In situ synthesis mechanism of ZrB2–ZrN composite

In situ ZrB2–ZrN composite was fabricated from Zr/BN powder blends by pressureless firing above 1600 °C with a pre-treatment in vacuum at 450 °C. The synthesis mechanism of ZrB2–ZrN composite was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. ZrN in the needle-like shape forms preferentially in the matrix at ca. 900 °C, followed by the appearance of ZrB, which is an intermediate phase in this reaction and will transform to ZrB2 later at a relatively high temperature. The initially formed ZrN is most probably a non-stoichiometric compound with vacancies in the nitrogen site, noted as ZrN1−x (x < 1), which incorporate then the nitrogen elements from the remains of BN and eventually transform to stoichiometric ZrN. A long time holding at the formation temperature of ZrN may promote the grain growth of needle-like ZrN and thus improve the mechanical properties of ZrB2–ZrN composite.