Optimization of diamond coated microdrills in aluminum alloy 7075 machining: A case study

• MCD and NCD films are deposited on balls and microdrills by HFCVD.
• The tribological properties between diamond films and aluminum alloy are studied.
• The cutting performance of MCD, NCD, DLC, and TiAlN coated microdrills is compared.
• NCD coated microdrill exhibits the longest working life in aluminum alloy machining.
• The NCD film thickness on microdrill is further optimized to be 4.5 μm.

Aluminum alloy 7075 is widely used for producing micro-scale heat sinks, micro-fluidic devices, micro-propellers and so on. This paper deals with optimizing microstructure and thickness of diamond coatings on microdrills used in 7075 aluminum alloy machining. Firstly, the friction tests between microcrystalline diamond (MCD), nanocrystalline diamond (NCD) films and aluminum alloy reveal that the stable coefficient of friction (COF) of MCD–aluminum alloy working pair is 0.240, much higher than that of NCD–aluminum alloy working pair (0.072). The decrease of COF is mainly attributed to the lower roughness of NCD films and the presence of more graphite or the non-diamond phases in NCD coatings. Afterwards, comparative cutting tests involving MCD, NCD, diamond-like coating (DLC) and TiAlN coated microdrills show that after drilling 200 holes, NCD coated microdrills exhibit the best cutting performance. Furthermore, NCD coated microdrills with coating thicknesses of 1 μm, 2 μm, 4.5 μm and 7 μm are fabricated and their cutting performance is studied in aluminum alloy machining. The cutting experiments show that the NCD coated microdrill with coating thickness of 4.5 μm shows the best cutting performance, exhibiting not only lowest flank wear and no tool tipping or chipping on the main cutting edges but also the highest quality of drilled holes because of the outstanding adhesive strength and wear resistance of the NCD coating.