Fine reactions and growth kinetics of grains during hot pressing in the Al2O3-TiN system

Abstract

(on the level of intergrain contacts) and also the growth kinetics of the grains during the hot pressing of the starting components, governing the physicomechanical properties of the products.This article gives the results of electron-microscopic studies of polycrystalline specimens in the AI20~-TiN system with concentrations of 0, i0, 20, 30, 50, and 100% TiN obtained from powders by hot pressing at 1973 K, and a pressing force of 9.807 N/mm =, and isothermal soaking of 3, 5, and 8 min. For comparison we used thermodynamic data [i] and the results of studies of these specimens.The microstructure of polycrystalline specimens was studied with the Tesla BS-500 transmission electron microscope, using platinum-carbon replicas, which were separated from the fresh fragment of the specimen by means of gelatine. The use of these replicas reinforces the contrast of the image, and also facilitates the use of platinum as the standard for determining the constant of the Tesla BS-500 apparatus, without which it is impossible to carry out the microphase analysis of the extracted specimen on replicas [2,3]. The constant of the apparatus determined with respect to platinum in the microdiffraction schedule was located within the range 11.7-11.85. The interpretation of the microdiffractions was done using data from [4].The size of the grains in polycrystalline specimens was evaluated by determining the diameters of equivalent spheres [5,6]. For this, each of the grains on the photomicrograph (negative) was measured in two mutually perpendicular directions (along the large and the small diameter of the grain) and a statistical series was compiled.The mean arithmetic value of the diameter of the grains in the po!ycrystalline specimen am was calculated according to the equation E~niwhere a i is the mean arithmetic diameter of the i-th grain, ni is the number of i-th grains~ Eni is the total number of all grains measured in the specimen. The quantitative assessment of the mean arithmetic values is considered to be valid With the measurement of at least i00 grains~ In our work the value for the grain diameter in specimens was determined for not less than 200-300 grains [7].As a result of the photomicrographic processing we obtained mean arithmetic diameters for the grains in the specimens for the AI2Os--TiN system. Table 1 shows the results of the analysis for extraction from the fresh surface of the specimens.In all the compositions we failed to extract grains of alumina, which may indicate the strong bonding with the oxide framework, and therefore the phase inversions in the alumina could not be examined.Lensovet Leningrad Institute of Technology. Translated from 0gneupory, No. 3, pp~ 15-18, March, 1988.