Using Microstructure to Attack the Brittle Nature of Silicon Nitride Ceramics

Abstract

The evolution of silicon nitride ceramics over the last two decades has brought about the advancement of materials which were first fabricated by the application of mechanical pressure and temperature (i.e., hot pressing) resulting in high flexure strengths (e.g., 700–800 MPa) but rather poor resistance to creep at temperatures of ~1200°C. At the same time, these ceramics remained quite brittle with fracture-toughness values of 4–5 MPa m½, such that strengths were very sensitive to flaw or crack sizes. As a result, measured strengths exhibited considerable scatter, as reflected by a low Weibull modulus. In the ensuing years, approaches were sought to develop more economical methods of fabricating silicon nitride components by densifying to near-net shape. Methods were also sought for increasing the elevated-temperature reliability by minimizing the additives employed to promote densification and by utilizing additives that produced more stable and refractory grain boundary phases. The application of gas-pressure sintering methods, utilizing gaseous environments of 10–100 atmospheres, led to the ability to produce dense near-net shaped components with very high fracture strengths (e.g., ≥1000 MPa). At the same time, advances in processing and additive chemistry, sometimes combined with additional fabrication methods (e.g., hot isostatic pressing), have resulted in ceramics with excellent creep resistances at temperatures in excess of 1300°C. Some of these silicon nitride ceramics exceed the elevated-temperature capability of superalloys by 200°C. The initial desire for light-weight ceramic components that could sustain tensile loads for high-temperature applications is, indeed, beginning to bear fruit. One of the most impressive examples of the development of a complexly shaped lightweight component is the silicon nitride turbocharger rotor used in a number of Japanese automobiles, which is currently manufactured at a cost approaching that of the opposing superalloy rotor and provides exceptionally high mechanical reliability and production yields. Currently, there are also earnest efforts to incorporate silicon nitride valves for engines, as well as in a variety of other components (e.g., combustion swirl chambers, valve-lifter pads, etc.). The acceptance and use of this class of brittle materials, which were once considered prohibitively expensive for fabrication into complex shapes and not suited for such applications, is a remarkable testimony of the progress that has been made.