Creep of Silicon Nitride

Creep behavior and creep mechanisms in advanced ceramics - particularly in sflicon nitride - have been summarized in several reviews. Messier et cd. [3] reviewed 

1) The use of commercial designations is for purposes of identiBcation only, and does not imply any endorsement by the National Institute of Standards and Technology, nor does it imply that the items mentioned are the best ones for the intended application. 

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The contribution of cavitation to tensile strain in vitreous-bonded silicon nitride exceeds 75-95%. Cavitation is the main creep mechanism in the tensile creep of silicon nitride. 

Figure 15.6 Experimental temperature-compensated strain rates versus stress plots for tensile creep of silicon nitride-based ceramics. The stress exponents n = 3 and n = 10 are...

Creep of Silicon Nitride Observed In Situ with Neutron Diffraction... 

W.E. Luecke and S.M. Wiederhom, "A New Model for Tensile Creep of Silicon Nitride", Journal of the American Ceramic Society, 82(10), 2769-2778 (1999)... 

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... 

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.

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.

Tatsuki Ohji and Yukihiko Yamauchi, Tensile Creep and Creep Rupture Behaviors of Silicon Nitride, J. Am. Ceram. Soc., 76[ 12], 3105-3112 (1993). 

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.

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... 

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