Microstructure Of

Pure Zr02 is polymorphic with a monolithic (m) structure up to approximately 1170°C; the structure is tetragonal (t) up to 2260°C and above this temperature it is cubic (c).

and MgO are most frequently used stabilizers in zirconia in order to suppress the transformation. One example of an acceptable microstructure is shown in Fig. 2.


I.K. Technology/I.K. Engineering has recently initiated transmission electron microcopy (TEM) studies of plasma sprayed ZrO^ microstructures 1.) Fig. 3 shows a typical TEM image of a 24% MgO stabilized coating. As is expected for materials with high cooling rate (10 C sec), as occurring during plasma spraying, the average grain size is about 0.4-0.8 um. This is much smaller than obtained by sintering zirconia.

The energy dispersive x-ray (EDX) spectrum stems from thin intergranular fiber seen in the figure. Such an amorphous Si-rich phase has previously been observed in sintered structures 2). Si impurities are common in Zr0_ and stem from the ZrSi04 precursor phase.

It is interesting to note that the crystalline grain size of the plasma sprayed structure lies in a size range (<1 um) where the tetragonal to monoclinic phase transformation is highly grain size dependent. It is assumed that this is done to the grain boundary free energies in the tetragonal and itonoclinic phases. From sintered zirconia structures the amorphous phase at grain boundary triple points seems to contribute toward a rounding off of sharp corners. This results in a reduction of the strain concentration at such points and consequently the effect of the tetragonal to monoclinic transformation toughening is reduced. This means that the K value will be smaller in the presence or the amorphous Si-phase than without.

Vfe have therefore taken action to produce PSZ powder free of Si as well as measures to obtain chemical homogeneity within the zirconia coating.

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