Production of sialons The start point for sialon production from silicon

10.4.2.4 Production of sialons The start point for sialon production from silicon

nitride powder at a temperature of 1800 °

C will lie in the vicinity of the bottom left-hand corner of Figure 10.4. Simultaneous replacement of N with O and Si with Al produces the desired ˇ’-phase which is represented by the narrow diagonal zone project-

ing towards the Al 4 O 6 corner. Such ‘alloying’ of the ceramic structure produces progressive and subtle changes in the structure of silicon nitride by altering the balance between covalent and ionic bonding forces. The resultant properties can be exceptional. Impor- tantly, the oxidation resistance and strength of sialons at temperatures above 1000 °

C are greatly superior to those of conventional silicon nitride. Relatively sim- ple fabrication procedures, similar to those used for oxide ceramics, can be adopted. Pressureless-sintering Figure 10.4 Si–Al–O–N behaviour diagram at 1800 ° C enables dense complex shapes of moderate size to be

(from Jack, 1987, pp. 259–88; reprinted by permission of

produced.

the American Ceramic Society) . ˇ -Si 3 N 4 powder is the principal constituent of the starting mixture for ‘alloying’. (As mentioned previ- ously, these particles usually carry a thin layer of

is shown in Figure 10.4. The ‘double reciprocal’ char- silica.) Although fine aluminium nitride would appear acteristic refers to the equivalent interplay of N/O and

to be an appropriate source of replacement aluminium, Si/Al along the vertical and horizontal axes, respec-

it readily hydrolyses, making it impracticable to use tively. It is necessarily assumed that the valency of the 4C 3C four elements is fixed (i.e. Si fabrication routes which involve aqueous solutions or , Al ,O and N ).

binders. One patented method for producing a ˇ 0 -sialon

(z D 1) solves this problem by reacting the silicon cations and 12 anions, the formulae for the other three

As the formula for the component Si 3 N 4 contains 12

components and for the various intermediate phases nitride (and its associated silica) with a specially- along the axes are expressed in the forms which give

prepared ‘polytypoid’. The phase relations for this

a similar charge balance (e.g. Si O method are shown in Figure 10.4.

The equivalent % of a given element in these formulae An addition of yttrium oxide to the mixture can be derived from the following equations:

3 6 rather than SiO 2 ).

causes an intergranular liquid phase to form during pressureless-sintering and encourage densification. By

Equivalent % oxygen controlling conditions, it is possible to induce this phase either to form a glass or to crystallize

D 100⊲atomic %O ð 2⊳ (devitrify). In sialons, as in many other ceramics, ⊲ atomic %O ð 2⊳ C ⊲atomic %N ð 3⊳

the final character of the intergranular phase has Equivalent % nitrogen

a great influence upon high-temperature strength. A structure of ˇ 0 grains C glass is strong and resists

D thermal shock at temperatures approaching 1000 ° C. However, at higher temperatures the glassy phase

Equivalent % aluminium deforms in a viscous manner and strength suffers. 100⊲atomic %Al ð 3⊳

Improved stability and strength can be achieved by a

D ⊲ atomic %Al ð 3⊳ C ⊲atomic %Si ð 4⊳

closely-controlled heat-treatment which transforms the glassy phase into crystals of yttrium-aluminium-garnet

Equivalent % silicon (YAG), as represented in the following equation:

Si

0 5 AlON 7 Y–Si–Al–O–N

Oxynitride contains 25 equivalent % oxygen and is located one

Thus the intermediate phase labelled 3/2 ⊲Si 2 N 2 O⊳

ˇ -sialon

glass quarter of the distance up the left-hand vertical scale.

⊲z D 1⊳

Si 5Cx Al O N 7Cx C Y 3 Al 5 O 12 An interesting feature of the diagram is the parallel

YAG sequence of phases near the aluminium nitride corner

Modified

ˇ 0 -sialon

(i.e. 27R, 21R, 12H, 15R and 8H). They are referred to as aluminium nitride ‘polytypoids’, or ‘polytypes’.

The two-phase structure of ˇ 0 grains C YAG is They have crystal structures that follow the pattern

extremely stable. It does not degrade in the presence of of wurtzite (hexagonal ZnS) and are generally stable,

molten metals and maintains strength and creep resis- refractory and oxidation-resistant.

tance up to a temperature of 1400 ° C.

Ceramics and glasses 329 More recent work has led to the production of

sialons from precursors other than ˇ-silicon nitride

(e.g. ˛ 0 -sialons from ˛-silicon nitride and O 0 -sialons

from oxynitrides). K. H. Jack proposed that ˛-silicon nitride, unlike the ˇ-form, is not a binary compound and should be regarded as an oxynitride, a defect struc- ture showing limited replacement of nitrogen by oxy- gen. The formula for its structural unit approximates

to SiN 3.9 O 0.1 . Dual-phase or composite structures have also been developed in which paired combinations of ˇ 0 -, ˛ 0 - and O 0 - phases provide enhancement of engi-

neering properties. Sometimes, as in ˛ 0 /ˇ 0 composites,

there is no glassy or crystalline intergranular phase. The sialon principle can be extended to some unusual natural waste materials. For instance, two siliceous materials, volcanic ash and burnt rice husks, have each been used in sinter mixes to produce sialons. Although such products are low grade, it has been proposed that they could find use as melt-resistant refractories.