Controlled devitrification of a glass It has long been appreciated that crystallization can

10.4.4.1 Controlled devitrification of a glass It has long been appreciated that crystallization can

take place in conventional glassy structures, particu- larly when they are heated. However, such crystalliza-

200nm

tion is initiated at relatively few sites and there is a tendency for crystals to grow perpendicular to the free

Figure 10.8 Electron micrograph of tetragonal-zirconia surface of the glass in a preferred manner. The result- polycrystal stabilized with 3 mol.% yttria (with

ing structure, being coarsely crystalline and strongly acknowledgement to M. G. Cain, Centre for Advanced

oriented, is mechanically weak and finds no practical Materials Technology, University of Warwick, UK) .

application.

332 Modern Physical Metallurgy and Materials Engineering The basic principle of glass-ceramic production is

that certain compositions of glass respond to con- trolled heat-treatment and can be converted, without distortion and with little dimensional change, from

a readily-shaped glass into a fine-grained crystalline ceramic possessing useful engineering properties. The key to this structural transformation, which takes place throughout the bulk of the glass (volume crystalliza- tion), is the presence of a nucleating agent, or catalyst, in the original formulation.

Controlled devitrification of a special glass involves two or more stages of heat-treatment (Figure 10.10). In the first stage, which can begin while the glass is cooling from the forming and shaping operation, hold-

Figure 10.11 Epitaxial growth of lithium metasilicate ing at a specific temperature for a definite time period

⊲LS D Li 2 SiO 3 ⊳ on a lithium orthophosphate seed crystal causes the catalyst to initiate the precipitation of large

⊲LP D LP 3 PO 4 ⊳ in SiO 2 2 2 O 3 glass containing numbers of nuclei throughout the glassy matrix. When

P 2 O 5 catalyst (from Headley and Loehmann, 1984, these seed regions reach a certain size, different species

pp. 620–25; reprinted by permission of the American Ceramic Society) of crystals may begin to grow upon them. Electron .

microscopy has demonstrated that epitaxial relation- ships exist between the first-formed crystals and suc- ceeding generations of crystals (Figure 10.11). Finally, in the second stage of heat-treatment (Figure 10.10), the structure is heated to a different temperature in order to induce further crystallization, crystal growth, crystal transitions and a gradual, almost complete, dis- appearance of the glassy matrix.

Control of time and temperature is essential dur- ing the production by heat-treatment of a glass- ceramic. Figure 10.12 provides a general guide to the temperature-dependence of nucleation and growth pro- cesses for any melt, irrespective of whether it forms

a glassy or crystalline solid. Curve N represents the rate of homogeneous nucleation; that is, the number of nuclei forming per second in each unit volume of glass. Curve G represents the rate of crystal growth (micron/second). Each curve has a peak value; for viscous glass-forming melts, these maxima are not very pronounced. With regard to the formation of a

Figure 10.12 Temperature-dependence of nucleation and growth processes (after Rawson, 1980) .

glass-ceramic, it is of prime importance to select a tem- perature which is close to the peak of the nucleation curve and then, in the second stage of heat-treatment, a temperature which does not encourage excessive grain growth. Usually the temperature chosen for the second stage is higher than that used for the first. A care- ful balance of conditions during heat-treatment will favour the production of the desired ultra-fine grain structure. Thus the rate of nucleation should be high, nuclei should be uniformly dispersed and the rate of crystal growth should not be excessive. Crystals are

Figure 10.10 Temperature/time schedule for producing a one micron or less in size; interlocking of these crystals glass-ceramic .

will enhance the mechanical strength.

Ceramics and glasses 333 Fundamental studies are complicated by the fact

have a strong fluxing action on silica. In addition, melt that the detailed mechanisms by which nuclei form

viscosity is important; alumina increases viscosity and in a homogeneous glassy matrix, and then develop

will tend to slow down melting and refining operations. into crystals, appear to be specific to each type

As composition control is a vital feature of glass- of glass. However, although spinodal decomposition

ceramics, it is essential to maintain melt composition is sometimes possible, it is usually regarded as a

reproducibly from batch to batch. Volatilization and nucleation and growth process. Although the exact

interaction between the melt and the refractory lining nature of the early stages of nucleation is highly

of the melting furnace can make this difficult. For debatable, the nuclei, once formed, enable hetero-

instance, high proportions of lithium oxide in the melt geneous nucleation of the major crystalline phases

will increase attack on the lining. The melt leaving the to take place. Metastable phases may form during

melting furnace has a temperature-dependent viscosity heat-treatment; such phases do not feature in phase

of 10 13 poise and the cooling mass can be worked (equilibrium) diagrams. Well-known crystalline phases

and shaped until its viscosity falls to about 10 8 poise. may appear at unexpected temperatures. Nominally

Although conventional glasses have a useful ‘long’ metastable phases may prove to be quite stable under

range of temperature over which they can be worked, service conditions. Furthermore, successful composi-

the composition of potential glass-ceramics restricts tions are usually multi-component in character and

(‘shortens’) this range, particularly when aluminium modifying oxides, particularly the catalyst, can have

oxide is present. As a consequence, the choice of

a significant effect upon the types of crystal produced shaping process may be restricted to gravity or and upon their transformation processes. For a given

centrifugal casting.

catalyst, a change in the heat-treatment process can As metastability is an essential feature of a glass- result in a change in the major crystalline phase(s).

ceramic, it is not surprising to find that certain Nucleation in glasses which use an oxide as catalyst

compositions tend to devitrify prematurely during often appears to be preceded by a process of separation

working. Oxides of aluminium, phosphorus and the into two solid glassy phases (metastable immiscibility).

alkali metals sodium and potassium inhibit devitrifica- These microphases differ in chemical composition and

tion in SiO 2 –LiO 2 glasses. On the other hand, the glass are therefore believed to have a desirable effect upon

should crystallise neither too quickly (during cooling of the ultimate grain size by favouring a high nucleation

the melt) nor too slowly (during heat-treatment). These density and reducing the growth rate of crystals. Phase

tendencies can be eliminated by adding oxides which separation may occur during either cooling of the melt

have a specific effect upon the strength of the glass net- or reheating and it is logical to presume that it is

work structure. For instance, lithium oxide introduces more likely to take place when two network-formers

non-bridging oxygen ions into a network of SiO 4 tetra- are present in the glass formulation. The production of

hedra and, by weakening it, favours crystallization. Vycor , a SiO 2 –B 2 O 3 –Na 2 O glass, also takes advantage of phase separation.