The residual ZrO2 shows that the reaction did not occur completely, however; the powder mixture did not contain any elemental Mg and all of Mg was in the form of MgO. This observation suggests that some of Mg may have been oxidized before the formation of ZrB2. The reason of oxidation may have been the interruption of the system and opening of the jar every 5-hour for taking sample. Because of the applied mechanical energy, Mg may have become very active, causing it to get oxidized quickly, when it came into contact with air during opening of the jar.