Abstract Silicon nitride cutting tool inserts with tool geometry TPGN 433 were pre-treated ultrasonically with different procedures before diamond deposition by microwave plasma chemical vapor deposition. The nucleation densities and deposition rates were compared with those of untreated inserts and inserts polished with diamond paste. An increase in diamond nucleation density was observed for samples with different pre-treatments relative to untreated substrates. A continuous diamond coating was obtained after deposition for 1 h for the sample polished with diamond paste before deposition while only discrete diamond crystals were obtained for samples without any pre-treatment. However, no significant differences in diamond crystal size, morphology and deposition rates were observed for different pre-treatments after long periods of deposition time. No significant difference in tool life was observed for the diamond-coated inserts with and without pre-treatment when tested with an Al-Si alloy.