Due to no ZrB2 formation after 20 h, milling time was increased to 30 h for uninterrupted milling. As can be seen from the XRD pattern of this experiment given in Fig. 7(a) that at the end of 30 h uninterrupted milling operation, the product contained ZrB2, MgO and ZrO2, indicating that Reaction (1) had occurred. The peaks of formed ZrB2 were more intense and narrower when compared to the results of the interrupted milling experiments. Besides, they were distinguished from the peaks of MgO. It can be deduced from these results that the reaction took place instantaneously, with a sudden increase in temperature.