LaB6 was incorporated into Si-B-C-N monoliths by mechanical alloying technique.
Thin BN(C) flakes are in situ formed in Si-B-C-N ceramics due to the LaB6 additive.
BN(C) flakes play a dominate role in improving the ceramic's toughness.
Solid-solution of La in BN(C) grains promotes in situ growth of BN(C) flakes.