Silicon creep behavior

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

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.

M. K. Cinibulk, G Thomas and S. M. Johnson, Strength and Creep-Behavior of Rare-Earth Disilicate Silicon-Nitride Ceramics, J. Am. Ceram. Soc. 75, 2050-55 (1992). 

Comparison of the creep behavior of silicon carbide fibers, pp. 27-36 in... 

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. 

Ohji, T., A. Nakahira, T. Hirano, and K. Niihara. 1995. Tensile creep behavior of alumina/silicon carbide nanocomposite. Journal of the American Ceramic Society 77(12) 3259-3262. ... 

R. Bodet, X. Bourant, J. Lamon, and R. Naslain, Tensile Creep Behavior of A Silicon-Carbide-Based Fibre with Low Oxygen Content, J. Mater. Sci., 30 (1995), 661-677. 

The scheme also assumes that the redistribution of the secondary phase changes the size distribution of cavities within the material, which is supported by experimental observations [23,46], Final proof of the cavitation creep model is the fact that Eq. (2) fits the experimental data over a wide range of stresses for both SN 88 and NT 154. Thus, the model shovm in Figure 13.16 provides an understanding of the overall creep behavior of silicon nitride. Cavitation is the main creep mechanism in silicon nitride, and in other similarly bonded ceramics. 

R. Bodet X. Bourrat J. Lamon and R. Naslain, Tensile creep behavior of a silicon carbide-based fiber with a low oxygen content J. Mat Sd., 30.661-677 (1995). 

L. A. Giannuzzi, K. L. Rugg and R. E. Tressler, High temperature strength and creep behavior of Carborundum alpha silicon carbide fibers, Ceram. Trans., 38,655-666 (1993). 

Q. Wei, et al., "Microstructural Changes Due to Heat-Treatment of Annealing and Their Effect on the Creep Behavior of Self-Reinforced Silicon Nitride Ceramics", A/a/. Sci. and Eng., A299,141-151 poi)... 

Q. Wei, et al., "High Temperature Uniaxial Creep Behavior of a Sintered In Situ Reinforced Silicon Nitride Ceramics", Ceramic Engineering Science Proceedings, ed. E. Usttlndag and G. Fischman, 20(3), 1999, The American Ceramic Society Westerville, OH, 463-470 G. Ziegler, "Thermo-Mechanical Properties of Silicon Nitride and Their Dependence on Microstructure", Mat. Sci. Forum, 47, 162-203 (1989)... 

An understanding of the creep behavior of ceramic materials is necessary in order to determine lifetime limits in applications where resistance to high temperatures is needed. Silicon carbide is one of the commercial ceramics that is used for various high-temperature structural applications. 

The Interaction between cyclic fatigue and creep of ceramic materials Is an area virtually unexplored. Although It has been demonstrated that the high-cycle fatigue life of silicon nitride can be enhanced by coaxing at high temperatures, little Is known about the influence of precoaxlng to the subsequent creep behavior. 

Exploratory creep tests were performed on tensile specimens of NT-154 silicon nitride at 1300 and 1370 C, These specimens were made from the same lot of the material, designated as CP-serles, used In the cyclic fatigue tests discussed In the last progress report, Creep strain was measured using the laser diffraction strain extensometer described elsewhere,2 Figure 7 shows the creep curves of specimens CP-28 and CP-10 tested at 1300 C under applied stress of 160 and 180 MPa, respectively. Both tests were shutdown Inadvertently due to equipment adjustments. The problems have been corrected since then. Therefore, the results of the tests do not represent the creep life of the specimens. However, there were sufficient data points to delineate the essential features of the creep behavior up to the steady-state phase which was clearly definable. Both curves showed a reversed Inflection at the transition between the primary and secondary creep phases, resulting In forming a bump In the otherwise smooth creep curves. 

Fig. 4. Compliance curves for creep tests of dilute silicone gels showing the effect of gel stiffness on compliance, a) t)q5ical creep behavior for different classes of materials b) time dependent strain of gels of var)dng stiffness under a constant stress of IPa c) creep compliance / of a polymer gel under varying stresses.

The unique high-temperature creep behavior of silicon oxycarbides was assumed to rely not only on the refractoriness of the carbon-containing glassy network Si O C, but also on their nano/microstructure, as described previously in this chapter. 

Silicon carbonitride ceramics were also studied with respect to their creep behavior at high temperatures (An, 1998 Shah, 2001 Zimmerman, 2002). Si-C-N samples show similar creep behavior and shear viscosity values with those determined for Si-O-C materials. Also in the case of SiCN, a creep hardening behavior was observed and was claimed to rely not necessarily on crystallization processes (as SiCN exhibits an improved crystallization resistance at high temperatures if compared to SiOC), but to a nanoscale densification creep mechanism (Shah, 2001). 

A typical tensile creep curve for a particulate reinforced ceramic matrix composite, siliconized silicon carbide (Si/SiC),28 is shown in Fig. 4.1. In comparison to the behavior of metals and metallic alloys, tertiary creep is suppressed in this material. There is only a slight upward curvature of the creep curve prior to failure. In many other ceramic matrix composites, tertiary... 

Fig. 4.1 Tensile creep curves for siliconized silicon carbide (Carborundum KX01). Over most of the data range, these data can be represented by a constant creep rate there is a short primary creep stage, and almost no tertiary creep. The rupture strain decreases with increasing creep rate. The strain to failure, =1.5%, indicates brittle behavior even at low rates of creep detormation. Figure from Ref. 28.

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