Evaluation of boron-doped-microcrystalline/nanocrystalline diamond composite coatings in drilling of CFRP

The drilling performance of four typical types of single-layer diamond coated drills and two-layer boron-doped-microcrystalline/nanocrystalline diamond (BDMC-NCD) composite coated drill is compared, and the growth duration ratio of the NCD layer to the BDMCD layer (N/B) is optimized. It is demonstrated that the BDMC-NCD film synthesized using proper N/B shows nice adhesion, moderate diamond quality and a smooth surface mostly composed of nano-sized diamond grains, so it shows the best comprehensive performance, including the thrust force reduction, the improvement of the hole quality, the slowing of the tool wear and coating peeling, and the elongation of the tool lifetime. Taking the time of the coating peeling into consideration, 3/9 is the optimum N/B for the BDMC-NCD coated drill, and such the drill also performs the longest lifetime using delamination factors (F-d) of 1.2 or 1.3 as the failure criteria. Moreover, the failure mode of the diamond coated drill is the flank wear when using F-d = 1.1 or 1.2 as the failure criteria, while the tool failure should be related to the coating peeling when F-d = 1.3.