We know from experience that ceramic materials are brittle and easily broken. This is one reason why ceramics have not been used as engineering materials. Fracture is the result of crack growth through the microstructure. It was Griffith who proposed that ceramics have intrinsic cracks which grow under applied stress. The concentration of the applied stress at the crack tip decreases the strength to a level of about 1% or less of the theoretical strength. If the crack starts to grow, strength decreases so sharply that a catastrophic fracture occurs.

In spite of the brittle nature of ceramics, their application as engineering materials was proposed in the 1960s because ceramic materials made of silicon nitride or carbide have higher strength at high temperatures than metals and oxide ceramics. Non-oxide ceramics have lower thermal-expansion-coefficients than oxides, resulting in better thermal shock resistance, which is one of the most important requirements for engineering ceramics.

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.

Silicon nitride is a highly covalent bonded compound which decomposes at 1877°C. Therefore, it is impossible to densify Si3N4 without sintering additives. Densification is achieved by liquid-phase sintering usually using metal oxides such as MgO, Y2O3, A12O3, and most of the rare-earth oxides as sintering additives. The oxides react with SiO2—always present at the surface of Si3N4 particles—to form an oxide melt and, with increasing temperature, an oxynitride melt by dissolution of Si3N4. The resulting microstructure consists of elongated Si3N4 needles embedded in a matrix of smaller equiaxed Si3N4 grains and a grain boundary phase, as shown in Figure 1. The amount and chemistry of the sintering aids determine the volume fraction of the grain boundary phase. The content required for complete densification depends on the sintering techniques: 2–5 vol% additives are sufficient if densification is supported by a high external pressure (hot pressing [HP] or hot isostatic pressing [HIP]); pressureless-sintered and gas-pressure-sintered (10-MPa nitrogen pressure) materials have additive contents of up to 15 vol%. Today, silicon nitride ceramics are regarded as a class of material comparable to steel. Different qualities depend on the size and shape of the silicon nitride grains and the amount and chemistry of the grain boundary phase. Materials with a high room-temperature strength exhibit a finegrained, elongated microstructure, while materials with a high fracture toughness are more coarse-grained. In both cases, a weak interface is required to induce transgranular fracture. (See Becher et al. in this issue.) Since all Si3N4 grains are completely wetted by the grain boundary phase, the interface strength is determined by the additive composition. Nevertheless, a contradiction arises between the development of high-strength and high-toughness Si3N4 ceramics and high-temperature resistant materials because the grain boundary phase is responsible for the excellent properties at low temperatures, but limits the properties at temperatures above its softening point.

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.

The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.

The improvement of mechanical properties by microstructural control has been one of the main topics of interest in the development of silicon nitride ceramics. Toughening, by developing an in situ composite or self-reinforced microstructure, has attracted particular attention.

Microstructural design is a key factor in the optimization of processing parameters. The microstructures of sintered materials are composed of silicon nitride grains and grain boundaries, which can be either crystalline, amorphous, or partially crystalline, depending on the composition, amount of sintering additives, and processing parameters. Silicon nitride ceramics have been fabricated with an addition of metal oxides and rare-earth oxides that form a liquid phase during sintering and accelerate grain boundary diffusion. The effect of composition of the glassy phase on the mechanical properties of ceramics is presented by Becher et al. and Hoffmann elsewhere in this issue. This article focuses specifically on the design and control of grain size.

As it is well recognized, many processing parameters affect grain growth behavior and the resulting microstructure. During sintering, the α- to β-phase transformation leads to a self-reinforcing microstructure on account of the anisotropic grain growth of the stable hexagonal β- Si3N4 phase. Therefore, α-rich powders are widely used for starting materials. Phase transformation accelerates anisotropic grain growth, resulting in an increase in the fracture toughness of Si3N4 ceramics. Kang and Han discuss the effect of phase transformation on nucleation and grain growth in an article in this issue. The effect of the grain-size distribution on microstructural development is described in this article, based on studies conducted mostly with β-Si3N4 powders.

Application of silicon carbide (SiC) as a structural material has been limited thus far by its low fracture toughness, even though, in comparison to other ceramic materials, SiC has superior high-temperature strength and creep, wear, corrosion, and oxidation resistance. For automotive applications, a higher fracture toughness is required. For example, the brittleness and catastrophic fracture behavior of SiC materials have resulted in limited use in automobile exhaust-valve systems and turbocharger rotors. High-density SiC bodies can be produced by a pressureless sintering process. However, the sintered bodies often include flaws which are related to processing, primarily, the presence of agglomerates and crystallographic defects in the starting powders. The importance of grain size and shape refinement in the improvement of mechanical properties has been recognized, and thus, processing procedures and sintering aid compositions have been examined extensively. However, one of the key factors is the “as-received” powder characterization (distribution of grain sizes, polytypes, and impurities) for producing sintered bodies of SiC with consistent physical properties.

A complexity in SiC materials is that SiC can form various crystal structures having essentially the same chemical composition but a differing number of stacking layers in the unit cell. This is commonly called a polytype. There is only one crystal structure with cubic symmetry, which is identified as 3C or the β-phase. At high temperature, the β-phase transforms to α-phases with hexagonal or rhombohedral symmetry, with 4H, 15R, and 6H (Ramsdell notation) being the major polytypes observed in SiC materials. Preference of the polytype selection during the β- to α-phase transformation is dependent on the chemistry of the sintering aids and metallic impurities in the grain boundaries.

The optimization of mechanical properties through microstructural design in metallurgical industries has been used successfully for many years. In comparison, the microstructural design in ceramics has been less successful, mainly due to the complexity and difficulties associated with low diffusivity (in their monolithic form) and to the ability to be sintered at elevated temperatures. For years, many structural ceramics could not be sintered to high densities (>95% of their theoretical density) without large quantities of sintering aids. A typical example is silicon carbide (SiC), which has a unique combination of properties, such as: high hardness and wear resistance, considerable strength at high temperatures, and excellent thermal conductivity, but, due to a high level of covalent bonding and consequently low diffusion rates, it cannot be sintered to high density (without the addition of sintering aids) even at temperatures approaching 3000°C.

Since the first successful sintering of SiC with small additions of boron and carbon in the early 1970s, remarkable progress has been made in developing a wide range of properties for the microstructure, including fracture toughness from 3–4 MPa m½ to 12–14 MPa m½ four-point-bending strength from 300 MPa to over 900 MPa, and thermal conductivity from 60 to 260 W/m K.4

This article presents the most recent developments in SiC ceramics, emphasizing optimization of mechanical properties through microstructure control. The discussion is limited to β-phase SiC pressureless sintering, using oxide additives.