Silicon nitride grades

Final Report 2004, EC/BBW Contract No. ICA-CT-2000-10020, FP5 INCO-Copemicus project LAMINATES (Silicon Nitride Based Laminar and Functionally Graded Ceramic Composites for Engineering Applications), project partners University of Warwick (UK), FCT Technologie (Germany), Institute for Problems of Materials Science (Ukraine), Materials Research Center Ltd (Ukraine), Institute for Problems of Strength (Ukraine), Institute of Chemical Physics (Armenia), Drexel University (USA), EMPA (Switzerland). 

Results on other composite materials are similar to those obtained by Morrell and Ashbee.56 Creep asymmetry has been demonstrated for two grades of siliconized silicon carbide,35,60,61 SiC whisker-reinforced silicon nitride,53 HIPed silicon nitride,29 and vitreous-bonded aluminum oxide.29 Again, stresses required to achieve the same creep rate were at least a factor of two greater in compression than in tension. In two grades of siliconized silicon carbide,35,58-61 the stress exponent changed from 4 at creep rates below... 

The behavior of cavities during deformation also depends on the refractoriness of the bonding matrix. In a recent study,33,34 stable cavities were observed to form at the grain boundaries of a grade of silicon nitride containing 4wt.% yttria, even though there was very tittle glass at these boundaries Fig. 4.14a. The cavities observed were reminiscent of Hull-... 

Fig. 4.14 Cavity formation in silicon nitride, (a) Lenticular cavities form at grain boundaries for grades of silicon nitride bonded by highly refractory glasses, (b) Crack-like cavities form at boundaries for grades of silicon nitride that are less refractory, (c) In addition to these interfacial cavities, more irregular, /urn-size cavities are also observed at multigrain junctions in silicon nitride. From Hockey and Wiederhom,94 Luecke et a/.101 and Wiederhom et al.100...

In Fig. 4.21, creep rupture data from a number of different grades of silicon nitride are plotted in a Monkman-Grant format.30,31,34,115 116 For purposes of comparison with metallic alloys, the temperature dependence of the Monkman-Grant curves has been ignored. As with the metallic alloys, the curves for all of the grades of material tend to plot within a relatively narrow band. These results imply that lifetime can be improved merely by improving creep rate the lower the creep rate, the longer the lifetime. 

Fig. 4.20 Monkman-Grant curves for two commercial grades of silicon nitride. Some grades give curves that are temperature-independent (a) AY6, SiC -reinforced others give a series of curves depending on temperature (b) NT154. The temperature independent curves have creep rate exponents, m, for the Monkman-Grant equation, tf = ce L, that are approximately 1, whereas the creep rate exponent for the temperature-dependent curves are greater than 1 e.g., 1.7 for NT154.

Fig. 4.21 Monkman-Grant curves for experimental and commercial grades of silicon nitride. The results from seven different studies are plotted on the same graph. With the exception of the data by Kossowsky et al., 116 data tend to plot within a relatively narrow band. Within a factor of three in lifetime prediction, all of these data can be represented by the dashed line. It is this line that is plotted in Fig. 4.17 with data from high temperature alloys.

FIGURE 1.22 Micrograph of silicon nitride powder, SN-EIO, from UBE Industries, ltd. [16] with sedigraph size distributions for various grades of this powder with 3 mV gm (EOS), 5 mVgm (E ), and 10 mVgm (ElO). 

Silicon carbide, SiC (i), and silicon nitride, Si3N4 (2), have been known for some time. Their properties, especially their high thermal and chemical stability, their hardness, and their high strength, have led to useful applications for both of these materials. The conventional methods for the preparation of SiC and SiaN4, the high-temperature reaction of fine-grade sand and coke (with additions of sawdust and NaCl) in an electric furnace (the... 

Metallurgical grade silicon is marketed in a coarsely crushed form or as a finely ground powder in different particle sizes. Powders with increased purity due to acid washing, particularly for the removal of metallic impurities, are specialty products. They are utilized, for example, in the manufacture of silicon nitride powder or reaction-bonded silicon nitride ceramic components and are therefore the starting materials for engineering ceramic specialties. 

Figure 13 Diagrams of a prototype of a MEMS drug delivery device incorporating multiple sealed compartments that can be opened on demand to deliver a drug dose. (A) Prime grade silicon wafers are sandwiched between two layers of silicon nitride and each device contained reservoirs that extended completely through the wafer. The devices also contain a cathode and an anode between which small electric potentials can be passed. (B) Each reservoir is square pyramidal in shape with one large and one small square opening. The reservoirs have a volume of approximately 25 nL and are sealed on the small square end with the anode which is a 0.3- im thick gold membrane. Abbreviation MEMS, microelectromechanical systems.

Characteristic for the late nineties was the increasing use of tough grades of silicon nitride, first of all when high feed rates are requested. With their rather low hardness, however, Si3N4 ceramics are outside the scope of the present discussion. 

The creep resistance of silicon nitride-containing Lu-doped additives was shown to be three to five orders of magnitude greater than that of earlier grades containing Y- and / or Yb-additives [74, 75]. This material has the potential for prolonged operation at... 

During recent decades, extensive efforts have been made to control the grain boundary phase and to improve the heat resistance of silicon nitride, and this has led to significant improvements in high-temperature mechanical reliability. For example, some grades of commercial silicon nitrides have shown excellent creep resistance, even at temperatures above 1400 °C (45, 46]. Subsequent XRD analyses of these materials have revealed Lu2Si207 and Lu4Si2N207 as secondary phases. 

Creep Mechanisms in Commercial Grades of Silicon Nitride... 

As shown in Figure 13.2 [21], the creep resistance of NT 154 at 150 MPa is much greater than that of earlier commercial grades of silicon nitride. Assuming a... 

Figure 13.2 Comparison ofthe creep performance in different grades of silicon nitride ceramics at 150 MPa [21],...

In this chapter, three grades of technologically mature, commercially available silicon nitride are considered. Because of their high quality, their availability on the open market, and their excellent high-temperature properties, these materials have been... 

SN 88 This is a commercial gas-pressure-sintered grade of silicon nitride produced by NGK Insulators, Ltd, Nagoya, Japan, for the hot-stage components of ceramic gas turbines. This grade of silicon nitride was extensively studied by Wiederhom et al. [20, 24, 30], by Lofaj et al. [15, 25-27, 34—37], and by Wereszczak et al. [38]. 

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