Abstract Composites of AlMgB 14 with TiB 2 display a positive deviation from the rule-of-mixtures for hardness and wear resistance  (Ahmed et al., 2006). This suggests exceptionally strong bonding between the two boride phases. A contributing factor to the strong bonding may be a close matching of surface energy in the two phases. A calculation was performed to estimate the surface energy and its temperature dependence in AlMgB 14, based on the critical crack energy release rate and assuming covalent bonding. Results predict that the surface energy of the two phases differs by about 0.4 J/m 2. Similar results were found for other transition metal diborides, and the surface energy of AlMgB 14 was close to that of TiC and TiN, two materials commonly used as sintering aids for TiB 2.