Some engineering and commercial ceramics

10.3 Some engineering and commercial ceramics

10.3.1 Alumina

Alumina (Al 2 O 3 ) is probably the most commonly used ceramic. It exists in two crystal forms, α -Al 2 O 3 and γ-Al 2 O 3 , but α-alumina, or corundum, is of engineering importance and is shown in Figure 10.2. The structure is basically cph with layers of close-packed O 2 − anions in ABABAB. . . sequence, with two-thirds of the octahedral sites filled with Al 3 + cations to give the stoichiometric ratio and a coordination 6:4. γ-Alumina has the O 2 − anions in an fcc arrangement giving a ‘defect’ spinel structure. It nevertheless has useful engineering properties in catalytic applications. The mechanical strength, hardness and wear resistance, coupled with the low density of alu- mina, has been utilized in a wide variety of applications – for example, rotating seals in water pumps and washing machines, cutting tools, agricultural spray nozzles, shaft bearings, ball valves in general engineering and hip-joint prosthetic devices, dental implants in biomaterial applications (Chapter 12). The exceptional insulating properties of the ceramic are widely used in the electrical and electronic industries as substrates for circuitry, sealed packaging for semiconductor microcir- cuits, radio-frequency-transmitting valve envelopes, waveguide windows and transistor housings.

The spark plug insulator exploits a wide range of alumina properties to fulfill its application in the very demanding conditions of engines, i.e. pressure, explosive temperatures, high voltage, corrosive gas environments, cyclic fatigue.

In all the above applications the alumina is processed to have a fine-grained structure (0.5–20 µm). Alumina may also be used in large tonnages in a coarse-grained form. Its high melting point (2050 ◦ C) and heat resistance make it appropriate for furnace refractories. In high-purity form it is used for thermocouple sheaths and for liquid metal filters.

10.3.2 Silica

Silica is a refractory ceramic widely used in steel and glass furnaces. The bricks are made by kiln-firing quartz around 1450 ◦

C to convert the quartz into a more open, less dense, form of mainly cristo- balite, with tridymite. These structures are shown in Table 10.3. The high-temperature modification

520 Physical Metallurgy and Advanced Materials Table 10.3 Principal crystalline forms of silica.

Form

Density (kg m −3 ) Cristobalite

Range of stability ( ◦ C)

Modifications

1470–1723 (m.p.)

Figure 10.3 Structure of β-cristobalite ( from Kingery, Bowen and Uhlmann, 1976; by permission of Wiley-Interscience).

β -cristobalite undergoes an α → β transformation at 270 ◦

C accompanied by a significant 3% volume expansion. To avoid thermal stress cracking, the structure has to be cooled very slowly below 700 ◦ C and in furnace use is usually kept above this temperature throughout the life of the furnace lining.

The structure of β-cristobalite is shown in Figure 10.3, with small Si 4 + cations located in a cubic arrangement similar to that of diamond. The larger O 2 − anions form SiO 4 4 − tetrahedra around each of the four occupied tetrahedral sites, with each Si 4 − equidistant between two anions. The regular network structure of corner sharing tetrahedra has a fourfold coordination of anions around a cation, with CN = 4:2 the overall coordination of the structure.

As shown in Figure 10.3, the oxygen anions are bigger than the cations and their grouping in space determines the form of the structure. As the anion and cation sizes become more nearly equal, the paired coordination numbers change to 6:3 and then 8:4, typical of the structure groups represented

by rutile (TiO 2 ) and fluorite (CaF 2 ) respectively.

10.3.3 Silicates

The silicates make up a variety of structures with a wide diversity of properties. A classification is

shown in Table 10.4. Their structure is based on the SiO 4 tetrahedron.

Non-metallics I – Ceramics, glass, glass-ceramics 521 Table 10.4 Classification of Silicate structures.

Type of silicate

(Si 4 ⫹ ⫹ Al 3 ⫹ ):O 2⫺ ∗ Arrangement

Examples

of tetrahedra †

Mineralogical Chemical name

name

Zircon, olivines, garnets Sorosilicate

Nesosilicate ‘Orthosilicate’ 1:4

Linear chains

Amphiboles, pyroxenes

Inosilicate ‘Metasilicate’

Flat sheets

Micas, kaolin, talc

Feldspars, zeolites, ultramarines

∗ Only includes A1 cations within tetrahedra. † ∆ represents a tetrahedron.

Zircon (ZrSiO 4 ) has the characteristic Si:O ratio of 1:4 for nesosilicates. Its structure comprises isolated SiO 4 4 − tetrahedra. Its main industrial use is in the pottery industry, supporting ceramic ware during kiln firing. The olivines (Mg 2 + ,Fe 2 + ) 2 SiO 4 4 − have compositions ranging from fosterite (Mg 2 SiO 4 , m.p. 1840 ◦

C) to Fe 2 SiO 4 (fayalite, m.p. 1200 ◦

C) with useful refractory properties. The garnets M 2 3 + M 3 2 + (SiO 4 ) 3 have divalent cations Ca 2 + , Mg 2 + , Mn 2 + or Fe 2 + and trivalent cations

Al 3 + , Cr 3 + , Fe 3 + or Ti 3 + and are extremely hard and colorful. Representatives of the double-strand linear chains of SiO 4 tetrahedra include the fiber-forming asbestiform minerals, amosite (brown asbestos), crocidolite (blue asbestos) and chrysolite (white asbestos). The backbone of the chain of Si 4 O 11 units runs parallel to the fiber direction and allows cleavage parallel to that direction. Amosite fibers have been used for high-temperature insulation and are acid resistant, but are brittle. Chrysolite fibers are strong and flexible, and can be woven for brakes, conveyor belts and asbestos/cement composites. Nowadays, much more is known about the health hazards of these minerals and their industrial manufacture, and use is carefully regulated.

Kaolinite is an example of a silicate with a layered structure (Si:O = 2:5). It has the chemical formula Al 2 Si 2 O 3 (OH) 4 and is the commonest clay mineral and a major constituent of china-clay.

A schematic representation of the layers is shown in Figure 10.4. In each layer a sheet of SiO 4 4 −

522 Physical Metallurgy and Advanced Materials

OH AI O, OH

Si O

Figure 10.4 Schematic representation of two layers of kaolinite structure ( from Evans, 1966, by permission of Cambridge University Press).

tetrahedra lies parallel to a sheet of AlO 2 (OH) 4 octahedra, with the two sheets sharing common O 2 − anions. There is strong ionic and covalent bonding within the layers but weak Van der Waals between the layers, which accounts for the softness, easy cleavage and moldability of the mineral. Another

layered structure includes talc (Mg 3 Si 4 O 10 (OH) 4 ), which has tetrahedral–tetrahedral layering, in contrast to the tetrahedral–octahedral layering of kaolinite. Another is mica, which allows perfect cleavage parallel to the silicate sheets.

Framework structures in which the SiO 4 4 − tetrahedra share all four corners to form a regular and extended three-dimensional network (see Table 10.4) include the feldspars and zeolites. The zeolites have an unusual open structure with tunnels and/or polyhedral cavities. They are used as molecular sieves when the tunnel dimensions are controlled to separate molecules of different sizes in a flowing

+ Si 4 + ):O 2 − ratio is 1:2 for zeolites to which water molecules (H 2 O) are loosely bound. Dehydrated zeolites are easily produced by heating the mineral without

gaseous mixture. Overall the (Al 3 +

altering the structure. In this condition it is used for absorbing gases (e.g. CO 2 , NH 3 ). Dehydrated zeolites have a large surface area/mass ratio and are increasingly used as catalysts in the petrochemical industry.

10.3.4 Perovskites, titanates and spinels

There are many other complex oxides, of which the perovskites, titanates and spinels are industrially important. Although CaTiO 3 is a prototype perovskite, the development of the ceramic superconduc- tors with a critical temperature T c ∼ 35 K, well above liquid hydrogen temperature, has brought the perovskite YBa 2 Cu 3 O 7 −x into prominence. The field has developed considerably in the last 10 years, with a wide range of new superconducting oxides (see Section 5.7.5) researched. Barium titanate has also been widely studied for its electrical properties. It is cubic below 120 ◦

C and tetragonal above. The new structure exhibits marked ferroelectric characteristics. The basic spinel MgAl 2 O 4 has also been widely developed to produce other valency II–III spinels, II–IV spinels

2 4 , Mg 2 Ge O 4 and Ag 2 Mo O 4 ). Most spinels are of the II–III type. The II–III MgAl 2 O 4 has a cubic unit cell comprising eight fcc sub-cells and overall contains

and I–VI spinels (e.g. Mg 2 + Al 3 + O 2 −

Non-metallics I – Ceramics, glass, glass-ceramics 523

32 O 2 − anions in fcc arrangement. The cations are distributed among the tetrahedral (CN = 4) and octahedral (CN = 6) interstices of these anions. Ferrites (NiFe 2 O 4 and CoFe 2 O 4 ) form a spinel structure in which the allocation of cations to tetrahedral and octahedral sites is reversed. This reversal produces new and significant magnetic and electrical characteristics. Thus, NiFe

2 O 4 may be written as Fe 3 + (Fe 3 + Ni 2 + )O −

4 , which indicates

that half the Fe cations are in tetrahedral sites and the remainder of Fe enter the octahedral sites

with Ni 2 + . The degree of reversal λ is influenced by heat treatment and cooling, varying between λ

= 0 and 0.5. Magnetite is an ‘inverse’ spinel with a formula Fe 3 + (Fe 3 + Fe 2 + )O 2 −

4 and λ = 0.5.

10.3.5 Silicon carbide

Silicon carbide is extremely hard (H v = 30 kg mm −2 ), has outstanding high-temperature strength, abrasive resistance, chemical inertness and high thermal conductivity. It can be manufactured in a wide variety of shapes and forms from powders and polycrystalline products to fibers and single- crystal whiskers. It exists in two structural forms, non-cubic α-SiC (hexagonal) and cubic β-SiC up to 2100 ◦

C. The main source of the material is the production of powder via the Acheson carbothermic process, which reacts silica sand with coke at a temperature of 2600 ◦

C with a heavy current passed through a graphite conductor (SiO 2 + 3C → α-SiC + 2CO). There are several methods of forming the powder, including drypressing, HIPing, slip casting, extrusion and injection molding. The main methods of firing are hot pressing (HP SiC), pressureless sintering (S SiC) and reaction sintering (Si SiC).

Because of its chemical inertness and high temperature strength, SiC has been particularly applied to smelting processes, e.g. zinc and iron. In zinc smelting these are SiC components such as distil- lation retorts, trays and the rotating condensation impellers which have to withstand the action of molten zinc and zinc vapor. In iron making, silicon carbide has been used to line the water-cooled bosh and stack zones of iron-smelting blast furnaces, where its high thermal conductivity and abrasion resistance are very important. At ambient temperatures, SiC is used in machine components subjected to abrasive wear (e.g. mechanical seals, bearings, slurry pump impellers, wire dies, fiber spinnerets). In high-temperature engineering, silicon carbide is now regarded, together with silicon nitride and the sialons, as a leading candidate material for service in heat engine designs which involve operation at temperatures in excess of 1000 ◦

C (e.g. glow plugs, turbocharger rotors, turbine blades and vanes, rocket nozzles). Glow plugs minimize the hazards of ‘flame-out’ in the gas turbine engines of aircraft. Their function is to reignite the fuel/air mixture. They must withstand considerable thermal shock

(Section 10.6.1), since temperature rises rapidly on start-up. An old established application makes use of the electrical conductivity in resistor elements for furnaces (e.g. Globars) which can operate in air up to 1650 ◦ C.

10.3.6 Silicon nitride

Silicon nitride (Si 3 N 4 ) is usually prepared by the reaction of silicon powder with nitrogen at 1200 ◦ C according to 3Si + 2N 2 → Si 3 N 4 , an exothermic reaction which needs careful temperature control. It exists in two crystalline hexagonal forms, α and β, which are predominantly covalent. The α-form consists of interleaved corrugated sheets of eight- and 12-membered rings of silicon and nitrogen atoms. Each nitrogen atom is bonded to three silicon atoms in a trigonal configuration and each silicon tetrahedrally bonded to four nitrogen atoms. In β the sheets are stacked in a regular manner to give void channels parallel to the c-axis. The final product has a high modulus (320 GN m −2 ), low thermal expansion coefficient (3 × 10 −6 K −1 ) and bulk density of ∼2700 kg m −3 . It is strong, hard, and resistant to wear and thermal shock. Its applications include cutting tools, bearings, foundry equipment, furnace parts, metal molding dies and heat engine components.

524 Physical Metallurgy and Advanced Materials

Figure 10.5 The crystal structure of β-Si 3 N 4 and β ′ -(Si, Al) 3 (O, N ) 4 :( • ) metal atom, (

non-metal atom ( from Jack, 1987, reprinted by permission of the American Ceramic Society).

Industrially there are a number of production routes for silicon nitride, reaction bonded (RBSN), hot-pressed silicon nitride (HPSN) and hot isostatic pressed silicon nitride (HIPSN). For HPSN the silicon nitride powder is mixed with a fluxing agent (MgO, Y 2 O 3 or Al 2 O 3 ) and induction heated in graphite dies at 1850 ◦

C. Hot pressing increases the bulk density and improves strength and corrosion resistance. HIPing at pressures up to 300 MN m −2 improves the density even further and closes pores, but with a substantial amount of shrinkage. Producing a fully dense material to precise dimensional tolerances unfortunately leads to some engineering limitations, which the sialons have overcome.

10.3.7 Sialons

The sialons are a very important group of engineering ceramics consisting of phases in Si–Al–O–N and related systems with additions of Li, Be, Mg, Ca, Sc, Y or La to produce M–Si–Al–O–N.

Structurally they are built up of (Si,Al)(O,N) 4 tetrahedra. β ′ -Sialon, a silicon–aluminum oxynitride,

is isostructural with β-Si 3 N 4 (Figure 10.5).

Four- and five-component systems are quite complex and are described in a behavior diagram represented in Figure 10.6, with all the concentrations expressed in equivalents. 1 The standard sialon square is derived from the tetrahedron made up of the four elements, Si, Al, O, N with the edges calibrated in equivalent percentages. The four components sit midway along a tetrahedron edge and form a square (Figure 10.7). The β ′ -sialon phase extends along a wide range of homogeneity and

varying hexagonal unit cell dimensions. The general formula is Si (24 −4z)+ Al 3z z + 6 −z O 2z z − N (24 −3z)− 8 −z , where z is the number of nitrogen atoms replaced by oxygen atoms. The value of z ranges from 0 to 4.

1 Equivalent % oxygen = 100 (at.% O × 2)/(at.% O ×2 + at.% N × 3). Equivalent % nitrogen = 100% − equivalent % oxygen.

Equivalent % aluminum = 100 (at.% Al × 3)/(at.% Al × 3 + at.% Si × 4). Equivalent % silicon = 100% − equivalent % aluminum.

Non-metallics I – Ceramics, glass, glass-ceramics 525

Si 3 O 6 6/13 (3Al 2 O 3 , 2SiO 2 ) Al 4 O 6

Equiv. %N

Liquid

4/7 (3Al 2 O 3 , AlN) 4/3 (Al 2 O 3 , AlN)

Si 3 N 4 1800⬚C

Al 4 N 4 Equiv. % Si

Equiv. % Al

Figure 10.6 Si–Al–O–N behavior diagram at 1800 ◦

C ( from Jack, 1987, reprinted by permission of the American Ceramic Society).

Al 4 N 4 % equivalent Al

% equivalent N

Figure 10.7 Relation between Si–Al–O–N tetrahedron and square Si 3 O 6 –Al 4 O 6 –Al 4 N 4 –Si 3 N 4 plane.

Compositions in the Si–Al–O–N system between β ′ -sialon and the Al 3 N 4 corner show six phases with structures based on wurtzite-type AlN. They are a series of polytypoids which exhibit changes in structural stacking in one dimension. These have the symbols 8H, 15H, 12H, 21R, 27R and 2H.

The metals Li, Be, Mg, Sc and others can replace Si and Al in polytypoids, extending the range of homogeneity.

β ′ -Sialons may be prepared in several ways with different powder mixes: (i) Si 3 N 4 –AlN–Al 2 O 3 , (ii) Si 3 N 4 –AlN–SiO 2 and (iii) Si 3 N 4 –SiO 2 –21R (a polytype). The nitride always has a thin surface oxide film so the problem is avoided with the polytypoid and produces a finer and more homoge- neous product. Relatively simple fabrication techniques are available, similar to those used for oxide ceramics, and pressureless sintering enables dense complex shapes to be made.

Yttrium β ′ -sialons are very successful cutting tool materials and used for wire-drawing dies, bear- ings and other hard-wearing applications. After densification with Y 2 O 3 low z-values of β ′ -sialon

526 Physical Metallurgy and Advanced Materials

1 kg load 2000 10 s indent time

Sialon

mm 1000 v (kgf

H Alumina 500

WC/Co

Temperature (⬚C)

Figure 10.8 Hot hardness of sialon, alumina and WC/Co cutting tool tips ( from Jack, 1987, reprinted by permission of the American Ceramic Society).

mixture when cooled from 1800 ◦

C give a β ′ -sialon with a grain boundary Y–Si–Al–O–N glass. This structure of (β ′ -sialon + glass) is strong and resists thermal shock at temperatures up to 1000 ◦ C. Controlled cooling, or heat treatment at about 1400 ◦

C, brings about a reaction of the matrix with the glass to give another β ′ -sialon and an intergranular Y–Al–garnet (YAG). This product has excellent oxidation and creep resistance.

The relative ease with which sialons can be manufactured into shapes and their ability to densify fully during sintering at temperatures ∼1800 ◦

C without the need for pressure makes them very attrac- tive engineering materials. Their strength, stability at high temperature, shock resistance, mechanical wear characteristics and ability to withstand molten metal are all special attributes. Figure 10.8 shows the superiority of sialon over cemented carbides for cutting tool tips, at higher cutting speeds and depths.

10.3.8 Zirconia

Zirconia (ZrO 2 ) exists in three crystalline forms, cubic, tetragonal and monoclinic (see Figure 10.9), and is able to harness the transitions to produce a material with reduced notch sensitivity and increased fracture toughness ( ∼20 MN m −3/2 ). The transformation-toughening property depends on retaining the tetragonal (t) phase to room temperature in a metastable condition. This is achieved by adding

Y 2 O 3 , MgO or CaO. In the presence of a crack propagating into or near a metastable region, the stress field at the crack tip causes the t-crystals to transform to the stable but less dense monoclinic phase m-ZrO 2 . The transformation is martensitic in character, with a 3–5% volume expansion, which helps to close the crack and relieve stresses at the tip. The phase diagram for zirconia-rich solid solutions is shown in Figure 10.9 for ZrO 2 –Y 2 O 3 . Three zirconia-based ceramics CSZ, TZP and PSZ are superimposed on the diagram. The CSZ ceramic is

Non-metallics I – Ceramics, glass, glass-ceramics 527

Temperature ( Monoclinic 500

Y 2 O 3 (% w/w)

Figure 10.9 Schematic phase diagram for ZrO 2 −Y 2 O 3 system: all phases depicted are solid solutions. TZP = tetragonal zirconia polycrystal, PSZ = partially stabilized zirconia, CSZ = cubic-stabilized zirconia.

200 nm

Figure 10.10 Electron micrograph of tetragonal zirconia polycrystal stabilized with 3 mol% yttria (with acknowledgement to M. G. Cain, Centre for Advanced Materials Technology, University of

Warwick, UK).

fully stabilized cubic and does not undergo transformation toughening but has other high temperature refractory uses. The TZP ceramic contains 2–4% Y 2 O 3 and is produced in fine-grained form by sintering in the tetragonal phase field, which is retained at room temperature (Figure 10.10). The PSZ ceramic, partially stabilized, has 8–10% Y 2 O 3 . Sintering in the temperature range 1650–1850 ◦ C produces a cubic solid solution which is then modified by heating at 1100–1450 ◦

C to precipitate t-crystals within the cubic c-grains. The morphology of the precipitated crystals could be changed from platey to cuboid or lenticular by modifying with CaO or MgO. At temperatures around 900– 1000 ◦

C, close to the t–m transition temperature, the toughening mechanism becomes less effective. Thermal cycling has the same effect of gradually reducing the fracture toughness. Fortunately, other

528 Physical Metallurgy and Advanced Materials

(a)

(b)

Figure 10.11 Crystalline (a) and non-crystalline (b) forms of the same composition ( from Kingery, Bowen and Uhlmann, 1976; by permission of Wiley-Interscience).

forms of stabilization, e.g. HfO 2 , have been developed which have better high-temperature fracture resistance properties. Apart from the fracture-toughening properties, zirconia has other good high-temperature ceramic properties, e.g. high melting point, chemical durability, high strength and hardness, and has many uses based on them. Also above 1200 ◦

2 C, ZrO becomes electrically conductive and is used as heating elements under oxidizing conditions. Its thermal expansion characteristics, which are similar to

metallic alloys, allow for its use as components in heat engines.