Boron carbide (B 4 C) is a hard, lightweight ceramic used in the nuclear and aerospace industries. Depositing a CVD diamond coating onto its surface may increase its wear resistance and hence its value as a structural material in advanced applications, such as gyroscope rotors. Hot filament CVD has been used to deposit 2-4-μm layers of polycrystalline diamond onto polished B 4 C blocks. We found that adhesion of the diamond coating was poor, with spontaneous delamination occurring. SEM and scanning Auger microscopy determined that boron from the surface of the B 4 C was diffusing into the CVD diamond to a depth of about 0.1 μm. This left a B-depleted, C-rich layer at the diamond/B 4 C interface which caused the delamination of the doped diamond film.