One of several non-oxide synthetic advanced ceramics, silicon nitride (Si3N4) is valued for its excellent resistance to thermal shock and its high temperature strength. In fact, it was these properties that made silicon nitride a promising material for use as ceramic engines in the energy crisis of the 1970s. Its high temperature strength and resistance to thermal shock lended it to being possible of producing an engine that could be run at temperatures that exceed the limits of metal-based internal combustion engines, thereby increasing its efficiency.
Unfortunately, silicon nitride’s $200 per kilogram price tag and catastrophic failure mechanism resulted in this research being a dead end. However, it is still a very interesting and important ceramic today.
There are a number of ways to produce silicon nitride. From a purely chemical standpoint, these reactions are the primary means of synthesizing Si3N4:
- 3Si + 2N2 = Si3N4
- 3SiO2 + 6C + 2N2 = Si3N4 + 6CO
- 3SiO(g) + 3CO + 2N2 = Si3N4 + 3CO2
- SiCl4(l) + 6NH3 = SiNH2 + 4NH4Cl
- SiNH2 = Si3N4 + N2 + 3H2
There are two common phases of silicon nitride, the α and β phases. α-Si3N4 is oxygen rich, so much so that it was long thought to be a two-phase composite material. β-Si3N4 is generally fibrous in morphology compared to the uniaxed α-Si3N4 phase, and is often transformed from α-Si3N4 into needles during the sintering process. However, pure β-Si3N4 does not sinter well and produces little to no grain growth and finds much more practical use as a fiber reinforcer in composites.
As previously mentioned, silicon nitride has thermal shock resistance superior to many other ceramics. Thermal shock resistance is a function of a material’s fracture strength (σ), thermal conductivity (K), Young’s modulus (E), and thermal expansivity (α). While silicon nitride’s mechanical properties are comparable to other advanced ceramics, its thermal expansivity (3 · 10-6 K-1) is superior to that of silicon carbide (4 · 10-6 K-1), another advanced ceramic noted for its good thermal properties.
Silicon nitride’s maximum operating temperature is governed by several reactions it undergoes at high temperatures (around 1900°C), particularly
Si3N4 = 3Si + 2N2
Si3N4 + 3SiO2 = 6SiO(g) + 2N2
The first reaction is simply the breakdown point of silicon nitride while the second reaction is an interaction between silicon nitride and its passivating silica layer which results in the volatization of solid material. However, the same passivating silica layer is what provides silicon nitride with its stability at room temperature; Si3N4 is thermodynamically unstable in the presence of oxygen due to silicon’s preference to be SiO2. The passivating SiO2 layer provides a kinetic barrier to oxidation of subsurface material, though, and results in silicon nitride’s metastability.
An even better kinetic barrier may arise from the passivating layer’s interaction with the silicon nitride below, producing a Si2N2O layer through which is harder still for oxygen to diffuse.
Because silicon nitride tends to react with its passivating SiO2 layer and preferentially undergo evaporation/condensation under sintering conditions, several interesting techniques have been developed in order to produce dense silicon nitride bodies. Reaction bonded silicon nitride involvings pressing pure silicon powder into a desired shape and firing it in a nitrogen furnace to induce the reaction between silicon and nitrogen.
The transformation of Si to Si3N4 is accompanied by a 22% volume increase, but this expansion is entirely into the body; that is, the surface silicon is nitrided first, creating a rigid shell that inhibits expansion. As the nitrogen infiltrates the pores between the silicon powder particles and reacts, the expansion of the silicon to silicon nitride transformation occurs at interparticle necks and pores, ultimately producing a body with up to 85% theoretical density.
This 15%+ porosity is due to the gas/solid nature of the reaction; without this porosity, internal regions would never be nitrided and would remain as a silicon core. It is also important to note that this process occurs near the melting point of silicon, and the nitridization reaction is exothermic. Thus, in order to prevent runaway heating and melting of the sample during the process, special furnaces that dynamically control the temperature ramp by gauging nitrogen consumption rates must be used. However, all of this effort results in a strongly bonded silicon nitride structure with fairly good creep resistance despite the porosity.
Hot pressing silicon nitride green bodies is another method of densifying bodies with simple geometry. MgO may be added to the silicon nitride to form an intermediate viscious liquid phase which promotes full density of the compact, and a combination of 2% Al2O3 and 5% Y2O3 can also be used to form a fluid glassy phase which fills grain boundaries. While this glassy phase greatly reduces the creep resistance of the resulting ceramics, it is the lesser of two evils in many applications that value a denser structure.
The addition of 3% Al2O3 and 8% Y2O3 also is used in the pressureless sintering of silicon nitride. By sintering under these reduced pressures, more liquid phase forms in the compact to help sintering. By starting with α-Si3N4 with not more than 5% β-Si3N4 present, the Y2O3 liquid can act as a solvent which dissolves the α phase and recrystallizes it as β phase on the β-Si3N4 seeds.
After the α phase is fully consumed, the result is a dense, body that possesses a glassy phase at the grain boundary but is intrinsically toughened by crack-bridging β-Si3N4 fibers; these have KIC values 2x - 3x greater than that of regular silicon nitride. Other additives to increase the strength of silicon nitride include Lu2O3.