Creep silicon nitrides

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). 

The covalently-bonded silicon carbide, silicon nitride, and sialons (alloys of Si3N4 and AI2O3) seem to be the best bet for high-temperature structural use. Their creep resistance... 

Lofaj F, Wiederhorn SM, Long GG, Jemian PR (2001) Tensile Creep in the next Generation Silicon Nitride. In Singh M, Jessen T (eds) 25th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures A (Ceram Eng Sci Proc 22). Am Ceram Soc, Westerville, OH, p 167... 

Ohji T (1994) Tensile Creep Rupture And Subcritical Crack Growth of Silicon Nitride. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. Kluwer Academic Publishers, Netherlands, p 339... 

Nixon, R.D., Koester, D.A., Chevacharoenkul, S. and Davis, R.F. Steady-state creep of hot-pressed SiC whisker-reinforced silicon nitride , Composites Sci. Tech., 37 (1990) 313-328. 

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... 

Melendez-Martinez, J J., Gomez-Garcla, D., and Dominguez-Rodriguez, A., Acritical analysis and a recent improvement of the two-dimensional model for solution-precipitation creep application to silicon nitride ceramics , Phil. Mag., 2004, 84, 2305-16. 

Jin, Q., Wilkinson, D.S. and Weatherly, G.C., (1999), High-resolution electron microscopy investigation of viscous flow creep in a high-purity silicon nitride , J. Am. Ceram. Soc., 82 (6), 1492-1496. 

Rendtel, A., Htibner, H., Hermann, M., Schubert, C., Silicon nitride/silicon carbide nanocomposite materials II, Hot strength, creep and oxidation resistance, J. Am. Ceram. Soc., 81(5), 1998, 1109-1120. 

B. J. Hockey, S. M. Wiederhorn, W. Liu, J. G. Baldoni,and S. T. Buljan, Tensile Creep of SiC Whisker-Reinforced Silicon Nitride, in Proc. 27th Auto. Tech. Dev. Contractors Coord. Meeting, Vol. P-230, Society of Automotive Engineers, Warrendale, PA, 1990, pp. 251-257. 

Fig. 4.2 Creep data on a commercial vintage of silicon nitride sintered with 4 w/o yttria. These data were taken from an early version of NT154. Later versions were heat treated to reduce the amount of primary creep. Primary creep occurs over the entire test period. Figure from Ref. 33.

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... 

Osborne Reynolds was the first to show that tightly packed granular solids expand their volume when deformed.84 This phenomenon is called dilatancy. It is well understood and is discussed in some detail in the literature on soil mechanics.85-87 In vitreous-bonded structural materials such as silicon nitride, dilatancy has been suggested as a contributing factor in the formation of cavities,88 and may be an important factor in the cavitation of ceramic matrix composites.64 Dilatancy has also been suggested as an important factor in controlling the creep and creep relaxation of glass-ceramics.89... 

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.

R. D. Nixon, D. A. Koester, S. Chevacharoenkul, and R. F. Davis, Steady State Creep of Hot Pressed SiC Whisker Reinforced Silicon Nitride, Comp. Sci. Tech., 37, 313-328 (1990). 

M. Giirtler and G. Grathwohl, Tensile Creep Testing of Sintered Silicon Nitride, in Proceedings of the Fourth International Conference on Creep and Fracture of Engineering Materials and Structures, Institute of Metals, London, U.K., 1990, pp. 39 408. 

---