in the fact that the structure of ~-Si3N 4 and AIN is built up respectively from tetrahedra SiN 4 and AIN4, and the aluminum may exist in the tetrahedral surroundings of the ions of oxygen; and furthermore, the tetrahedra SiN 4, AIN 4, and AIO 4 are close in size. This creates the possibility of substitution of the silicon by aluminum and of nitrogen by oxygen in the structure ~-Si3N 4 and the formation of solid solutions of substitution on the basis of ~-Si3N4, consisting of tetrahedra (Si, AI) (O, N)4 , named ~'-sialons.In the present work we studied the formation and structure of sialon, obtained by the method of hot pressing, in the system Si3N4-AI203-AIN.To establish the region of extension of the monophase ~'-sialon solid solution in the stated system, we prepared a mixture having the composition (l-x) Si3N 4 + x (AI203"AIN). The starting components consisted of pure finely dispersed powdered ~-Si3N4, AIN, and AI203.These compositions (in the range 0 ~ x ~ 0.9) were carefully mixed and subjected to hot pressing at 1750~ soaking for 30 min, and using a pressure of 1.5 N/mm z.The phase composition of the specimens was determined by x-ray and crystal-optical methods. For this we used recordings on the DRON-I.5 diffractometer in CuK~ radiation (Ni filter) directly from the plane of the cross sections of the fired specimens. The parameters of the cell for the ~'-sialon were determined by the scanning method using lines (i01) and (210) on the hexagonal structure for the calculations. The determination errors for the parameters were • nm. The refractive indices were located with the MIN-7 microscope in immersion preparations.The x-ray studies of the fired specimens in the system Si3N4-AI203-AIN (compositions of which were selected along the lines with a ratio of cation/anion = 3/4) for establishing the extent of the region for the ~'-sialon showed that over the section (I -x) Si3N 4 + x (AI203"AIN) right up to x = 0.8 there forms a monophase region of hexagonal solid solutions on the basis of ~-Si3N 4 (~'-sialon). On the diffractograms of these samples we observed a shift in the reflexes typical of the structure of ~-Si3N 4 toward the smaller angle...