Erosive wear resistance of NCD coatings produced by pulsed microwave discharges

Erosion tests on nanocrystalline diamond (NCD) films are relevant not only for the evaluation of the erosive wear resistance, anticipating applications where coated materials are exposed to particle impacts, but also as a way to evaluate their adhesion to the substrates. NCD films were grown on Si3N4 ceramic by microwave plasma assisted deposition in continuous (CW) and pulsed (PW-50 Hz and PW-500 Hz) discharge modes in argon-rich gas mixture. The films grown in PW modes presented lower crystallite size and lower surface roughness than those grown in CW one, while the use of CF4 plasma pre-treatment of the substrate lead to further film homogeneity. The erosive wear resistance of NCD was evaluated by solid particle impact using SiC (45–250 μm size) as erodent material, with selected parameters accordingly to Hertzian stress field calculations. Film weight loss was undetectable until delamination took place. When tested with 150 μm SiC particles, the CF4 plasma pre-treated substrates yield a three-fold increase (15 min) in delamination time comparing to untreated specimens, while samples coated under PW-50 Hz conditions presented a six times lower erosion rate compared to CW ones. It is believed that the improved nucleation behaviour by the use of PW mode and its higher homogeneity on the CF4 plasma pre-treated samples decrease the flaw population on the diamond/substrate interface, leading to improved adhesion levels.