Fundamental Issues in the Sintering of Ceramic Particulate Composites
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FUNDAMENTAL
ISSUES IN THE SINTERING PARTICULATE COMPOSITES
MOHAMED N. RAHAMAN University of Missouri-Rolla, Rolla, Missouri 65401
OF
CERAMIC
Department of Ceramic Engineering,
ABSTRACT The sintering rate of a ceramic powder matrix can be reduced significantly by the presence of rigid inclusions. The factors that lead to this reduced sinterability have been the subject of much debate in the last 5-10 years. Recent work indicate that the reduction in the matrix sinterability is a processing-related problem. The main factors that control the sinterability of ceramic particulate composites include the packing of the matrix immediately surrounding the inclusions and interactions between the inclusions which constrain the matrix. A processing method which avoids the difficulties associated with these two factors is outlined. The method involves the synthesis of coated inclusion particles. Composites containing up to 40 volume percent inclusions can be freely sintered in the solid state to full density. INTRODUCTION Ceramic particulate composites can offer important performance advantages compared to single phase ceramics. The potential for improved performance is, however, offset by the increased difficulties in forming these materials with the high density and controlled microstructure normally required for advanced technological applications. In particular, it has been commonly observed that the conventional sintering of ceramic particulate composites is impeded significantly, even at modest volume fractions of the dispersed inclusions [1-3). The reduction in the sintering rate has been attributed to various factors, including high viscoelastic backstresses within the matrix due to mismatch in the shrinkage rates between the matrix and the rigid inclusions [4,5], the development of crack-like defects promoted by the presence of the dispersed inclusions either in the compaction process or during sintering of the composite [6), the formation of a rigid, contiguous network of touching or neartouching inclusions [7], the effects of clustering of the inclusion phase [8], microstructural anisotropy in the matrix due to the different transient fields in the radial and hoop directions [9), and the competition between coarsening and densification processes [9]. while theoretical models [10,11] have indicated that the viscoelastic backstresses within the matrix are too small to account for the observed reduction in the sintering rate, the significance of the other factors has remained somewhat ambiguous. In the present paper, we review our recent investigations into the factors that control the sintering of ceramic particulate composites. The composite system of interest consists of a fine-grained matrix and coarse, inert inclusions. These Mat. Res. Soc. Symp. Proc. Vol. 249. ยง1992 Materials Research Society
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investigations, described in the next section, indicate that the origins of the reduced sintering rates are processing-related. First, we identify the factors that control the conventional sintering of model com
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