engleski

In situ high temperature study of ZrO2 ball milled to nanometric sizes

Nanostructured ZrO2 was prepared by high-energy ball-milling at different conditions. This technique decreases particle sizes to nano dimensions and generally induces change of crystal structure. The samples were sintered at high temperatures with the aim to investigate their behavior during the sintering and properties of the final products. The milling was performed in air using Fritsch planetary ball mill with zirconia (94% ZrO2) milling assembly. Two different powder-to-ball weight ratios (R) were used: R1=1:50 and R2=1:10. The milling time varied from 1 min to 10 hours. Raman spectroscopy (RS) was use as a main method for investigating structural changes of ZrO2 because it has an advantage to resolve easily tetragonal and cubic structure. The microstructure of milled powders was additionally studied by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray powder diffraction (XRD). The sintering process was monitored in situ at high temperature (300 to 1400  C) by RS and XRD. The kinetics of microstructural changes during the milling was dependent on powder-to-ball weight ratio. The diminution to nanometric particle and crystallites sizes was observed by TEM and XRD, respectively. It is well known that monoclinic ZrO2 during milling partially transform to tetragonal phase  1-3 . However, in our milling experiment the transformation to tetragonal ZrO2 was not observed when milling with R=1:10 was used for 10 h. Even after prolonged milling with R1=1:50, Raman spectra indicate presence of traces (only if any) of tetragonal phase. The important difference of the present milling experiment compared to the previous experiments [1-3] is in the usage of zirconia ball-milling assembly, which reduces the influence of an additional material, involved in system as contamination due to wearing of milling media. It was concluded that impurities introduced during milling had more influence on the stabilization of tetragonal ZrO2 than particle size (surface energy), emphasized in the earlier works  1, 2 . During sintering of prolonged milled samples only the amorphization was observed by RS in situ at high temperature. On the other hand, during sintering of un-milled sample the diffusionless martensitic phase transition to tetragonal form was clearly observed at temperatures higher than 1200 °C. The reason for the obtained differences will be discussed.

ZrO2; high-energy ball-milling; nanostructured; high temperature sintering

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