Silicon Nitride Ceramics

Silicon nitride as a structural material

Silicon nitride is one of the most promising ceramic materials for high temperature structural components. The superior thermal and mechanical properties of this material are due to the highly covalent nature of their chemical bonds. In particular Si₃N₄ has high strength at high temperatures, good thermal stress resistance due to the low thermal coeffecient of expansion, and relatively good resistance to oxidation compared to other high temperature structural materials. This combination of properties can be used to increase operating temperatures of engines, thus increasing their efficiency. Potential applications for which Si₃N₄ is under consideration are the all ceramic gas turbine engine or in the replacement of metallic components in internal combustion engines. Moreover, other engineering applications are also under consideration, such as energy conversion systems, industrial heat exchangers, wear resistant materials in metals processing, ball and roller bearings, and inserts for cutting tools.

The high toughness of silicon nitride can be attributed to its unique microstructure. Because of the highly covalent nature of the Si-N bonds and the low diffusivity of Si or even N at higher temperatures, Si₃N₄ cannot be sintered to its full theoretical density. Hence sintering additives such as Y₂O₃ , Al₂O₃ or Mg₂O₃ are adedd to the Si₃N₄ powder which acts as a medium for liquid phase sintering. The resulting microstructure consists of equi-axed grains of alpha Si₃N₄ , and columnar grains of beta Si₃N₄ It is this unique microstructure consisting of elongated beta-Si₃N₄ grains which is responsible for the pronounced toughness of Si₃N₄. The columnar beta Si₃N₄ grains bridge cracks growing through the microstructure and shield the crack tip from the far field applied loads.

Work at Stanford: Fatigue in Si₃N₄ at ambient and elevated temperatures

Experimental Procedures at Ambient Temperatures

We study subcritical crack growth behavior under a range of applied loads and environments using fracture mechanics based techniques. Crack length at ambient temperatures is measured using a DC potential drop technique across a Ni-Cr film deposited on our specimen surface. Changes in potential caused by crack growth can be direcly calibrated to crack lengths in-situ from which stress intensities can be calculated at the crack tip. Knowing the variations in crack lengths with time, crack velocities or crack growth/cycle is plotted as a function of the applied stress intensity as is common in most fatigue literature

Cyclic Fatigue

Cyclic fatigue accounts for nearly 80% of all failures in service and is one of the most important considerations for reliability of ceramic components. Traditionally, ceramics were thought to be immune to cyclic loading because of their brittle nature, however as ceramics have been externally toughened, cyclic fatigue effects have been observed in a number of ceramic microstructures. This generally involves a degradation of some toughening mechanism. Our Si₃N₄ materials are also susceptible to such effects which lead to much faster crack growth rates compared to static loading conditions. We believe that frictional wear mechanisms degrade the interface between the grains and the matrix resulting in a reduced bridging capacity under cyclic loading conditions. Ample evidence of such effects are seen on our fracture surfaces where we see the presence of extensive wear debris.

Effect of Environment

We are also investigating the effect of different environments on crack growth rates in these materials. These interactions generally involve a dissociative chemisorption process of an environmental molecule with the crack tip. So far our studies show that environmental species plays an insignificant role in promoting or exacerbating subcritical crack growth in these materials.

High Temperature Fatigue

As these materials are intended for high temperature use where they will experience temperatures upto 1370 C, we are investigating subcritical crack growth at elevated temperatures in the range of 900-1200 C. We have developed at Stanford a unique computer controlled capability of testing compact tension specimens at these temperatures. By utlizing state of the art, high temperature extensometery we are able to measure crack lengths at these temperatures in-situ. These are correlated with optical measurements made by a telescope mounted on an XYZ translational stage. Our specially designed furnace allows close optical access of the sample at these high temperatures and a very uniform hot zone where we can monitor surface temperature on the specimen to about a degree. So far we have obtained cyclic fatigue data using this experimental appartus on a few commercial silicon nitride ceramics. We see a substantial decrease in toughness and cyclic fatigue resistance at these high temperatures as has been reported in the literature. We are also concurrently attempting to model the micromechanics at elevated temperatures.