Applications of silicon carbide cular bond is strong at high temperatures and helps to

10.4.5.3 Applications of silicon carbide cular bond is strong at high temperatures and helps to

Silicon carbide has been the subject of continuous improve thermal shock resistance.

development since it was first produced by the then- Specialized and often costly processing methods are remarkable Acheson process in 1891. used to produce fine dense ceramics for demanding 1 Now it is avail-

engineering applications. The methods available for able in a wide variety of forms which range from monolithics to single-crystal filaments (whiskers). It

forming silicon carbide powders include dry-pressing, is used for metal-machining, refractories and heating HIPing, slip-casting, extrusion and injection-moulding.

elements in furnaces, chemical plant, heat exchangers, The last process requires an expendable polymeric

heat engines, etc.

binder and is particularly attractive in cases where long The extreme hardness (2500 –2800 kgf mm 2 ; production runs of complex shapes are envisaged (e.g.

Knoop indenter) of its particles and their ability automotive applications). With regard to firing, the

to retain their cutting edges at high contact main methods are akin to those developed for silicon

C) quickly established silicon nitride; each, in its own way, is intended to maximize

temperatures (circa 1000 °

carbide grits as important grinding media. The the quality of interparticle bonding. They include hot-

comparatively high cost of silicon carbide refractories pressing (HP SiC), pressureless-sintering (S SiC) and

is generally justified by their outstanding high- reaction-sintering (Si SiC).

temperature strength, chemical inertness, abrasion The hot-pressing method for producing ˛-SiC

resistance and high thermal conductivity. In the New blanks of high density was originally developed by the

Jersey process for producing high-purity zinc, SiC is Norton Co., USA. A small amount of additive (boron

used for components such as distillation retorts, trays carbide ⊲B 4 C⊳ or a mixture of alumina and aluminium)

and the rotating condensation impellers which have to plays a key role while the carbide grains are being

withstand the action of molten zinc and zinc vapour. heated ⊲>2000 ° C⊳ and compressed in induction-heated

In iron-making, silicon carbide has been used to line graphite dies. It has been suggested in the case

the water-cooled bosh and stack zones of iron-smelting of HP SiC (and S SiC) that the boron encourages

blast furnaces, where its high thermal conductivity and grain boundary/surface diffusion and that the carbon

abrasion-resistance are very relevant. However, it can breaks down the silica layers which contaminate grain

be attacked by certain molten slags, particularly those surfaces. These additives leave an intergranular residue

rich in iron oxides. This characteristic is illustrated which determines the high-temperature service ceiling.

by experience with the skid rails which support steel The hot-pressed blanks usually require mechanical

billets in reheating furnaces. This type of furnace finishing (e.g. diamond machining). Production of

C. Water-cooled complex shapes by hot-pressing is therefore expensive.

operates at a temperature of 1250 °

steel rails have traditionally been used but warp, The pressureless-sintering route (for S SiC) uses

wear rapidly and tend to form ‘cold spots’ where extremely fine silicon carbide powders of low oxygen

content. Again, an additive is necessary (B 1

Carborundum Co. Ltd: The Americans E. G. Acheson and minium C carbon) in order to promote densification.

4 C or alu-

W. A. McCallister gave the name Carborundum to their The mixture is cold-pressed and then fired at approxi-

new material, assuming that it was a combination of carbon

and corundum ⊲Al 2 O 3 ⊳ mately 2000 . Even after its true chemical identity C in an inert atmosphere. was established, Acheson retained the name, regarding it as The REFEL process for producing siliconized sili-

‘phonetic, of pleasing effect in print, even though a trifle con carbide (Si SiC) was developed by the UKAEA

lengthy’.

336 Modern Physical Metallurgy and Materials Engineering they contact the billets. Replacement with uncooled

made at the same end. Elements should be of reason- silicon carbide rails solved these problems but it was

able diameter/length and be neither too fragile nor too found that iron oxide scale from the billets could

massive.

melt and attack silicon carbide. Some improvement The presence of impurity atoms in silicon carbide was achieved by flush-mounting the rails in the

enables electron flow to take place; it is accordingly furnace floor.

classed as an extrinsic semiconductor. The electrical Silicon carbide also has a key role in recent designs

resistance of silicon carbide is very temperature- of radiant tube heaters in gas-fired furnaces. The com-

dependent, decreasing from room temperature to bination of a 60 kW recuperative burner and a radi-

C and then slowly increasing with further rise ant tube (1.4 m long ð 170 mm diameter) made from

in temperature. Because of this characteristic and Si 3 N 4 -bonded silicon carbide is shown in Figure 10.13.

the great sensitivity of cold-resistance to traces of This British Gas design allows outgoing combustion

impurities, a typical production procedure is to check products to preheat incoming air, giving high thermal

the resistance of each element (in air) with an electrical efficiency, and also keeps these gases separate from the

load per cm 2 of radiating surface which is equivalent atmosphere within the furnace chamber. The maximum

to a typical operating surface temperature (e.g. surface temperature for the radiant tube is 1350 °

15.5 W cm 2 and 1070 C. ° C). This nominal resistance Silicon carbide is electrically conductive and care

is then used to calculate the number and size of has to be taken when it is used as a refractory in the

elements required. As the element ages, its resistance structure of electrometallurgical plant. However, the

slowly increases. A constant rate of energy input to the combination of electrical conductivity and refractori-

furnace is maintained by increasing the voltage applied across the elements (e.g. by multi-tap transformer).

ness offers special advantages. For example, silicon Specialized forms of silicon carbide now find carbide resistor elements have been used since about

widespread use in engineering. At ambient temper- 1930 in indirect resistance-heated furnaces throughout

atures, they serve in machine components subjected industry (e.g. Globars). These elements act as energy-

to abrasive wear (e.g. mechanical seals, bearings, conversion devices, heating the furnace charge by radi-

slurry pump impellers, wire dies, fibre spinnerets). ation and convection. They can operate in air and inert

In high-temperature engineering, silicon carbide is gas atmospheres at temperatures up to 1650 °

now regarded, together with silicon nitride and the certain conditions can shorten their life (e.g. carbon

C but

sialons, as a leading candidate material for service pick-up from hydrocarbon gases, oxidation by water

in heat engine designs which involve operation at vapour). Service life also tends to decrease as the oper-

C (e.g. glow plugs, ating temperature is increased. Provided that service

temperatures in excess of 1000 °

turbocharger rotors, turbine blades and vanes, rocket conditions are not too severe, a life of at least 10 000 h

nozzles). Glow plugs minimize the hazards of ‘flame- can be anticipated. A double-helical heating section

out’ in the gas turbine engines of aircraft. Their func- is available as an alternative to the standard cylindri-

tion is to reignite the fuel/air mixture. They must cal shape and allows both electrical connections to be

withstand considerable thermal shock; for instance,

Figure 10.13 Operating principle of the ceramic radiant tube (from Wedge, Jan 1987, pp. 36–8; by courtesy of the Institute of Materials) .

Ceramics and glasses 337 on engine start-up the temperature rises from ambi-

ent to 1600 °

C in 20 s and falls to 900 °

C in less than

1 s if flame-out occurs. Si 3 N 4 -bonded silicon carbide

performed well in this application. Large-scale uti- lization in gas-turbine and diesel engines has been greatly inhibited by the inherent brittleness of silicon carbide. Addition of a second phase to the structure, the composite approach, is regarded as a likely way to solve this problem of low fracture toughness. In a more general sense, it is accepted that the methodology and practice of non-destructive evaluation (NDE) and proof-testing for ceramic components require further refinement.