REFINEMENT OF HREM IMAGE PROCESSING ANALYSIS OF DEFECTS IN MECHANICALLY ALLOYED ZrO2-Y2O3 POWDERS
A. M. Tonejc1, A. Tonejc1, G.W. Farrants2, S. Hovmller3
1 Faculty of
Sciencesand Mathematics, Department of Physics, University of
Zagreb, Bijenieka c. 32, POBox 162, 10001 Zagreb, Croatia
2 Calidris, Sollentuna, Sweden
3 Department of Structural Chemistry, Stockholm
University, Stockholm, Sweden
Keywords: Mechanical
Alloying, Ball Milling, HRTEM Image Processing, Zirconia, Yttria
Partially stabilized zirconia (PSZ-mixture of tetragonal and cubic zirconia or a mixture of other ceramic materials and metastable tetragonal zirconia) is generally recognized as the ceramics material with the most useful mechanical properties and it is named "ceramic steel" 1,2. The stabilisation of the high temperature cubic phase at room temperature is possible by alloying zirconia with other oxides such as MnO, NiO, Cr2O3, Fe2O3, Y2O3, Ce2O3. However, a very high temperature (over 10000C) of calcination or sintering is required for preparation of solid solutions of zirconia with these oxides 3,4.
High energy ball milling (mechanical alloying) 5,6 has become a widely used processing method for producing intermetallic compounds, extended solid solutions (even of immiscible elements), amorphous alloys and nanocrystalline materials 7. However, high energy ball milling (BM) was initially applied mainly to produce alloys formed from pure metals or intermetallic compounds, and only recently 8 has it been shown that mechanical alloying can also be used for ceramic materials (ZrO2-Ce2O3 system).
Also, high energy ball milling could be equivalent to thermal processes in achieving high temperature phase transitions 9. Recently, it was reported [10,11] that it is possible to synthesise ZrO2-10mol% Y2O3 cubic solid solutions as the end product of mechanical alloying, although in first stage of alloying the tetragonal phase appeared.
It is generally believed that alloying and amorphization are brought about by a solid state reaction during mechanical alloying, as a mixture of thin layers [12]. This is the same mechanism as that which occurs in thin films.
In the present work we have investigated using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) alloying process in m-ZrO2- 10 mol% Y2O3 powders using HRTEM image processing. We present results obtained by applying the CRISP program [13] to analyse HRTEM photographs of mechanically alloyed materials. We focused our investigation to following regions: a) a grain boundary region; b) to the region of stacking faults near grain boundary; c) the region of overlapping layers of zirconia and yttria [14].
Fourier filtering revealed on the
atomic level one possible sequence of alloying that occurred in
the grain boundary, on stacking faults and in the overlapping
layers. Performing Fourier filtering with different filtering
mask, i. e. making refinement of filtering analyses, we are able
to isolate separate planes introduced into correct order of
particular family of m-ZrO2 lattice. The introduced
planes weather belonging to m-ZrO2 or Y2O3,
could be regarded as dislocations introduced into perfect m-ZrO2
lattice and as they are identified in corresponding FT from whom
the filtering was performed it is possible to give interpretation
about transformation of mechanical alloying observed in
particular HRTEM image.