Phase Trasnsformation in Nano Particles of Zirconia

3.7.3. Phase Trasnsformation in Nano Particles of Zirconia

Compared to the materials like silicon carbide and alumina, there is a peculiarity or specialty in zirconia. In zirconia, there is a structural transformation from monoclinic to tetragonal phase, which is a subject of intensive research in the field of materials science all over the world, since zirconia has many interesting and important application as engineering materials. In order to speed up the transformation process to make a stabilized zirconia in the tetragonal crystalline form, many dopants like calcia, ceria, yttria, etc. are commonly used. Since the advent of nano-materials based on zirconia or rather since the beginning of the so-called discovery of various processes and methods to prepare nano particles of zirconia, we have been naturally thinking on how best we can exploit the nano properties of zirconia on this stabilization of ‘structural phase transformation’ process.

Simple classical ideas like ‘thermodynamics’ is applied to enquire about the difficulty in the retention of tetragonal phase at lower temperature [64]. It has been found by Subbarao et al [65] that the transformation from tetragonal to monoclinic zirconia is 'martensitic' in nature, which is basically a ‘diffusionless’ athermal transformation involving ‘cooperative movement’ of the atoms taking place within a very short distance. It has also been observed that the stabilization of the high temperature tetragonal form cannot be achieved by any quenching mechanism below the transformation temperature [65-67]. However, it is known that the surface energy of the monoclinic form is higher than that of the tetragonal form. Hence, the ‘difference in surface energy’ can play an ‘importat role’ in that it must become more pronounced as the particle size becomes smaller and smaller, i.e. towards the nano-range. This is an important ‘clue’ for scientific research.

Like in many scientific phenomena involving ‘criticality’, there must be a ‘critical’ particle size of zirconia below which this ‘difference in the surface energy’ precludes the tetragonal (T) → monoclinic (M) transformation. But, there is another problem in that if the 'transformation' involving finite volume changes is suppressed, like in the case of zirconia particles dispersed in another ceramic matrix, there is

a kind of strain inside the structure. Thus, this ‘strain energy effect’ associated with the ‘expansion’ of transforming zirconia particles also become important. Lange [64] has suggested that the ‘micro-crack- ing’ of the matrix around the zirconia particles and the 'twinning' involving the particles themselves can cause a 'neutralization of a part of this ‘strain energy’.

Agarwal’s group at BHU has done some important work in this feld for the overall energy bal- ance during ‘T → M’ transformation [68]. They have considered the thermodynamics of the system as Δ G Tot = – ΔG chem + ΔU S + ΔU Strain with ΔU S being the total change in surface energy and ΔU Strain is the total strain energy. These workers also considered various parameters of importance, and came up with

an equation for the critical particle size for zirconia (D C ) as follows : 6( γ−γ+ M g tt g TT γ+ g cc γ )T 0

Δ H chem (T 0 − T) T [(1 F ) / F ] U − 0 − c c Δ Strain

where, γ M , γ T = specific surface energies for monoclinic and tetragonal phases, t and c represent the twinning and microcracking for the geometric factor (g) and surface energy (γ), T 0 = equilibrium tem- perature, i.e. T 0 – T = degree of undercooling, and F is a factor by which ΔU Strain is reduced.

NANO PARTICLES OF ALUMINA AND ZIRCONIA

141 The above equation explains the complex interaction of various factors like temperature, strain

energy, microcracking and twinning effects, and finallyof the particle size. It also shows that the zirconia particles of suitable size range, i.e. possibly in the nano range, and dispersed in a properly selected matrix can retain tetragonal form on cooling below its bulk phase transformation temperature. Then, it can be made to transform under the influence of an applied ‘tensile stress’, which would eventually reduce the strain energy of the matrix, i.e. ΔU Strain . Thus, the particles of a given size, which help in the retention of the tetragonal form even on cooling, can be made to transform to monoclinic form by applying stresses in excess of the critical value.

It has been shown by Lange [64] that the above action leads to the ‘absorption’ of energy in the form of ‘elastic strain’, which can be obviously related to the strain of transformation. Hence, the ‘ab- sorption’ can be brought about just at the point of crack extension., which eventually contribute towards the enhancement of fracture toughness of the zirconia materials, as said earlier, since the whole process depends on some ‘criticality, the critical size depends on the ‘constraints’ that can be produced by providing the following conditions :

1. The dopant oxide is present in the concentration range, which is less than that needed for the complete ‘stabilization’ of the cubic phase.

2. The cubic phase is heat-treated in order to develop a two-phase ceramics.