Phase instability in ZrO2–TiB2 composites

2.5 mol% Y2O3-stabilised ZrO2 composites with 30 vol.% TiB2 particles, hot pressed in vacuum at 1450 °C, were investigated by means of quantitative electron energy loss spectroscopy (EELS) with a nanobeam analytical field emission gun transmission electron microscope (FEG-TEM). The experimental results revealed that B is diffusing out of the titanium boride grains, forming textured polycrystalline BN agglomerates leaving behind non-stoichiometric TiB2−x single crystal grains. The formation of BN was calculated to be thermodynamically favourable under the actual hot pressing conditions. Boron was also detected in the amorphous intergranular phase, together with Y, Si, O, Al, but not in the ZrO2 grains. Ti was only detected in the amorphous material in-between the TiB2−x grains and the micro-textured BN grains, but not in the zirconia grains nor in the intergranular amorphous phase in between. There are indications that Ti forms facetted carbide precipitates inside the TiB2−x grains. Zr was not detected in the titanium boride grains, nor in the intergranular amorphous phase in-between the zirconia grains.