The effect of TiB2 precursors on properties of composite zirconia powders synthesized by the in situ method

Waldemar Pyda, Norbert Moskała, Mirosław M. Bućko Akademia Górniczo-Hutnicza, Wydział Inżynierii Materiałowej i Ceramiki, al. Mickiewicza 30, 30-059 Kraków, Poland

Quarterly No. 4, 2008 pages 354-359

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abstract An in situ method was used to prepare zirconia powders containing nonoxide particles. The method involved a reaction among TiO2 dissolved in the zirconia solid solution, elementary boron or boron oxide originated from H3BO3 dehydration and carbon originated from pyrolysis of phenol-formaldehyde resin. The zirconia nanopowder stabilized with 2.5 mol. % Y2O3 and doped with 18 mol. % TiO2 was used. The nanopowder was prepared by using a co-precipitation method followed by hydrothermal crystallization of a zirconia hydrogel for 4 h at 240ºC under 3.4 MPa. Uniform mixtures of zirconia nanopowder, phenol-formaldehyde resin and boric acids or elementary boron were preheated for 0.5 h at 800ºC in flow of argon and then heat treated for 4 hrs at the temperatures ranging from 1100ºC to 1600ºC under 2•10–4 mbar. The effect of TiB2 precursors on properties of the resultant zirconia composite powders was studied. The phase composition of the powders and a structure of the selected nonoxide phase were determined byX-ray diffractometry. Crystallite sizes of TiB2 and TiC particles were determined from (011) and (002) X-ray line broadening. Specific surface area was measured by the BET method. The following phases were detected in the studied powders: monoclinic, tetragonal and cubic zirconia polymorphs, TiB2, TiC, ZrB2, ZrSi, YBO3 i Y2O3, Zr(C,B)-I, Zr(C,B)-II, Zr(C,B)-III. The last three phases are isostructural between each other and with ZrC and TiC. They are most probably solid solutions of carbon in ZrB or boron in TiC. TiB2 and TiC were main inclusion phases observed. The powders originated from boric acid contained TiB2 beside a significant amount of TiC. The powders originated from elementary boron showed mainly TiB2 embedded in the zirconia phases. In case of the powder synthesized at 1300ºC there were only TiB2 inclusions present. The amount of secondary phases such as ZrB2, Zr(C,B)-I, Zr(C,B)-II and Zr(C,B)-III increased with temperature reaching maximums at 1600ºC. The monoclinic phase content decreased with temperature from a value of ~80 vol. % to < 30 vol. % independently on the TiB2 precursor used. Simultaneously, the sum of tetragonal and cubic zirconia increased. The elementary cell volume measurements indicated an increase of the TiB2 and TiC cell volume with temperature when the boric acid was used. The TiB2 cell volume remained practically unchanged when the elementary boron was used. This confirms that the studied secondary phases were the solid solutions or nonstoichiometric phases of composition depended on the crystallization conditions and boron precursor used. TiB2 crystallites increased the size by 3 times with synthesis temperature when elementary boron was used. In case of the zirconia powders of boron acid origin, the TiB2 crystallites increased only by 50% in size. It can be concluded that boron acid favours crystallization of the TiB2 and TiC nanocrystallites of 6080 nm and 2540 nm in size, respectively and elementary boron favours only TiB2 nanocrystallites of 60145 nm in size. Keywords: composite powder, composites, TiB2, ZrO2, in situ synthesis