Composite Si3N4 ceramic

Silicon nitride has the composition Si3N4 and its chemical bonding is predominantly covalent. Si3N4 represents the backbone of silicon nitride ceramics, a class of ceramic materials which, because of their exceptional profile of properties, are gaining increasing acceptance in engineering applications. 

This approach has been applied for Si-Al-O-N ceramics. The results can explain the existence of the amorphous phase in the grain boundaries (Fig. 24), depending on the overall composition of the system ([equ% O] = 16, 12, 8 and [equ% Al] = 10.6) and on the strength of interaction (Ap). In a strict sense it must be concluded that the conventional phase diagrams of Si3N4 ceramics are incomplete representations of materials containing an amorphous intergranular phase. 

The specific heat of Si3N4 ceramics is in the temperature range 293 up to 1200 K [Cp (293 K) = 0.67 KJ (K kg)-1] nearly independent of the composition of the additives. The isobaric specific heat values agree well with the isochoric specific heat calculated by Debye s theory. Also the Dulong Petit s rule can applied as an approximation of the Cv values [25 J(K mol)-1] at temperatures >1100 K [371]. From the Cp values at around 100 K the amount of the amorphous grain boundary phase can be calculated [371]. 

In principle, the amount, composition and degree of crystallisation of the grain boundary phase are key factors which must be considered for successful development of Si3N4 ceramics for applications at elevated temperatures. It is... 

Fig. 34a-d. Oxide layers on Si3N4 ceramics oxidised at 1500 °C. a HIP-SN (no additives 2500 h), b SSN (Y2O3/AI2O3 additives 1000 h), c SSN (Y203 additive 5000 h), d Si3N4/MoSi2 composite (Y203 additive 5000 h)... 

The oxidation behaviour of Si3N4 ceramics strongly depends on impurities in the gas atmosphere. Impurities like alkaline or alkaline earth metals, S02, and vanadium drastically decrease oxidation [431, 433, 434]. The main influence of the different impurities is caused by a change of the viscosity or the destruction of the oxide scale, accelerating the diffusion of oxygen or water vapour into the ceramic and increasing the corrosion. Of coarse, the effect strongly depends on temperature and gas composition. 

To improve, mechanical or electrical properties of Si3N4 ceramics different types of composite materials have been developed (Table 16). Among them Si3N4/TiN and Si3N4/SiC are the most extensively investigated. 

The composites have an improved wear behaviour under load [580-582], In comparison to monolithic Si3N4 ceramics, the differences at 800 °C were more pronounced than at room temperature. This was attributed to the formation of TiOx layers, which are known as solid lubricants [580, 581],... 

Summarising the data about the Si3N4/SiC composites, it can be stated that no significant increase in the mechanical properties at room temperature has been achieved which can not be realised in monolithic Si3N4 ceramics. However, at high temperatures substantially improved long term behaviour connected with a change in the oxidation mechanism can be realised. 

The possibility to produce Si3N4 ceramics of precise composition and high purity from low molecular weight, inorganic or organoelement precursors is a topic of increasing relevance for small and thin components. 

The coefficient of thermal expansion (CTE) of composite materials usually follows the simple rule of mixtures (or more complex models), based on the CTE of the respective components, their volume fraction and the volume fraction of interfacial phases. Based on these models, a Si3N4-Si3N4(w) composite should possess a similar CTE to monolithic Si3N4 ceramic (3.2 x 10 6/°C). obviously, the chemical composition of the sintering additive will have a certain influence but should remain within the variations observed for monolithic Si3N4. 

Jia, D.C., Zhou, Y. and Lei, T.C. Thermal shock resistance of SiC whiskers reinforced Si3N4 ceramic composites , Ceramics International, 22 (1996) 107-112. 

Baldacim, S.A., Santos, C. Silva, O.M.M. and Silva, C.R.M. Ceramics composites Si3N4-SiC(w) containing rare earth concentrate (CRE) as sintering aids , Mat. Sci. 

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