Abstract The effect of Si 3 N 4 secondary phases on chemical vapour deposition (CVD) diamond film growth was analyzed. Silicon nitride substrates were obtained by pressureless sintering, placing the green samples inside a powder bed of Si 3 N 4 /BN. Local variations in the sintering atmosphere led to samples with different grey colouration as well as chemical and physical characteristics, determined by X-ray diffraction and thermal conductivity tests, which affected the diamond film growth. A complete characterization of the films, including thickness, average crystal size, surface roughness, texture and adhesion, was done. The Si 3 N 4 substrate with glassier phase gave thicker diamond films, with smaller crystal sizes and better film adhesion to the substrate than the diamond films grown on ceramic substrates with less vitreous phase.