Engineering applications of sialons The relative ease with which sialons can be shaped is

10.4.2.5 Engineering applications of sialons The relative ease with which sialons can be shaped is

one of their outstanding characteristics. Viable shap- ing techniques include pressing (uniaxial, isostatic), extrusion, slip-casting and injection-moulding; their variety has been a great stimulus to the search for novel engineering applications. Similarly, their ability to densify fully during sintering at temperatures in the order of 1800 °

C, without need of pressure application, favours the production of complex shapes. However, due allowance must be made for the large amount of linear shrinkage (20–25%) which occurs as a result of liquid phase formation during sintering. Although final machining with diamond grit, ultrasonic energy or laser beam energy is possible, the very high hard- ness of sialons encourages adoption of a near-net-shape approach to design. As with many other engineering ceramics, sialon components are extremely sensitive to shape and it is generally appreciated that a change in curvature or section can frequently improve service performance. The structure of a sialon is, of course, the main determinant of its properties. Fortunately, sialons are very responsive to ‘alloying’ and combinations of attributes such as strength, stability at high tempera- tures, resistance to thermal shock, mechanical wear and molten metals can be developed in order to withstand onerous working conditions.

During metal-machining, tool tips are subjected to highly destructive and complex conditions which include high local temperatures and thermal shock, high stresses and impact loading, and degradation by wear. At a test temperature of 1000 °

C, the indentation

hardness of ˇ 0 -sialon (C glass) is much greater than that of either alumina or cobalt-bonded tungsten car- bide (Figure 10.5). The introduction of tool tips made from this sialon was a notable success. They were found to have a longer edge life than conventional tungsten carbide inserts, could remove metal at high speed with large depths of cut and could tolerate the shocks, mechanical and thermal, of interrupted cutting.

Figure 10.5 Hot hardness of sialon, alumina and WC/Co cutting tool tips (from Jack, 1987, pp. 259–88; reprinted by permission of the American Ceramic Society) .

The strength and wear resistance of sialons led to their use in the metal-working operations of extrusion (hot- and cold-) and tube-drawing. In each process, the relative movement of the metal stock through the die aperture should be fast with low friction and mini- mal die wear, producing closely dimensioned bar/tube with a smooth and sound surface texture. Sialon die inserts have been successfully used for both fer- rous and non-ferrous metals and alloys, challenging the long-established use of tungsten carbide inserts. Sialons have also been used for the plugs (captive or floating) which control bore size during certain tube-drawing operations. It appears that the absence of metallic microconstituents in sialons obviates the risk of momentary adhesion or ‘pick-up’ between dies and/or plugs and the metal being shaped. Sialon tools have made it possible to reduce the problems normally associated with the drawing of difficult alloys such as stainless steels.

The endurance of sialons at high temperatures and in the presence of invasive molten metal or slag has led to their use as furnace and crucible refractories. On a smaller scale, sialons have been used for com- ponents in electrical machines for welding (e.g. gas shrouds, locating pins for the workpiece). These appli- cations can demand resistance to thermal shock and wear, electrical insulation, great strength as well as immunity to attack by molten metal spatter. Sialons have proved superior to previous materials (alumina, hardened steel) and have greatly extended the service life of these small but vital machine components.

330 Modern Physical Metallurgy and Materials Engineering The search for greater efficiency in automotive

engines, petrol and diesel, has focused attention on regions of the engine that are subjected to the most severe conditions of heat and wear. Sialons have been adopted for pre-combustion chambers in indi- rect diesel engines. Replacement of metal with ceramic also improves the power/weight ratio. It is still possible that the original goal of researches on silicon nitride and sialons, the ceramic gas turbine, will eventually be achieved.

Figure 10.6 Crack propagating into grains of t-zirconia,

10.4.3 Zirconia

causing them to transform into m-zirconia . Zirconium oxide (ZrO 2 ) has a very high melting point

⊲ 2680 ° C⊳, chemical durability and is hard and strong; because of these properties, it has long been used for refractory containers and as an abrasive medium. At temperatures above 1200 °

C, it becomes electrically

conductive and is used for heating elements in furnaces operating with oxidizing atmospheres. Zirconia-based materials have similar thermal expansion characteris- tics to metallic alloys and can be usefully integrated with metallic components in heat engines. In addition to these established applications, it has been found practicable to harness the structural transitions of zir- conia, thereby reducing notch-sensitivity and raising

fracture toughness values into the 15–20 MN m band, thus providing a new class of toughened ceram- ics. This approach is an alternative to increasing the toughness of a ceramic by either (1) adding filaments or (2) introducing microcracks that will blunt the tip of a propagating crack.

Zirconia is polymorphic, existing in three crystalline forms; their interrelation, in order of decreasing tem- perature, is as follows:

Melt ↽ Cubic ↽

Tetragonal

2680 ° C c 2370 ° C t

950 ° C Figure 10.7 Schematic phase diagram for ZrO 2 –Y 2 O 3 ↽

Monoclinic system: all phases depicted are solid solutions. TZP D

1150 ° C m

tetragonal zirconia polycrystal, PSZ D partially-stabilized zirconia, CSZ D cubic-stabilized zirconia .

The technique of transformation-toughening hinges upon stabilizing the high-temperature tetragonal (t) form so that it is metastable at room temperature.

diagram for the zirconia-rich end of the ZrO 2 –Y 2 O 3 Stabilization, partial or whole, is achieved by adding

system (Figure 10.7). The same principles apply in certain oxides (Y 2 O 3 , MgO, CaO) to zirconia. In

a very general sense to the other two binary sys- the metastable condition, the surrounding structure

tems, ZrO 2 –MgO and ZrO 2 –CaO. Yttria is partic- opposes the expansive transition from t- to m-forms.

ularly effective as a stabilizer. Three zirconia-based In the event of a propagating crack passing into or

types of ceramic have been superimposed upon the near metastable regions, the concentrated stress field

diagram; CSZ, TZP and PSZ. The term CSZ refers at the crack tip enables t-crystals of zirconia-rich

to material with a fully-stabilized cubic (not tetrag- solid solution to transform into stable, but less dense,

onal) crystal structure which cannot take advantage m-ZrO 2 (Figure 10.6). The transformation is marten-

of the toughening transformation. It is used for fur- sitic in character. The associated volumetric expan-

nace refractories and crucibles. The version known sion (3–5% v/v) tends to close the crack and relieve

as tetragonal zirconia polycrystal (TZP) contains the stresses at its tip. This transformation mechanism is

least amount of oxide additive (e.g. 2–4 mol% Y 2 O 3 ) primarily responsible for the beneficial toughening

and is produced in a fine-grained form by sinter- effect of a metastable phase within the microstructure.

ing and densifying ultra-fine powder in the tem- The relative stability of zirconia-rich solid solutions

perature range 1350–1500 ° C; such temperatures are can be conveniently expressed in terms of the phase

well within the phase field for the tetragonal solid

Ceramics and glasses 331 solution (Figure 10.7). After cooling to room temper-

ature, the structure is essentially single-phase, consist-

ing of very fine grains (¾0.2–1 µ m) of t-ZrO 2 which

make this material several times stronger than other types of zirconia-toughened ceramics. A typical TZP microstructure, as revealed by electron microscopy, is shown in Figure 10.8. Added oxide(s) and sili- cate impurities form an intergranular phase which can promote liquid-phase sintering during consolidation. (A similar effect is utilized in the production of silicon nitride.)

In partially-stabilized zirconia (PSZ), small t-crys- tals are dispersed as a precipitate throughout a matrix of coarser cubic grains. Zirconia is mixed with 8–10

mol% additive (MgO, CaO or Y 2 O 3 ) and heat-

treated in two stages (Figure 10.7). Sintering in the temperature range 1650–1850 °

C produces a parent

solid solution with a cubic structure which is then modified by heating in the range 1100–1450 °

C. This

second treatment induces a precipitation of coherent t-crystals (¾200 nm in size) within the c-grains. The morphology of the precipitate depends upon the nature

10 mm

of the added solute (e.g. ZrO 2 –MgO, ZrO 2 –CaO and

ZrO 2 –Y 2 O 3 solid solutions produce lenticular, cuboid Figure 10.9 Duplex structure of ZT ⊲Al 2 O 3 ⊳ consisting of and platey crystals, respectively). The average size of

alumina and t-zirconia grains (back-scattered electron precipitate crystals is determined by the conditions of

image). (from Green, 1984, p. 84; by permission of Marcel temperature and time adopted during heat-treatment in

Dekker Inc.) .

the crucial ‘t C c’ field of the phase diagram. In the third example of transformation-toughening,

So far, we have concentrated upon mechanical t-zirconia grains are dispersed in a dissimilar ceramic

behaviour at or below ambient temperature. If the tem-

perature of a zirconia-toughened material is raised to are dispersed among alumina grains (Figure 10.9).

matrix; for example, in ZT(A) or ZT⊲Al 2 O 3 ⊳ they

C, which is close to the t-m transition tem- An intergranular distribution of the metastable phase

perature, the toughening mechanism tends to become results when conventional processing methods are used

ineffective. In addition, thermal cycling in service but it has also been found possible to produce an

tends to induce the t-m transition at temperatures in the intragranular distribution. As with PSZ materials, the

C and the toughening property is grad- size of metastable particles and matrix grains must be

range 800–900 °

ually lost. This tendency for fracture toughness to fall carefully controlled and balanced.

as the service temperature increases has naturally led to the investigation of alternative forms of stabilization in systems which have much higher transformation tem-

peratures (e.g. ZT⊲HfO 2 ⊳ ). Intergranular residues (e.g. in TZP), despite their beneficial effect during sintering, become easier to deform as the temperature rises and the material then suffers loss of strength and resistance to creep.