Silicon nitride liquid-phase sintering

Keywords Silicon nitride, liquid phase sintering, SiAIONs, a-phase, P-phase, grain boundary glass, oxynitride glasses, microstructural engineering, microstructure-property relationships. 

A new type of liquid-phase sintered SiC using yttria [1314-36-9J, Y20, as the oxide additive and submicrometer SiC powder for enhanced densification, produces a material which can be densified without the application of pressure (13). This material, sintered from cold isostatically pressed billets, appears to be comparable to silicon nitride in strength and fracture toughness. 

Due to the tailored properties of liquid phase sintered silicon carbide (LPSSiC) it is used as dewatering elements in the paper machinery and as rings for highly stressed gas seals. It is a price competitive alternative to silicon nitride materials and outperforms alumina and tungsten carbide materials. In addition, LPSSiC is proposed as neutral matrix in ceramic matrix composites containing plutonium to burn the world s stockpiles of military plutonium in thermal or fast reactors [278]. 

A low cost-high performance silicon nitride-BAS composite can be fabricated via pressureless sintering. In this composite, the BAS glass-ceramic serves as an effective liquid-phase-sintering aid, to attain full densification and complete the a-(3 Si,iN4 phase transformation, and remains as a structural matrix that is reinforced by the silicon nitride whiskers, Si3N4 whiskers grow in random directions in a near completely crystallized matrix of hex-acelsian BAS. 

Silicon nitride powder particles are contaminated by layers of silica on their surfaces. Oxide additives MxOy mixed with silicon nitride react with this silica and some of the nitride itself at sintering temperatures to form an oxynitride liquid which promotes liquid phase sintering and densification by solution-precipitation [1, 6, 10]. The a-Si3N4 dissolves in the liquid and is... 

A systematic study of pressureless sintering kinetics for silicon nitride ceramics [10] applied the Kingery liquid-phase sintering model [17] in which three stages are identified, as shown schematically by the log shrinkage - log time plot of Fig. 5. The stages are ... 

Figure 5. Schematic log shrinkage - log time plot showing 3 stages of liquid phase sintering of silicon nitride ceramics according to model of Kingery [17],...

H.J. Choi, J.G. Lee, and Y.W. Kim, Oxidation Behaviour of Liquid-Phase Sintered Silicon Carbide with Aluminium Nitride and Rare-Earth Oxides (RerOs, where Re=Y, Er, Yb), J.Am.Ceram.Soc., 85(9), 2281-86 (2002). 

As with silicon nitride, solid forms of silicon carbide are made by one of three processes sintered, hot-pressed, or reaction-sintered. Sintering requires use of an additive such as alumina, carbon, or boron to promote liquid-phase sintering. Hot pressing is done at 2000°C and results in a very hard, dense material. Reaction sintering occurs when a mixture of SiC powder and carbon is heated in contact with molten silicon. The reaction results in a nearly complete conversion to SiC, although according to Schwartz, most materials made by this process contain an excess of carhon or silicon. ... 

In the sintering of such materials as silicon nitride, a silica-rich liquid phase is formed which remains in the sintered body as an intra-granular glass, but this phase, while leading to consolidation, can also lead to a deterioration in the high-temperature mechanical properties. 

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