Silicon nitride sintering aids

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Silicon nitride (95% of a-modification) with 2 wt% of alumina and 5 wt% of yttria as sintering aids (layers with index 1, bulk compressive stress)... 

The values of the creep parameters (p, n and Q) identifying the superplastic behaviour of ceramic-related materials are not unique to such materials, nor to the same type of materials. As shown in the review papers, these parameters are very similar in tension as in compression in zirconia-based materials (probably the most widely studied ceramics in the widest experimental conditions), although that depends strongly on the purity of the ceramics 5,7 however, their behaviour seems to be very different in compression than in tension when an aid-sintering phase is necessary during the processing, as in silicon carbide and silicon nitride ceramics.8... 

Y NMR has also been reported extensively from yttrium sialon ceramics. Yttria is an important sintering aid for densifying silicon nitride. The properties of the sialon produced are determined by the nature of the grain boundary phase, which is usually a crystalline or glassy yttrium sialon. The often disordered nature of such phases can mean that broader 89Y resonances are... 

Hot gas corrosion behavior of various silicon-based non-oxide ceramic materials (e.g., silicon carbide, silicon nitride, etc.) can vary widely depending on the stoichiometry, structure and sintering aids. Silicon carbide materials exhibit excellent corrosion resistance towards sulfur-containing atmospheres even at a high temperature around 1,400X [Fdrthmann and Naoumidis, 1990]. 

Fig. 19. Creep behavior of silicon nitride ceramics. The composition of this ceramic contains 7 vol % Y3A15Oi2 as a sintering aid. The specimens were hot-pressed at 1750°C under a pressure of 20 Mpa. The holding times are marked on the curve. This curve indicates that crystallization of the grain-boundary phase improves the creep resistance. Garnet crystals were observed at the grain boundaries after annealing [3/]. , Sample A60 , Sample B60 A, Sample B150, x, failure.

Magnesium nitride has been used as a sintering aid for silicon nitride powder [178]. It can be prepared by the low pressure CVD of a mixture of elemental metal vapor, ammonia and nitrogen [178]. 

Silicon nitride has found many applications in which structural features play an important role. Conventionally derived silicon nitride is fairly difficult to sinter, and pressureless sintering requires large amounts of sintering aids (e.g., 8% AI2O3 and 2% Y2O3). The first chemical route , which has been applied for the formation of silicon nitride at comparably low temperature, is diimid precipitation, already reported in 1830 [36]. 

The composition of the IGF is important in determining overall creep behavior. For example, using Y2O3 as a sintering aid for silicon nitride ceramics has been found to be superior to using MgO. Other important aspects of the microstructure are the grain size and the volume fraction of liquid present. 

R. D. Nixon and P. K. Mehrotra, Optimization of Sintering Aids in Silicon Nitrides for Cutting Tool Application , Ceramic Engineering < Science Proceedings, 18, 1997, 457-464. 

Yang W, Hojo J, Enomoto N, Tanaka Y, Inada M (2013) Influence of sintering aid on the tranlucency of spark plasma-sintered silicon nitride ceramics. J Am Ceram Soc 96 2556-2561... 

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. 

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