Pressure-sintered silicon nitride

Material type Reaction bonded silicon nitride (RBSN) Sintered RBSN (SRBSN) Sintered silicon nitride (SSN) Gas pressure sintered silicon nitride (GPSN) Hot-pressed silicon nitride (HPSN) Sinter HIP silicon nitride (Sinter-HIPSN) HIP-ed silicon nitride HIP-SN... 

Urashima, K., Tajima, Y, and Watanabe, M. (1992) R-curveand Fatigue behavior of Gas Pressure Sintered Silicon Nitride, in Fracture Mechanics of Ceramics, vol. 9 (ed. R.C. Bradt), Plenum Press, New York, USA, pp. 235-310. 

Figure 16.13 Relationship between thermal conductivity and four-point bending strength of various types of high-thermal conductivity Si3N4 fabricated by different routes. GPSN = gas-pressure-sintered silicon nitride SRBSN = sintered reaction-bonded silicon nitride [62].

Since the 1970s, the search for improved materials has led to a better understanding of the role of additives in the densification and microstructural development of silicon nitride-based ceramics and the consequences for final properties [1, 6]. Improvements in powder manufacture and forming techniques and the development of alternative firing processes has led to a complete family of materials including RBSN, HPSN, sintered silicon nitride (SSN), sintered reaction-bonded silicon nitride (SRBSN), gas-pressure sintered silicon nitride (GPSSN), hot isostatically pressed silicon nitride (HIPSN) and silicon nitride alloys or solid solutions termed SiAlONs, based on their elemental components. 

Tiegs, T. N., et al.. Effects of Processing Parameters on Densification and Mechanical Properties of Gas-Pressure Sintered Silicon Nitride, in Ceramic Engineering and Science Proc., Am. Ceram. Soc. Publishers, pp. 677-685 (1994)... 

ZAN. A form of pressure-sintered silicon nitride using ZrN and AIN powders as sintering aids, (referred to as AZ powders). 

Two important groups of ceramics have silicon nitride SijN as the main component Both are characterized by better toughness than ceramics in general. The first product is pressure-sintered silicon nitride SijN (Figure 44.4). 

It is not always possible to obtain a low-porosity body by pressureless sintering , i.e. by sintering at atmospheric pressure. For example, difficulties are experienced with silicon nitride and silicon carbide. More commonly it may prove difficult to combine the complete elimination of porosity with the maintenance of small crystal size. These problems can usually by overcome by hot-pressing, i.e. sintering under pressure between punches in a die, as shown in Fig. 8.9. The pressure now provides the major part of the driving force eliminating porosity and the temperature can be kept at a level at which crystal growth is minimized. 

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. 

The solid SisN4 forms as a smoke. It is captured as a powder, mixed with a carefully controlled amount of MgO additive, placed in an enclosed mold, and sintered at 1850°C under a pressure of 230 atm. The resulting ceramic shrinks to nearly full density (no pores). Because the material does not flow well (to fill a complex mold completely), only simple shapes are possible. Hot-pressed silicon nitride is impressively tough and can be machined only with great difficulty and with diamond tools. 

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.

K. R. Lai and T. Y. Tien, Kinetics of j3-Si3N4 grain growth in silicon nitride ceramics sintered under high nitrogen pressure. J. Am. Ceram. Soc. (in press). 

In general, covalently bonded materials are difficult to sinter because of their inherently low value of self-diffusivity. However, using microwave heating, silicon nitride with 20 wt.% yttria-doped zirconia has been sintered at 1400°C in a nitrogen atmosphere at a pressure 0.1 MPa. In contrast, via a conventionally heated hot isostatic press, a sintering temperature of 1850°C and a nitrogen pressure of about 180 MPa were required to densify the same silicon nitride/zirconia composition as completely as was achieved by microwave heating at 1400°C. ... 

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]. 

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