po Microstructure of Engineering Ceramics 
ad im 
Zoltan Lendés, Dugan Galusek, Pavol Saj galik 
Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia 
Introduction 
isan Advanced engineering ceramics are polycrystalline materials, which consist of major crystalline and 
minor amorphous/crystalline phases. Their volume/shape distribution dictated by the microstructure 
4 Metallography/ affect mechanical and physical properties of ceramic body. The relationship between microstructure 
and properties is not completely clear and still is intensively studied. Ambition of present paper is to 
dent Light and show the potential, which is hidden in the microstructural design of ceramic body in respect to the 
1981 by Elsevier selected mechanical properties as the wear resistance, strength, fracture toughness and reliability 
| are. The examples of three types of ceramic microstructures will be shown. The alumina-based 
ceramics is chosen in order to show the role of relationship minor/major phases with respect to the 
microstructure and wear resistance. The design of silicon nitride based materials with respect to the 
high strength and fracture toughness will be also presented. The example of conscious introduction 
, of residual stresses to the ceramic body and their effect on the mechanical properties will be 
AL-TEC und bei discussed. 
A. Catan für die 
cher vom Max - Role of microstructure 
Besides the physical/chemical properties of the major crystalline phase, the microstructure is a key 
factor determining the global properties of polycrystalline ceramic material. Generally is accepted 
that the high-strength ceramic material should have a fine microstructure (mean grain size < 1 jim) 
without macrodefects. The coarse microstructure (>> 1 pm) with a needle/platelet-like grain shape 
result in a high fracture toughness. The material with optimum of both mentioned mechanical 
properties must always be a compromise between these two limiting microstructures. It means that 
material engineer producing the ceramic material must always keep in mind the final application. 
The strength and fracture toughness are only the examples of whole variety of mechanical 
properties depending on the microstructure. General relationship between the microstructure and 
mechanical properties is not simple due to many parameters defining the microstructure. The most 
important of them are as follows: 
» Grain size distribution of major and minor phases 
Volume fraction of crystalline and amorphous phases 
Shape distribution of grains 
Chemical composition of phases 
Size, distribution and chemistry of defects 
Presence of impurities 
» Residual stresses, etc. 
Above-mentioned factors affecting the microstructure are the tools, which help to tailor the ceramic 
material for the particular application. Three examples of these tailored microstructures engineering 
applications are shown in the following three paragraphs. 
Alumina based ceramics 
Polycrystalline aluminas with markedly different microstructures were prepared by solid state 
sintering (SSS) or by liquid phase sintering (LPS) with the addition of magnesium silicate, Fig. 1. 
The addition of MgSiOs inhibited the growth of grains significantly larger than the mean, and 
provided materials with a narrower grain size distribution. The milder densification conditions used 
for materials containing magnesium silicate also resulted in smaller alumina mean grain size, and 
narrower grain size distributions, Fig. 1. 
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