Werkstoffe der technischen Keramik

3.4.3.1 Carbides

3.4.3.1.1 Silicon carbide

By far the most important carbide ceramics are materials based on silicon carbide (SiC). Diverse types are manufactured, depending on the intended purpose, but all are characterised by the typical properties of silicon carbide, such as

• very high hardness,
• corrosion resistance, even at high temperatures,
• high resistance to wear,
• high strength, even at high temperatures,
• resistance to oxidation even at very high temperatures,
• good thermal shock resistance,
• low thermal expansion,
• very high thermal conductivity,
• good tribological properties and
• semiconductivity.
  

The typical properties mentioned above are more or less pronounced in the different varieties of the material. Depending on the manufacturing technique, it is necessary to distinguish between self-bonded and second-phase bonded silicon carbide ceramics, as well as between open porous and dense types.

The type and proportion of the bonding are decisive in determining the corresponding characteristic properties of the silicon carbide ceramic.

Carbide

Silicate bonded silicon carbide is manufactured from coarse and medium grained SiC powders, sintered with 5 to 15 % aluminosilicate binder in air. However, strength, corrosion resistance, and above all the high-temperature characteristics, are determined by the silicate binding matrix, and lie below those of non-oxide bonded SiC ceramics. The binding matrix begins to soften at very high application temperatures, and the material begins to deform under stress. The advantage lies in the comparatively low manufacturing cost. Applications for this material include, for example, plate stackers used in the manufacture of porcelain.

Figure 30: Microstructure of a coarse, silicate-bonded silicon carbide

Figure 31: Microstructure of a fine-grained, silicate-bonded silicon carbide

Liquid-phase sintered silicon carbide (LPSIC) is a dense material containing SiC, a mixed oxynitride SiC phase, and an oxide secondary phase. The material is manufactured from silicon carbide powder and various mixtures of oxide ceramic powders, often based on aluminium oxide. The oxide components are responsible here for the density which, at approx. 3.24 g/cm3, is somewhat higher than that of SSIC. The components are compressed in a pressure sintering procedure at a pressure of 20-30 MPa and a temperature of more than 2,000°C.
The material is also characterised by a fine-grained matrix with grain sizes < 2 µm, by being almost entirely free from pores, by very high strength and fracture toughness.
LPSIC lies somewhere between SSIC and the high strength and toughness of Si3N4, from the point of view of mechanical properties,

Figure 32: Microstructure of liquid-phase sintered silicon carbide

Pressureless sintered silicon carbide (SSIC) is produced using very fine SiC powder containing sintering additives. It is processed using forming methods typical for other ceramics and sintered at 2,000 to 2,200° C in an inert gas atmosphere. As well as fine-grained versions, with grain sizes < 5 ?m, coarse-grained versions with grain sizes of up to 1.5 mm are available. SSIC is distinguished by high strength that stays nearly constant up to very high temperatures (approximately 1,600° C), maintaining that strength over long periods!
This material displays an extremely high corrosion resistance in acidic and basic media, and this too is maintained up to very high temperatures. The coarse-grained versions offer particular advantages. These properties are outstanding among high-temperature ceramics, and are complemented by high thermal shock resistance, high thermal conductivity, high resistance to wear, and a hardness close to that of diamond. Thus, SSIC is ideal for extremely demanding applications, for example, slip ring seals in chemical pumps, bearing bushes, high temperature burner nozzles, or as kiln furniture for very high application temperatures. The use of SSIC with graphite inclusions improves the performance of tribological systems.

Figure 33: Microstructure of an SSIC (unetched)

Figure 34: Microstructure of coarse-grained SSIC (etched)

Hot-pressed silicon carbide (HPSIC) and hot isostatic pressed silicon carbide (HIPSIC) exhibit even better mechanical specifications compared to pressureless sintered SSIC, since the products are nearly free of pores due to the application of mechanical pressures reaching up to about 2,000 bar. The axial (HP) and the isostatic (HIP) pressing techniques limit the parts to be made to relatively simple or small geometries, and involve greater expense compared with pressureless sintering. As a result, HPSIC and HIPSIC are used exclusively in the most demanding applications.

Reaction bonded silicon infiltrated silicon carbide (SISIC) is composed of approximately 85 to 94 % SiC and correspondingly 15 to 6 % metallic silicon (Si). SISIC has practically no residual porosity.

This is achieved by infiltrating a formed part of silicon carbide and carbon with metallic silicon. The reaction between the liquid silicon and the carbon leads to SiC bonding between SiC grains, and the remaining pore volume is filled with metallic silicon. The advantage of this technique is that, in contrast to powder sintering technologies, no shrinkage takes place during the infiltration process. In this way unusually large parts with very precise dimensions can be manufactured. The application of SISIC is limited to approximately 1,380° C due to the melting point of metallic silicon. Below this temperature, SISIC exhibits very high strength and corrosion resistance combined with good thermal shock resistance and wear resistance. SISIC is thus ideal as a material for highly stressed kiln furniture (beams, rolls, supports etc.) and various burner parts for direct and indirect combustion (flame tubes, recuperators and jet pipes).
It is also useful in machine construction for components that must be highly resistant to wear and to corrosion (slip ring seals).

Figure 35: Microstructure of SISIC

Figure 36: Microstructure of coarse-grained SISIC

Recrystallized silicon carbide (RSIC) is a pure silicon carbide material with approximately 11 to 15 % open porosity. This material is sintered at very high temperatures from 2,300 to 2,500° C, at which a mixture of extremely fine and coarse grains is converted to a compact SiC matrix without shrinkage. As a result of its open porosity, RSIC possesses lower strength in comparison to dense silicon carbide ceramics.

Figure 37: Microstructure of RSIC

Due to its porosity, RSIC demonstrates outstanding thermal shock resistance. Analogous to SISIC, this shrinkage-free sintering technique allows the manufacture of large parts that are used primarily as heavy duty kiln furniture (beams, rollers, supports etc.), for example in the porcelain industry. Due to its open porosity, this material does not resist oxidation over long periods, and is subject to a certain amount of corrosion when applied as kiln furniture or as a heating element. The maximum application temperature lies between 1,600 and 1,650° C.

Nitride bonded silicon carbide (NSIC) is a porous material, having a porosity of between 10 and 15 %, of which between 1 and 5 % is opened porosity. The manufacturing process is shrinkage-free, and involves a moulded body of silicon carbide granulate and metallic silicon powder being nitrided in an atmosphere of nitrogen at approx. 1,400 °C. The initially metallic silicon changes here to silicon nitride, creating a bond between the silicon carbide grains. The material is then exposed to an oxidising atmosphere at a temperature above 1,200 °C. In this way a thin oxidation layer of glass is created.

Figure 38: Microstructure of an NSIC

The effect of the matrix of silicon nitride is that workpieces made from NSIC are only wetted with great difficulty by molten non-ferrous metals.
The bending strength of NSIC his greater by approx. 100%, because the pore size is smaller than that of RSIC. It is also more resistant to oxidisation, while the tougher surface means that it does not deform during its service life. This material is particularly suitable for use in highly stressed kiln furniture at up to 1,500°C.

3.4.3.1.2 Boron carbide

Boron carbide ceramics (B4C) are manufactured, similarly to silicon carbide ceramics, from sub-micron B4C powder in an inert gas atmosphere at temperatures above 2,000° C without pressure (SBC), hot pressed (HPBC), or hot isostatic pressed (HIPBC). Boron carbide ceramics are distinguished by their outstanding hardness, only exceeded by cubic boron nitrides and diamond. The mechanical properties of boron carbide ceramics are similar to those of silicon carbide, with the exception of comparatively high wear resistance. The combination of very low density (2.51 g/cm3), high mechanical strength and high elastic modulus makes this type of ceramic particularly attractive in the field of ballistic protection. Boron carbide ceramics can only be used to a maximum of 1,000° C in oxidising atmospheres, since it oxidises very rapidly at higher temperatures.

Borcarbid