On some distinctive features of the structure of composite ceramic materials obtained by the method of directed reaction impregnation

Abstract

The method of directed reaction impregnation (DRI), known as a Lanxide process in foreign countries, provides a wide spectrum of ceramic composite materials with a variable content of metallic phase, which possess high physicomechanical properties. An important advantage of the method is the absence of shrinkage, which diminishes substantially the amount of heat-treatment rejects and the cost of the finishing mechanical treatment. The structural characteristics of composites obtained by impregnation of the pore space of \-SiC and A12O3 ceramic matrices with an aluminum alloy with 5 wt.% Si and 2 wt.% Mg are presented. The former matrix is obtained by free pouring of a granular powder of silicon carbide, and the latter matrix is represented by a densely sintered alumina preform with through cylindrical channels. The process is conducted at 1150 – 1200‡C for 24 h in air. The structure of the composites bears Al2O3 crystals and a mixed Al/Al2O3 phase formed from the metallic melt. The microhardness of the phases that compose the structure of the composite with a silicon carbide matrix is determined (HV = 28.4 – 73.2 GPa for Al2O3,HV = 10.3 – 19.7 GPa for SiC, andHV = 0.5 – 0.8 GPa for A1/A12O3). Directed reaction impregnation through cylindrical channels of the aluminum oxide matrix yields highly porous fibers with a typical dimple structure. The impregnation process in this composite is considered from the standpoint of equilibrium (the Le Chatelier principle). Its occurrence in different stages is considered from the standpoint of thermodynamics.