Unit 11 Advanced Structural Ceramics
Structural components derived from engineering ceramics are used as monoliths, coatings and composites in conjunction with or as replacements for metals when applications rely on mechanical behavior of the ceramics and their refractory properties, that is chemical resistance to the working environment. Nickel superalloys are currently the main high temperature materials for components such as combustors in gas turbine engines. They have melting points around 1573K and a maximum working temperature near 1300K. Compared with metals, ceramics are generally more resistant to oxidation, corrosion, creep and wear in addition to being better thermal insulators. They have higher melting points and greater strength than superalloys at elevated temperature so that a major potential application, particularly for silicon nitride, is in gas turbine and reciprocating engines where operating temperatures higher than attainable with metals can result in greater efficiencies. This enhanced strength is shown in figure for hot pressed silicon nitride (HPSN), hot pressed silicon carbide (HPSC), hot isostatically pressed silicon nitride (HIPSN), sintered silicon nitride (SSN), sintered silicon carbide (SSC), reaction bonded silicon nitride (RBSN) and reaction bonded silicon carbide (RBSC). Although ceramics offer improvements in engine efficiency, incorporation of silicon nitride over the past three decades has been slow, mainly because of the difficulty in reproducible fabrication of dense components to close dimensional tolerances.
Silicon nitride occurs in two phases, the α and the β forms. The β form consists of SiN4 tetrahedra joined together by sharing corners in a three dimensional network. It is possible to replace silicon by aluminum and maintain charge neutrality in the crystal lattice by substitution of nitrogen with oxygen. The resulting solid solutions in the Si-Al-O-N system are known as β-Si3N4 over the composition range Si6-bAlbObN8-b (0<b<4). They exhibit mechanical behavior similar to β-Si3N4 and have some features of aluminum oxide. However, in contrast with Al2O3, which consists of sixcoordinated Al, β-Si3N4 contains Al that is four coordinated by oxygen and this results in an enhanced Al-O bond strength compared with the oxide. Unlike Si3N4, β'-sialons can be densified readily by pressureless sintering and they have been put into commercial production by Lucas Cookson Syalon Limited. Syalon components include automotive parts such as valves, valve guides and seats, tappets, rocker inserts and precombustion chambers in addition to weld shrouds, location pints, extrusion dies, tube drawing dies and plugs.
As far as material properties are concerned, the ceramics is assumed to provide very good thermal shock resistance. Aluminum titanate, for example, with its extremely low thermal expansion coefficient, develops outstanding thermal shock resistance. Aluminum titanate is used as port liners in some automobile engines because its low thermal conductivity (2 Wm[-1]K[-1]) reduces heat flow to the cylinder block and hence the amount of cooling required. Glass ceramics have applications in cooking utensils, tableware, heat exchangers, vacuum tube components and missile radomes. Partially stabilised zirconia was developed in 1975 and is particularly suited for withstanding mechanical and thermal shock because of its high fracture toughness. Examples are dies for extrusion of copper and aluminum tubes, diesel engine cam follower faces, valve guides, cylinder liners and piston caps, wear and corrosion resistant nozzles in papermaking equipment, wear resistant inserts such as tabletting dies as well as scissors and knives.
Not all ceramic components require high temperature strength. The high Yound's modulus (550 GPa) of titanium diboride, TiB2, makes it useful for armour plating whereas ceramics are suitable materials in seals because of their chemical resistance. Hence sintered silicon carbide is used for mechanical seals and sliding bearings whereas boron nitride, which is not wetted by glass and liquid metals, constitutes break rings in the horizontal continuous casting process for steels. Boron carbide, a harder ceramic than SiC, is suited to wear resistant applications such as grit blasting nozzles whereas Si3N4 is also used as ball bearings.
An established industrial use for engineering ceramics is as cutting tools for steels where high temperature hardness of sialons and zirconia toughened alumina together with their low reactivity towards metals are desirable properties. Cutting tools are subject to high mechanical, chemical and thermal loads on their cutting edges during maching operations. On average, 45kW power is transferred to an in attack cutter with a contact surface of only 2mm[2]. Particularly in the first cut, any interruptions and change in cutting depth give rise to a complex dynamic force that can reach a level of some 5000N. At high cutting speeds, the temperature of the cutting edge may exceed 1000C. Obviously, metals are liable to undergo plastic deformation, diffusion phenomena between tool and workpiece, and oxidation induced scaling at such temperatures. Ceramic cutting tools experience no such problems and are therefore insensitive to high cutting speeds and accordingly high cutting edge temperatures.
The latest material development is aimed at further increasing the strength of such ceramics through the incorporation of fibers or whiskers. Whisker reinforced ceramic composites hold the promise of quasi ductile strength behavior. The whisker rerinforcing principle is based on activation of the following mechanisms: transfer of stress from the matrix material to the high strength reinforcing components through the introduction of whiskers with high Young's moduli, generating compressive stresses in the matrix by introducing whiskers with higher thermal expansion coefficients, impeding crack propagation through the whiskers, diverting the cracks along the whisker matrix interface, and inducing microcracking by differences in the mechanical and thermal properties of the whiskers and the matrix. However, the nonding forces between the whiskers out of the matrix istead of tearing them apart. In whisker reinforced ceramics, the mechanisms described may occur in superimposition.
Selected from:" High Tech Ceramics", P. Vincenzini, Elsevier 1987
Words and Expressions
superalloy 超合金
reciprocate 来回
utensil 器皿
shroud 覆盖
thermal shock resistance 抗热震
radome 整流罩
cam 凸轮
corrosion resistant 抗腐蚀的
armour 盔甲
grit 磨料
whisker 晶须
ductile 延展的
impede 阻碍
superimposition 重叠
