Reinforced silicon nitride 
Generally the reinforced silicon nitride ceramics is prepared by the liquid phase sintering of eqiaxed 
starting @x-Si3N4 powder which transforms to the needle-like B-SisN4 during densification. The 
phase transformation, and thus the final microstructure is influenced by the chemistry of sintering 
additives, amount of B-SizN, seeds in starting powder and the heat treatment. Seeding of the starting 
powder with B-Si3sNy is very effective method of modifying the final microstructure but brings 
increased requirements on the processing because of tendency to agglomerate. Example in Fig. 3 
shows the material with agglomerates of elongated B-Si3N4 seeds (Fig. 3a) which have strength of 
676 MPa and Weibull modulus 7. The material with the same composition, prepared by the same 
heating regime, but free of critical defects has strength of 930 MPa and Weibull modulus of 16, [2]. 
ure toughness of 
‚scale mechanical 
materials, since 
ie of single grain 
et erosive wear of 
microfracture and 
ular silicate films 
eral loss in these 
here may also be Fig. 3: Reinforced silicon nitride ceramics. 
: 450 MPa) in de a) microstructure with B-Si3N4 agglomerate; 6 = 512 MPa, m = 6, 
ies on b) defect free microstructure; 6 = 930 MPa, m = 19. 
[line phases 
on win from The different heating regime result in a different amount of elongated grains in the final 
gagation a by microstructure as it is generally known. For example, the material containing 48 vol. % of grains 
x Employing the with aspect ration > 4 has a fracture toughness 8.1 MPa.m'”, on the other hand the material with the 
en dives same composition but different heating regime containing only 11 vol. % of such grains has fracture 
ni with the toughness only 6.1 MPa.m'?, [3] 
Layered silicon nitride based composites 
Effect of the microstructure in layered ceramic composites can be considered on two different size 
levels. The first one is on the level of grain size, which was discussed in the previous paragraph. 
The second level, which is on the size of layer thickness influences the composite property through 
the residual stresses. The level of residual stresses depends on the factors as follows: 
Thermal expansion coefficient, thickness, Young’s modulus and sintering behaviour of individual 
layers. In case of conscious design of layered material remarkable improvement of mechanical 
properties can be achieved. An example of layered ceramic composite is shown in Fig. 4. While the 
monolithic material has strength of 650 MPa and fracture toughness of 4.8 MPa.m'?, the optimised 
layered composite has strength of 1200 MPa and fracture toughness almost 10 MPa.m'?, [4]. 
Moreover, this layered composite is more flaw tolerant as the monolithic ones. The presence of 
compressive residual stress in the layer containing large defect diminishes the critical stress 
concentrated on it. A consequence of this phenomenon is a more flaw tolerant material. 
The experiences of seeding the starting mixture by B-Si3N4 can be applied also in the case of 
lavered materials. The seed particles can be strongly aligned during the tape casting process. The 
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