The mechanical properties of chemically vapor‐deposited boron carbides (B4C) with varied B/C ratios were investigated as a function of composition. The maximum hardness, H, and fracture toughness, K1c, were observed at an almost stoichiometric composition. For nonstoichiometric B4C (B/C>4), H and K1cdecreased with increasing B content, suggesting that excess B diminishes the bond strength in the B4C structure. The decrease in H and K1Cat B /C<4 was attributed to free C in the microstructur