Sintering

Sintering is the process in which a heat treatment is used to covert a green body into a rigid, polycrystalline solid, which is usually very dense. For a nanoceramic powder system, the high surface area leads to a high total surface energy. Since the main driving force for sintering is surface energy reduction [92], higher surface energy means higher driving force and allows sintering processes to occur at lower temperatures. However, the high surface free energy brings a high driving force for grain growth during the sintering process, resulting in difficulties in retaining a nanostructure. It is necessary to find methods to control the grain growth during sintering. Table 2.5 [4] summarises some of the commonly used sintering method to process ceramic armour. The advantages and disadvantages are listed as well.

Table 2.5 Sintering processes for ceramic armour materials [4]

Process Advantages Disadvantages

Hot pressing Lower temperature,

2.5.3.1 Methods to control grain growth

There are four approaches that have been used to control the grain growth and to improve densification [90]: 1) the addition of solutes or glass phase, 2) very rapid

firing (e.g. microwave sintering), 3) pressure-assisted techniques (e.g. hot pressing) and 4) two-stage sintering. The type 1) approach is not suitable for armour material processing since the solute or glass phase significantly harm the desired properties of the final products, hence will not be discussed in detail.

For the type 2) approach, the reduction of grain growth time can result in an effective control of the grain size. Microwave sintering is a method to provide faster processing time and lower sintering temperatures than for conventional sintering [100]. According to the study of Brosnan et al. [100], a fast heating rate of 45-60˚C/min was achieved using microwave sintering of MgO-doped alumina. In addition, 95% density was reached at 1350˚C using microwave heating whilst the same density can only be achieved at 1600˚C using conventional heating. However, the rapid sintering requires extremely homogeneous green compacts and physically small components to avoid different densification rates at different locations and excessive thermal gradients [90].

By using hot pressing (type 3), samples can be sintered at much lower temperature under high pressure in order to prevent grain growth at high temperatures. Liao et al.

[43] sintered nano α-alumina with grain size <50 nm and density >98% from a nano γ-alumina green body. During sintering the γ to α transformation temperature decreased from 1075˚C at ambient pressure to about 460˚C at 8 GPa. Grain growth was limited by the low sintering temperature and a multiplicity of nucleation events in the parent γ phase at high pressure created a nanoscale α grain size. The hot pressing method effectively solved the problem of exaggerated grain growth of γ-alumina during transformation. However, it brought the major disadvantage of higher processing costs and limited component shape capability.

Type 4), two-stage sintering, is another effective method to control the grain growth of alumina [86,101]. Two-stage sintering was introduced as a method by Chen and Wang [102] for the sintering of nanostructured yttria (Y2O3) ceramics. The key elements in this process are a) heating the green bodies to a high initial temperature, T1,for a very short time to conduct the first-step sintering, b) achieving a high density at T1 (>75%) to render pores unstable and c) quickly lowering the temperature to a lower temperature, T2,to conduct the second-stage of sintering, during which there

was only densification and no grain growth according to the authors (Figure 2.40 (a) [102]). The resultant products were observed to have full density with a mean grain size of lower than 100 nm. The suppression of grain growth, but not densification, was considered to be achieved by the suppression of grain boundary migration whilst keeping grain-boundary diffusion active [102,103]. According to the kinetic study of this sintering method[103], there was a kinetic window for the second stage of sintering (Figure 2.40 (b) [103]). For each grain size, products with full density and no grain growth could only be obtained at the second-stage sintering temperatures, which were within the kinetic window.

Figure 2.40 (a) Grain size of Y₂O₃ in two-stage sintering (Heating schedule shown in inset). Note that the grain size remained approx. constant in the second sintering step

[102]. (b) Kinetic window for the second stage sintering of Y₂O₃. Solid symbols when full density was attained without grain growth; data above the upper boundary had

grain growth. Data below the lower boundary did not fully densify. The triangle represents a one-step sintering experiment at the temperature shown [103].

As reported by some other authors [104], two-stage sintering has been proved to be effective in reducing grain growth during densification, although not all the final ceramics retain a nanostructure, which is probably due to the vary agglomerated nature of some of the precursor nanopowders used. According to a recent study of Binner et al. [34], the application of two-stage sintering successfully controlled the grain size of 3YSZ products to less than 100 nm. However, grain growth still occurred at densities above about 97%, although with a much lower grain growth

a) b)

rate, which suggested that the two-stage sintering reduced the grain growth but could not fully restrain it.

2.5.3.2 Sintering of ZTA

Regarding to the sintering of ZTA, the zirconia inclusions in the ZTA have been found to suppress the grain growth of the alumina matrix [105,106]. Lange et al.

[107] found that, with the zirconia content above 5 vol%, a majority (or all) of the 4-point grain junctions are occupied by the zirconia inclusions. The latter exhibits sufficient self-diffusion to move with the alumina 4-grain junctions during grain growth and, therefore, exerts a dragging force at the junction points to limit the grain growth. On the other hand, when the zirconia content is below ~2.5 vol%, the zirconia inclusions located in the junction points were not sufficient and abnormal grain growth occurred.

In addition to the effect of the grain growth inhibition, the zirconia addition in the ZTA also affects the sintering temperatures. According to the study of Bodisova et al.

[108], in the two-stage sintering of pure alumina, the optimal temperatures are T1, 1400-1500˚C, and T2, 1150˚C. In comparison, the two-stage sintering temperature for ZTA, as found by Wang et al. [106] is about 1400-1450˚C (T1) and 1350-1400˚C (T2). The relatively higher T2 is considered to be caused by the effect of the zirconia, which increased the activation energy of the densification of alumina. Wang and Raj [109] reported that the activation energy of ZTA with zirconia content from 5-95 vol%

remains in the range of 700±100 kJ/mol, whilst the activation energy of pure alumina is 440±45 kJ/mol. The increase of the activation energy increases the energy required to activate grain boundary diffusion, therefore a higher temperature needs to be used during the second stage of the ZTA sintering process

3 EXPERIMENTAL