Silicon carbide (SiC) is one of the toughest and strongest ceramic materials, maintaining strength even at elevated temperatures. Available as CVD (chemical vapor deposition), reaction bonded or sintered forms.
Pressureless sintered, hot pressing and reaction sintering SiC is known for its excellent acid and alkali resistance; however, its flexural strengths tend to be low due to poor fracture toughness and coarse grain structures.
Thermal Conductivity
Silicon carbide (SiC) is an extremely hard, strong ceramic material with excellent thermal conductivity properties and low specific thermal conductivity, ideal for turbine components in jet aircraft due to its durability and satellite subsystems requiring radiation resistance. Due to its light density (approx. 3g/cm3) it also contributes to fuel efficiency and performance; sintered SiC is generally preferred over reaction bonded SiC for such uses.
The DC resistivity of CS samples increased with increasing sintering temperature, yet decreased below 1900 degC due to uneven distribution of liquid phases that prevented carrier transport between SiC grains. Conversely, as more additives were added the opposite occurred – polished surfaces showed even distribution of liquid phases at higher concentrations which contributed towards densification of samples produced from this method.
Noteworthy was that resistivity of RS specimens were greater than those prepared with lower sintering temperatures; this may be attributable to oxygen-free sintering conditions employed for RS preforms, which prevent reactions between SiC powder and SiO2, which have detrimental effects on sintering process. Furthermore, SEM fracture surface analysis for samples prepared at 1550 degC revealed more sintering necks indicating secondary phases present due to lower temperatures of sintering.
Mechanical Properties
Sintered sic is an extremely hard and durable material designed to withstand both abrasive wear and cavitation wear, as well as thermal shock resistance, making it suitable for jet engine components where temperature regulation is crucial, satellite subsystems where radiation protection is key, as well as low density (3-3.2g/cm3) that reduces aircraft weight for improved performance and fuel efficiency. It is often specified for use in jet engine components requiring low temperatures as well as satellite subsystems where radiation protection must be prioritized. Due to these characteristics it is often specified for use in jet engine components where temperatures must remain low while satellite subsystems require radiation shielding of delicate electronics from radiation radiation from spacecraft subsystems where delicate electronics must be protected from radiation from satellite subsystems.
Reaction bonded sic is produced using reactive liquid silicon injected into porous ceramic bodies to react with carbon and form silicon carbide, which then bonds with existing a-SiC particles to produce reaction bonded sic, often at less cost and strength than direct sintered. Reaction bonded sic may not last as long or be as strong and tough.
HECs with SiC whiskers enhance fracture toughness through crack-bridging and deflection mechanisms, increasing fracture toughness by 67% of theoretical density, while those containing whiskers achieve 92% density at 1600 degC.
Samples sintered at 1950 degC with 5 weight percent YAG had an extremely fine microstructure, featuring compact internal structures with small grain sizes and few pores that significantly enhanced bending strength and fracture toughness. The addition of YAG contributes to increasing density by speeding mass transfer of SiC; additionally, larger proportions of crystal boundaries between grains helped prevent crack expansion resulting in improved bending strength and fracture toughness.
Microstructure
Silicon carbide (SiC) has long been considered an excellent material choice for aerospace components such as jet engine turbine blades and landing gears, thanks to its exceptional temperature stability, radiation resistance, low weight and thermal conductivity ranging from 32 W/m/K up to 490 W/m/K depending on microstructure and fabrication conditions – the latter factor also affects density, damage from radiation exposure and mechanical properties of SiC ceramics.
SiC powders can be produced through reaction bonding or pressureless sintered processes such as hot pressing. The resulting microstructures vary significantly and significantly affect mechanical properties: Reaction bonded SiC has a platelet-like alpha phase microstructure with occasional macroscopic pores (10 – 100m), giving rise to relatively low fracture toughness; in comparison, Hexoloy SA-type pressureless sintered SiC boasts finer microstructure with few pores for superior bending strength, fracture toughness and chemical resistance than reaction bonded SiC.
Hexoloy SA features an intriguing microstructure consisting of occasional platelet-like alpha phase crystals scattered among an equiaxed matrix of alpha grains with finely distributed microporosity and rare macropores, all combined into an extremely pure state able to withstand rigorous corrosion tests. Furthermore, Hexoloy SA’s X-ray diffraction patterns indicate the predominant grain growth mechanism has switched from interface reaction to atom diffusion; and its diffraction spectra show distinct Mo and Ti peaks with higher intensities than its bright phase counterpart, suggesting lower oxygen content within its gray phase counterpart compared with its bright counterpart.
Heat Resistance
Sintered sic has many industrial applications due to its combination of strength, hardness, corrosion and oxidation resistance and lightweight nature. From pump seals for challenging environments such as mining to counterweights to stabilize reactor components; sintered sic ceramics provide heavy-duty wear protection in high temperature conditions and environments with high wear rates.
Sic is an ideal material for protective gear due to its high density and hardness; such as ballistic armour plates that protect soldiers against projectiles fired from firearms and rifles. Furthermore, its thermal conductivity helps decrease heat load on vehicles and power engines.
Pressureless sintered silicon carbide is an economical, high-grade material with excellent mechanical properties that is produced in various shapes and sizes for production. Able to withstand temperatures up to 1650 degC, pressureless sintered silicon carbide makes an excellent choice for precision parts such as those found in high-end kiln equipment.
Reaction bonded silicon carbide (RBSiC) is produced through reaction sintering, in which liquid silicon or silicon alloy is infiltrated into a porous carbon-containing ceramic body in order to react with existing a-SiC particles and form b-SiC particles. This method provides lower sintering temperatures, shorter sintering times, and near net size formation of final products.
However, the sintering process can lead to reactions between a-SiC and b-SiC that result in new phases or volatile components that damage ceramic structures. Sintering additives must therefore be carefully chosen in order to maximize densification of b-SiC and speed up its sintering.
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