Cutting Performances of MCD, SMCD, NCD and MCD/NCD Coated Tools in High-speed Milling of Hot Bending Graphite Molds

Abstract The machining accuracy and service life of high-speed milling tools have a great influence on the surface roughness and dimensional precision of hot bending graphite molds used in the manufacture of 3D curved glass such as protective screens and rear covers of 3C electronic products. The microcrystalline diamond (MCD), submicrocrystalline diamond (SMCD), nanocrystalline diamond (NCD) and micro/nano-crystalline composite diamond (MCD/NCD) coatings were deposited on commercial corner radius end milling tools by using a hot filament chemical vapor deposition (HFCVD) equipment. The morphologies, phase structures and adhesive strength of various types of diamond coatings were systematically tested by Field emission scanning electron microscopy (FESEM), Raman spectroscopy and Rockwell indentation tests respectively. Besides, the tribological performances of various diamond coatings were assessed by the ball-on-disk rotational friction tester. Composite MCD/NCD coating exhibits good friction properties, superior wear resistance and satisfactory material removal rate during the process of sliding against graphite. Further high-speed milling experiments of graphite using bare WC-Co, MCD, SMCD, NCD and MCD/NCD coated milling tools were carried out to characterize their corresponding cutting performances and wear mechanism. Tool wear progressions of different kinds of milling tools with the cutting length were obtained. Moreover, tool wear morphologies were acquired by FESEM and machined surface roughness of graphite were measured by 3D surface profile meter. Results indicate that MCD/NCD coated milling tool exhibits excellent cutting performances compared to the other milling tools due to the comprehensive properties of better surface smoothness and higher adhesive strength with WC-Co substrates. What’s more, the machining dimensional accuracy and machined surface roughness during high-speed milling of hot bending graphite molds using MCD/NCD coated milling tool could meet the strict requirements of 3C electronic products even without the need or with less need for subsequent lapping and polishing.